The IPFIREWALL (IPFW) is a FreeBSD sponsored firewall software application authored and maintained by FreeBSD volunteer staff members. It uses the legacy stateless rules and a legacy rule coding technique to achieve what is referred to as Simple Stateful logic.
The IPFW sample ruleset (found in /etc/rc.firewall and /etc/rc.firewall6) in the standard FreeBSD install is rather simple and it is not expected to be used directly without modifications. The example does not use stateful filtering, which is beneficial in most setups, so it will not be used as base for this section.
The IPFW stateless rule syntax is empowered with technically sophisticated selection capabilities which far surpasses the knowledge level of the customary firewall installer. IPFW is targeted at the professional user or the advanced technical computer hobbyist who have advanced packet selection requirements. A high degree of detailed knowledge into how different protocols use and create their unique packet header information is necessary before the power of the IPFW rules can be unleashed. Providing that level of explanation is out of the scope of this section of the Handbook.
IPFW is composed of seven components, the primary component is the kernel firewall filter rule processor and its integrated packet accounting facility, the logging facility, the divert rule which triggers the NAT facility, and the advanced special purpose facilities, the dummynet traffic shaper facilities, the fwd rule forward facility, the bridge facility, and the ipstealth facility. IPFW supports both IPv4 and IPv6.
IPFW is included in the basic FreeBSD install as a separate run time loadable module. The system will dynamically load the kernel module when the rc.conf statement firewall_enable="YES" is used. There is no need to compile IPFW into the FreeBSD kernel.
After rebooting your system with firewall_enable="YES" in rc.conf the following white highlighted message is displayed on the screen as part of the boot process:
ipfw2 initialized, divert disabled, rule-based forwarding disabled, default to deny, logging disabled
The loadable module does have logging ability compiled in. To enable logging and set the verbose logging limit, there is a knob that can be set in /etc/sysctl.conf. By adding these statements, logging will be enabled on future reboots:
net.inet.ip.fw.verbose=1 net.inet.ip.fw.verbose_limit=5
It is not a mandatory requirement to enable IPFW by compiling the following options into the FreeBSD kernel. It is presented here as background information only.
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 ruleset.
options IPFIREWALL_VERBOSE_LIMIT=5
Limits the number of packets logged through syslogd(8) on a per entry basis. This option may be used in hostile environments, when firewall activity logging is desired. 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 the firewall is being set up for the first time.
options IPDIVERT
This enables the use of NAT functionality.
Note: The firewall will block all incoming and outgoing packets if either the IPFIREWALL_DEFAULT_TO_ACCEPT kernel option or a rule to explicitly allow these connections are missing.
Enable the firewall:
firewall_enable="YES"
To select one of the default firewall types provided by FreeBSD, select one by reading the /etc/rc.firewall file and place it in the following:
firewall_type="open"
Available values for this setting are:
open — pass all traffic.
client — will protect only this machine.
simple — protect the whole network.
closed — entirely disables IP traffic except for the loopback interface.
UNKNOWN — disables the loading of firewall rules.
filename — absolute path of file containing firewall rules.
It is possible to use two different ways to load custom rules for ipfw firewall. One is by setting firewall_type variable to absolute path of file, which contains firewall rules without any command-line options for ipfw(8) itself. The following is a simple example of a ruleset file that blocksall incoming and outgoing traffic:
add deny in add deny out
On the other hand, it is possible to set the firewall_script variable to the absolute path of an executable script that includes ipfw commands being executed at system boot time. A valid ruleset script that would be equivalent to the ruleset file shown above would be the following:
#!/bin/sh ipfw -q flush ipfw add deny in ipfw add deny out
Note: If firewall_type is set to either client or simple, the default rules found in /etc/rc.firewall should be reviewed to fit to the configuration of the given machine. Also note that the examples used in this chapter expect that the firewall_script is set to /etc/ipfw.rules.
Enable logging:
firewall_logging="YES"
Warning: The only thing that the
firewall_loggingvariable will do is setting thenet.inet.ip.fw.verbosesysctl variable to the value of 1 (see Section 31.6.1). There is no rc.conf variable to set log limitations, but it can be set via sysctl variable, manually or from the /etc/sysctl.conf file:net.inet.ip.fw.verbose_limit=5
If your machine is acting as a gateway, i.e., providing Network Address Translation (NAT) via natd(8), please refer to Section 32.10 for information regarding the required /etc/rc.conf options.
