(\\$1 NOT IN IPFW)

.. .Dd August 13, 2002 .Dt IPFW 8 .Os .Sh NAME .Nm ipfw .Nd IP firewall and traffic shaper control program .Sh SYNOPSIS .Nm .Op Fl q .Cm add .Ar rule .Nm .Op Fl adeftNS rq Cm list | show .Op Ar number ... .Nm .Op Fl f | q .Cm flush .Nm .Op Fl q rq Cm delete | zero | resetlog .Op Cm set .Op Ar number ...

p .Nm .Cm set Oo Cm disable Ar number ... Oc Op Cm enable Ar number ... .Nm .Cm set move .Op Cm rule .Ar number Cm to Ar number .Nm .Cm set swap Ar number number .Nm .Cm set show

p .Nm rq Cm pipe | queue .Ar number .Cm config .Ar config-options .Nm .Op Fl s Op Ar field rq Cm pipe | queue rq Cm delete | list | show .Op Ar number ...

p .Nm .Op Fl q .Oo .Fl p Ar preproc .Oo Fl D .Ar macro Ns Op = Ns Ar value .Oc .Op Fl U Ar macro .Oc .Ar pathname .Sh DESCRIPTION The .Nm utility is the user interface for controlling the .Xr ipfw 4 firewall and the .Xr dummynet 4 traffic shaper in .Fx .

p .Em NOTE: this manual page refers to the newer version of .Nm introduced in July 2002, also known as .Nm ipfw2 . The commands listed here are a superset of the old firewall, which we will call .Nm ipfw1 when it is necessary to distinguish between the two.

p .Nm ipfw2 is standard in .Fx CURRENT, whereas .Fx STABLE still uses .Nm ipfw1 unless the kernel is compiled with .Cm options IPFW2 , and .Nm /sbin/ipfw and .Nm /usr/lib/libalias are recompiled with .Cm -DIPFW2 and reinstalled (the same effect can be achieved by adding .Cm IPFW2=TRUE to .Nm /etc/make.conf before a buildworld).

p See the .Sx IPFW2 ENHANCEMENTS Section for a list of features which are not present in .Nm ipfw1 . This list can also be useful to revise your rules and write them more efficiently.

p An .Nm configuration, or .Em ruleset , is made of a list of .Em rules numbered from 1 to 65535. Packets are passed to .Nm in a number of different places in the protocol stack (depending on the source and destination of the packet, it is possible that .Nm is invoked multiple times on the same packet). The packet passed to the firewall is compared against each of the rules in the firewall .Em ruleset . When a match is found, the action corresponding to the matching rule is performed.

p Depending on the action and certain system settings, packets can be reinjected into the firewall at some rule after the matching one for further processing.

p An .Nm ruleset always includes a .Em default rule (numbered 65535) which cannot be modified, and matches all packets. The action associated with the .Em default rule can be either .Cm deny or .Cm allow depending on how the kernel is configured.

p If the ruleset includes one or more rules with the .Cm keep-state or .Cm limit option, then .Nm assumes a .Em stateful behaviour, i.e. upon a match it will create dynamic rules matching the exact parameters (addresses and ports) of the matching packet.

p These dynamic rules, which have a limited lifetime, are checked at the first occurrence of a .Cm check-state , .Cm keep-state or .Cm limit rule, and are typically used to open the firewall on-demand to legitimate traffic only. See the .Sx STATEFUL FIREWALL and .Sx EXAMPLES Sections below for more information on the stateful behaviour of .Nm .

p All rules (including dynamic ones) have a few associated counters: a packet count, a byte count, a log count and a timestamp indicating the time of the last match. Counters can be displayed or reset with .Nm commands.

p Rules can be added with the .Cm add command; deleted individually or in groups with the .Cm delete command, and globally with the .Cm flush command; displayed, optionally with the content of the counters, using the .Cm show and .Cm list commands. Finally, counters can be reset with the .Cm zero and .Cm resetlog commands.

p Also, each rule belongs to one of 32 different .Em sets , and there are .Nm commands to atomically manipulate sets, such as enable, disable, swap sets, move all rules in a set to another one, delete all rules in a set. These can be useful to install temporary configurations, or to test them. See Section .Sx SETS OF RULES for more information on .Em sets .

