30.5. IPFILTER (IPF)

IPFILTER, also known as IPF, is a cross-platform, open source firewall which has been ported to several operating systems, including FreeBSD, NetBSD, OpenBSD, and Solaris™.

IPFILTER is a kernel-side firewall and NAT mechanism that can be controlled and monitored by userland programs. Firewall rules can be set or deleted using ipf, NAT rules can be set or deleted using ipnat, run-time statistics for the kernel parts of IPFILTER can be printed using ipfstat, and ipmon can be used to log IPFILTER actions to the system log files.

IPF was originally written using a rule processing logic of the last matching rule wins and only used stateless rules. Since then, IPF has been enhanced to include the quick and keep state options.

For a detailed explanation of the legacy rules processing method, refer to 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 IPFilter mailing list is available at http://marc.info/?l=ipfilter.

This section of the Handbook focuses on IPF as it pertains to FreeBSD. It provides examples of rules that contain the quick and keep state options.

30.5.1. Enabling IPF

IPF is included in the basic FreeBSD install as a kernel loadable module, meaning that a custom kernel is not needed in order to enable IPF.

For users who prefer to statically compile IPF support into a custom kernel, refer to the instructions in Chapter 9, Configuring the FreeBSD Kernel. The following kernel options are available:

options IPFILTER
options IPFILTER_LOG
options IPFILTER_LOOKUP
options IPFILTER_DEFAULT_BLOCK

where options IPFILTER enables support for IPFILTER, options IPFILTER_LOG enables IPF logging using the ipl packet logging pseudo-device for every rule that has the log keyword, IPFILTER_LOOKUP enables IP pools in order to speed up IP lookups, and options IPFILTER_DEFAULT_BLOCK changes the default behavior so that any packet not matching a firewall pass rule gets blocked.

To configure the system to enable IPF at boot time, add the following entries to /etc/rc.conf. These entries will also enable logging and default pass all. To change the default policy to block all without compiling a custom kernel, remember to add a block all rule at the end of the ruleset.

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 NAT functionality is needed, also add these lines:

gateway_enable="YES"              # Enable as LAN gateway
ipnat_enable="YES"                # Start ipnat function
ipnat_rules="/etc/ipnat.rules"    # rules definition file for ipnat

Then, to start IPF now:

# service ipfilter start

To load the firewall rules, specify the name of the ruleset file using ipf. The following command can be used to replace the currently running firewall rules:

# ipf -Fa -f /etc/ipf.rules

where -Fa flushes all the internal rules tables and -f specifies the file containing the rules to load.

This provides the ability to make changes to a custom ruleset and update the running firewall with a fresh copy of the rules without having to reboot the system. This method is convenient for testing new rules as the procedure can be executed as many times as needed.

Refer to ipf(8) for details on the other flags available with this command.

30.5.2. IPF Rule Syntax

This section describes the IPF rule syntax used to create stateful rules. When creating rules, keep in mind that unless the quick keyword appears in a rule, every rule is read in order, with the last matching rule being the one that is applied. This means that even if the first rule to match a packet is a pass, if there is a later matching rule that is a block, the packet will be dropped. Sample rulesets can be found in /usr/share/examples/ipfilter.

When creating rules, a # character is used to mark the start of a comment and may appear at the end of a rule, to explain that rule's function, or on its own line. Any blank lines are ignored.

The keywords which are used in rules must be written in a specific order, from left to right. Some keywords are mandatory while others are optional. Some keywords have sub-options which may be keywords themselves and also include more sub-options. The keyword order is as follows, where the words shown in uppercase represent a variable and the words shown in lowercase must precede the variable that follows it:

ACTION DIRECTION OPTIONS proto PROTO_TYPE from SRC_ADDR SRC_PORT to DST_ADDR DST_PORT TCP_FLAG|ICMP_TYPE keep state STATE

This section describes each of these keywords and their options. It is not an exhaustive list of every possible option. Refer to ipf(5) for a complete description of the rule syntax that can be used when creating IPF rules and examples for using each keyword.