The ipfw command is the normal vehicle for making manual single rule additions or deletions to the active firewall internal rules while it is running. The problem with using this method is once your system is shutdown or halted all the rules that were added, 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 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 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
The next example lists accounting information, the 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 the rule with number NUM:
# ipfw zero NUM
A ruleset 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 ruleset processes both the packets arriving from the public Internet, as well as the packets originating from the system as a response to them. Each TCP/IP service (i.e.: telnet, www, mail, etc.) is predefined by its protocol and privileged (listening) port. Packets destined for a specific service, originate from the source address using an unprivileged (high order) port and target the specific service port on the destination address. All the above parameters (i.e., ports and addresses) can be used as selection criteria to create rules which will pass or block services.
When a packet enters the firewall it is compared against the first rule in the ruleset and progresses one rule at a time moving from top to bottom of the set in ascending rule number sequence order. When the packet matches the selection parameters of a rule, the rules' action field value is executed and the search of the ruleset 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 ruleset.
Warning: Be careful when working with firewall rules, as it is easy to end up locking yourself out.
The rule syntax presented here has been simplified to what is necessary to create a standard inclusive type firewall ruleset. 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 is associated with a rule_number in the range 1..65535.
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 ruleset, 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(8) 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 checked 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
Any other protocol names found in /etc/protocols are also recognized and may be used. The value specified is the 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 (CIDR notation), or as single dotted IP address numeric form. This is a mandatory requirement. The net-mgmt/ipcalc port may be used to ease up the calculations. Additional information is available in the utility's web page: http://jodies.de/ipcalc.
port number
For protocols which support port numbers (such as TCP and UDP), it is mandatory to code the port number of the service that will be matched. 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 one or the other is coded 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 the same rule. The limit option 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 matching capabilities 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.
The check-state option 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 dynamically 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 ruleset 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 checking 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 and 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 ruleset will be logged, 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 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 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 messages 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(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 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 syslogd with a phrase like the following:
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 them. This method is very convenient in testing new rules as the procedure can be executed as many times as needed. Being a script, symbolic substitution can be used to code frequent used values and substitute them in multiple rules. This is shown in the following example.
The script syntax used here is compatible with the sh(1), csh(1), 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 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 the /etc/ipfw.rules file, the rules could be reloaded by entering the following 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 ruleset 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. Firewalls designed to protect entire network segments, have at minimum two interfaces which must 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 to place the rules that authorize and control access of the outbound and inbound connections. This can be your user PPP tun0 interface or your NIC that is connected to your DSL or cable modem.
In cases where one or more than one NICs are connected to a private LAN 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 blocking and logging all packets on that interface and direction.
The Outbound section in the following ruleset 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. The first is that malicious packets may be partial matches for legitimate traffic. These packets have to be discarded rather than allowed in, based on their partial matches against allow rules. The second reason is that known and uninteresting rejects may be blocked silently, rather than being caught and logged by the last rules in the section. The final rule in each section, blocks and logs all packets and can be used to create the legal evidence needed to prosecute the people who are attacking your system.
Another thing that should be taken care of, is to insure there is no response returned for any of the undesirable stuff. Invalid packets should just get dropped and vanish. This way the attacker has no knowledge if his packets have reached your system. The less the attackers can learn about your system, the more secure it is. Packets with unrecognized port numbers may be looked up in /etc/services/ or go to http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers and do a port number lookup to find the purpose of the particular port number is. Check out this link for port numbers used by Trojans: http://www.sans.org/security-resources/idfaq/oddports.php.
The following non-NATed ruleset is a complete inclusive type ruleset. It is safe to use this ruleset on your own systems. Just comment out any pass rules for services that are not required. To avoid logging undesired messages, add a deny rule in the inbound section. The dc0 interface will have to be changed in every rule, with the actual name of the interface (NIC) that connects your system to the public Internet. For user PPP, this would be tun0.
There is a noticeable 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
# destined 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)
# Check packets originating from the public Internet
# destined 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 ruleset coding logic. The positioning of the check-state, and divert natd rules in the ruleset 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 each rule is numbered, so the skipto rule number knows exactly where it is jumping to.
The following is an uncommented example of one coding method, selected here to explain the sequence of the packet flow through the rulesets.
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 reached 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 specify the direction the packet is going (i.e.: outbound or inbound) and the interface. Also notice that 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 are transmitted over port 80. So the 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, 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, where a response packet is generated and sent back. This new packet enters the top of the ruleset. 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 is 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 an 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 then matched against all the nasty things that need to be checked 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. The packet generated as a response, is recognized by the check-state as belonging to anexisting session conversation. It is then sent to rule 500 for NATing and released to the 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)
# Check session start requests originating from behind the
# firewall on the private network or from this gateway server
# destined 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)
# Check packets originating from the public Internet
# destined 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 ###############################