IPFW is a stateful firewall written for FreeBSD which supports both IPv4 and IPv6. It is comprised of several components: the kernel firewall filter rule processor and its integrated packet accounting facility, the logging facility, NAT, the dummynet(4) traffic shaper, a forward facility, a bridge facility, and an ipstealth facility.
FreeBSD provides a sample ruleset in
/etc/rc.firewall which defines several
firewall types for common scenarios to assist novice users in
generating an appropriate ruleset.
IPFW provides a powerful syntax which
advanced users can use to craft customized rulesets that meet
the security requirements of a given environment.
This section describes how to enable IPFW, provides an overview of its rule syntax, and demonstrates several rulesets for common configuration scenarios.
IPFW is included in the basic FreeBSD install as a kernel loadable module, meaning that a custom kernel is not needed in order to enable IPFW.
For those users who wish to statically compile IPFW support into a custom kernel, refer to the instructions in Chapter 9, Configuring the FreeBSD Kernel. The following options are available for the custom kernel configuration file:
options IPFIREWALL # enables IPFW options IPFIREWALL_VERBOSE # enables logging for rules with log keyword options IPFIREWALL_VERBOSE_LIMIT=5 # limits number of logged packets per-entry options IPFIREWALL_DEFAULT_TO_ACCEPT # sets default policy to pass what is not explicitly denied options IPDIVERT # enables NAT
To configure the system to enable
IPFW at boot time, add the
following entry to /etc/rc.conf:
firewall_enable="YES"
To use one of the default firewall types provided by FreeBSD, add another line which specifies the type:
firewall_type="open"
The available types are:
open: passes all traffic.
client: protects only this
machine.
simple: protects the whole
network.
closed: entirely disables IP
traffic except for the loopback interface.
workstation: protects only this
machine using stateful rules.
UNKNOWN: disables the loading of
firewall rules.
filename
If firewall_type is set to either
client or simple,
modify the default rules found in
/etc/rc.firewall to fit the
configuration of the system.
Note that the filename type is used to
load a custom ruleset.
An alternate way to load a custom ruleset is to set the
firewall_script variable to the absolute
path of an executable script that
includes IPFW commands. The
examples used in this section assume that the
firewall_script is set to
/etc/ipfw.rules:
firewall_script="/etc/ipfw.rules"
To enable logging, include this line:
firewall_logging="YES"
There is no /etc/rc.conf variable to
set logging limits. To limit the number of times a rule is
logged per connection attempt, specify the number using this
line in /etc/sysctl.conf:
net.inet.ip.fw.verbose_limit=5After saving the needed edits, start the firewall. To
enable logging limits now, also set the
sysctl value specified above:
#service ipfw start#sysctl net.inet.ip.fw.verbose_limit=5
When a packet enters the IPFW
firewall, it is compared against the first rule in the ruleset
and progresses one rule at a time, moving from top to bottom
in sequence. When the packet matches the selection parameters
of a rule, the rule's action is executed and the search of the
ruleset terminates for that packet. This is referred to as
“first match wins”. 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 silently discards them. However,
if the packet matches a rule that contains the
count, skipto, or
tee keywords, the search continues. Refer
to ipfw(8) for details on how these keywords affect rule
processing.
When creating an
IPFW rule, keywords must be
written in the following order. Some keywords are mandatory
while other keywords are optional. The words shown in
uppercase represent a variable and the words shown in
lowercase must precede the variable that follows it. The
# symbol is used to mark the start of a
comment and may appear at the end of a rule or on its own
line. Blank lines are ignored.
CMD RULE_NUMBER set SET_NUMBER ACTION log
LOG_AMOUNT PROTO from SRC SRC_PORT to DST DST_PORT
OPTIONS
This section provides an overview of these keywords and their options. It is not an exhaustive list of every possible option. Refer to ipfw(8) for a complete description of the rule syntax that can be used when creating IPFW rules.
Every rule must start with
ipfw add.
Each rule is associated with a number from
1 to
65534. The number is used to
indicate the order of rule processing. Multiple rules
can have the same number, in which case they are applied
according to the order in which they have been
added.
