FreeBSD Handbook - Chapter 30 Firewalls
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30.1 Introduction
Firewalls make it possible to filter incoming and outgoing traffic that flows through your system. A firewall can use one or more sets of “rules” to inspect the network packets as they come in or go out of your network connections and either allows the traffic through or blocks it. The rules of a firewall can inspect one or more characteristics of the packets, including but not limited to the protocol type, the source or destination host address, and the source or destination port.
Firewalls can greatly enhance the security of a host or a network. They can be used to do one or more of the following things:
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To protect and insulate the applications, services and machines of your internal network from unwanted traffic coming in from the public Internet.
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To limit or disable access from hosts of the internal network to services of the public Internet.
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To support network address translation (NAT), which allows your internal network to use private IP addresses and share a single connection to the public Internet (either with a single IP address or by a shared pool of automatically assigned public addresses).
After reading this chapter, you will know:
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How to properly define packet filtering rules.
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The differences between the firewalls built into FreeBSD.
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How to use and configure the OpenBSD PF firewall.
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How to use and configure IPFILTER.
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How to use and configure IPFW.
Before reading this chapter, you should:
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Understand basic FreeBSD and Internet concepts.
30.2 Firewall Concepts
There are two basic ways to create firewall rulesets: “inclusive” or “exclusive”. An exclusive firewall allows all traffic through except for the traffic matching the ruleset. An inclusive firewall does the reverse. It only allows traffic matching the rules through and blocks everything else.
Inclusive firewalls are generally safer than exclusive firewalls because they significantly reduce the risk of allowing unwanted traffic to pass through the firewall.
Security can be tightened further using a “stateful firewall”. With a stateful firewall the firewall keeps track of which connections are opened through the firewall and will only allow traffic through which either matches an existing connection or opens a new one. The disadvantage of a stateful firewall is that it can be vulnerable to Denial of Service (DoS) attacks if a lot of new connections are opened very fast. With most firewalls it is possible to use a combination of stateful and non-stateful behavior to make an optimal firewall for the site.
30.3 Firewall Packages
FreeBSD has three different firewall packages built into the base system. They are: IPFILTER (also known as IPF), IPFIREWALL (also known as IPFW), and OpenBSD’s PacketFilter (also known as PF). FreeBSD also has two built in packages for traffic shaping (basically controlling bandwidth usage): altq(4) and dummynet(4). Dummynet has traditionally been closely tied with IPFW, and ALTQ with PF. Traffic shaping for IPFILTER can currently be done with IPFILTER for NAT and filtering and IPFW with dummynet(4) or by using PF with ALTQ. IPFW, and PF all use rules to control the access of packets to and from your system, although they go about it different ways and have different rule syntaxes.
The reason that FreeBSD has multiple built in firewall packages is that different people have different requirements and preferences. No single firewall package is the best.
The author prefers IPFILTER because its stateful rules are much less complicated to use in a NAT environment and it has a built in ftp proxy that simplifies the rules to allow secure outbound FTP usage.
Since all firewalls are based on inspecting the values of selected packet control fields, the creator of the firewall rulesets must have an understanding of how TCP/IP works, what the different values in the packet control fields are and how these values are used in a normal session conversation. For a good explanation go to: http://www.ipprimer.com/overview.cfm.
30.4 The OpenBSD Packet Filter (PF) and ALTQ
Revised and updated by John Ferrell.
As of July 2003 the OpenBSD firewall software application known as PF was ported to FreeBSD and made available in the FreeBSD Ports Collection. Released in 2004, FreeBSD 5.3 was the first release that contained PF as an integrated part of the base system. PF is a complete, full-featured firewall that has optional support for ALTQ (Alternate Queuing). ALTQ provides Quality of Service (QoS) functionality.
The OpenBSD Project does an outstanding job of maintaining the PF FAQ. As such, this section of the Handbook will focus on PF as it pertains to FreeBSD while providing some general information regarding usage. For detailed usage information please refer to the PF FAQ.
More information about PF for FreeBSD can be found at http://pf4freebsd.love2party.net/.
30.4.1 Using the PF loadable kernel module
Since the release of FreeBSD 5.3, PF has been included in the basic install as a separate run time loadable module. The system will dynamically load the PF kernel module when the rc.conf(5) statement pf_enable="YES" is present. However, the PF module will not load if the system cannot find a PF ruleset configuration file. The default location is /etc/pf.conf. If your PF ruleset is located somewhere else put pf_rules="/path/pf.rules" to your /etc/rc.conf configuration file to specify the location.
Note: As of FreeBSD 7.0 the sample pf.conf that was in /etc/ has been moved to /usr/share/examples/pf/. For FreeBSD versions prior to 7.0 there is an /etc/pf.conf by default.
The PF module can also be loaded manually from the command line:
# kldload pf.ko
The loadable module was created with pflog(4) enabled which provides support for logging. If you need other PF features you will need to compile PF support into the kernel.
30.4.2 PF kernel options
While it is not necessary that you compile PF support into the FreeBSD kernel, you may want to do so to take advantage of one of PF’s advanced features that is not included in the loadable module, namely pfsync(4), which is a pseudo-device that exposes certain changes to the state table used by PF. It can be paired with carp(4) to create failover firewalls using PF. More information on CARP can be found in chapter 29 of the handbook.
The PF kernel options can be found in /usr/src/sys/conf/NOTES and are reproduced below:
device pf device pflog device pfsync
The device pf option enables support for the “Packet Filter” firewall (pf(4)).
The device pflog option enables the optional pflog(4) pseudo network device which can be used to log traffic to a bpf(4) descriptor. The pflogd(8) daemon can be used to store the logging information to disk.
The device pfsync option enables the optional pfsync(4) pseudo-network device that is used to monitor “state changes”.
30.4.3 Available rc.conf Options
The following rc.conf(5) statements configure PF and pflog(4) at boot:
pf_enable="YES" # Enable PF (load module if required) pf_rules="/etc/pf.conf" # rules definition file for pf pf_flags="" # additional flags for pfctl startup pflog_enable="YES" # start pflogd(8) pflog_logfile="/var/log/pflog" # where pflogd should store the logfile pflog_flags="" # additional flags for pflogd startup
If you have a LAN behind this firewall and have to forward packets for the computers on the LAN or want to do NAT, you will need the following option as well:
gateway_enable="YES" # Enable as LAN gateway
30.4.4 Creating Filtering Rules
PF reads its configuration rules from pf.conf(5) (/etc/pf.conf by default) and it modifies, drops, or passes packets according to the rules or definitions specified there. The FreeBSD installation includes several sample files located in /usr/share/examples/pf/. Please refer to the PF FAQ for complete coverage of PF rulesets.
Warning: When browsing the PF FAQ, please keep in mind that different versions of FreeBSD contain different versions of PF:
The FreeBSD packet filter mailing list is a good place to ask questions about configuring and running the PF firewall. Do not forget to check the mailing list archives before asking questions!
30.4.5 Working with PF
Use pfctl(8) to control PF. Below are some useful commands (be sure to review the pfctl(8) man page for all available options):
Command
Purpose
pfctl -e
Enable PF
pfctl -d
Disable PF
pfctl -F all -f /etc/pf.conf
Flush all rules (nat, filter, state, table, etc.) and reload from the file /etc/pf.conf
pfctl -s [ rules | nat | state ]
Report on the filter rules, nat rules, or state table
pfctl -vnf /etc/pf.conf
Check /etc/pf.conf for errors, but do not load ruleset
30.4.6 Enabling ALTQ
ALTQ is only available by compiling support for it into the FreeBSD kernel. ALTQ is not supported by all of the available network card drivers. Please see the altq(4) manual page for a list of drivers that are supported in your release of FreeBSD.
The following kernel options will enable ALTQ and add additional functionality:
options ALTQ options ALTQ_CBQ # Class Bases Queuing (CBQ) options ALTQ_RED # Random Early Detection (RED) options ALTQ_RIO # RED In/Out options ALTQ_HFSC # Hierarchical Packet Scheduler (HFSC) options ALTQ_PRIQ # Priority Queuing (PRIQ) options ALTQ_NOPCC # Required for SMP build
options ALTQ enables the ALTQ framework.
options ALTQ_CBQ enables Class Based Queuing (CBQ). CBQ allows you to divide a connection’s bandwidth into different classes or queues to prioritize traffic based on filter rules.
options ALTQ_RED enables Random Early Detection (RED). RED is used to avoid network congestion. RED does this by measuring the length of the queue and comparing it to the minimum and maximum thresholds for the queue. If the queue is over the maximum all new packets will be dropped. True to its name, RED drops packets from different connections randomly.
options ALTQ_RIO enables Random Early Detection In and Out.
options ALTQ_HFSC enables the Hierarchical Fair Service Curve Packet Scheduler. For more information about HFSC see: http://www-2.cs.cmu.edu/~hzhang/HFSC/main.html.
options ALTQ_PRIQ enables Priority Queuing (PRIQ). PRIQ will always pass traffic that is in a higher queue first.
options ALTQ_NOPCC enables SMP support for ALTQ. This option is required on SMP systems.
30.5 The IPFILTER (IPF) Firewall
Note: This section is work in progress. The contents might not be accurate at all times.
The author of IPFILTER is Darren Reed. IPFILTER is not operating system dependent: it is an open source application and has been ported to FreeBSD, NetBSD, OpenBSD, SunOS, HP/UX, and Solaris operating systems. IPFILTER is actively being supported and maintained, with updated versions being released regularly.
IPFILTER is based on a kernel-side firewall and NAT mechanism that can be controlled and monitored by userland interface programs. The firewall rules can be set or deleted with the ipf(8) utility. The NAT rules can be set or deleted with the ipnat(1) utility. The ipfstat(8) utility can print run-time statistics for the kernel parts of IPFILTER. The ipmon(8) program can log IPFILTER actions to the system log files.
IPF was originally written using a rule processing logic of “the last matching rule wins” and used only stateless type of rules. Over time IPF has been enhanced to include a “quick” option and a stateful “keep state” option which drastically modernized the rules processing logic. IPF’s official documentation covers the legacy rule coding parameters and the legacy rule file processing logic. The modernized functions are only included as additional options, completely understating their benefits in producing a far superior secure firewall.