p The following options are available: l -tag -width indent t Fl a While listing, show counter values. The .Cm show command just implies this option. t Fl d While listing, show dynamic rules in addition to static ones. t Fl e While listing, if the .Fl d option was specified, also show expired dynamic rules. t Fl f Don't ask for confirmation for commands that can cause problems if misused, .No i.e. Cm flush . If there is no tty associated with the process, this is implied. t Fl N Try to resolve addresses and service names in output. t Fl q While .Cm add Ns ing , .Cm zero Ns ing , .Cm resetlog Ns ging or .Cm flush Ns ing , be quiet about actions (implies .Fl f ) . This is useful for adjusting rules by executing multiple .Nm commands in a script (e.g., .Ql sh /etc/rc.firewall ) , or by processing a file of many .Nm rules, across a remote login session. If a .Cm flush is performed in normal (verbose) mode (with the default kernel configuration), it prints a message. Because all rules are flushed, the message might not be delivered to the login session, causing the remote login session to be closed and the remainder of the ruleset is not processed. Access to the console would then be required to recover. t Fl S While listing rules, show the .Em set each rule belongs to. If this flag is not specified, disabled rules will not be listed. t Fl s Op Ar field While listing pipes, sort according to one of the four counters (total and current packets or bytes). t Fl t While listing, show last match timestamp. .El

p To ease configuration, rules can be put into a file which is processed using .Nm as shown in the first synopsis line. An absolute .Ar pathname must be used. The file will be read line by line and applied as arguments to the .Nm utility.

p Optionally, a preprocessor can be specified using .Fl p Ar preproc where .Ar pathname is to be piped through. Useful preprocessors include .Xr cpp 1 and .Xr m4 1 . If .Ar preproc doesn't start with a slash

q Ql / as its first character, the usual .Ev PATH name search is performed. Care should be taken with this in environments where not all filesystems are mounted (yet) by the time .Nm is being run (e.g. when they are mounted over NFS). Once .Fl p has been specified, optional .Fl D and .Fl U specifications can follow and will be passed on to the preprocessor. This allows for flexible configuration files (like conditionalizing them on the local hostname) and the use of macros to centralize frequently required arguments like IP addresses.

p The .Nm .Cm pipe and .Cm queue commands are used to configure the traffic shaper, as shown in the .Sx TRAFFIC SHAPER CONFIGURATION Section below. .Sh PACKET FLOW .Nm can be invoked from multiple places in the protocol stack, under control of several system parameters, and it is important to understand when this occurs in order to design a proper ruleset. The places where .Nm is invoked are listed below, together with the sysctl variables which control its invocation. d -literal -offset indent ^ to upper layers V | | +----------->-----------+ ^ V [ip_input] [ip_output] net.inet.ip.fw.enable=1 | | ^ V [ether_demux] [ether_output_frame] net.link.ether.ipfw=1 | | +-->--[bdg_forward]-->--+ net.link.ether.bridge_ipfw=1 ^ V | to devices | .Ed

p As can be noted from the above picture, the number of times the same packet goes through the firewall can vary between 0 and 4 depending o packet source and destination, and system configuration. In each of these places, the packet is passed to .Nm with all (and only) the fields that belong to that level. That is, incoming packets will include the MAC header when .Nm is invoked from .Cm ether_demux() , but the same packets will have the MAC header stripped off when .Nm is invoked from .Cm ip_input() .

The complete ruleset is always used, irrespective of the place where .Nm is invoked, or the source of the packet. If a rule contains some match patterns or actions which are not valid for the place of invokation (e.g. trying to match a MAC header when .Nm is called from .Cm ip_input() ) the rule will simply not match. It is thus responsibility of the programmer, if necessary, to write a suitable ruleset to differentiate among the possible places. .Cm skipto rules can be useful here, as an example: d -literal -offset indent # packets from ether_demux or bdg_forward ipfw add 10 skipto 1000 all from any to any layer2 in # packets from ip_input ipfw add 10 skipto 2000 all from any to any not layer2 in # packets from ip_output ipfw add 10 skipto 3000 all from any to any not layer2 out # packets from ether_output_frame ipfw add 10 skipto 4000 all from any to any layer2 out .Ed

p (yes, at the moment there is no way to differentiate between ether_demux and bdg_forward). .Sh RULE FORMAT The format of .Nm rules is the following: d -ragged -offset indent .Op Ar rule_number .Op Cm set Ar set_number .Op Cm prob Ar match_probability

