The IPFIREWALL (IPFW) is a FreeBSD sponsored firewall software application authored
and maintained by FreeBSD volunteer staff members. It uses the legacy stateless rules and
a legacy rule coding technique to achieve what is referred to as Simple Stateful
logic.
The IPFW stateless rule syntax is empowered with technically sophisticated selection
capabilities which far surpasses the knowledge level of the customary firewall installer.
IPFW is targeted at the professional user or the advanced technical computer hobbyist who
have advanced packet selection requirements. A high degree of detailed knowledge into how
different protocols use and create their unique packet header information is necessary
before the power of the IPFW rules can be unleashed. Providing that level of explanation
is out of the scope of this section of the handbook.
IPFW is composed of seven components, the primary component is the kernel firewall
filter rule processor and its integrated packet accounting facility, the logging
facility, the 'divert' rule which triggers the NAT
facility, and the advanced special purpose facilities, the dummynet traffic shaper
facilities, the 'fwd rule' forward facility, the bridge facility, and the ipstealth
facility.
It is not a mandatory requirement that you enable IPFW by compiling the following
options into the FreeBSD kernel unless you need NAT
function. It is presented here as background information.
options IPFIREWALL
This option enables IPFW as part of the kernel
options IPFIREWALL_VERBOSE
Enables logging of packets that pass through IPFW and have the 'log' keyword specified
in the rule set.
options IPFIREWALL_VERBOSE_LIMIT=5
Limits the number of packets logged through syslogd(8) on a per
entry basis. You may wish to use this option in hostile environments which you want to
log firewall activity. This will close a possible denial of service attack via syslog
flooding.
options IPFIREWALL_DEFAULT_TO_ACCEPT
This option will allow everything to pass through the firewall by default, which is a
good idea when you are first setting up your firewall.
options IPV6FIREWALL
options IPV6FIREWALL_VERBOSE
options IPV6FIREWALL_VERBOSE_LIMIT
options IPV6FIREWALL_DEFAULT_TO_ACCEPT
These options are exactly the same as the IPv4 options but they are for IPv6. If you
do not use IPv6 you might want to use IPV6FIREWALL without any rules to block all
IPv6
options IPDIVERT
This enables the use of NAT functionality.
Note: If you do not include IPFIREWALL_DEFAULT_TO_ACCEPT or set your rules to
allow incoming packets you will block all packets going to and from this machine.
The ipfw command is the normal vehicle for making manual single rule additions or
deletions to the firewall active internal rules while it is running. The problem with
using this method is once your system is shutdown or halted all the rules you added or
changed or deleted are lost. Writing all your rules in a file and using that file to load
the rules at boot time, or to replace in mass the currently running firewall rules with
changes you made to the files content is the recommended method used here.
The ipfw command is still a very useful to display the running firewall rules to the
console screen. The IPFW accounting facility dynamically creates a counter for each rule
that counts each packet that matches the rule. During the process of testing a rule,
listing the rule with its counter is the one of the ways of determining if the rule is
functioning.
To list all the rules in sequence:
# ipfw list
To list all the rules with a time stamp of when the last time the rule was
matched:
# ipfw -t list
To list the accounting information, packet count for matched rules along with the
rules themselves. The first column is the rule number, followed by the number of outgoing
matched packets, followed by the number of incoming matched packets, and then the rule
itself.
# ipfw -a list
List the dynamic rules in addition to the static rules:
# ipfw -d list
Also show the expired dynamic rules:
# ipfw -d -e list
Zero the counters:
# ipfw zero
Zero the counters for just rule NUM:
# ipfw zero NUM
A rule set is a group of ipfw rules coded to allow or deny packets based on the values
contained in the packet. The bi-directional exchange of packets between hosts comprises a
session conversation. The firewall rule set processes the packet twice: once on its
arrival from the public Internet host and again as it leaves for its return trip back to
the public Internet host. Each tcp/ip service (i.e. telnet, www, mail, etc.) is
predefined by its protocol, and port number. This is the basic selection criteria used to
create rules which will allow or deny services.