ACTION

The action keyword indicates what to do with the packet if it matches that rule. Every rule must have an action. The following actions are recognized:

block: drops the packet.

pass: allows the packet.

log: generates a log record.

count: counts the number of packets and bytes which can provide an indication of how often a rule is used.

auth: queues the packet for further processing by another program.

call: provides access to functions built into IPF that allow more complex actions.

decapsulate: removes any headers in order to process the contents of the packet.

DIRECTION

Next, each rule must explicitly state the direction of traffic using one of these keywords:

in: the rule is applied against an inbound packet.

out: the rule is applied against an outbound packet.

all: the rule applies to either direction.

If the system has multiple interfaces, the interface can be specified along with the direction. An example would be in on fxp0.

OPTIONS

Options are optional. However, if multiple options are specified, they must be used in the order shown here.

log: when performing the specified ACTION, the contents of the packet's headers will be written to the ipl(4) packet log pseudo-device.

quick: if a packet matches this rule, the ACTION specified by the rule occurs and no further processing of any following rules will occur for this packet.

on: must be followed by the interface name as displayed by ifconfig(8). The rule will only match if the packet is going through the specified interface in the specified direction.

When using 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, this option is recommended so that only the triggering packet is logged and not every packet which matches the stateful connection.

Additional options are available to specify error return messages. Refer to ipf(5) for more details.

PROTO_TYPE

The protocol type is optional. However, it is mandatory if the rule needs to specify a SRC_PORT or a DST_PORT as it defines the type of protocol. When specifying the type of protocol, use the proto keyword followed by either a protocol number or name from /etc/protocols. Example protocol names include tcp, udp, or icmp. If PROTO_TYPE is specified but no SRC_PORT or DST_PORT is specified, all port numbers for that protocol will match that rule.

SRC_ADDR

The from keyword is mandatory and is followed by a keyword which represents the source of the packet. The source can be a hostname, an IP address followed by the CIDR mask, an address pool, or the keyword all. Refer to ipf(5) for examples.

There is no way to match ranges of IP addresses which do not express themselves easily using the dotted numeric form / mask-length notation. The net-mgmt/ipcalc package or port may be used to ease the calculation of the CIDR mask. Additional information is available at the utility's web page: http://jodies.de/ipcalc.

SRC_PORT

The port number of the source is optional. However, if it is used, it requires PROTO_TYPE to be first defined in the rule. The port number must also be preceded by the proto keyword.

A number of different comparison operators are supported: = (equal to), != (not equal to), < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to).

To specify port ranges, place the two port numbers between <> (less than and greater than ), >< (greater than and less than ), or : (greater than or equal to and less than or equal to).

DST_ADDR

The to keyword is mandatory and is followed by a keyword which represents the destination of the packet. Similar to SRC_ADDR, it can be a hostname, an IP address followed by the CIDR mask, an address pool, or the keyword all.

DST_PORT

Similar to SRC_PORT, the port number of the destination is optional. However, if it is used, it requires PROTO_TYPE to be first defined in the rule. The port number must also be preceded by the proto keyword.

TCP_FLAG|ICMP_TYPE

If tcp is specifed as the PROTO_TYPE, flags can be specified as letters, where each letter represents one of the possible TCP flags used to determine the state of a connection. Possible values are: S (SYN), A (ACK), P (PSH), F (FIN), U (URG), R (RST), C (CWN), and E (ECN).

If icmp is specifed as the PROTO_TYPE, the ICMP type to match can be specified. Refer to ipf(5) for the allowable types.

STATE

If a pass rule contains keep state, IPF will add an entry to its dynamic state table and allow subsequent packets that match the connection. IPF can track state for TCP, UDP, and ICMP sessions. Any packet that IPF can be certain is part of an active session, even if it is a different protocol, will be allowed.