.Ar " " action .Op Cm log Op Cm logamount Ar number .Ar body .Ed

p where the body of the rule specifies which information is used for filtering packets, among the following:

p l -tag -width "Source and dest. addresses and ports" -offset XXX -compact t Layer-2 header fields When available t IPv4 Protocol TCP, UDP, ICMP, etc. t Source and dest. addresses and ports t Direction See Section .Sx PACKET FLOW t Transmit and receive interface By name or address t Misc. IP header fields Version, type of service, datagram length, identification, fragment flag (non-zero IP offset), Time To Live t IP options t Misc. TCP header fields TCP flags (SYN, FIN, ACK, RST, etc.), sequence number, acknowledgment number, window t TCP options t ICMP types for ICMP packets t User/group ID When the packet can be associate to a local socket. .El

p Note that some of the above information, e.g. source MAC or IP addresses and TCP/UDP ports, could easily be spoofed, so filtering on those fields alone might not guarantee the desired results. l -tag -width indent t Ar rule_number Each rule is associated with a .Ar rule_number in the range 1..65535, with the latter reserved for the .Em default rule. Rules are checked sequentially by rule number. Multiple rules can have the same number, in which case they are checked (and listed) according to the order in which they have been added. If a rule is entered without specifying a number, the kernel will assign one in such a way that the rule becomes the last one before the .Em default rule. Automatic rule numbers are assigned by incrementing the last non-default rule number by the value of the sysctl variable .Ar net.inet.ip.fw.autoinc_step which defaults to 100. If this is not possible (e.g. because we would go beyond the maximum allowed rule number), the same number of the last non-default value is used instead. t Cm set Ar set_number Each rule is associated to a .Ar set_number in the range 0..31, with the latter reserved for the .Em default rule. Sets can be individually disabled and enabled, so this parameter is of fundamental importance for atomic ruleset manipulation. It can be also used to simplify deletion of groups of rules. If a rule is entered without specifying a set number, set 0 will be used. t Cm prob Ar match_probability A match is only declared with the specified probability (floating point number between 0 and 1). This can be useful for a number of applications such as random packet drop or (in conjunction with .Xr dummynet 4 ) to simulate the effect of multiple paths leading to out-of-order packet delivery. t Cm log Op Cm logamount Ar number When a packet matches a rule with the .Cm log keyword, a message will be logged to .Xr syslogd 8 with a .Dv LOG_SECURITY facility. The logging only occurs if the sysctl variable .Em net.inet.ip.fw.verbose is set to 1 (which is the default when the kernel is compiled with .Dv IPFIREWALL_VERBOSE ) and the number of packets logged so far for that particular rule does not exceed ther .Cm logamount parameter. If no .Cm logamount is specified, the limit is taken from the sysctl variable .Em net.inet.ip.fw.verbose_limit . In both cases, a value of 0 removes the logging limit.

p Once the limit is reached, logging can be re-enabled by clearing the logging counter or the packet counter for that entry, see the .Cm resetlog command.

p .El .Ss RULE ACTIONS A rule can be associated with one of the following actions, which will be executed when the packet matches the body of the rule. l -tag -width indent t Cm allow | accept | pass | permit Allow packets that match rule. The search terminates. t Cm check-state Checks the packet against the dynamic ruleset. If a match is found, execute the action associated with the rule which generated this dynamic rule, otherwise move to the next rule.

.Cm Check-state rules do not have a body. If no .Cm check-state rule is found, the dynamic ruleset is checked at the first .Cm keep-state or .Cm limit rule. t Cm count Update counters for all packets that match rule. The search continues with the next rule. t Cm deny | drop Discard packets that match this rule. The search terminates. t Cm divert Ar port Divert packets that match this rule to the .Xr divert 4 socket bound to port .Ar port . The search terminates. t Cm fwd | forward Ar ipaddr Ns Op , Ns Ar port Change the next-hop on matching packets to .Ar ipaddr , which can be an IP address in dotted quad or a host name. The search terminates if this rule matches.

p If .Ar ipaddr is a local address, then matching packets will be forwarded to .Ar port (or the port number in the packet if one is not specified in the rule) on the local machine.

