24.5 The IPFILTER (IPF) Firewall

24.5.1 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.

24.5.2 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 (/usr/src/sys/arch/conf/LINT for FreeBSD 4.X) 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.

24.5.3 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

24.5.4 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 24.5.9.

24.5.5 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.

24.5.6 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.

24.5.7 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 (local0 in 4.X) as the “facility” name. All IPMON logged data goes to security (local0 in 4.X). 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 for FreeBSD 5.X and later:

security.* /var/log/ipfilter.log

Or add the following statement to /etc/syslog.conf for FreeBSD 4.X:

local0.* /var/log/ipfilter.log

The security.* (local0 for 4.X) 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 (killall -HUP syslogd in FreeBSD 4.X).

Do not forget to change /etc/newsyslog.conf to rotate the new log you just created above.

24.5.8 The Format of Logged Messages

Messages generated by ipmon consist of data fields separated by white space. Fields common to all messages are:

1. The date of packet receipt.

2. 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).

3. The name of the interface the packet was processed on, e.g. dc0.

4. The group and rule number of the rule, e.g. @0:17.

These can be viewed with ipfstat -in.

1. 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.

2. 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.

3. PR followed by the protocol name or number, e.g. PR tcp.

4. 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.

24.5.9 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:

• 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.

• 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.

24.5.10 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.

24.5.11 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

24.5.12 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.

24.5.13 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 #####################################

24.5.14 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:

24.5.15 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

24.5.16 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.

24.5.17 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.

24.5.18 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"

24.5.19 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.

map dc0 192.168.1.0/24 -> 0/32

map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp 20000:60000

map dc0 192.168.1.0/24 -> 0/32 portmap tcp/udp auto

map dc0 192.168.1.0/24 -> 204.134.75.1

map dc0 192.168.1.0/24 -> 204.134.75.1-10

map dc0 192.168.1.0/24 -> 204.134.75.0/24

24.5.20 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 0/32 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

24.5.21 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.

map dc0 10.0.10.0/29 -> 0/32 proxy port 21 ftp/tcp

map dc0 0.0.0.0/0 -> 0/32 proxy port 21 ftp/tcp

map dc0 10.0.10.0/29 -> 0/32

# 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

block in quick on rl0 proto tcp from any to any port = 21