In IPF, packets destined to go out through the interface connected to the public Internet are first checked against the dynamic state table. If the packet matches the next expected packet comprising an active session conversation, it exits the firewall and the state of the session conversation flow is updated in the dynamic state table. Packets that do not belong to an already active session are checked against the outbound ruleset. Packets coming in from the interface connected to the public Internet are first checked against the dynamic state table. If the packet matches the next expected packet comprising an active session, it exits the firewall and the state of the session conversation flow is updated in the dynamic state table. Packets that do not belong to an already active session are checked against the inbound ruleset.

Several keywords can be added after keep state. If used, these keywords set various options that control stateful filtering, such as setting connection limits or connection age. Refer to ipf(5) for the list of available options and their descriptions.

30.5.3. Example Ruleset

This section demonstrates how to create an example ruleset which only allows services matching pass rules and blocks all others.

FreeBSD uses the loopback interface (lo0) and the IP address 127.0.0.1 for internal communication. The firewall ruleset must contain rules to allow free movement of these internally used packets:

# no restrictions on loopback interface
pass in quick on lo0 all
pass out quick on lo0 all

The public interface connected to the Internet is used to authorize and control access of all outbound and inbound connections. If one or more interfaces are cabled to private networks, those internal interfaces may require rules to allow packets originating from the LAN to flow between the internal networks or to the interface attached to the Internet. The ruleset should be organized into three major sections: any trusted internal interfaces, outbound connections through the public interface, and inbound connections through the public interface.

These two rules allow all traffic to pass through a trusted LAN interface named xl0:

# no restrictions on inside LAN interface for private network
pass out quick on xl0 all
pass in quick on xl0 all

The rules for the public interface's outbound and inbound 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 for that interface and direction.

This set of rules defines the outbound section of the public interface named dc0. These rules keep state and identify the specific services that internal systems are authorized for public Internet access. All the rules use quick and specify the appropriate port numbers and, where applicable, destination addresses.

# interface facing Internet (outbound)
# Matches session start requests originating from or behind the
# firewall, destined for the Internet.

# Allow outbound access to public DNS servers.
# Replace x.x.x. with address listed in /etc/resolv.conf.
# Repeat for each DNS server.
pass out quick on dc0 proto tcp from any to x.x.x. port = 53 flags S keep state
pass out quick on dc0 proto udp from any to xxx port = 53 keep state

# Allow access to ISP's specified DHCP server for cable or DSL networks.
# Use the first rule, then check log for the IP address of DHCP server.
# Then, uncomment the second rule, replace z.z.z.z with the IP address,
# and comment out the 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 HTTP and HTTPS
pass out quick on dc0 proto tcp from any to any port = 80 flags S keep state
pass out quick on dc0 proto tcp from any to any port = 443 flags S keep state

# Allow email
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 NTP
pass out quick on dc0 proto tcp from any to any port = 37 flags S keep state

# Allow FTP
pass out quick on dc0 proto tcp from any to any port = 21 flags S keep state

# Allow SSH
pass out quick on dc0 proto tcp from any to any port = 22 flags S keep state

# Allow ping
pass out quick on dc0 proto icmp from any to any icmp-type 8 keep state

# Block and log everything else
block out log first quick on dc0 all

This example of the rules in the inbound section of the public interface blocks all undesirable packets first. This reduces the number of packets that are logged by the last rule.

# interface facing Internet (inbound)
# 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 fragments and too short tcp packets
block in quick on dc0 all with frags
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 OS fingerprint attempts and log first occurrence
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 and ident
block in quick on dc0 proto icmp all icmp-type 8
block in quick on dc0 proto tcp from any to any port = 113

# Block incoming Netbios services
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

Any time there are logged messages on a rule with the log first option, run ipfstat -hio to evaluate how many times the rule has been matched. A large number of matches may indicate that the system is under attack.

The rest of the rules in the inbound section define which connections are allowed to be initiated from the Internet. The last rule denies all connections which were not explicitly allowed by previous rules in this section.