When a packet enters the firewall it is compared against the first rule in the rule
set and progress one rule at a time moving from top to bottom of the set in ascending
rule number sequence order. When the packet matches a rule selection parameters, the
rules action field value is executed and the search of the rule set terminates for that
packet. This is referred to as “the first match wins” search method. If the
packet does not match any of the rules, it gets caught by the mandatory ipfw default
rule, number 65535 which denies all packets and discards them without any reply back to
the originating destination.
Note: The search continues after count, skipto and tee rules.
The instructions contained here are based on using rules that contain the stateful
'keep state', 'limit', 'in'/'out', and via options. This is the basic framework for
coding an inclusive type firewall rule set.
An inclusive firewall only allows services matching the rules through. This way you
can control what services can originate behind the firewall destine for the public
Internet and also control the services which can originate from the public Internet
accessing your private network. Everything else is denied by default design. Inclusive
firewalls are much, much more secure than exclusive firewall rule sets and is the only
rule set type covered here in.
Warning: When working with the firewall rules be careful, you can end up
locking your self out.
The rule syntax presented here has been simplified to what is necessary to create a
standard inclusive type firewall rule set. For a complete rule syntax description see the
ipfw(8) manual
page.
Rules contain keywords: these keywords have to be coded in a specific order from left
to right on the line. Keywords are identified in bold type. Some keywords have
sub-options which may be keywords them selves and also include more sub-options.
# is used to mark the start of a comment and may appear at
the end of a rule line or on its own lines. Blank lines are ignored.
CMD RULE_NUMBER ACTION LOGGING SELECTION
STATEFUL
Each new rule has to be prefixed with add to add the
rule to the internal table.
Each rule has to have a rule number to go with it.
A rule can be associated with one of the following actions, which will be executed
when the packet matches the selection criterion of the rule.
allow | accept | pass | permit
These all mean the same thing which is to allow packets that match the rule to exit
the firewall rule processing. The search terminates at this rule.
check-state
Checks the packet against the dynamic rules table. If a match is found, execute the
action associated with the rule which generated this dynamic rule, otherwise move to the
next rule. The check-state rule does not have selection criterion. If no check-state rule
is present in the rule set, the dynamic rules table is checked at the first keep-state or
limit rule.
deny | drop
Both words mean the same thing which is to discard packets that match this rule. The
search terminates.
log or logamount
When a packet matches a rule with the log keyword, a message will be logged to syslogd
with a facility name of SECURITY. The logging only occurs if the number of packets logged
so far for that particular rule does not exceed the logamount parameter. If no logamount
is specified, the limit is taken from the sysctl variable net.inet.ip.fw.verbose_limit.
In both cases, a value of zero removes the logging limit. Once the limit is reached,
logging can be re-enabled by clearing the logging counter or the packet counter for that
rule, see the ipfw reset log command.
Note: Logging is done after all other packet matching conditions have been
successfully verified, and before performing the final action (accept, deny) on the
packet. It is up to you to decide which rules you want to enable logging on.
The keywords described in this section are used to describe attributes of the packet
to be interrogated when determining whether rules match the packet or not. The following
general-purpose attributes are provided for matching, and must be used in this order:
udp | tcp | icmp
or any protocol names found in /etc/protocols are recognized
and may be used. The value specified is protocol to be matched against. This is a
mandatory requirement.
from src to dst
The from and to keywords are used to match against IP addresses. Rules must specify
BOTH source and destination parameters. any is a special keyword
that matches any IP address. me is a special keyword that
matches any IP address configured on an interface in your FreeBSD system to represent the
PC the firewall is running on (i.e. this box) as in 'from me to any' or 'from any to me'
or 'from 0.0.0.0/0 to any' or 'from any to 0.0.0.0/0' or 'from 0.0.0.0 to any' or 'from
any to 0.0.0.0' or 'from me to 0.0.0.0'. IP addresses are specified as a dotted IP
address numeric form/mask-length, or as single dotted IP address numeric form. This is a
mandatory requirement. See this link for help on writing mask-lengths. https://jodies.de/ipcalc
port number
For protocols which support port numbers (such as TCP and UDP). It is mandatory that you code the port number of
the service you want to match on. Service names (from /etc/services) may be used instead of numeric port values.