If .Ar ipaddr is not a local address, then the port number (if specified) is ignored, and the packet will be forwarded to the remote address, using the route as found in the local routing table for that IP.

A .Ar fwd rule will not match layer-2 packets (those received on ether_input, ether_output, or bridged).

The .Cm fwd action does not change the contents of the packet at all. In particular, the destination address remains unmodified, so packets forwarded to another system will usually be rejected by that system unless there is a matching rule on that system to capture them. For packets forwarded locally, the local address of the socket will be set to the original destination address of the packet. This makes the .Xr netstat 1 entry look rather weird but is intended for use with transparent proxy servers. t Cm pipe Ar pipe_nr Pass packet to a .Xr dummynet 4 .Dq pipe (for bandwidth limitation, delay, etc.). See the .Sx TRAFFIC SHAPER CONFIGURATION Section for further information. The search terminates; however, on exit from the pipe and if the .Xr sysctl 8 variable .Em net.inet.ip.fw.one_pass is not set, the packet is passed again to the firewall code starting from the next rule. t Cm queue Ar queue_nr Pass packet to a .Xr dummynet 4 .Dq queue (for bandwidth limitation using WF2Q). t Cm reject (Deprecated). Synonym for .Cm unreach host . t Cm reset Discard packets that match this rule, and if the packet is a TCP packet, try to send a TCP reset (RST) notice. The search terminates. t Cm skipto Ar number Skip all subsequent rules numbered less than .Ar number . The search continues with the first rule numbered .Ar number or higher. t Cm tee Ar port Send a copy of packets matching this rule to the .Xr divert 4 socket bound to port .Ar port . The search terminates and the original packet is accepted (but see Section .Sx BUGS below). t Cm unreach Ar code Discard packets that match this rule, and try to send an ICMP unreachable notice with code .Ar code , where .Ar code is a number from 0 to 255, or one of these aliases: .Cm net , host , protocol , port , .Cm needfrag , srcfail , net-unknown , host-unknown , .Cm isolated , net-prohib , host-prohib , tosnet , .Cm toshost , filter-prohib , host-precedence or .Cm precedence-cutoff . The search terminates. .El .Ss RULE BODY The body of a rule contains zero or more patterns (such as specific source and destination addresses or ports, protocol options, incoming or outgoing interfaces, etc.) that the packet must match in order to be recognised. In general, the patterns are connected by (implicit) .Em and connectives -- i.e. all must match in order for the rule to match. Individual patterns can be prefixed by the .Em not keyword to reverse the result of the match, as in

p .Dl "ipfw add 100 allow ip from not 1.2.3.4 to any"

p Additionally, sets of alternative match patterns ( .Em or-blocks ) can be constructed by putting the patterns in lists enclosed between parentheses ( ) or braces { }, and using .Cm or connectives as follows:

p .Dl "ipfw add 100 allow ip from { x or not y or z } to any"

p Only one level of parentheses is allowed. Beware that most shells have special meanings for parentheses or braces, so it is advisable to put a \ in front of them.

p The body of a rule must in general comprise a source and destination addres specifier. The keyword .Ar any can be used in various places to specify that the content of a required field is irrelevant.

p The general rule body format is one of the following: d -ragged -offset indent .Ar proto .Cm from Ar src .Cm to Ar dst .Op Ar options

.Cm MAC Ar dst-mac src-mac mac-type .Op Cm from Ar src Cm to Ar dst .Op Ar options .Ed

p where the second format allows you to specify MAC header fields instead (or in addition) of the IPv4 header fields.