Each rule is associated with a set number from
0 to 31.
Sets can be individually disabled or enabled, making it
possible to quickly add or delete a set of rules. If a
SET_NUMBER is not specified, the rule will be added to
set 0.
A rule can be associated with one of the following actions. The specified action will be executed when the packet matches the selection criterion of the rule.
allow | accept | pass |
permit: these keywords are equivalent and
allow packets that match the rule.
check-state: checks the
packet against the dynamic state table. If a match is
found, execute the action associated with the rule which
generated this dynamic rule, otherwise move to the next
rule. A check-state rule does not
have selection criterion. If no
check-state rule is present in the
ruleset, the dynamic rules table is checked at the first
keep-state or
limit rule.
count: updates counters for
all packets that match the rule. The search continues
with the next rule.
deny | drop: either word
silently discards packets that match this rule.
Additional actions are available. Refer to ipfw(8) for details.
When a packet matches a rule with the
log keyword, a message will be logged
to syslogd(8) with a facility name of
SECURITY. Logging only occurs if the
number of packets logged for that particular rule does
not exceed a specified LOG_AMOUNT. If no
LOG_AMOUNT is specified, the limit is taken from the
value of
net.inet.ip.fw.verbose_limit. 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,
using ipfw reset log.
Logging is done after all other packet matching conditions have been met, and before performing the final action on the packet. The administrator decides which rules to enable logging on.
This optional value can be used to specify any
protocol name or number found in
/etc/protocols.
The from keyword must be followed
by the source address or a keyword that represents the
source address. An address can be represented by
any, me (any
address configured on an interface on this system),
me6, (any IPv6
address configured on an interface on this system), or
table followed by the number of a
lookup table which contains a list of addresses. When
specifying an IP address, it can be
optionally followed by its CIDR mask
or subnet mask. For example,
1.2.3.4/25 or
1.2.3.4:255.255.255.128.
An optional source port can be specified using the
port number or name from
/etc/services.
The to keyword must be followed
by the destination address or a keyword that represents
the destination address. The same keywords and
addresses described in the SRC section can be used to
describe the destination.
An optional destination port can be specified using
the port number or name from
/etc/services.
Several keywords can follow the source and
destination. As the name suggests, OPTIONS are
optional. Commonly used options include
in or out, which
specify the direction of packet flow,
icmptypes followed by the type of
ICMP message, and
keep-state.
When a keep-state rule is
matched, the firewall will create a dynamic rule which
matches bidirectional traffic between the source and
destination addresses and ports using the same
protocol.
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 type of
attack with IPFW, use
limit. This option limits the number
of simultaneous sessions by checking the open dynamic
rules, counting the number of times this rule and
IP address combination occurred. If
this count is greater than the value specified by
limit, the packet is
discarded.
Dozens of OPTIONS are available. Refer to ipfw(8) for a description of each available option.
This section demonstrates how to create an example
stateful firewall ruleset script named
/etc/ipfw.rules. In this example, all
connection rules use in or
out to clarify the direction. They also
use via
interface-name to specify
the interface the packet is traveling over.
When first creating or testing a firewall ruleset, consider temporarily setting this tunable:
net.inet.ip.fw.default_to_accept="1"
This sets the default policy of ipfw(8) to be more
permissive than the default deny ip from any to
any, making it slightly more difficult to get
locked out of the system right after a reboot.
The firewall script begins by indicating that it is a
Bourne shell script and flushes any existing rules. It then
creates the cmd variable so that
ipfw add does not have to be typed at the
beginning of every rule. It also defines the
pif variable which represents the name of
the interface that is attached to the Internet.