# Allow traffic in from ISP's DHCP server. Replace z.z.z.z with
# the same IP address used in the outbound section.
pass in quick on dc0 proto udp from z.z.z.z to any port = 68 keep state

# Allow public connections to specified internal web server
pass in quick on dc0 proto tcp from any to x.x.x.x port = 80 flags S keep state

# Block and log only first occurrence of all remaining traffic.
block in log first quick on dc0 all

30.5.4. Configuring NAT

To enable NAT, add these statements to /etc/rc.conf and specify the name of the file containing the NAT rules:

gateway_enable="YES"
ipnat_enable="YES"
ipnat_rules="/etc/ipnat.rules"

NAT rules are flexible and can accomplish many different things to fit the needs of both commercial and home users. The rule syntax presented here has been simplified to demonstrate common usage. For a complete rule syntax description, refer to ipnat(5).

The basic syntax for a NAT rule is as follows, where map starts the rule and IF should be replaced with the name of the external interface:

map IF LAN_IP_RANGE -> PUBLIC_ADDRESS

The LAN_IP_RANGE is the range of IP addresses used by internal clients. Usually, it is a private address range such as 192.168.1.0/24. The PUBLIC_ADDRESS can either be the static external IP address or the keyword 0/32 which represents the IP address assigned to IF.

In IPF, when a packet arrives at the firewall from the LAN with a public destination, it first passes through the outbound rules of the firewall ruleset. Then, the packet is passed to the NAT ruleset which is read from the top down, where the first matching rule wins. IPF tests each NAT rule against the packet's interface name and source IP address. When a packet's interface name matches a NAT rule, the packet's source IP address in the private LAN is checked to see if it falls within the IP address range specified in LAN_IP_RANGE. On a match, the packet has its source IP address rewritten with the public IP address specified by PUBLIC_ADDRESS. IPF posts an entry in its internal NAT table so that when the packet returns from the Internet, it can be mapped back to its original private IP address before being passed to the firewall rules for further processing.

For networks that have large numbers of internal systems or multiple subnets, the process of funneling every private IP address into a single public IP address becomes a resource problem. Two methods are available to relieve this issue.

The first method is to assign a range of ports to use as source ports. By adding the portmap keyword, NAT can be directed to only use source ports in the specified range:

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

Alternately, use the auto keyword which tells NAT to determine the ports that are available for use:

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

The second method is to use a pool of public addresses. This is useful when there are too many LAN addresses to fit into a single public address and a block of public IP addresses is available. These public addresses can be used as a pool from which NAT selects an IP address as a packet's address is mapped on its way out.

The range of public IP addresses can be specified using a netmask or CIDR notation. These two rules are equivalent:

map dc0 192.168.1.0/24 -> 204.134.75.0/255.255.255.0
map dc0 192.168.1.0/24 -> 204.134.75.0/24

A common practice is to have a publically accessible web server or mail server segregated to an internal network segment. The traffic from these servers still has to undergo NAT, but port redirection is needed to direct inbound traffic to the correct server. For example, to map a web server using the internal address 10.0.10.25 to its public IP address of 20.20.20.5, use this rule:

rdr dc0 20.20.20.5/32 port 80 -> 10.0.10.25 port 80

If it is the only web server, this rule would also work as it redirects all external HTTP requests to 10.0.10.25:

rdr dc0 0.0.0.0/0 port 80 -> 10.0.10.25 port 80

IPF has a built in FTP proxy which can be used with NAT. It monitors all outbound traffic for active or passive FTP connection requests and dynamically creates temporary filter rules containing the port number used by the FTP data channel. This eliminates the need to open large ranges of high order ports for FTP connections.

In this example, the first rule calls the proxy for outbound FTP traffic from the internal LAN. The second rule passes the FTP traffic from the firewall to the Internet, and the third rule handles all non-FTP traffic from the internal LAN:

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

The FTP map rules go before the NAT rule so that when a packet matches an FTP rule, the FTP proxy creates temporary filter rules to let the FTP session packets pass and undergo NAT. All LAN packets that are not FTP will not match the FTP rules but will undergo NAT if they match the third rule.