in | out
Matches incoming or outgoing packets, respectively. The in and out are keywords and it
is mandatory that you code one or the other as part of your rule matching criterion.
via IF
Matches packets going through the interface specified by exact name. The via keyword causes the interface to always be checked as part of the
match process.
setup
This is a mandatory keyword that identifies the session start request for TCP packets.
keep-state
This is a mandatory keyword. Upon a match, the firewall will create a dynamic rule,
whose default behavior is to match bidirectional traffic between source and destination
IP/port using the same protocol.
limit {src-addr | src-port | dst-addr | dst-port}
The firewall will only allow N connections with
the same set of parameters as specified in the rule. One or more of source and
destination addresses and ports can be specified. The 'limit' and 'keep-state' can not be
used on same rule. Limit provides the same stateful function as 'keep-state' plus its own
functions.
Stateful filtering treats traffic as a bi-directional exchange of packets comprising a
session conversation. It has the interrogation abilities to determine if the session
conversation between the originating sender and the destination are following the valid
procedure of bi-directional packet exchange. Any packets that do not properly fit the
session conversation template are automatically rejected as impostors.
'check-state' is used to identify where in the IPFW rules set the packet is to be
tested against the dynamic rules facility. On a match the packet exits the firewall to
continue on its way and a new rule is dynamic created for the next anticipated packet
being exchanged during this bi-directional session conversation. On a no match the packet
advances to the next rule in the rule set for testing.
The dynamic rules facility is vulnerable to resource depletion from a SYN-flood attack
which would open a huge number of dynamic rules. To counter this attack, FreeBSD added
another new option named limit. This option is used to limit the number of simultaneous
session conversations by interrogating the rules source or destinations fields as
directed by the limit option and using the packet's IP address found there, in a search
of the open dynamic rules counting the number of times this rule and IP address
combination occurred, if this count is greater that the value specified on the limit
option, the packet is discarded.
The benefits of logging are obvious: it provides the ability to review after the fact
the rules you activated logging on which provides information like, what packets had been
dropped, what addresses they came from, where they were going, giving you a significant
edge in tracking down attackers.
Even with the logging facility enabled, IPFW will not generate any rule logging on
it's own. The firewall administrator decides what rules in the rule set he wants to log
and adds the log verb to those rules. Normally only deny rules are logged, like the deny
rule for incoming ICMP pings. It is very customary to
duplicate the ipfw default deny everything rule with the log verb included as your last
rule in the rule set. This way you get to see all the packets that did not match any of
the rules in the rule set.
Logging is a two edged sword, if you are not careful, you can lose yourself in the
over abundance of log data and fill your disk up with growing log files. DoS attacks that
fill up disk drives is one of the oldest attacks around. These log message are not only
written to syslogd, but also are displayed on the root console screen and soon become
very annoying.
The IPFIREWALL_VERBOSE_LIMIT=5 kernel option limits the
number of consecutive messages sent to the system logger syslogd, concerning the packet
matching of a given rule. When this option is enabled in the kernel, the number of
consecutive messages concerning a particular rule is capped at the number specified.
There is nothing to be gained from 200 log messages saying the same identical thing. For
instance, five consecutive messages concerning a particular rule would be logged to
syslogd, the remainder identical consecutive messages would be counted and posted to the
syslogd with a phrase like this:
last message repeated 45 times
All logged packets messages are written by default to /var/log/security file, which is defined in the /etc/syslog.conf file.
Most experienced IPFW users create a file containing the rules and code them in a
manner compatible with running them as a script. The major benefit of doing this is the
firewall rules can be refreshed in mass without the need of rebooting the system to
activate the new rules. This method is very convenient in testing new rules as the
procedure can be executed as many times as needed. Being a script, you can use symbolic
substitution to code frequent used values and substitution them in multiple rules. You
will see this in the following example.