The instructions contained in this section are based on using rules that contain the “quick” option and the stateful “keep state” option. This is the basic framework for coding an inclusive firewall rule set.
An inclusive firewall only allows packets matching the rules to pass through. This way you can control what services can originate behind the firewall destined for the public Internet and also control the services which can originate from the public Internet accessing your private network. Everything else is blocked and logged by default design. Inclusive firewalls are much, much more secure than exclusive firewall rule sets and is the only rule set type covered herein.
For detailed explanation of the legacy rules processing method see: http://www.obfuscation.org/ipf/ipf-howto.html#TOC_1 and http://coombs.anu.edu.au/~avalon/ip-filter.html.
The IPF FAQ is at http://www.phildev.net/ipf/index.html.
A searchable archive of the open-source IPFilter mailing list is available at http://marc.theaimsgroup.com/?l=ipfilter.
30.5.1 Enabling IPF
IPF is included in the basic FreeBSD install as a separate run time loadable module. The system will dynamically load the IPF kernel loadable module when the rc.conf statement ipfilter_enable="YES" is used. The loadable module was created with logging enabled and the default pass all options. You do not need to compile IPF into the FreeBSD kernel just to change the default to block all, you can do that by just coding a block all rule at the end of your rule set.
30.5.2 Kernel options
It is not a mandatory requirement that you enable IPF by compiling the following options into the FreeBSD kernel. It is only presented here as background information. Compiling IPF into the kernel causes the loadable module to never be used.
Sample kernel config IPF option statements are in the /usr/src/sys/conf/NOTES kernel source and are reproduced here:
options IPFILTER options IPFILTER_LOG options IPFILTER_DEFAULT_BLOCK
options IPFILTER enables support for the “IPFILTER” firewall.
options IPFILTER_LOG enables the option to have IPF log traffic by writing to the ipl packet logging pseudo–device for every rule that has the log keyword.
options IPFILTER_DEFAULT_BLOCK changes the default behavior so any packet not matching a firewall pass rule gets blocked.
These settings will take effect only after you have built and installed a kernel with them set.
30.5.3 Available rc.conf Options
You need the following statements in /etc/rc.conf to activate IPF at boot time:
ipfilter_enable="YES" # Start ipf firewall
ipfilter_rules="/etc/ipf.rules" # loads rules definition text file
ipmon_enable="YES" # Start IP monitor log
ipmon_flags="-Ds" # D = start as daemon
# s = log to syslog
# v = log tcp window, ack, seq
# n = map IP & port to names
If you have a LAN behind this firewall that uses the reserved private IP address ranges, then you need to add the following to enable NAT functionality:
gateway_enable="YES" # Enable as LAN gateway ipnat_enable="YES" # Start ipnat function ipnat_rules="/etc/ipnat.rules" # rules definition file for ipnat
30.5.4 IPF
The ipf command is used to load your rules file. Normally you create a file containing your custom rules and use this command to replace in mass the currently running firewall internal rules:
# ipf -Fa -f /etc/ipf.rules
-Fa means flush all internal rules tables.
-f means this is the file to read for the rules to load.
This gives you the ability to make changes to your custom rules file, run the above IPF command, and thus update the running firewall with a fresh copy of all the rules without having to reboot the system. This method is very convenient for testing new rules as the procedure can be executed as many times as needed.
See the ipf(8) manual page for details on the other flags available with this command.
The ipf(8) command expects the rules file to be a standard text file. It will not accept a rules file written as a script with symbolic substitution.
There is a way to build IPF rules that utilizes the power of script symbolic substitution. For more information, see Section 30.5.9.
30.5.5 IPFSTAT
The default behavior of ipfstat(8) is to retrieve and display the totals of the accumulated statistics gathered as a result of applying the user coded rules against packets going in and out of the firewall since it was last started, or since the last time the accumulators were reset to zero by the ipf -Z command.
See the ipfstat(8) manual page for details.
The default ipfstat(8) command output will look something like this:
input packets: blocked 99286 passed 1255609 nomatch 14686 counted 0 output packets: blocked 4200 passed 1284345 nomatch 14687 counted 0 input packets logged: blocked 99286 passed 0 output packets logged: blocked 0 passed 0 packets logged: input 0 output 0 log failures: input 3898 output 0 fragment state(in): kept 0 lost 0 fragment state(out): kept 0 lost 0 packet state(in): kept 169364 lost 0 packet state(out): kept 431395 lost 0 ICMP replies: 0 TCP RSTs sent: 0 Result cache hits(in): 1215208 (out): 1098963 IN Pullups succeeded: 2 failed: 0 OUT Pullups succeeded: 0 failed: 0 Fastroute successes: 0 failures: 0 TCP cksum fails(in): 0 (out): 0 Packet log flags set: (0)
When supplied with either -i for inbound or -o for outbound, it will retrieve and display the appropriate list of filter rules currently installed and in use by the kernel.
ipfstat -in displays the inbound internal rules table with rule number.
ipfstat -on displays the outbound internal rules table with the rule number.
The output will look something like this:
@1 pass out on xl0 from any to any @2 block out on dc0 from any to any @3 pass out quick on dc0 proto tcp/udp from any to any keep state
ipfstat -ih displays the inbound internal rules table, prefixing each rule with a count of how many times the rule was matched.
ipfstat -oh displays the outbound internal rules table, prefixing each rule with a count of how many times the rule was matched.
The output will look something like this:
2451423 pass out on xl0 from any to any 354727 block out on dc0 from any to any 430918 pass out quick on dc0 proto tcp/udp from any to any keep state
One of the most important functions of the ipfstat command is the -t flag which displays the state table in a way similar to the way top(1) shows the FreeBSD running process table. When your firewall is under attack this function gives you the ability to identify, drill down to, and see the attacking packets. The optional sub-flags give the ability to select the destination or source IP, port, or protocol that you want to monitor in real time. See the ipfstat(8) manual page for details.
30.5.6 IPMON
In order for ipmon to work properly, the kernel option IPFILTER_LOG must be turned on. This command has two different modes that it can be used in. Native mode is the default mode when you type the command on the command line without the -D flag.
Daemon mode is for when you want to have a continuous system log file available so that you can review logging of past events. This is how FreeBSD and IPFILTER are configured to work together. FreeBSD has a built in facility to automatically rotate system logs. That is why outputting the log information to syslogd is better than the default of outputting to a regular file. In the default rc.conf file you see the ipmon_flags statement uses the -Ds flags:
ipmon_flags="-Ds" # D = start as daemon
# s = log to syslog
# v = log tcp window, ack, seq
# n = map IP & port to names
The benefits of logging are obvious. It provides the ability to review, after the fact, information such as which packets had been dropped, what addresses they came from and where they were going. These all give you a significant edge in tracking down attackers.
Even with the logging facility enabled, IPF will not generate any rule logging on its own. The firewall administrator decides what rules in the rule set he wants to log and adds the log keyword to those rules. Normally only deny rules are logged.
It is very customary to include a default deny everything rule with the log keyword included as your last rule in the rule set. This way you get to see all the packets that did not match any of the rules in the rule set.
30.5.7 IPMON Logging
Syslogd uses its own special method for segregation of log data. It uses special groupings called “facility” and “level”. IPMON in -Ds mode uses security as the “facility” name. All IPMON logged data goes to security The following levels can be used to further segregate the logged data if desired:
LOG_INFO - packets logged using the "log" keyword as the action rather than pass or block. LOG_NOTICE - packets logged which are also passed LOG_WARNING - packets logged which are also blocked LOG_ERR - packets which have been logged and which can be considered short
To setup IPFILTER to log all data to /var/log/ipfilter.log, you will need to create the file. The following command will do that:
# touch /var/log/ipfilter.log
The syslog function is controlled by definition statements in the /etc/syslog.conf file. The syslog.conf file offers considerable flexibility in how syslog will deal with system messages issued by software applications like IPF.
Add the following statement to /etc/syslog.conf:
security.* /var/log/ipfilter.log
The security.* means to write all the logged messages to the coded file location.
To activate the changes to /etc/syslog.conf you can reboot or bump the syslog task into re-reading /etc/syslog.conf by running /etc/rc.d/syslogd reload
Do not forget to change /etc/newsyslog.conf to rotate the new log you just created above.
30.5.8 The Format of Logged Messages
Messages generated by ipmon consist of data fields separated by white space. Fields common to all messages are:
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The date of packet receipt.
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The time of packet receipt. This is in the form HH:MM:SS.F, for hours, minutes, seconds, and fractions of a second (which can be several digits long).
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The name of the interface the packet was processed on, e.g. dc0.
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The group and rule number of the rule, e.g. @0:17.
These can be viewed with ipfstat -in.
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The action: p for passed, b for blocked, S for a short packet, n did not match any rules, L for a log rule. The order of precedence in showing flags is: S, p, b, n, L. A capital P or B means that the packet has been logged due to a global logging setting, not a particular rule.
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The addresses. This is actually three fields: the source address and port (separated by a comma), the -> symbol, and the destination address and port. 209.53.17.22,80 -> 198.73.220.17,1722.
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PR followed by the protocol name or number, e.g. PR tcp.
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len followed by the header length and total length of the packet, e.g. len 20 40.
If the packet is a TCP packet, there will be an additional field starting with a hyphen followed by letters corresponding to any flags that were set. See the ipmon(8) manual page for a list of letters and their flags.
If the packet is an ICMP packet, there will be two fields at the end, the first always being “ICMP”, and the next being the ICMP message and sub-message type, separated by a slash, e.g. ICMP 3/3 for a port unreachable message.
30.5.9 Building the Rule Script with Symbolic Substitution
Some experienced IPF users create a file containing the rules and code them in a manner compatible with running them as a script with symbolic substitution. The major benefit of doing this is that you only have to change the value associated with the symbolic name and when the script is run all the rules containing the symbolic name will have the value substituted in the rules. Being a script, you can use symbolic substitution to code frequently used values and substitute them in multiple rules. You will see this in the following example.
The script syntax used here is compatible with the sh, csh, and tcsh shells.