#!/bin/sh # Flush out the list before we begin. ipfw -q -f flush # Set rules command prefix cmd="ipfw -q add" pif="dc0" # interface name of NIC attached to Internet
The first two rules allow all traffic on the trusted internal interface and on the loopback interface:
# Change xl0 to 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
The next rule allows the packet through if it matches an existing entry in the dynamic rules table:
$cmd 00101 check-state
The next set of rules defines which stateful connections internal systems can create to hosts on the Internet:
# Allow access to public DNS # Replace x.x.x.x with the IP address of a public DNS server # and repeat for each DNS server in /etc/resolv.conf $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 access to ISP's DHCP server for cable/DSL configurations. # Use the first rule and check log for IP address. # Then, uncomment the second rule, input the IP address, and delete the 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 outbound HTTP and HTTPS connections $cmd 00200 allow tcp from any to any 80 out via $pif setup keep-state $cmd 00220 allow tcp from any to any 443 out via $pif setup keep-state # Allow outbound email connections $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 outbound ping $cmd 00250 allow icmp from any to any out via $pif keep-state # Allow outbound NTP $cmd 00260 allow tcp from any to any 37 out via $pif setup keep-state # Allow outbound SSH $cmd 00280 allow tcp from any to any 22 out via $pif setup keep-state # deny and log all other outbound connections $cmd 00299 deny log all from any to any out via $pif
The next set of rules controls connections from Internet
hosts to the internal network. It starts by denying packets
typically associated with attacks and then explicitly allows
specific types of connections. All the authorized services
that originate from the Internet use limit
to prevent flooding.
# 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 services. $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 fragments $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 $pif # Allow traffic from ISP's DHCP server. # Replace x.x.x.x with the same IP address used in rule 00120. #$cmd 00360 allow udp from any to x.x.x.x 67 in via $pif keep-state # Allow HTTP connections to internal web server $cmd 00400 allow tcp from any to me 80 in via $pif setup limit src-addr 2 # Allow inbound SSH connections $cmd 00410 allow tcp from any to me 22 in via $pif setup limit src-addr 2 # Reject and log all other incoming connections $cmd 00499 deny log all from any to any in via $pif
The last rule logs all packets that do not match any of the rules in the ruleset:
# Everything else is denied and logged $cmd 00999 deny log all from any to any
FreeBSD's built-in NAT daemon, natd(8), works in conjunction with IPFW to provide network address translation. This can be used to provide an Internet Connection Sharing solution so that several internal computers can connect to the Internet using a single IP address.
To do this, the FreeBSD machine connected to the Internet must act as a gateway. This system must have two NICs, where one is connected to the Internet and the other is connected to the internal LAN. Each machine connected to the LAN should be assigned an IP address in the private network space, as defined by RFC 1918, and have the default gateway set to the natd(8) system's internal IP address.
Some additional configuration is needed in order to
activate the NAT function of
IPFW. If the system has a custom
kernel, the kernel configuration file needs to include
option IPDIVERT along with the other
IPFIREWALL options described in Section 30.4.1, “Enabling IPFW”.
To enable NAT support at boot time, the
following must be in /etc/rc.conf:
gateway_enable="YES" # enables the gateway natd_enable="YES" # enables NAT natd_interface="rl0" # specify interface name of NIC attached to Internet natd_flags="-dynamic -m" # -m = preserve port numbers; additional options are listed in natd(8)
It is also possible to specify a configuration file which contains the options to pass to natd(8):
natd_flags="-f /etc/natd.conf"
The specified file must contain a list of configuration options, one per line. For example:
redirect_port tcp 192.168.0.2:6667 6667 redirect_port tcp 192.168.0.3:80 80
For more information about this configuration file, consult natd(8).
Next, add the NAT rules to the firewall
ruleset. When the rulest contains stateful rules, the
positioning of the NAT rules is critical
and the skipto action is used. The
skipto action requires a rule number so
that it knows which rule to jump to.