The script syntax used here is compatible with the 'sh', 'csh', 'tcsh' shells.
Symbolic substitution fields are prefixed with a dollar sign $. Symbolic fields do not
have the $ prefix. The value to populate the Symbolic field must be enclosed to "double
quotes".
Start your rules file like this:
############### start of example ipfw rules script #############
#
ipfw -q -f flush # Delete all rules
# Set defaults
oif="tun0" # out interface
odns="192.0.2.11" # ISP's DNS server IP address
cmd="ipfw -q add " # build rule prefix
ks="keep-state" # just too lazy to key this each time
$cmd 00500 check-state
$cmd 00502 deny all from any to any frag
$cmd 00501 deny tcp from any to any established
$cmd 00600 allow tcp from any to any 80 out via $oif setup $ks
$cmd 00610 allow tcp from any to $odns 53 out via $oif setup $ks
$cmd 00611 allow udp from any to $odns 53 out via $oif $ks
################### End of example ipfw rules script ############
That is all there is to it. The rules are not important in this example, how the
Symbolic substitution field are populated and used are.
If the above example was in /etc/ipfw.rules file, you could
reload these rules by entering on the command line.
# sh /etc/ipfw.rules
The /etc/ipfw.rules file could be located anywhere you want
and the file could be named any thing you would like.
The same thing could also be accomplished by running these commands by hand:
# ipfw -q -f flush
# ipfw -q add check-state
# ipfw -q add deny all from any to any frag
# ipfw -q add deny tcp from any to any established
# ipfw -q add allow tcp from any to any 80 out via tun0 setup keep-state
# ipfw -q add allow tcp from any to 192.0.2.11 53 out via tun0 setup keep-state
# ipfw -q add 00611 allow udp from any to 192.0.2.11 53 out via tun0 keep-state
The following non-NATed rule set is an example of
how to code a very secure 'inclusive' type of firewall. An inclusive firewall only allows
services matching pass rules through and blocks all other by default. All firewalls have
at the minimum two interfaces which have to have rules to allow the firewall to
function.
All UNIX® flavored operating systems, FreeBSD
included, are designed to use interface lo0 and IP address
127.0.0.1 for internal communication with in the operating
system. The firewall rules must contain rules to allow free unmolested movement of these
special internally used packets.
The interface which faces the public Internet, is the one which you code your rules to
authorize and control access out to the public Internet and access requests arriving from
the public Internet. This can be your ppp tun0 interface or
your NIC that is connected to your DSL or cable modem.
In cases where one or more than one NIC are connected to a private LANs behind the
firewall, those interfaces must have rules coded to allow free unmolested movement of
packets originating from those LAN interfaces.
The rules should be first organized into three major sections, all the free unmolested
interfaces, public interface outbound, and the public interface inbound.
The order of the rules in each of the public interface sections should be in order of
the most used rules being placed before less often used rules with the last rule in the
section being a block log all packets on that interface and direction.
The Outbound section in the following rule set only contains 'allow' rules which
contain selection values that uniquely identify the service that is authorized for public
Internet access. All the rules have the, proto, port, in/out, via and keep state option
coded. The 'proto tcp' rules have the 'setup' option included to identify the start
session request as the trigger packet to be posted to the keep state stateful table.
The Inbound section has all the blocking of undesirable packets first for two
different reasons. First is these things being blocked may be part of an otherwise valid
packet which may be allowed in by the later authorized service rules. Second reason is
that by having a rule that explicitly blocks selected packets that I receive on an
infrequent bases and do not want to see in the log, this keeps them from being caught by
the last rule in the section which blocks and logs all packets which have fallen through
the rules. The last rule in the section which blocks and logs all packets is how you
create the legal evidence needed to prosecute the people who are attacking your
system.
Another thing you should take note of, is there is no response returned for any of the
undesirable stuff, their packets just get dropped and vanish. This way the attackers has
no knowledge if his packets have reached your system. The less the attackers can learn
about your system the more secure it is. When you log packets with port numbers you do
not recognize, look the numbers up in /etc/services/ or go to
https://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: https://www.simovits.com/trojans/trojans.html.