Symbolic substitution fields are prefixed with a dollar sign: $.
Symbolic fields do not have the $ prefix.
The value to populate the symbolic field must be enclosed with double quotes (").
Start your rule file with something like this:
############# Start of IPF rules script ######################## oif="dc0" # name of the outbound interface odns="192.0.2.11" # ISP's DNS server IP address myip="192.0.2.7" # my static IP address from ISP ks="keep state" fks="flags S keep state" # You can choose between building /etc/ipf.rules file # from this script or running this script "as is". # # Uncomment only one line and comment out another. # # 1) This can be used for building /etc/ipf.rules: #cat > /etc/ipf.rules << EOF # # 2) This can be used to run script "as is": /sbin/ipf -Fa -f - << EOF # Allow out access to my ISP's Domain name server. pass out quick on $oif proto tcp from any to $odns port = 53 $fks pass out quick on $oif proto udp from any to $odns port = 53 $ks # Allow out non-secure standard www function pass out quick on $oif proto tcp from $myip to any port = 80 $fks # Allow out secure www function https over TLS SSL pass out quick on $oif proto tcp from $myip to any port = 443 $fks EOF ################## End of IPF rules script ########################
That is all there is to it. The rules are not important in this example; how the symbolic substitution fields are populated and used are. If the above example was in a file named /etc/ipf.rules.script, you could reload these rules by entering the following command:
# sh /etc/ipf.rules.script
There is one problem with using a rules file with embedded symbolics: IPF does not understand symbolic substitution, and cannot read such scripts directly.
This script can be used in one of two ways:
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Uncomment the line that begins with cat, and comment out the line that begins with /sbin/ipf. Place ipfilter_enable="YES" into /etc/rc.conf as usual, and run script once after each modification to create or update /etc/ipf.rules.
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Disable IPFILTER in system startup scripts by adding ipfilter_enable="NO" (this is default value) into /etc/rc.conf file.
Add a script like the following to your /usr/local/etc/rc.d/ startup directory. The script should have an obvious name like ipf.loadrules.sh. The .sh extension is mandatory.
#!/bin/sh sh /etc/ipf.rules.script
The permissions on this script file must be read, write, execute for owner root.
# chmod 700 /usr/local/etc/rc.d/ipf.loadrules.sh
Now, when your system boots, your IPF rules will be loaded.
30.5.10 IPF Rule Sets
A rule set is a group of ipf rules coded to pass or block packets based on the values contained in the packet. The bi-directional exchange of packets between hosts comprises a session conversation. The firewall rule set processes the packet two times, once on its arrival from the public Internet host and again as it leaves for its return trip back to the public Internet host. Each TCP/IP service (i.e. telnet, www, mail, etc.) is predefined by its protocol, source and destination IP address, or the source and destination port number. This is the basic selection criteria used to create rules which will pass or block services.
IPF was originally written using a rules processing logic of “the last matching rule wins” and used only stateless rules. Over time IPF has been enhanced to include a “quick” option and a stateful “keep state” option which drastically modernized the rule processing logic.
The instructions contained in this section are based on using rules that contain the “quick” option and the stateful “keep state” option. This is the basic framework for coding an inclusive firewall rule set.
An inclusive firewall only allows services matching the rules through. This way you can control what services can originate behind the firewall destined for the public Internet and also control the services which can originate from the public Internet accessing your private network. Everything else is blocked and logged by default design. Inclusive firewalls are much, much securer than exclusive firewall rule sets and is the only rule set type covered herein.
Warning: When working with the firewall rules, be very careful. Some configurations will lock you out of the server. To be on the safe side, you may wish to consider performing the initial firewall configuration from the local console rather than doing it remotely e.g. via ssh.
30.5.11 Rule Syntax
The rule syntax presented here has been simplified to only address the modern stateful rule context and “first matching rule wins” logic. For the complete legacy rule syntax description see the ipf(8) manual page.
A # character is used to mark the start of a comment and may appear at the end of a rule line or on its own line. Blank lines are ignored.
Rules contain keywords. These keywords have to be coded in a specific order from left to right on the line. Keywords are identified in bold type. Some keywords have sub-options which may be keywords themselves and also include more sub-options. Each of the headings in the below syntax has a bold section header which expands on the content.
ACTION IN-OUT OPTIONS SELECTION STATEFUL PROTO SRC_ADDR,DST_ADDR OBJECT PORT_NUM TCP_FLAG STATEFUL
ACTION = block | pass
IN-OUT = in | out
OPTIONS = log | quick | on interface-name
SELECTION = proto value | source/destination IP | port = number | flags flag-value
PROTO = tcp/udp | udp | tcp | icmp
SRC_ADD,DST_ADDR = all | from object to object
OBJECT = IP address | any
PORT_NUM = port number
TCP_FLAG = S
STATEFUL = keep state
30.5.11.1 ACTION
The action indicates what to do with the packet if it matches the rest of the filter rule. Each rule must have a action. The following actions are recognized:
block indicates that the packet should be dropped if the selection parameters match the packet.
pass indicates that the packet should exit the firewall if the selection parameters match the packet.
30.5.11.2 IN-OUT
A mandatory requirement is that each filter rule explicitly state which side of the I/O it is to be used on. The next keyword must be either in or out and one or the other has to be coded or the rule will not pass syntax checks.
in means this rule is being applied against an inbound packet which has just been received on the interface facing the public Internet.
out means this rule is being applied against an outbound packet destined for the interface facing the public Internet.
30.5.11.3 OPTIONS
Note: These options must be used in the order shown here.
log indicates that the packet header will be written to the ipl log (as described in the LOGGING section below) if the selection parameters match the packet.
quick indicates that if the selection parameters match the packet, this rule will be the last rule checked, allowing a “short-circuit” path to avoid processing any following rules for this packet. This option is a mandatory requirement for the modernized rules processing logic.
on indicates the interface name to be incorporated into the selection parameters. Interface names are as displayed by ifconfig(8). Using this option, the rule will only match if the packet is going through that interface in the specified direction (in/out). This option is a mandatory requirement for the modernized rules processing logic.
When a packet is logged, the headers of the packet are written to the IPL packet logging pseudo-device. Immediately following the log keyword, the following qualifiers may be used (in this order):
body indicates that the first 128 bytes of the packet contents will be logged after the headers.
first If the log keyword is being used in conjunction with a “keep state” option, it is recommended that this option is also applied so that only the triggering packet is logged and not every packet which thereafter matches the “keep state” information.
30.5.11.4 SELECTION
The keywords described in this section are used to describe attributes of the packet to be interrogated when determining whether rules match or not. There is a keyword subject, and it has sub-option keywords, one of which has to be selected. The following general-purpose attributes are provided for matching, and must be used in this order:
30.5.11.5 PROTO
proto is the subject keyword and must be coded along with one of its corresponding keyword sub-option values. The value allows a specific protocol to be matched against. This option is a mandatory requirement for the modernized rules processing logic.
tcp/udp | udp | tcp | icmp or any protocol names found in /etc/protocols are recognized and may be used. The special protocol keyword tcp/udp may be used to match either a TCP or a UDP packet, and has been added as a convenience to save duplication of otherwise identical rules.
30.5.11.6 SRC_ADDR/DST_ADDR
The all keyword is essentially a synonym for “from any to any” with no other match parameters.
from src to dst: the from and to keywords are used to match against IP addresses. Rules must specify BOTH source and destination parameters. any is a special keyword that matches any IP address. Examples of use: “from any to any” or “from 0.0.0.0/0 to any” or “from any to 0.0.0.0/0” or “from 0.0.0.0 to any” or “from any to 0.0.0.0”.
IP addresses may be specified as a dotted IP address numeric form/mask-length, or as single dotted IP address numeric form.
There is no way to match ranges of IP addresses which do not express themselves easily as mask-length. See this web page for help on writing mask-length: http://jodies.de/ipcalc.
30.5.11.7 PORT
If a port match is included, for either or both of source and destination, then it is only applied to TCP and UDP packets. When composing port comparisons, either the service name from /etc/services or an integer port number may be used. When the port appears as part of the from object, it matches the source port number; when it appears as part of the to object, it matches the destination port number. The use of the port option with the to object is a mandatory requirement for the modernized rules processing logic. Example of use: “from any to any port = 80”
Port comparisons may be done in a number of forms, with a number of comparison operators, or port ranges may be specified.
port "=" | "!=" | "<" | ">" | "<=" | ">=" | "eq" | "ne" | "lt" | "gt" | "le" | "ge".
To specify port ranges, port "<>" | "><"
Warning: Following the source and destination matching parameters, the following two parameters are mandatory requirements for the modernized rules processing logic.
30.5.11.8 TCP_FLAG
Flags are only effective for TCP filtering. The letters represents one of the possible flags that can be interrogated in the TCP packet header.
The modernized rules processing logic uses the flags S parameter to identify the tcp session start request.
30.5.11.9 STATEFUL
keep state indicates that on a pass rule, any packets that match the rules selection parameters should activate the stateful filtering facility.
Note: This option is a mandatory requirement for the modernized rules processing logic.
30.5.12 Stateful Filtering
Stateful filtering treats traffic as a bi-directional exchange of packets comprising a session conversation. When activated, keep-state dynamically generates internal rules for each anticipated packet being exchanged during the bi-directional session conversation. It has the interrogation abilities to determine if the session conversation between the originating sender and the destination are following the valid procedure of bi-directional packet exchange. Any packets that do not properly fit the session conversation template are automatically rejected as impostors.
Keep state will also allow ICMP packets related to a TCP or UDP session through. So if you get ICMP type 3 code 4 in response to some web surfing allowed out by a keep state rule, they will be automatically allowed in. Any packet that IPF can be certain is part of an active session, even if it is a different protocol, will be let in.
What happens is:
Packets destined to go out the interface connected to the public Internet are first checked against the dynamic state table, if the packet matches the next expected packet comprising in a active session conversation, then it exits the firewall and the state of the session conversation flow is updated in the dynamic state table, the remaining packets get checked against the outbound rule set.