p Rule fields have the following meaning: l -tag -width indent t Ar proto An IPv4 protocol specified by number or name (for a complete list see

a /etc/protocols ) . The .Cm ip or .Cm all keywords mean any protocol will match. t Ar src No and Ar dst : A single .Ar ip address , or an .Em or-block containing one or more of them, optionally followed by .Em port numbers. t Ar ip address : An address (or set of addresses) specified in one of the following ways, optionally preceded by a .Cm not operator: l -tag -width indent t Cm any matches any IP address. t Cm me matches any IP address configured on an interface in the system. The address list is evaluated at the time the packet is analysed. t Ar numeric-ip | hostname Matches a single IPv4 address, specified as dotted-quad or a hostname. Hostnames are resolved at the time the rule is added to the firewall list. t Ar addr Ns / Ns Ar masklen Matches all addresses with base .Ar addr (specified as a dotted quad or a hostname) and mask width of .Cm masklen bits. As an example, 1.2.3.4/25 will match all IP numbers from 1.2.3.0 to 1.2.3.127 . t Ar addr Ns / Ns Ar masklen Ns Cm { Ns Ar num,num,... Ns Cm } Matches all addresses with base address .Ar addr (specified as a dotted quad or a hostname) and whose last byte is in the list between braces { } . Note that there must be no spaces between braces, commas and numbers. The .Ar masklen field is used to limit the size of the set of addresses, and can have any value between 24 and 32.

As an example, an address specified as 1.2.3.4/24{128,35,55,89} will match the following IP addresses:

1.2.3.128 1.2.3.35 1.2.3.55 1.2.3.89 .

This format is particularly useful to handle sparse address sets within a single rule. Because the matching occurs using a bitmask, it takes constant time and dramatically reduces the complexity of rulesets. .El t port numbers With protocols which support port numbers (such as TCP and UDP), optional .Cm ports may be specified as one or more ports or port ranges, separated by commas but no spaces, and an optional .Cm not operator: d -ragged -offset indent .Op Cm not rq Ar port | port Ns - Ns Ar port Ns .Op , Ns Ar ... .Ed

p The .Ql - notation specifies a range of ports (including boundaries).

p Service names (from

a /etc/services ) may be used instead of numeric port values. The length of the port list is limited to 14 ports or ranges, though you can also use port ranges within an .Em or-block to build essentially unlimited lists:

p .Dl "ipfw add allow tcp from any { 1-20,30-50 or 500-600 } to any"

p

p A backslash

q Ql \e can be used to escape the dash

q Ql - character in a service name:

p .Dl "ipfw add count tcp from any ftp\e\e-data-ftp to any"

p Fragmented packets which have a non-zero offset (i.e. not the first fragment) will never match a rule which has one or more port specifications. See the .Cm frag option for details on matching fragmented packets. t dst-mac, src-mac Destination and source MAC addresses, specified as groups of hex digits separated by commas, and optionally followed by a mask indicating how many bits are significant:

p .Dl "ipfw add allow MAC 10:20:30:40:50:60/30 any any

p Note that the order of MAC addresses (destination first, source second) is the same as on the wire, but the opposite of the one used for IP addresses. t mac-type The value of the Ethernet Type field, specified in the same way as .Cm port numbers (i.e. one or more comma-separated single values or ranges). You can use symbolic names for known values such as .Em vlan , ipv4, ipv6 . The values can be enter as decimal or hexadecimal, but they are always printed as hexadecimal (unless the .Cm -N option is used, in which case symbolic resolution will be attempted). .El .Ss RULE OPTIONS Additional match patterns can be used within rules. Zero or more of these so-called .Em options can be present in a rule, optionally prefixed by the .Cm not operand, and possibly grouped into .Em or-blocks .

p Note that there is an ambiguity in the syntax: in a rule of the form

p .Dl "ipfw add allow ip from any to any { in or layer2 }"

p the or-block could contain either port lists or options. To remove the ambiguity, one should specify a destination port, which can be done by either using the keyword .Cm any or an empty or-block .Cm { } e.g.:

p .Dl "ipfw add allow ip from any to any any { in or layer2 }"

p The following options are available: l -tag -width indent t Cm bridged Matches only bridged packets. t Cm established TCP packets only. Match packets that have the RST or ACK bits set. t Cm frag Match if the packet is a fragment and this is not the first fragment of the datagram. .Cm frag may not be used in conjunction with either .Cm tcpflags or TCP/UDP port specifications. t Cm gid Ar group Match all TCP or UDP packets sent by or received for a .Ar group . A .Ar group may be matched by name or identification number. t Cm icmptypes Ar types ICMP packets only. Match if the ICMP type is in the list .Ar types . The list may be specified as any combination of ranges or individual types separated by commas. The supported ICMP types are:

p echo reply

q Cm 0 , destination unreachable

q Cm 3 , source quench

q Cm 4 , redirect

q Cm 5 , echo request

q Cm 8 , router advertisement

q Cm 9 , router solicitation

q Cm 10 , time-to-live exceeded

q Cm 11 , IP header bad

q Cm 12 , timestamp request

q Cm 13 , timestamp reply

q Cm 14 , information request

q Cm 15 , information reply

q Cm 16 , address mask request

q Cm 17 and address mask reply

q Cm 18 . t Cm in | out Only match incoming or outgoing packets, respectively. .Cm in and .Cm out are mutually exclusive (in fact, .Cm out is implemented as .Cm not in ). t Cm ipid Ar id Match if the identification of IP datagram is .Ar id . t Cm iplen Ar len Match if the total length of a packet, including header and data, is .Ar len bytes. t Cm ipoptions Ar spec Match if the IP header contains the comma separated list of options specified in .Ar spec . The supported IP options are:

p .Cm ssrr (strict source route), .Cm lsrr (loose source route), .Cm rr (record packet route) and .Cm ts (timestamp). The absence of a particular option may be denoted with a .Ql ! . t Cm ipprecedence Ar precedence Match if the numeric value of IP datagram's precedence is equal to .Ar precedence . t Cm iptos Ar spec Match if the IP header contains the comma separated list of service types specified in .Ar spec . The supported IP types of service are:

p .Cm lowdelay

q Dv IPTOS_LOWDELAY , .Cm throughput

q Dv IPTOS_THROUGHPUT , .Cm reliability

q Dv IPTOS_RELIABILITY , .Cm mincost

q Dv IPTOS_MINCOST , .Cm congestion

q Dv IPTOS_CE . The absence of a particular type may be denoted with a .Ql ! . t Cm ipttl Ar ttl Match if the time to live of IP datagram is .Ar ttl . t Cm ipversion Ar ver Match if the IP header version is .Ar ver . t Cm keep-state Upon a match, the firewall will create a dynamic rule, whose default behaviour is to matching bidirectional traffic between source and destination IP/port using the same protocol. The rule has a limited lifetime (controlled by a set of .Xr sysctl 8 variables), and the lifetime is refreshed every time a matching packet is found. t Cm layer2 Matches only layer2 packets, i.e. those passed to .Nm from ether_demux() and ether_output_frame(). t Cm limit Bro Cm src-addr | src-port | dst-addr | dst-port Brc Ar N The firewall will only allow .Ar 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. t Cm recv | xmit | via Brq Ar ifX | Ar if Ns Cm * | Ar ipno | Ar any Packet must be received, transmitted or be going through, respectively, the interface specified by exact name ( .Ar ifX ), by device name ( .Ar if Ns Cm * ), by IP address, or through some interface.

p The .Cm via keyword causes the interface to always be checked. If .Cm recv or .Cm xmit is used instead of .Cm via , then only the receive or transmit interface (respectively) is checked. By specifying both, it is possible to match packets based on both receive and transmit interface, e.g.:

p .Dl "ipfw add deny ip from any to any out recv ed0 xmit ed1"

p The .Cm recv interface can be tested on either incoming or outgoing packets, while the .Cm xmit interface can only be tested on outgoing packets. So .Cm out is required (and .Cm in is invalid) whenever .Cm xmit is used.

p A packet may not have a receive or transmit interface: packets originating from the local host have no receive interface, while packets destined for the local host have no transmit interface. t Cm setup TCP packets only. Match packets that have the SYN bit set but no ACK bit. This is the short form of .Dq Li tcpflags syn,!ack . t Cm tcpack Ar ack TCP packets only. Match if the TCP header acknowledgment number field is set to .Ar ack . t Cm tcpflags Ar spec TCP packets only. Match if the TCP header contains the comma separated list of flags specified in .Ar spec . The supported TCP flags are:

p .Cm fin , .Cm syn , .Cm rst , .Cm psh , .Cm ack and .Cm urg . The absence of a particular flag may be denoted with a .Ql ! . A rule which contains a .Cm tcpflags specification can never match a fragmented packet which has a non-zero offset. See the .Cm frag option for details on matching fragmented packets. t Cm tcpseq Ar seq TCP packets only. Match if the TCP header sequence number field is set to .Ar seq . t Cm tcpwin Ar win TCP packets only. Match if the TCP header window field is set to .Ar win . t Cm tcpoptions Ar spec TCP packets only. Match if the TCP header contains the comma separated list of options specified in .Ar spec . The supported TCP options are:

p .Cm mss (maximum segment size), .Cm window (tcp window advertisement), .Cm sack (selective ack), .Cm ts (rfc1323 timestamp) and .Cm cc (rfc1644 t/tcp connection count). The absence of a particular option may be denoted with a .Ql ! . t Cm uid Ar user Match all TCP or UDP packets sent by or received for a .Ar user . A .Ar user may be matched by name or identification number. .El .Sh SETS OF RULES Each rule belongs to one of 32 different .Em sets , numbered 0 to 31. Set 31 is reserved for the default rule.

p By default, rules are put in set 0, unless you use the .Cm set N attribute when entering a new rule. Sets can be individually and atomically enabled or disabled, so this mechanism permits an easy way to store multiple configurations of the firewall and quickly (and atomically) switch between them. The command to enable/disable sets is

p .Nm .Cm set disable Ar number ... Op Cm enable Ar number ...

p where multiple .Cm enable or .Cm disable sections can be specified. Command execution is atomic on all the sets specified in the command. By default, all sets are enabled.

p When you disable a set, its rules behave as if they were not existing in the firewall configuration, with only one exception: l -bullet t dynamic rules created from a rule before it had been disabled will still be active until they expire. In order to delete dynamic rules you have to explicitly delete the parent rule which generated them; .El The set number of rules can be changed with the command

p .Nm .Cm set move rq Cm rule Ar rule-number | old-set .Cm to Ar new-set

p Also, you can atomically swap two rulesets with the command

p .Nm .Cm set swap Ar first-set second-set

p See the .Sx EXAMPLES Section on some possible uses of sets of rules. .Sh STATEFUL FIREWALL Stateful operation is a way for the firewall to dynamically create rules for specific flows when packets that match a given pattern are detected. Support for stateful operation comes through the .Cm check-state , keep-state and .Cm limit options of .Nm rules.

p Dynamic rules are created when a packet matches a .Cm keep-state or .Cm limit rule, causing the creation of a .Em dynamic rule which will match all and only packets with a given .Em protocol between a .Em src-ip/src-port dst-ip/dst-port pair of addresses ( .Em src and .Em dst are used here only to denote the initial match addresses, but they are completely equivalent afterwards). Dynamic rules will be checked at the first .Cm check-state, keep-state or .Cm limit occurrence, and the action performed upon a match will be the same as in the parent rule.

p Note that no additional attributes other than protocol and IP addresses and ports are checked on dynamic rules.

p The typical use of dynamic rules is to keep a closed firewall configuration, but let the first TCP SYN packet from the inside network install a dynamic rule for the flow so that packets belonging to that session will be allowed through the firewall:

p .Dl "ipfw add check-state" .Dl "ipfw add allow tcp from my-subnet to any setup" .Dl "ipfw add deny tcp from any to any"

p A similar approach can be used for UDP, where an UDP packet coming from the inside will install a dynamic rule to let the response through the firewall:

p .Dl "ipfw add check-state" .Dl "ipfw add allow udp from my-subnet to any" .Dl "ipfw add deny udp from any to any"

p Dynamic rules expire after some time, which depends on the status of the flow and the setting of some .Cm sysctl variables. See Section .Sx SYSCTL VARIABLES for more details. For TCP sessions, dynamic rules can be instructed to periodically send keepalive packets to refresh the state of the rule when it is about to expire.

p See Section .Sx EXAMPLES for more examples on how to use dynamic rules. .Sh TRAFFIC SHAPER CONFIGURATION .Nm is also the user interface for the .Xr dummynet 4 traffic shaper. The shaper operates by dividing packets into .Em flows according to a user-specified mask on different fields of the IP header. Packets belonging to the same flow are then passed to two different objects, named .Em pipe or .Em queue .

p A .Em pipe emulates a link with given bandwidth, propagation delay, queue size and packet loss rate. Packets transit through the pipe according to its parameters.