The following non-NATed rule set is a complete
inclusive type ruleset. You can not go wrong using this rule set for you own. Just
comment out any pass rules for services you do not want. If you see messages in your log
that you want to stop seeing just add a deny rule in the inbound section. You have to
change the 'dc0' interface name in every rule to the interface name of the NIC that
connects your system to the public Internet. For user ppp it would be 'tun0'.
You will see a pattern in the usage of these rules.
-
All statements that are a request to start a session to the public Internet use
keep-state.
-
All the authorized services that originate from the public Internet have the limit
option to stop flooding.
-
All rules use in or out to clarify direction.
-
All rules use via interface name to specify the interface the packet is traveling
over.
The following rules go into /etc/ipfw.rules.
################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
pif="dc0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Not needed unless you have LAN.
# Change xl0 to your LAN NIC interface name
#################################################################
#$cmd 00005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
$cmd 00010 allow all from any to any via lo0
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
$cmd 00015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network or from this gateway server
# destine for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP.s DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
$cmd 00110 allow tcp from any to x.x.x.x 53 out via $pif setup keep-state
$cmd 00111 allow udp from any to x.x.x.x 53 out via $pif keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
# This rule is not needed for .user ppp. connection to the public Internet.
# so you can delete this whole group.
# Use the following rule and check log for IP address.
# Then put IP address in commented out rule & delete first rule
$cmd 00120 allow log udp from any to any 67 out via $pif keep-state
#$cmd 00120 allow udp from any to x.x.x.x 67 out via $pif keep-state
# Allow out non-secure standard www function
$cmd 00200 allow tcp from any to any 80 out via $pif setup keep-state
# Allow out secure www function https over TLS SSL
$cmd 00220 allow tcp from any to any 443 out via $pif setup keep-state
# Allow out send & get email function
$cmd 00230 allow tcp from any to any 25 out via $pif setup keep-state
$cmd 00231 allow tcp from any to any 110 out via $pif setup keep-state
# Allow out FBSD (make install & CVSUP) functions
# Basically give user root "GOD" privileges.
$cmd 00240 allow tcp from me to any out via $pif setup keep-state uid root
# Allow out ping
$cmd 00250 allow icmp from any to any out via $pif keep-state
# Allow out Time
$cmd 00260 allow tcp from any to any 37 out via $pif setup keep-state
# Allow out nntp news (i.e. news groups)
$cmd 00270 allow tcp from any to any 119 out via $pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
$cmd 00280 allow tcp from any to any 22 out via $pif setup keep-state
# Allow out whois
$cmd 00290 allow tcp from any to any 43 out via $pif setup keep-state
# deny and log everything else that.s trying to get out.
# This rule enforces the block all by default logic.
$cmd 00299 deny log all from any to any out via $pif
#################################################################
# Interface facing Public Internet (Inbound Section)
# Interrogate packets originating from the public Internet
# destine for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
$cmd 00300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP
$cmd 00301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP
$cmd 00302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP
$cmd 00303 deny all from 127.0.0.0/8 to any in via $pif #loopback
$cmd 00304 deny all from 0.0.0.0/8 to any in via $pif #loopback
$cmd 00305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config
$cmd 00306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs
$cmd 00307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster interconnect
$cmd 00308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast
# Deny public pings
$cmd 00310 deny icmp from any to any in via $pif
# Deny ident
$cmd 00315 deny tcp from any to any 113 in via $pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
$cmd 00320 deny tcp from any to any 137 in via $pif
$cmd 00321 deny tcp from any to any 138 in via $pif
$cmd 00322 deny tcp from any to any 139 in via $pif
$cmd 00323 deny tcp from any to any 81 in via $pif
# Deny any late arriving packets
$cmd 00330 deny all from any to any frag in via $pif
# Deny ACK packets that did not match the dynamic rule table
$cmd 00332 deny tcp from any to any established in via $pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP.s DHCP server as it.s the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for .user ppp. type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
#$cmd 00360 allow udp from any to x.x.x.x 67 in via $pif keep-state
# Allow in standard www function because I have apache server
$cmd 00400 allow tcp from any to me 80 in via $pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
$cmd 00410 allow tcp from any to me 22 in via $pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID & PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
$cmd 00420 allow tcp from any to me 23 in via $pif setup limit src-addr 2
# Reject & Log all incoming connections from the outside
$cmd 00499 deny log all from any to any in via $pif
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
$cmd 00999 deny log all from any to any
################ End of IPFW rules file ###############################
There are some additional configuration statements that need to be enabled to activate
the NAT function of IPFW. The kernel source needs
'option IPDIVERT' statement added to the other IPFIREWALL statements compiled into a
custom kernel.