Packets coming in to the interface connected to the public Internet are first checked against the dynamic state table, if the packet matches the next expected packet comprising a active session conversation, then it exits the firewall and the state of the session conversation flow is updated in the dynamic state table, the remaining packets get checked against the inbound rule set.
When the conversation completes it is removed from the dynamic state table.
Stateful filtering allows you to focus on blocking/passing new sessions. If the new session is passed, all its subsequent packets will be allowed through automatically and any impostors automatically rejected. If a new session is blocked, none of its subsequent packets will be allowed through. Stateful filtering has technically advanced interrogation abilities capable of defending against the flood of different attack methods currently employed by attackers.
30.5.13 Inclusive Rule Set Example
The following rule set is an example of how to code a very secure inclusive type of firewall. An inclusive firewall only allows services matching pass rules through and blocks all other by default. All firewalls have at the minimum two interfaces which have to have rules to allow the firewall to function.
All UNIX flavored systems including FreeBSD are designed to use interface lo0 and IP address 127.0.0.1 for internal communication within the operating system. The firewall rules must contain rules to allow free unmolested movement of these special internally used packets.
The interface which faces the public Internet is the one where you place your rules to authorize and control access out to the public Internet and access requests arriving from the public Internet. This can be your user PPP tun0 interface or your NIC that is connected to your DSL or cable modem.
In cases where one or more NICs are cabled to private LANs behind the firewall, those interfaces must have a rule coded to allow free unmolested movement of packets originating from those LAN interfaces.
The rules should be first organized into three major sections: all the free unmolested interfaces, the public interface outbound, and the public interface inbound.
The rules in each of the public interface sections should have the most frequently matched rules placed before less commonly matched rules, with the last rule in the section blocking and logging all packets on that interface and direction.
The Outbound section in the following rule set only contains ‘pass’ rules which contain selection values that uniquely identify the service that is authorized for public Internet access. All the rules have the ‘quick’, ‘on’, ‘proto’, ‘port’, and ‘keep state’ option coded. The ‘proto tcp’ rules have the ‘flag’ option included to identify the session start request as the triggering packet to activate the stateful facility.
The Inbound section has all the blocking of undesirable packets first, for two different reasons. The first is that these things being blocked may be part of an otherwise valid packet which may be allowed in by the later authorized service rules. The second reason is that by having a rule that explicitly blocks selected packets that I receive on an infrequent basis and that I do not want to see in the log, they will not be caught by the last rule in the section which blocks and logs all packets which have fallen through the rules. The last rule in the section which blocks and logs all packets is how you create the legal evidence needed to prosecute the people who are attacking your system.
Another thing you should take note of, is there is no response returned for any of the undesirable stuff, their packets just get dropped and vanish. This way the attacker has no knowledge if his packets have reached your system. The less the attackers can learn about your system, the more time they must invest before actually doing something bad. The inbound ‘nmap OS fingerprint’ attempts rule I log the first occurrence because this is something a attacker would do.
Any time you see log messages on a rule with ‘log first’. You should do an ipfstat -hio command to see the number of times the rule has been matched so you know if you are being flooded, i.e. under attack.
When you log packets with port numbers you do not recognize, look it up in /etc/services or go to http://www.securitystats.com/tools/portsearch.php and do a port number lookup to find what the purpose of that port number is.
Check out this link for port numbers used by Trojans http://www.simovits.com/trojans/trojans.html.
The following rule set is a complete very secure ‘inclusive’ type of firewall rule set that I have used on my system. You can not go wrong using this rule set for your own. Just comment out any pass rules for services that you do not want to authorize.
If you see messages in your log that you want to stop seeing just add a block rule in the inbound section.
You have to change the dc0 interface name in every rule to the interface name of the Nic card that connects your system to the public Internet. For user PPP it would be tun0.
Add the following statements to /etc/ipf.rules:
################################################################# # No restrictions on Inside LAN Interface for private network # Not needed unless you have LAN ################################################################# #pass out quick on xl0 all #pass in quick on xl0 all ################################################################# # No restrictions on Loopback Interface ################################################################# pass in quick on lo0 all pass out quick on lo0 all ################################################################# # Interface facing Public Internet (Outbound Section) # Interrogate session start requests originating from behind the # firewall on the private network # or from this gateway server destine for the public Internet. ################################################################# # Allow out access to my ISP's Domain name server. # xxx must be the IP address of your ISP's DNS. # Dup these lines if your ISP has more than one DNS server # Get the IP addresses from /etc/resolv.conf file pass out quick on dc0 proto tcp from any to xxx port = 53 flags S keep state pass out quick on dc0 proto udp from any to xxx port = 53 keep state # Allow out access to my ISP's DHCP server for cable or DSL networks. # This rule is not needed for 'user ppp' type connection to the # public Internet, so you can delete this whole group. # Use the following rule and check log for IP address. # Then put IP address in commented out rule & delete first rule pass out log quick on dc0 proto udp from any to any port = 67 keep state #pass out quick on dc0 proto udp from any to z.z.z.z port = 67 keep state # Allow out non-secure standard www function pass out quick on dc0 proto tcp from any to any port = 80 flags S keep state # Allow out secure www function https over TLS SSL pass out quick on dc0 proto tcp from any to any port = 443 flags S keep state # Allow out send & get email function pass out quick on dc0 proto tcp from any to any port = 110 flags S keep state pass out quick on dc0 proto tcp from any to any port = 25 flags S keep state # Allow out Time pass out quick on dc0 proto tcp from any to any port = 37 flags S keep state # Allow out nntp news pass out quick on dc0 proto tcp from any to any port = 119 flags S keep state # Allow out gateway & LAN users non-secure FTP ( both passive & active modes) # This function uses the IPNAT built in FTP proxy function coded in # the nat rules file to make this single rule function correctly. # If you want to use the pkg_add command to install application packages # on your gateway system you need this rule. pass out quick on dc0 proto tcp from any to any port = 21 flags S keep state # Allow out secure FTP, Telnet, and SCP # This function is using SSH (secure shell) pass out quick on dc0 proto tcp from any to any port = 22 flags S keep state # Allow out non-secure Telnet pass out quick on dc0 proto tcp from any to any port = 23 flags S keep state # Allow out FBSD CVSUP function pass out quick on dc0 proto tcp from any to any port = 5999 flags S keep state # Allow out ping to public Internet pass out quick on dc0 proto icmp from any to any icmp-type 8 keep state # Allow out whois for LAN PC to public Internet pass out quick on dc0 proto tcp from any to any port = 43 flags S keep state # Block and log only the first occurrence of everything # else that's trying to get out. # This rule enforces the block all by default logic. block out log first quick on dc0 all ################################################################# # Interface facing Public Internet (Inbound Section) # Interrogate packets originating from the public Internet # destine for this gateway server or the private network. ################################################################# # Block all inbound traffic from non-routable or reserved address spaces block in quick on dc0 from 192.168.0.0/16 to any #RFC 1918 private IP block in quick on dc0 from 172.16.0.0/12 to any #RFC 1918 private IP block in quick on dc0 from 10.0.0.0/8 to any #RFC 1918 private IP block in quick on dc0 from 127.0.0.0/8 to any #loopback block in quick on dc0 from 0.0.0.0/8 to any #loopback block in quick on dc0 from 169.254.0.0/16 to any #DHCP auto-config block in quick on dc0 from 192.0.2.0/24 to any #reserved for docs block in quick on dc0 from 204.152.64.0/23 to any #Sun cluster interconnect block in quick on dc0 from 224.0.0.0/3 to any #Class D & E multicast ##### Block a bunch of different nasty things. ############ # That I do not want to see in the log # Block frags block in quick on dc0 all with frags # Block short tcp packets block in quick on dc0 proto tcp all with short # block source routed packets block in quick on dc0 all with opt lsrr block in quick on dc0 all with opt ssrr # Block nmap OS fingerprint attempts # Log first occurrence of these so I can get their IP address block in log first quick on dc0 proto tcp from any to any flags FUP # Block anything with special options block in quick on dc0 all with ipopts # Block public pings block in quick on dc0 proto icmp all icmp-type 8 # Block ident block in quick on dc0 proto tcp from any to any port = 113 # Block all Netbios service. 137=name, 138=datagram, 139=session # Netbios is MS/Windows sharing services. # Block MS/Windows hosts2 name server requests 81 block in log first quick on dc0 proto tcp/udp from any to any port = 137 block in log first quick on dc0 proto tcp/udp from any to any port = 138 block in log first quick on dc0 proto tcp/udp from any to any port = 139 block in log first quick on dc0 proto tcp/udp from any to any port = 81 # Allow traffic in from ISP's DHCP server. This rule must contain # the IP address of your ISP's DHCP server as it's the only # authorized source to send this packet type. Only necessary for # cable or DSL configurations. This rule is not needed for # 'user ppp' type connection to the public Internet. # This is the same IP address you captured and # used in the outbound section. pass in quick on dc0 proto udp from z.z.z.z to any port = 68 keep state # Allow in standard www function because I have apache server pass in quick on dc0 proto tcp from any to any port = 80 flags S keep state # Allow in non-secure Telnet session from public Internet # labeled non-secure because ID/PW passed over public Internet as clear text. # Delete this sample group if you do not have telnet server enabled. #pass in quick on dc0 proto tcp from any to any port = 23 flags S keep state # Allow in secure FTP, Telnet, and SCP from public Internet # This function is using SSH (secure shell) pass in quick on dc0 proto tcp from any to any port = 22 flags S keep state # Block and log only first occurrence of all remaining traffic # coming into the firewall. The logging of only the first # occurrence stops a .denial of service. attack targeted # at filling up your log file space. # This rule enforces the block all by default logic. block in log first quick on dc0 all ################### End of rules file #####################################
30.5.14 NAT
NAT stands for Network Address Translation. To those familiar with Linux, this concept is called IP Masquerading; NAT and IP Masquerading are the same thing. One of the many things the IPF NAT function enables is the ability to have a private Local Area Network (LAN) behind the firewall sharing a single ISP assigned IP address on the public Internet.
You may ask why would someone want to do this. ISPs normally assign a dynamic IP address to their non-commercial users. Dynamic means that the IP address can be different each time you dial in and log on to your ISP, or for cable and DSL modem users when you power off and then power on your modems you can get assigned a different IP address. This IP address is how you are known to the public Internet.