The following example builds upon the firewall ruleset
shown in the previous section. It adds some additional
entries and modifies some existing rules in order to configure
the firewall for NAT. It starts by adding
some additional variables which represent the rule number to
skip to, the keep-state option, and a list
of TCP ports which will be used to reduce
the number of rules:
#!/bin/sh ipfw -q -f flush cmd="ipfw -q add" skip="skipto 500" pif=dc0 ks="keep-state" good_tcpo="22,25,37,53,80,443,110"
The inbound NAT rule is inserted
after the two rules which allow all
traffic on the trusted internal interface and on the loopback
interface and before the
check-state rule. It is important that the
rule number selected for this NAT rule, in
this example 100, is higher than the first
two rules and lower than the check-state
rule:
$cmd 005 allow all from any to any via xl0 # exclude LAN traffic $cmd 010 allow all from any to any via lo0 # exclude loopback traffic $cmd 100 divert natd ip from any to any in via $pif # NAT any inbound packets # Allow the packet through if it has an existing entry in the dynamic rules table $cmd 101 check-state
The outbound rules are modified to replace the
allow action with the
$skip variable, indicating that rule
processing will continue at rule 500. The
seven tcp rules have been replaced by rule
125 as the
$good_tcpo variable contains the
seven allowed outbound ports.
# Authorized outbound packets $cmd 120 $skip udp from any to x.x.x.x 53 out via $pif $ks $cmd 121 $skip udp from any to x.x.x.x 67 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
The inbound rules remain the same, except for the very
last rule which removes the via $pif in
order to catch both inbound and outbound rules. The
NAT rule must follow this last outbound
rule, must have a higher number than that last rule, and the
rule number must be referenced by the
skipto action. In this ruleset, rule
number 500 diverts all packets which match
the outbound rules to natd(8) for
NAT processing. The next rule allows any
packet which has undergone NAT processing
to pass.
$cmd 499 deny log all from any to any $cmd 500 divert natd ip from any to any out via $pif # skipto location for outbound stateful rules $cmd 510 allow ip from any to any
In this example, rules 100,
101, 125,
500, and 510 control the
address translation of the outbound and inbound packets so
that the entries in the dynamic state table always register
the private LAN IP
address.
Consider an internal web browser which initializes a new
outbound HTTP session over port 80. When
the first outbound packet enters the firewall, it does not
match rule 100 because it is headed out
rather than in. It passes rule 101 because
this is the first packet and it has not been posted to the
dynamic state table yet. The packet finally matches rule
125 as it is outbound on an allowed port
and has a source IP address from the
internal LAN. On matching this rule, two
actions take place. First, the keep-state
action adds an entry to the dynamic state table and the
specified action, skipto rule 500, is
executed. Next, the packet undergoes NAT
and is sent out to the Internet. This packet makes its way to
the destination web server, where a response packet is
generated and sent back. This new packet enters the top of
the ruleset. It matches rule 100 and has
its destination IP address mapped back to
the original internal address. It then is processed by the
check-state rule, is found in the table as
an existing session, and is released to the
LAN.
On the inbound side, the ruleset has to deny bad packets
and allow only authorized services. A packet which matches an
inbound rule is posted to the dynamic state table and the
packet is released to the LAN. The packet
generated as a response is recognized by the
check-state rule as belonging to an
existing session. It is then sent to rule
500 to undergo
NAT before being released to the outbound
interface.
The drawback with natd(8) is that the LAN clients are not accessible from the Internet. Clients on the LAN can make outgoing connections to the world but cannot receive incoming ones. This presents a problem if trying to run Internet services on one of the LAN client machines. A simple way around this is to redirect selected Internet ports on the natd(8) machine to a LAN client.
For example, an IRC server runs on
client A and a web server runs on
client B. For this to work
properly, connections received on ports 6667
(IRC) and 80 (HTTP)
must be redirected to the respective machines.
The syntax for -redirect_port is as
follows:
-redirect_port proto targetIP:targetPORT[-targetPORT]
[aliasIP:]aliasPORT[-aliasPORT]
[remoteIP[:remotePORT[-remotePORT]]]In the above example, the argument should be:
-redirect_port tcp 192.168.0.2:6667 6667
-redirect_port tcp 192.168.0.3:80 80This redirects the proper TCP ports to the LAN client machines.
Port ranges over individual ports can be indicated with
-redirect_port. For example,
tcp 192.168.0.2:2000-3000
2000-3000 would redirect all connections
received on ports 2000 to 3000 to ports 2000 to 3000 on
client A.