In addition to the normal IPFW options in /etc/rc.conf, the
following are needed.
natd_enable="YES" # Enable NATD function
natd_interface="rl0" # interface name of public Internet NIC
natd_flags="-dynamic -m" # -m = preserve port numbers if possible
Utilizing stateful rules with divert natd rule (Network Address Translation) greatly
complicates the rule set coding logic. The positioning of the check-state, and 'divert
natd' rules in the rule set becomes very critical. This is no longer a simple
fall-through logic flow. A new action type is used, called 'skipto'. To use the skipto
command it is mandatory that you number each rule so you know exactly where the skipto
rule number is you are really jumping to.
The following is an uncommented example of one coding method, selected here to explain
the sequence of the packet flow through the rule sets.
The processing flow starts with the first rule from the top of the rule file and
progress one rule at a time deeper into the file until the end is reach or the packet
being tested to the selection criteria matches and the packet is released out of the
firewall. It is important to take notice of the location of rule numbers 100 101, 450,
500, and 510. These rules control the translation of the outbound and inbound packets so
their entries in the keep-state dynamic table always register the private LAN IP address.
Next notice that all the allow and deny rules specified the direction the packet is going
(IE outbound or inbound) and the interface. Also notice that all the start outbound
session requests all skipto rule 500 for the network address translation.
Lets say a LAN user uses their web browser to get a web page. Web pages use port 80 to
communicate over. So the packet enters the firewall, It does not match 100 because it is
headed out not in. It passes rule 101 because this is the first packet so it has not been
posted to the keep-state dynamic table yet. The packet finally comes to rule 125 a
matches. It is outbound through the NIC facing the public Internet. The packet still has
it's source IP address as a private LAN IP address. On the match to this rule, two
actions take place. The keep-state option will post this rule into the keep-state dynamic
rules table and the specified action is executed. The action is part of the info posted
to the dynamic table. In this case it is "skipto rule 500". Rule 500 NATs the packet IP address and out it goes. Remember this, this
is very important. This packet makes its way to the destination and returns and enters
the top of the rule set. This time it does match rule 100 and has it destination IP
address mapped back to its corresponding LAN IP address. It then is processed by the
check-state rule, it's found in the table as an existing session conversation and
released to the LAN. It goes to the LAN PC that sent it and a new packet is sent
requesting another segment of the data from the remote server. This time it gets checked
by the check-state rule and its outbound entry is found, the associated action, 'skipto
500', is executed. The packet jumps to rule 500 gets NATed and released on it's way out.
On the inbound side, everything coming in that is part of an existing session
conversation is being automatically handled by the check-state rule and the properly
placed divert natd rules. All we have to address is denying all the bad packets and only
allowing in the authorized services. Lets say there is a apache server running on the
firewall box and we want people on the public Internet to be able to access the local web
site. The new inbound start request packet matches rule 100 and its IP address is mapped
to LAN IP for the firewall box. The packet is them matched against all the nasty things
we want to check for and finally matches against rule 425. On a match two things occur.
The packet rule is posted to the keep-state dynamic table but this time any new session
requests originating from that source IP address is limited to 2. This defends against
DoS attacks of service running on the specified port number. The action is allow so the
packet is released to the LAN. On return the check-state rule recognizes the packet as
belonging to an existing session conversation sends it to rule 500 for NATing and released to outbound interface.