Now lets say you have five PCs at home and each one needs Internet access. You would have to pay your ISP for an individual Internet account for each PC and have five phone lines.
With NAT you only need a single account with your ISP, then cable your other four PCs to a switch and the switch to the NIC in your FreeBSD system which is going to service your LAN as a gateway. NAT will automatically translate the private LAN IP address for each separate PC on the LAN to the single public IP address as it exits the firewall bound for the public Internet. It also does the reverse translation for returning packets.
NAT is most often accomplished without the approval, or knowledge, of your ISP and in most cases is grounds for your ISP terminating your account if found out. Commercial users pay a lot more for their Internet connection and usually get assigned a block of static IP address which never change. The ISP also expects and consents to their Commercial customers using NAT for their internal private LANs.
There is a special range of IP addresses reserved for NATed private LAN IP address. According to RFC 1918, you can use the following IP ranges for private nets which will never be routed directly to the public Internet:
Start IP 10.0.0.0
-
Ending IP 10.255.255.255
Start IP 172.16.0.0
-
Ending IP 172.31.255.255
Start IP 192.168.0.0
-
Ending IP 192.168.255.255
30.5.15 IPNAT
NAT rules are loaded by using the ipnat command. Typically the NAT rules are stored in /etc/ipnat.rules. See ipnat(1) for details.
When changing the NAT rules after NAT has been started, make your changes to the file containing the NAT rules, then run ipnat command with the -CF flags to delete the internal in use NAT rules and flush the contents of the translation table of all active entries.
To reload the NAT rules issue a command like this:
# ipnat -CF -f /etc/ipnat.rules
To display some statistics about your NAT, use this command:
# ipnat -s
To list the NAT table’s current mappings, use this command:
# ipnat -l
To turn verbose mode on, and display information relating to rule processing and active rules/table entries:
# ipnat -v
30.5.16 IPNAT Rules
NAT rules are very flexible and can accomplish many different things to fit the needs of commercial and home users.
The rule syntax presented here has been simplified to what is most commonly used in a non-commercial environment. For a complete rule syntax description see the ipnat(5) manual page.
The syntax for a NAT rule looks something like this:
map IF LAN_IP_RANGE -> PUBLIC_ADDRESS
The keyword map starts the rule.
Replace IF with the external interface.
The LAN_IP_RANGE is what your internal clients use for IP Addressing, usually this is something like 192.168.1.0/24.
The PUBLIC_ADDRESS can either be the external IP address or the special keyword 0/32, which means to use the IP address assigned to IF.
30.5.17 How NAT works
A packet arrives at the firewall from the LAN with a public destination. It passes through the outbound filter rules, NAT gets his turn at the packet and applies its rules top down, first matching rule wins. NAT tests each of its rules against the packets interface name and source IP address. When a packets interface name matches a NAT rule then the [source IP address, i.e. private LAN IP address] of the packet is checked to see if it falls within the IP address range specified to the left of the arrow symbol on the NAT rule. On a match the packet has its source IP address rewritten with the public IP address obtained by the 0/32 keyword. NAT posts a entry in its internal NAT table so when the packet returns from the public Internet it can be mapped back to its original private IP address and then passed to the filter rules for processing.
30.5.18 Enabling IPNAT
To enable IPNAT add these statements to /etc/rc.conf.
To enable your machine to route traffic between interfaces:
gateway_enable="YES"
To start IPNAT automatically each time:
ipnat_enable="YES"
To specify where to load the IPNAT rules from:
ipnat_rules="/etc/ipnat.rules"
30.5.19 NAT for a very large LAN
For networks that have large numbers of PC’s on the LAN or networks with more than a single LAN, the process of funneling all those private IP addresses into a single public IP address becomes a resource problem that may cause problems with the same port numbers being used many times across many NATed LAN PC’s, causing collisions. There are two ways to relieve this resource problem.
30.5.19.1 Assigning Ports to Use
A normal NAT rule would look like:
map dc0 192.168.1.0/24 -> 0/32
In the above rule the packet’s source port is unchanged as the packet passes through IPNAT. By adding the portmap keyword you can tell IPNAT to only use source ports in a range. For example the following rule will tell IPNAT to modify the source port to be within that range:
map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp 20000:60000
Additionally we can make things even easier by using the auto keyword to tell IPNAT to determine by itself which ports are available to use:
map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp auto
30.5.19.2 Using a pool of public addresses
In very large LANs there comes a point where there are just too many LAN addresses to fit into a single public address. If a block of public IP addresses is available, you can use these addresses as a “pool”, and let IPNAT pick one of the public IP addresses as packet-addresses are mapped on their way out.
For example, instead of mapping all packets through a single public IP address, as in:
map dc0 192.168.1.0/24 -> 204.134.75.1
A range of public IP addresses can be specified either with a netmask:
map dc0 192.168.1.0/24 -> 204.134.75.0/255.255.255.0
or using CIDR notation:
map dc0 192.168.1.0/24 -> 204.134.75.0/24
30.5.20 Port Redirection
A very common practice is to have a web server, email server, database server and DNS server each segregated to a different PC on the LAN. In this case the traffic from these servers still have to be NATed, but there has to be some way to direct the inbound traffic to the correct LAN PCs. IPNAT has the redirection facilities of NAT to solve this problem. Lets say you have your web server on LAN address 10.0.10.25 and your single public IP address is 20.20.20.5 you would code the rule like this:
rdr dc0 20.20.20.5/32 port 80 -> 10.0.10.25 port 80
or:
rdr dc0 0.0.0.0/0 port 80 -> 10.0.10.25 port 80
or for a LAN DNS Server on LAN address of 10.0.10.33 that needs to receive public DNS requests:
rdr dc0 20.20.20.5/32 port 53 -> 10.0.10.33 port 53 udp
30.5.21 FTP and NAT
FTP is a dinosaur left over from the time before the Internet as it is known today, when research universities were leased lined together and FTP was used to share files among research Scientists. This was a time when data security was not a consideration. Over the years the FTP protocol became buried into the backbone of the emerging Internet and its username and password being sent in clear text was never changed to address new security concerns. FTP has two flavors, it can run in active mode or passive mode. The difference is in how the data channel is acquired. Passive mode is more secure as the data channel is acquired be the ordinal ftp session requester. For a real good explanation of FTP and the different modes see http://www.slacksite.com/other/ftp.html.
30.5.21.1 IPNAT Rules
IPNAT has a special built in FTP proxy option which can be specified on the NAT map rule. It can monitor all outbound packet traffic for FTP active or passive start session requests and dynamically create temporary filter rules containing only the port number really in use for the data channel. This eliminates the security risk FTP normally exposes the firewall to from having large ranges of high order port numbers open.
This rule will handle all the traffic for the internal LAN:
map dc0 10.0.10.0/29 -> 0/32 proxy port 21 ftp/tcp
This rule handles the FTP traffic from the gateway:
map dc0 0.0.0.0/0 -> 0/32 proxy port 21 ftp/tcp
This rule handles all non-FTP traffic from the internal LAN:
map dc0 10.0.10.0/29 -> 0/32
The FTP map rule goes before our regular map rule. All packets are tested against the first rule from the top. Matches on interface name, then private LAN source IP address, and then is it a FTP packet. If all that matches then the special FTP proxy creates temp filter rules to let the FTP session packets pass in and out, in addition to also NATing the FTP packets. All LAN packets that are not FTP do not match the first rule and fall through to the third rule and are tested, matching on interface and source IP, then are NATed.
30.5.21.2 IPNAT FTP Filter Rules
Only one filter rule is needed for FTP if the NAT FTP proxy is used.
Without the FTP Proxy you will need the following three rules:
# Allow out LAN PC client FTP to public Internet # Active and passive modes pass out quick on rl0 proto tcp from any to any port = 21 flags S keep state # Allow out passive mode data channel high order port numbers pass out quick on rl0 proto tcp from any to any port > 1024 flags S keep state # Active mode let data channel in from FTP server pass in quick on rl0 proto tcp from any to any port = 20 flags S keep state
30.6 IPFW
The IPFIREWALL (IPFW) is a FreeBSD sponsored firewall software application authored and maintained by FreeBSD volunteer staff members. It uses the legacy stateless rules and a legacy rule coding technique to achieve what is referred to as Simple Stateful logic.
The IPFW sample rule set (found in /etc/rc.firewall and /etc/rc.firewall6) in the standard FreeBSD install is rather simple and it is not expected that it used directly without modifications. The example does not use stateful filtering, which is beneficial in most setups, so it will not be used as base for this section.
The IPFW stateless rule syntax is empowered with technically sophisticated selection capabilities which far surpasses the knowledge level of the customary firewall installer. IPFW is targeted at the professional user or the advanced technical computer hobbyist who have advanced packet selection requirements. A high degree of detailed knowledge into how different protocols use and create their unique packet header information is necessary before the power of the IPFW rules can be unleashed. Providing that level of explanation is out of the scope of this section of the handbook.
IPFW is composed of seven components, the primary component is the kernel firewall filter rule processor and its integrated packet accounting facility, the logging facility, the ‘divert’ rule which triggers the NAT facility, and the advanced special purpose facilities, the dummynet traffic shaper facilities, the ‘fwd rule’ forward facility, the bridge facility, and the ipstealth facility. IPFW supports both IPv4 and IPv6.
30.6.1 Enabling IPFW
IPFW is included in the basic FreeBSD install as a separate run time loadable module. The system will dynamically load the kernel module when the rc.conf statement firewall_enable="YES" is used. You do not need to compile IPFW into the FreeBSD kernel unless you want NAT function enabled.