Without the FTP proxy, the following firewall rules would instead be needed. Note that without the proxy, all ports above 1024 need to be allowed:

# 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

Whenever the file containing the NAT rules is edited, run ipnat with -CF to delete the current NAT rules and flush the contents of the dynamic translation table. Include -f and specify the name of the NAT ruleset to load:

# ipnat -CF -f /etc/ipnat.rules

To display the NAT statistics:

# ipnat -s

To list the NAT table's current mappings:

# ipnat -l

To turn verbose mode on and display information relating to rule processing and active rules and table entries:

# ipnat -v

30.5.5. Viewing IPF Statistics

IPF includes ipfstat(8) which can be used to retrieve and display statistics which are gathered as packets match rules as they go through the firewall. Statistics are accumulated since the firewall was last started or since the last time they were reset to zero using ipf -Z.

The default ipfstat output looks 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)

Several options are available. When supplied with either -i for inbound or -o for outbound, the command will retrieve and display the appropriate list of filter rules currently installed and in use by the kernel. To also see the rule numbers, include -n. For example, ipfstat -on displays the outbound rules table with rule numbers:

@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

Include -h to prefix each rule with a count of how many times the rule was matched. For example, ipfstat -oh displays the outbound internal rules table, prefixing each rule with its usage count:

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

To display the state table in a format similar to top(1), use ipfstat -t. When the firewall is under attack, this option provides the ability to identify and see the attacking packets. The optional sub-flags give the ability to select the destination or source IP, port, or protocol to be monitored in real time. Refer to ipfstat(8) for details.

30.5.6. IPF Logging

IPF provides ipmon, which can be used to write the firewall's logging information in a human readable format. It requires that options IPFILTER_LOG be first added to a custom kernel using the instructions in Chapter 9, Configuring the FreeBSD Kernel.

This command is typically run in daemon mode in order to provide a continuous system log file so that logging of past events may be reviewed. Since FreeBSD has a built in syslogd(8) facility to automatically rotate system logs, the default rc.conf ipmon_flags statement uses -Ds:

ipmon_flags="-Ds" # D = start as daemon
                  # s = log to syslog
                  # v = log tcp window, ack, seq
                  # n = map IP & port to names

Logging provides the ability to review, after the fact, information such as which packets were dropped, what addresses they came from, and where they were going. This information is useful in tracking down attackers.

Once the logging facility is enabled in rc.conf and started with service ipmon start, IPF will only log the rules which contain the log keyword. The firewall administrator decides which rules in the ruleset should be logged and normally only deny rules are logged. It is customary to include the log keyword in the last rule in the ruleset. This makes it possible to see all the packets that did not match any of the rules in the ruleset.

By default, ipmon -Ds mode uses local0 as the logging facility. The following logging levels can be used to further segregate the logged data:

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 due to an incomplete header

In order to setup IPF to log all data to /var/log/ipfilter.log, first create the empty file:

# touch /var/log/ipfilter.log

Then, to write all logged messages to the specified file, add the following statement to /etc/syslog.conf:

local0.* /var/log/ipfilter.log

To activate the changes and instruct syslogd(8) to read the modified /etc/syslog.conf, run service syslogd reload.

Do not forget to edit /etc/newsyslog.conf to rotate the new log file.

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.

  3. The name of the interface that processed the packet.

  4. The group and rule number of the rule in the format @0:17.

  5. The action: p for passed, b for blocked, S for a short packet, n did not match any rules, and L for a log rule.

  6. The addresses written as three fields: the source address and port separated by a comma, the -> symbol, and the destination address and port. For example: 209.53.17.22,80 -> 198.73.220.17,1722.

  7. PR followed by the protocol name or number: for example, PR tcp.

  8. len followed by the header length and total length of the packet: for example, 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. Refer to ipf(5) 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. For example: icmp 3/3 for a port unreachable message.

All FreeBSD documents are available for download at http://ftp.FreeBSD.org/pub/FreeBSD/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
Send questions about this document to <freebsd-doc@FreeBSD.org>.