Example Ruleset #1:
#!/bin/sh
cmd="ipfw -q add"
skip="skipto 500"
pif=rl0
ks="keep-state"
good_tcpo="22,25,37,43,53,80,443,110,119"
ipfw -q -f flush
$cmd 002 allow all from any to any via xl0 # exclude LAN traffic
$cmd 003 allow all from any to any via lo0 # exclude loopback traffic
$cmd 100 divert natd ip from any to any in via $pif
$cmd 101 check-state
# Authorized outbound packets
$cmd 120 $skip udp from any to xx.168.240.2 53 out via $pif $ks
$cmd 121 $skip udp from any to xx.168.240.5 53 out via $pif $ks
$cmd 125 $skip tcp from any to any $good_tcpo out via $pif setup $ks
$cmd 130 $skip icmp from any to any out via $pif $ks
$cmd 135 $skip udp from any to any 123 out via $pif $ks
# Deny all inbound traffic from non-routable reserved address spaces
$cmd 300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP
$cmd 301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP
$cmd 302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP
$cmd 303 deny all from 127.0.0.0/8 to any in via $pif #loopback
$cmd 304 deny all from 0.0.0.0/8 to any in via $pif #loopback
$cmd 305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config
$cmd 306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs
$cmd 307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster
$cmd 308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast
# Authorized inbound packets
$cmd 400 allow udp from xx.70.207.54 to any 68 in $ks
$cmd 420 allow tcp from any to me 80 in via $pif setup limit src-addr 1
$cmd 450 deny log ip from any to any
# This is skipto location for outbound stateful rules
$cmd 500 divert natd ip from any to any out via $pif
$cmd 510 allow ip from any to any
######################## end of rules ##################
The following is pretty much the same as above, but uses a self documenting coding
style full of description comments to help the inexperienced IPFW rule writer to better
understand what the rules are doing.
Example Ruleset #2:
#!/bin/sh
################ Start of IPFW rules file ###############################
# Flush out the list before we begin.
ipfw -q -f flush
# Set rules command prefix
cmd="ipfw -q add"
skip="skipto 800"