After rebooting your system with firewall_enable="YES" in rc.conf the following white highlighted message is displayed on the screen as part of the boot process:
ipfw2 initialized, divert disabled, rule-based forwarding disabled, default to deny, logging disabled
The loadable module does have logging ability compiled in. To enable logging and set the verbose logging limit, there is a knob you can set in /etc/sysctl.conf by adding these statements, logging will be enabled on future reboots:
net.inet.ip.fw.verbose=1 net.inet.ip.fw.verbose_limit=5
30.6.2 Kernel Options
It is not a mandatory requirement that you enable IPFW by compiling the following options into the FreeBSD kernel unless you need NAT function. It is presented here as background information.
options IPFIREWALL
This option enables IPFW as part of the kernel
options IPFIREWALL_VERBOSE
Enables logging of packets that pass through IPFW and have the ‘log’ keyword specified in the rule set.
options IPFIREWALL_VERBOSE_LIMIT=5
Limits the number of packets logged through syslogd(8) on a per entry basis. You may wish to use this option in hostile environments which you want to log firewall activity. This will close a possible denial of service attack via syslog flooding.
options IPFIREWALL_DEFAULT_TO_ACCEPT
This option will allow everything to pass through the firewall by default, which is a good idea when you are first setting up your firewall.
options IPDIVERT
This enables the use of NAT functionality.
Note: If you do not include IPFIREWALL_DEFAULT_TO_ACCEPT or set your rules to allow incoming packets you will block all packets going to and from this machine.
30.6.3 /etc/rc.conf Options
Enable the firewall:
firewall_enable="YES"
To select one of the default firewall types provided by FreeBSD, select one by reading the /etc/rc.firewall file and place it in the following:
firewall_type="open"
Available values for this setting are:
-
open — pass all traffic.
-
client — will protect only this machine.
-
simple — protect the whole network.
-
closed — entirely disables IP traffic except for the loopback interface.
-
UNKNOWN — disables the loading of firewall rules.
-
filename — absolute path of file containing firewall rules.
It is possible to use two different ways to load custom rules for ipfw firewall. One is by setting firewall_type variable to absolute path of file, which contains firewall rules without any command-line options for ipfw(8) itself. A simple example of ruleset file can be following:
add block in all add block out all
On the other hand, it is possible to set firewall_script variable to absolute path of executable script that includes ipfw commands being executed at system boot time. A valid ruleset script that would be equivalent to the ruleset file shown above would be following:
#!/bin/sh ipfw -q flush ipfw add block in all ipfw add block out all
Note: If firewall_type is set to either client or simple, the default rules found in /etc/rc.firewall should be reviewed to fit to the configuration of the given machine. Also note that the examples used in this chapter expect that the firewall_script is set to /etc/ipfw.rules.
Enable logging:
firewall_logging="YES"
Warning: The only thing that the
firewall_loggingvariable will do is setting thenet.inet.ip.fw.verbosesysctl variable to the value of 1 (see Section 30.6.1). There is no rc.conf variable to set log limitations, but it can be set via sysctl variable, manually or from the /etc/sysctl.conf file:net.inet.ip.fw.verbose_limit=5
If your machine is acting as a gateway, i.e. providing Network Address Translation (NAT) via natd(8), please refer to Section 31.9 for information regarding the required /etc/rc.conf options.
30.6.4 The IPFW Command
The ipfw command is the normal vehicle for making manual single rule additions or deletions to the firewall active internal rules while it is running. The problem with using this method is once your system is shutdown or halted all the rules you added or changed or deleted are lost. Writing all your rules in a file and using that file to load the rules at boot time, or to replace in mass the currently running firewall rules with changes you made to the files content is the recommended method used here.
The ipfw command is still a very useful to display the running firewall rules to the console screen. The IPFW accounting facility dynamically creates a counter for each rule that counts each packet that matches the rule. During the process of testing a rule, listing the rule with its counter is the one of the ways of determining if the rule is functioning.
To list all the rules in sequence:
# ipfw list
To list all the rules with a time stamp of when the last time the rule was matched:
# ipfw -t list
To list the accounting information, packet count for matched rules along with the rules themselves. The first column is the rule number, followed by the number of outgoing matched packets, followed by the number of incoming matched packets, and then the rule itself.
# ipfw -a list
List the dynamic rules in addition to the static rules:
# ipfw -d list
Also show the expired dynamic rules:
# ipfw -d -e list
Zero the counters:
# ipfw zero
Zero the counters for just rule NUM:
# ipfw zero NUM
30.6.5 IPFW Rule Sets
A rule set is a group of ipfw rules coded to allow or deny packets based on the values contained in the packet. The bi-directional exchange of packets between hosts comprises a session conversation. The firewall rule set processes the packet twice: once on its arrival from the public Internet host and again as it leaves for its return trip back to the public Internet host. Each tcp/ip service (i.e. telnet, www, mail, etc.) is predefined by its protocol, and port number. This is the basic selection criteria used to create rules which will allow or deny services.
When a packet enters the firewall it is compared against the first rule in the rule set and progress one rule at a time moving from top to bottom of the set in ascending rule number sequence order. When the packet matches a rule selection parameters, the rules action field value is executed and the search of the rule set terminates for that packet. This is referred to as “the first match wins” search method. If the packet does not match any of the rules, it gets caught by the mandatory ipfw default rule, number 65535 which denies all packets and discards them without any reply back to the originating destination.
Note: The search continues after count, skipto and tee rules.
The instructions contained here are based on using rules that contain the stateful ‘keep state’, ‘limit’, ‘in’/'out’, and via options. This is the basic framework for coding an inclusive type firewall rule set.
An inclusive firewall only allows services matching the rules through. This way you can control what services can originate behind the firewall destine for the public Internet and also control the services which can originate from the public Internet accessing your private network. Everything else is denied by default design. Inclusive firewalls are much, much more secure than exclusive firewall rule sets and is the only rule set type covered here in.
Warning: When working with the firewall rules be careful, you can end up locking your self out.
30.6.5.1 Rule Syntax
The rule syntax presented here has been simplified to what is necessary to create a standard inclusive type firewall rule set. For a complete rule syntax description see the ipfw(8) manual page.
Rules contain keywords: these keywords have to be coded in a specific order from left to right on the line. Keywords are identified in bold type. Some keywords have sub-options which may be keywords them selves and also include more sub-options.
# is used to mark the start of a comment and may appear at the end of a rule line or on its own lines. Blank lines are ignored.
CMD RULE_NUMBER ACTION LOGGING SELECTION STATEFUL
30.6.5.1.1 CMD
Each new rule has to be prefixed with add to add the rule to the internal table.
30.6.5.1.2 RULE_NUMBER
Each rule has to have a rule number to go with it.
30.6.5.1.3 ACTION
A rule can be associated with one of the following actions, which will be executed when the packet matches the selection criterion of the rule.
allow | accept | pass | permit
These all mean the same thing which is to allow packets that match the rule to exit the firewall rule processing. The search terminates at this rule.
check-state
Checks the packet against the dynamic rules table. If a match is found, execute the action associated with the rule which generated this dynamic rule, otherwise move to the next rule. The check-state rule does not have selection criterion. If no check-state rule is present in the rule set, the dynamic rules table is checked at the first keep-state or limit rule.
deny | drop
Both words mean the same thing which is to discard packets that match this rule. The search terminates.
30.6.5.1.4 Logging
log or logamount
When a packet matches a rule with the log keyword, a message will be logged to syslogd with a facility name of SECURITY. The logging only occurs if the number of packets logged so far for that particular rule does not exceed the logamount parameter. If no logamount is specified, the limit is taken from the sysctl variable net.inet.ip.fw.verbose_limit. In both cases, a value of zero removes the logging limit. Once the limit is reached, logging can be re-enabled by clearing the logging counter or the packet counter for that rule, see the ipfw reset log command.
Note: Logging is done after all other packet matching conditions have been successfully verified, and before performing the final action (accept, deny) on the packet. It is up to you to decide which rules you want to enable logging on.
30.6.5.1.5 Selection
The keywords described in this section are used to describe attributes of the packet to be interrogated when determining whether rules match the packet or not. The following general-purpose attributes are provided for matching, and must be used in this order:
udp | tcp | icmp
or any protocol names found in /etc/protocols are recognized and may be used. The value specified is protocol to be matched against. This is a mandatory requirement.
from src to dst
The from and to keywords are used to match against IP addresses. Rules must specify BOTH source and destination parameters. any is a special keyword that matches any IP address. me is a special keyword that matches any IP address configured on an interface in your FreeBSD system to represent the PC the firewall is running on (i.e. this box) as in ‘from me to any’ or ‘from any to me’ or ‘from 0.0.0.0/0 to any’ or ‘from any to 0.0.0.0/0′ or ‘from 0.0.0.0 to any’ or ‘from any to 0.0.0.0′ or ‘from me to 0.0.0.0′. IP addresses are specified as a dotted IP address numeric form/mask-length, or as single dotted IP address numeric form. This is a mandatory requirement. See this link for help on writing mask-lengths. http://jodies.de/ipcalc
port number
For protocols which support port numbers (such as TCP and UDP). It is mandatory that you code the port number of the service you want to match on. Service names (from /etc/services) may be used instead of numeric port values.
in | out
Matches incoming or outgoing packets, respectively. The in and out are keywords and it is mandatory that you code one or the other as part of your rule matching criterion.
via IF
Matches packets going through the interface specified by exact name. The via keyword causes the interface to always be checked as part of the match process.
setup
This is a mandatory keyword that identifies the session start request for TCP packets.
keep-state
This is a mandatory keyword. Upon a match, the firewall will create a dynamic rule, whose default behavior is to match bidirectional traffic between source and destination IP/port using the same protocol.
limit {src-addr | src-port | dst-addr | dst-port}
The firewall will only allow N connections with the same set of parameters as specified in the rule. One or more of source and destination addresses and ports can be specified. The ‘limit’ and ‘keep-state’ can not be used on same rule. Limit provides the same stateful function as ‘keep-state’ plus its own functions.
30.6.5.2 Stateful Rule Option
Stateful filtering treats traffic as a bi-directional exchange of packets comprising a session conversation. It has the interrogation abilities to determine if the session conversation between the originating sender and the destination are following the valid procedure of bi-directional packet exchange. Any packets that do not properly fit the session conversation template are automatically rejected as impostors.
‘check-state’ is used to identify where in the IPFW rules set the packet is to be tested against the dynamic rules facility. On a match the packet exits the firewall to continue on its way and a new rule is dynamic created for the next anticipated packet being exchanged during this bi-directional session conversation. On a no match the packet advances to the next rule in the rule set for testing.