These options can be used when directly running
natd(8), placed within the
natd_flags="" option in
/etc/rc.conf, or passed via a
configuration file.
For further configuration options, consult natd(8)
Address redirection is useful if more than one
IP address is available. Each
LAN client can be assigned its own
external IP address by natd(8),
which will then rewrite outgoing packets from the
LAN clients with the proper external
IP address and redirects all traffic
incoming on that particular IP address
back to the specific LAN client. This is
also known as static NAT. For example,
if IP addresses 128.1.1.1, 128.1.1.2, and 128.1.1.3 are available,
128.1.1.1 can be
used as the natd(8) machine's external
IP address, while 128.1.1.2 and 128.1.1.3 are forwarded
back to LAN clients
A and
B.
The -redirect_address syntax is as
follows:
-redirect_address localIP publicIP
| localIP | The internal IP address of the LAN client. |
| publicIP | The external IP address corresponding to the LAN client. |
In the example, this argument would read:
-redirect_address 192.168.0.2 128.1.1.2 -redirect_address 192.168.0.3 128.1.1.3
Like -redirect_port, these arguments
are placed within the natd_flags=""
option of /etc/rc.conf, or passed via a
configuration file. With address redirection, there is no
need for port redirection since all data received on a
particular IP address is
redirected.
The external IP addresses on the natd(8) machine must be active and aliased to the external interface. Refer to rc.conf(5) for details.
ipfw can be used to make manual,
single rule additions or deletions to the active firewall
while it is running. The problem with using this method is
that all the changes are lost when the system reboots. It is
recommended to instead write all the rules in a file and to
use that file to load the rules at boot time and to replace
the currently running firewall rules whenever that file
changes.
ipfw is a useful way 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 one way to
determine if the rule is functioning as expected.
To list all the running rules in sequence:
#ipfw list
To list all the running rules with a time stamp of when the last time the rule was matched:
#ipfw -t list
The next example lists accounting information and the packet count for matched rules along with the rules themselves. The first column is the rule number, followed by the number of matched packets and bytes, followed by the rule itself.
#ipfw -a list
To list dynamic rules in addition to static rules:
#ipfw -d list
To also show the expired dynamic rules:
#ipfw -d -e list
To zero the counters:
#ipfw zero
To zero the counters for just the rule with number
NUM:
#ipfw zeroNUM
Even with the logging facility enabled,
IPFW will not generate any rule
logging on its own. The firewall administrator decides
which rules in the ruleset will be logged, and adds the
log keyword to those rules. Normally
only deny rules are logged. It is customary to duplicate
the “ipfw default deny everything” rule with
the log keyword included as the last rule
in the ruleset. This way, it is possible to see all the
packets that did not match any of the rules in the
ruleset.
Logging is a two edged sword. If one is not careful, an over abundance of log data or a DoS attack can fill the disk with log files. Log messages are not only written to syslogd, but also are displayed on the root console screen and soon become annoying.
The IPFIREWALL_VERBOSE_LIMIT=5
kernel option limits the number of consecutive messages
sent to syslogd(8), 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 identical log messages. With this option
set to five,
five consecutive messages concerning a particular rule
would be logged to syslogd and
the remainder identical consecutive messages would be
counted and posted to syslogd
with a phrase like the following:
last message repeated 45 times
All logged packets messages are written by default to
/var/log/security, which is
defined in /etc/syslog.conf.
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 them. This method is convenient in testing new rules as the procedure can be executed as many times as needed. Being a script, symbolic substitution can be used for frequently used values to be substituted into multiple rules.
This example script is compatible with the syntax used by the sh(1), csh(1), and tcsh(1) 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 in double quotes ("").
Start the 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 ############
The rules are not important as the focus of this example is how the symbolic substitution fields are populated.
If the above example was in
/etc/ipfw.rules, the rules could be
reloaded by the following command:
#sh /etc/ipfw.rules
/etc/ipfw.rules can be located
anywhere and the file can have any name.
The same thing could 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
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 <[email protected]>.
Send questions about this document to <[email protected]>.