pif="rl0" # public interface name of NIC
# facing the public Internet
#################################################################
# No restrictions on Inside LAN Interface for private network
# Change xl0 to your LAN NIC interface name
#################################################################
$cmd 005 allow all from any to any via xl0
#################################################################
# No restrictions on Loopback Interface
#################################################################
$cmd 010 allow all from any to any via lo0
#################################################################
# check if packet is inbound and nat address if it is
#################################################################
$cmd 014 divert natd ip from any to any in via $pif
#################################################################
# Allow the packet through if it has previous been added to the
# the "dynamic" rules table by a allow keep-state statement.
#################################################################
$cmd 015 check-state
#################################################################
# Interface facing Public Internet (Outbound Section)
# Interrogate session start requests originating from behind the
# firewall on the private network or from this gateway server
# destine for the public Internet.
#################################################################
# Allow out access to my ISP's Domain name server.
# x.x.x.x must be the IP address of your ISP's DNS
# Dup these lines if your ISP has more than one DNS server
# Get the IP addresses from /etc/resolv.conf file
$cmd 020 $skip tcp from any to x.x.x.x 53 out via $pif setup keep-state
# Allow out access to my ISP's DHCP server for cable/DSL configurations.
$cmd 030 $skip udp from any to x.x.x.x 67 out via $pif keep-state
# Allow out non-secure standard www function
$cmd 040 $skip tcp from any to any 80 out via $pif setup keep-state
# Allow out secure www function https over TLS SSL
$cmd 050 $skip tcp from any to any 443 out via $pif setup keep-state
# Allow out send & get email function
$cmd 060 $skip tcp from any to any 25 out via $pif setup keep-state
$cmd 061 $skip tcp from any to any 110 out via $pif setup keep-state
# Allow out FreeBSD (make install & CVSUP) functions
# Basically give user root "GOD" privileges.
$cmd 070 $skip tcp from me to any out via $pif setup keep-state uid root
# Allow out ping
$cmd 080 $skip icmp from any to any out via $pif keep-state
# Allow out Time
$cmd 090 $skip tcp from any to any 37 out via $pif setup keep-state
# Allow out nntp news (i.e. news groups)
$cmd 100 $skip tcp from any to any 119 out via $pif setup keep-state
# Allow out secure FTP, Telnet, and SCP
# This function is using SSH (secure shell)
$cmd 110 $skip tcp from any to any 22 out via $pif setup keep-state
# Allow out whois
$cmd 120 $skip tcp from any to any 43 out via $pif setup keep-state
# Allow ntp time server
$cmd 130 $skip udp from any to any 123 out via $pif keep-state
#################################################################
# Interface facing Public Internet (Inbound Section)
# Interrogate packets originating from the public Internet
# destine for this gateway server or the private network.
#################################################################
# Deny all inbound traffic from non-routable reserved address spaces
$cmd 300 deny all from 192.168.0.0/16 to any in via $pif #RFC 1918 private IP
$cmd 301 deny all from 172.16.0.0/12 to any in via $pif #RFC 1918 private IP
$cmd 302 deny all from 10.0.0.0/8 to any in via $pif #RFC 1918 private IP
$cmd 303 deny all from 127.0.0.0/8 to any in via $pif #loopback
$cmd 304 deny all from 0.0.0.0/8 to any in via $pif #loopback
$cmd 305 deny all from 169.254.0.0/16 to any in via $pif #DHCP auto-config
$cmd 306 deny all from 192.0.2.0/24 to any in via $pif #reserved for docs
$cmd 307 deny all from 204.152.64.0/23 to any in via $pif #Sun cluster
$cmd 308 deny all from 224.0.0.0/3 to any in via $pif #Class D & E multicast
# Deny ident
$cmd 315 deny tcp from any to any 113 in via $pif
# Deny all Netbios service. 137=name, 138=datagram, 139=session
# Netbios is MS/Windows sharing services.
# Block MS/Windows hosts2 name server requests 81
$cmd 320 deny tcp from any to any 137 in via $pif
$cmd 321 deny tcp from any to any 138 in via $pif
$cmd 322 deny tcp from any to any 139 in via $pif
$cmd 323 deny tcp from any to any 81 in via $pif
# Deny any late arriving packets
$cmd 330 deny all from any to any frag in via $pif
# Deny ACK packets that did not match the dynamic rule table
$cmd 332 deny tcp from any to any established in via $pif
# Allow traffic in from ISP's DHCP server. This rule must contain
# the IP address of your ISP's DHCP server as it's the only
# authorized source to send this packet type.
# Only necessary for cable or DSL configurations.
# This rule is not needed for 'user ppp' type connection to
# the public Internet. This is the same IP address you captured
# and used in the outbound section.
$cmd 360 allow udp from x.x.x.x to any 68 in via $pif keep-state
# Allow in standard www function because I have Apache server
$cmd 370 allow tcp from any to me 80 in via $pif setup limit src-addr 2
# Allow in secure FTP, Telnet, and SCP from public Internet
$cmd 380 allow tcp from any to me 22 in via $pif setup limit src-addr 2
# Allow in non-secure Telnet session from public Internet
# labeled non-secure because ID & PW are passed over public
# Internet as clear text.
# Delete this sample group if you do not have telnet server enabled.
$cmd 390 allow tcp from any to me 23 in via $pif setup limit src-addr 2
# Reject & Log all unauthorized incoming connections from the public Internet
$cmd 400 deny log all from any to any in via $pif
# Reject & Log all unauthorized out going connections to the public Internet
$cmd 450 deny log all from any to any out via $pif
# This is skipto location for outbound stateful rules
$cmd 800 divert natd ip from any to any out via $pif
$cmd 801 allow ip from any to any
# Everything else is denied by default
# deny and log all packets that fell through to see what they are
$cmd 999 deny log all from any to any
################ End of IPFW rules file ###############################