The dynamic rules facility is vulnerable to resource depletion from a SYN-flood attack which would open a huge number of dynamic rules. To counter this attack, FreeBSD added another new option named limit. This option is used to limit the number of simultaneous session conversations by interrogating the rules source or destinations fields as directed by the limit option and using the packet’s IP address found there, in a search of the open dynamic rules counting the number of times this rule and IP address combination occurred, if this count is greater that the value specified on the limit option, the packet is discarded.
30.6.5.3 Logging Firewall Messages
The benefits of logging are obvious: it provides the ability to review after the fact the rules you activated logging on which provides information like, what packets had been dropped, what addresses they came from, where they were going, giving you a significant edge in tracking down attackers.
Even with the logging facility enabled, IPFW will not generate any rule logging on it’s own. The firewall administrator decides what rules in the rule set he wants to log and adds the log verb to those rules. Normally only deny rules are logged, like the deny rule for incoming ICMP pings. It is very customary to duplicate the ipfw default deny everything rule with the log verb included as your last rule in the rule set. This way you get to see all the packets that did not match any of the rules in the rule set.
Logging is a two edged sword, if you are not careful, you can lose yourself in the over abundance of log data and fill your disk up with growing log files. DoS attacks that fill up disk drives is one of the oldest attacks around. These log message are not only written to syslogd, but also are displayed on the root console screen and soon become very annoying.
The IPFIREWALL_VERBOSE_LIMIT=5 kernel option limits the number of consecutive messages sent to the system logger syslogd, concerning the packet matching of a given rule. When this option is enabled in the kernel, the number of consecutive messages concerning a particular rule is capped at the number specified. There is nothing to be gained from 200 log messages saying the same identical thing. For instance, five consecutive messages concerning a particular rule would be logged to syslogd, the remainder identical consecutive messages would be counted and posted to the syslogd with a phrase like this:
last message repeated 45 times
All logged packets messages are written by default to /var/log/security file, which is defined in the /etc/syslog.conf file.
30.6.5.4 Building a Rule Script
Most experienced IPFW users create a file containing the rules and code them in a manner compatible with running them as a script. The major benefit of doing this is the firewall rules can be refreshed in mass without the need of rebooting the system to activate the new rules. This method is very convenient in testing new rules as the procedure can be executed as many times as needed. Being a script, you can use symbolic substitution to code frequent used values and substitution them in multiple rules. You will see this in the following example.
The script syntax used here is compatible with the ’sh’, ‘csh’, ‘tcsh’ shells. Symbolic substitution fields are prefixed with a dollar sign $. Symbolic fields do not have the $ prefix. The value to populate the Symbolic field must be enclosed to "double quotes".
Start your rules file like this:
############### start of example ipfw rules script ############# # ipfw -q -f flush # Delete all rules # Set defaults oif="tun0" # out interface odns="192.0.2.11" # ISP's DNS server IP address cmd="ipfw -q add " # build rule prefix ks="keep-state" # just too lazy to key this each time $cmd 00500 check-state $cmd 00502 deny all from any to any frag $cmd 00501 deny tcp from any to any established $cmd 00600 allow tcp from any to any 80 out via $oif setup $ks $cmd 00610 allow tcp from any to $odns 53 out via $oif setup $ks $cmd 00611 allow udp from any to $odns 53 out via $oif $ks ################### End of example ipfw rules script ############
That is all there is to it. The rules are not important in this example, how the Symbolic substitution field are populated and used are.
If the above example was in /etc/ipfw.rules file, you could reload these rules by entering on the command line.
# sh /etc/ipfw.rules
The /etc/ipfw.rules file could be located anywhere you want and the file could be named any thing you would like.
The same thing could also be accomplished by running these commands by hand:
# ipfw -q -f flush # ipfw -q add check-state # ipfw -q add deny all from any to any frag # ipfw -q add deny tcp from any to any established # ipfw -q add allow tcp from any to any 80 out via tun0 setup keep-state # ipfw -q add allow tcp from any to 192.0.2.11 53 out via tun0 setup keep-state # ipfw -q add 00611 allow udp from any to 192.0.2.11 53 out via tun0 keep-state
30.6.5.5 Stateful Ruleset
The following non-NATed rule set is an example of how to code a very secure ‘inclusive’ type of firewall. An inclusive firewall only allows services matching pass rules through and blocks all other by default. All firewalls have at the minimum two interfaces which have to have rules to allow the firewall to function.
All UNIX flavored operating systems, FreeBSD included, are designed to use interface lo0 and IP address 127.0.0.1 for internal communication with in the operating system. The firewall rules must contain rules to allow free unmolested movement of these special internally used packets.
The interface which faces the public Internet, is the one which you code your rules to authorize and control access out to the public Internet and access requests arriving from the public Internet. This can be your ppp tun0 interface or your NIC that is connected to your DSL or cable modem.
In cases where one or more than one NIC are connected to a private LANs behind the firewall, those interfaces must have rules coded to allow free unmolested movement of packets originating from those LAN interfaces.
The rules should be first organized into three major sections, all the free unmolested interfaces, public interface outbound, and the public interface inbound.
The order of the rules in each of the public interface sections should be in order of the most used rules being placed before less often used rules with the last rule in the section being a block log all packets on that interface and direction.
The Outbound section in the following rule set only contains ‘allow’ rules which contain selection values that uniquely identify the service that is authorized for public Internet access. All the rules have the, proto, port, in/out, via and keep state option coded. The ‘proto tcp’ rules have the ’setup’ option included to identify the start session request as the trigger packet to be posted to the keep state stateful table.
The Inbound section has all the blocking of undesirable packets first for two different reasons. First is these things being blocked may be part of an otherwise valid packet which may be allowed in by the later authorized service rules. Second reason is that by having a rule that explicitly blocks selected packets that I receive on an infrequent bases and do not want to see in the log, this keeps them from being caught by the last rule in the section which blocks and logs all packets which have fallen through the rules. The last rule in the section which blocks and logs all packets is how you create the legal evidence needed to prosecute the people who are attacking your system.
Another thing you should take note of, is there is no response returned for any of the undesirable stuff, their packets just get dropped and vanish. This way the attackers has no knowledge if his packets have reached your system. The less the attackers can learn about your system the more secure it is. When you log packets with port numbers you do not recognize, look the numbers up in /etc/services/ or go to http://www.securitystats.com/tools/portsearch.php and do a port number lookup to find what the purpose of that port number is. Check out this link for port numbers used by Trojans: http://www.simovits.com/trojans/trojans.html.
30.6.5.6 An Example Inclusive Ruleset
The following non-NATed rule set is a complete inclusive type ruleset. You can not go wrong using this rule set for you own. Just comment out any pass rules for services you do not want. If you see messages in your log that you want to stop seeing just add a deny rule in the inbound section. You have to change the ‘dc0′ interface name in every rule to the interface name of the NIC that connects your system to the public Internet. For user ppp it would be ‘tun0′.
You will see a pattern in the usage of these rules.
-
All statements that are a request to start a session to the public Internet use keep-state.
-
All the authorized services that originate from the public Internet have the limit option to stop flooding.
-
All rules use in or out to clarify direction.
-
All rules use via interface name to specify the interface the packet is traveling over.
The following rules go into /etc/ipfw.rules.
################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
pif="dc0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Not needed unless you have LAN.
# Change xl0 to your LAN NIC interface name
#################################################################
#$cmd 00005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
$cmd 00010 allow all from any to any via lo0
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
$cmd 00015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network or from this gateway server
# destine for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP.s DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
$cmd 00110 allow tcp from any to x.x.x.x 53 out via $pif setup keep-state
$cmd 00111 allow udp from any to x.x.x.x 53 out via $pif keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
# This rule is not needed for .user ppp. connection to the public Internet.
# so you can delete this whole group.
# Use the following rule and check log for IP address.
# Then put IP address in commented out rule & delete first rule
$cmd 00120 allow log udp from any to any 67 out via $pif keep-state
#$cmd 00120 allow udp from any to x.x.x.x 67 out via $pif keep-state
# Allow out non-secure standard www function
$cmd 00200 allow tcp from any to any 80 out via $pif setup keep-state
# Allow out secure www function https over TLS SSL
$cmd 00220 allow tcp from any to any 443 out via $pif setup keep-state
# Allow out send & get email function
$cmd 00230 allow tcp from any to any 25 out via $pif setup keep-state
$cmd 00231 allow tcp from any to any 110 out via $pif setup keep-state
# Allow out FBSD (make install & CVSUP) functions
# Basically give user root "GOD" privileges.
$cmd 00240 allow tcp from me to any out via $pif setup keep-state uid root
# Allow out ping
$cmd 00250 allow icmp from any to any out via $pif keep-state
# Allow out Time
$cmd 00260 allow tcp from any to any 37 out via $pif setup keep-state
# Allow out nntp news (i.e. news groups)
$cmd 00270 allow tcp from any to any 119 out via $pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
$cmd 00280 allow tcp from any to any 22 out via $pif setup keep-state
# Allow out whois
$cmd 00290 allow tcp from any to any 43 out via $pif setup keep-state
# deny and log everything else that.s trying to get out.
# This rule enforces the block all by default logic.
$cmd 00299 deny log all from any to any out via $pif
#################################################################
# Interface facing Public Internet (Inbound Section)
# Interrogate packets originating from the public Internet
# destine for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
$cmd 00300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP
$cmd 00301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP
$cmd 00302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP
$cmd 00303 deny all from 127.0.0.0/8 to any in via $pif #loopback
$cmd 00304 deny all from 0.0.0.0/8 to any in via $pif #loopback
$cmd 00305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config
$cmd 00306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs
$cmd 00307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster interconnect
$cmd 00308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast
# Deny public pings
$cmd 00310 deny icmp from any to any in via $pif
# Deny ident
$cmd 00315 deny tcp from any to any 113 in via $pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
$cmd 00320 deny tcp from any to any 137 in via $pif
$cmd 00321 deny tcp from any to any 138 in via $pif
$cmd 00322 deny tcp from any to any 139 in via $pif
$cmd 00323 deny tcp from any to any 81 in via $pif
# Deny any late arriving packets
$cmd 00330 deny all from any to any frag in via $pif
# Deny ACK packets that did not match the dynamic rule table
$cmd 00332 deny tcp from any to any established in via $pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP.s DHCP server as it.s the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for .user ppp. type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
#$cmd 00360 allow udp from any to x.x.x.x 67 in via $pif keep-state
# Allow in standard www function because I have apache server
$cmd 00400 allow tcp from any to me 80 in via $pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
$cmd 00410 allow tcp from any to me 22 in via $pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID & PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
$cmd 00420 allow tcp from any to me 23 in via $pif setup limit src-addr 2
# Reject & Log all incoming connections from the outside
$cmd 00499 deny log all from any to any in via $pif
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
$cmd 00999 deny log all from any to any
################ End of IPFW rules file ###############################
30.6.5.7 An Example NAT and Stateful Ruleset
There are some additional configuration statements that need to be enabled to activate the NAT function of IPFW. The kernel source needs ‘option IPDIVERT’ statement added to the other IPFIREWALL statements compiled into a custom kernel.
In addition to the normal IPFW options in /etc/rc.conf, the following are needed.
natd_enable="YES" # Enable NATD function natd_interface="rl0" # interface name of public Internet NIC natd_flags="-dynamic -m" # -m = preserve port numbers if possible
Utilizing stateful rules with divert natd rule (Network Address Translation) greatly complicates the rule set coding logic. The positioning of the check-state, and ‘divert natd’ rules in the rule set becomes very critical. This is no longer a simple fall-through logic flow. A new action type is used, called ’skipto’. To use the skipto command it is mandatory that you number each rule so you know exactly where the skipto rule number is you are really jumping to.
The following is an uncommented example of one coding method, selected here to explain the sequence of the packet flow through the rule sets.
The processing flow starts with the first rule from the top of the rule file and progress one rule at a time deeper into the file until the end is reach or the packet being tested to the selection criteria matches and the packet is released out of the firewall. It is important to take notice of the location of rule numbers 100 101, 450, 500, and 510. These rules control the translation of the outbound and inbound packets so their entries in the keep-state dynamic table always register the private LAN IP address. Next notice that all the allow and deny rules specified the direction the packet is going (IE outbound or inbound) and the interface. Also notice that all the start outbound session requests all skipto rule 500 for the network address translation.
Lets say a LAN user uses their web browser to get a web page. Web pages use port 80 to communicate over. So the packet enters the firewall, It does not match 100 because it is headed out not in. It passes rule 101 because this is the first packet so it has not been posted to the keep-state dynamic table yet. The packet finally comes to rule 125 a matches. It is outbound through the NIC facing the public Internet. The packet still has it’s source IP address as a private LAN IP address. On the match to this rule, two actions take place. The keep-state option will post this rule into the keep-state dynamic rules table and the specified action is executed. The action is part of the info posted to the dynamic table. In this case it is "skipto rule 500". Rule 500 NATs the packet IP address and out it goes. Remember this, this is very important. This packet makes its way to the destination and returns and enters the top of the rule set. This time it does match rule 100 and has it destination IP address mapped back to its corresponding LAN IP address. It then is processed by the check-state rule, it’s found in the table as an existing session conversation and released to the LAN. It goes to the LAN PC that sent it and a new packet is sent requesting another segment of the data from the remote server. This time it gets checked by the check-state rule and its outbound entry is found, the associated action, ’skipto 500′, is executed. The packet jumps to rule 500 gets NATed and released on it’s way out.
On the inbound side, everything coming in that is part of an existing session conversation is being automatically handled by the check-state rule and the properly placed divert natd rules. All we have to address is denying all the bad packets and only allowing in the authorized services. Lets say there is a apache server running on the firewall box and we want people on the public Internet to be able to access the local web site. The new inbound start request packet matches rule 100 and its IP address is mapped to LAN IP for the firewall box. The packet is them matched against all the nasty things we want to check for and finally matches against rule 425. On a match two things occur. The packet rule is posted to the keep-state dynamic table but this time any new session requests originating from that source IP address is limited to 2. This defends against DoS attacks of service running on the specified port number. The action is allow so the packet is released to the LAN. On return the check-state rule recognizes the packet as belonging to an existing session conversation sends it to rule 500 for NATing and released to outbound interface.
Example Ruleset #1:
#!/bin/sh cmd="ipfw -q add" skip="skipto 500" pif=rl0 ks="keep-state" good_tcpo="22,25,37,43,53,80,443,110,119" ipfw -q -f flush $cmd 002 allow all from any to any via xl0 # exclude LAN traffic $cmd 003 allow all from any to any via lo0 # exclude loopback traffic $cmd 100 divert natd ip from any to any in via $pif $cmd 101 check-state # Authorized outbound packets $cmd 120 $skip udp from any to xx.168.240.2 53 out via $pif $ks $cmd 121 $skip udp from any to xx.168.240.5 53 out via $pif $ks $cmd 125 $skip tcp from any to any $good_tcpo out via $pif setup $ks $cmd 130 $skip icmp from any to any out via $pif $ks $cmd 135 $skip udp from any to any 123 out via $pif $ks # Deny all inbound traffic from non-routable reserved address spaces $cmd 300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP $cmd 301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP $cmd 302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP $cmd 303 deny all from 127.0.0.0/8 to any in via $pif #loopback $cmd 304 deny all from 0.0.0.0/8 to any in via $pif #loopback $cmd 305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config $cmd 306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs $cmd 307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster $cmd 308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast # Authorized inbound packets $cmd 400 allow udp from xx.70.207.54 to any 68 in $ks $cmd 420 allow tcp from any to me 80 in via $pif setup limit src-addr 1 $cmd 450 deny log ip from any to any # This is skipto location for outbound stateful rules $cmd 500 divert natd ip from any to any out via $pif $cmd 510 allow ip from any to any ######################## end of rules ##################
The following is pretty much the same as above, but uses a self documenting coding style full of description comments to help the inexperienced IPFW rule writer to better understand what the rules are doing.
Example Ruleset #2:
#!/bin/sh
################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
skip="skipto 800"
pif="rl0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Change xl0 to your LAN NIC interface name
#################################################################
$cmd 005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
$cmd 010 allow all from any to any via lo0
#################################################################
# check if packet is inbound and nat address if it is
#################################################################
$cmd 014 divert natd ip from any to any in via $pif
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
$cmd 015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network or from this gateway server
# destine for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP's DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
$cmd 020 $skip tcp from any to x.x.x.x 53 out via $pif setup keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
$cmd 030 $skip udp from any to x.x.x.x 67 out via $pif keep-state
# Allow out non-secure standard www function
$cmd 040 $skip tcp from any to any 80 out via $pif setup keep-state
# Allow out secure www function https over TLS SSL
$cmd 050 $skip tcp from any to any 443 out via $pif setup keep-state
# Allow out send & get email function
$cmd 060 $skip tcp from any to any 25 out via $pif setup keep-state
$cmd 061 $skip tcp from any to any 110 out via $pif setup keep-state
# Allow out FreeBSD (make install & CVSUP) functions
# Basically give user root "GOD" privileges.
$cmd 070 $skip tcp from me to any out via $pif setup keep-state uid root
# Allow out ping
$cmd 080 $skip icmp from any to any out via $pif keep-state
# Allow out Time
$cmd 090 $skip tcp from any to any 37 out via $pif setup keep-state
# Allow out nntp news (i.e. news groups)
$cmd 100 $skip tcp from any to any 119 out via $pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
$cmd 110 $skip tcp from any to any 22 out via $pif setup keep-state
# Allow out whois
$cmd 120 $skip tcp from any to any 43 out via $pif setup keep-state
# Allow ntp time server
$cmd 130 $skip udp from any to any 123 out via $pif keep-state
#################################################################
# Interface facing Public Internet (Inbound Section)
# Interrogate packets originating from the public Internet
# destine for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
$cmd 300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP
$cmd 301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP
$cmd 302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP
$cmd 303 deny all from 127.0.0.0/8 to any in via $pif #loopback
$cmd 304 deny all from 0.0.0.0/8 to any in via $pif #loopback
$cmd 305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config
$cmd 306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs
$cmd 307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster
$cmd 308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast
# Deny ident
$cmd 315 deny tcp from any to any 113 in via $pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
$cmd 320 deny tcp from any to any 137 in via $pif
$cmd 321 deny tcp from any to any 138 in via $pif
$cmd 322 deny tcp from any to any 139 in via $pif
$cmd 323 deny tcp from any to any 81 in via $pif
# Deny any late arriving packets
$cmd 330 deny all from any to any frag in via $pif
# Deny ACK packets that did not match the dynamic rule table
$cmd 332 deny tcp from any to any established in via $pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP's DHCP server as it's the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for 'user ppp' type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
$cmd 360 allow udp from x.x.x.x to any 68 in via $pif keep-state
# Allow in standard www function because I have Apache server
$cmd 370 allow tcp from any to me 80 in via $pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
$cmd 380 allow tcp from any to me 22 in via $pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID & PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
$cmd 390 allow tcp from any to me 23 in via $pif setup limit src-addr 2
# Reject & Log all unauthorized incoming connections from the public Internet
$cmd 400 deny log all from any to any in via $pif
# Reject & Log all unauthorized out going connections to the public Internet
$cmd 450 deny log all from any to any out via $pif
# This is skipto location for outbound stateful rules
$cmd 800 divert natd ip from any to any out via $pif
$cmd 801 allow ip from any to any
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
$cmd 999 deny log all from any to any
################ End of IPFW rules file ###############################
Tags: account, Apache, archive, bsd, cron, database, domain, domain name, email, freebsd, FreeBSD Handbook, ftp, netbsd, openbsd, password, php, pop, software, ssh, ssl, telnet, trojan
Category: core
Module: sys
Announced: 2003-09-23
Credits: Person@
Affects: All releases of 
CVE Name: CVE-XXXX-XXXX
For general information regarding 
II. Problem Description
III. Impact
IV. Workaround
V. Solution
VI. Correction details
VII. References