p A .Em queue is an abstraction used to implement the WF2Q+ (Worst-case Fair Weighted Fair Queueing) policy. The queue associates to each flow a weight and a reference pipe. Then, all flows linked to the same pipe are scheduled at the rate fixed by the pipe according to the WF2Q+ policy.

p The .Nm pipe configuration format is the following: d -ragged -offset indent .Cm pipe Ar number Cm config Ar pipe-configuration .Ed

p The .Nm queue configuration format is the following: d -ragged -offset indent .Cm queue Ar number Cm config Ar queue-configuration .Ed

p The following parameters can be configured for a pipe:

p l -tag -width indent -compact t Cm bw Ar bandwidth | device Bandwidth, measured in .Sm off .Op Cm K | M rq Cm bit/s | Byte/s . .Sm on

p A value of 0 (default) means unlimited bandwidth. The unit must follow immediately the number, as in

p .Dl "ipfw pipe 1 config bw 300Kbit/s"

p If a device name is specified instead of a numeric value, then the transmit clock is supplied by the specified device. At the moment only the .Xr tun 4 device supports this functionality, for use in conjunction with .Xr ppp 8 .

p t Cm delay Ar ms-delay Propagation delay, measured in milliseconds. The value is rounded to the next multiple of the clock tick (typically 10ms, but it is a good practice to run kernels with .Dq "options HZ=1000" to reduce the granularity to 1ms or less). Default value is 0, meaning no delay. .El

p The following parameters can be configured for a queue:

p l -tag -width indent -compact t Cm pipe Ar pipe_nr Connects a queue to the specified pipe. Multiple queues (usually with different weights) can be connected to the same pipe, which specifies the aggregate rate for the set of queues.

p t Cm weight Ar weight Specifies the weight to be used for flows matching this queue. The weight must be in the range 1..100, and defaults to 1. .El

p Finally, the following parameters can be configured for both pipes and queues:

p l -tag -width indent -compact

p t Cm buckets Ar hash-table-size Specifies the size of the hash table used for storing the various queues. Default value is 64 controlled by the .Xr sysctl 8 variable .Em net.inet.ip.dummynet.hash_size , allowed range is 16 to 1024.

p t Cm mask Ar mask-specifier The .Xr dummynet 4 lets you to create per-flow queues. A flow identifier is constructed by masking the IP addresses, ports and protocol types as specified in the pipe configuration. Packets with the same identifier after masking fall into the same queue. Available mask specifiers are a combination of the following: .Cm dst-ip Ar mask , .Cm src-ip Ar mask , .Cm dst-port Ar mask , .Cm src-port Ar mask , .Cm proto Ar mask or .Cm all , where the latter means all bits in all fields are significant. When used within a .Ar pipe configuration, each flow is assigned a rate equal to the rate of the pipe. When used within a .Ar queue configuration, each flow is assigned a weight equal to the weight of the queue, and all flows insisting on the same pipe share bandwidth proportionally to their weight.

p t Cm noerror When a packet is dropped by a dummynet queue or pipe, the error is normally reported to the caller routine in the kernel, in the same way as it happens when a device queue fills up. Setting this option reports the packet as successfully delivered, which can be needed for some experimental setups where you want to simulate loss or congestion at a remote router.

p t Cm plr Ar packet-loss-rate Packet loss rate. Argument .Ar packet-loss-rate is a floating-point number between 0 and 1, with 0 meaning no loss, 1 meaning 100% loss. The loss rate is internally represented on 31 bits.

p t Cm queue Brq Ar slots | size Ns Cm Kbytes Queue size, in .Ar slots or .Cm KBytes . Default value is 50 slots, which is the typical queue size for Ethernet devices. Note that for slow speed links you should keep the queue size short or your traffic might be affected by a significant queueing delay. E.g., 50 max-sized ethernet packets (1500 bytes) mean 600Kbit or 20s of queue on a 30Kbit/s pipe. Even worse effect can result if you get packets from an interface with a much larger MTU, e.g. the loopback interface with its 16KB packets.

p t Cm red | gred Ar w_q Ns / Ns Ar min_th Ns / Ns Ar max_th Ns / Ns Ar max_p Make use of the RED (Random Early Detection) queue management algorithm. .Ar w_q and .Ar max_p are floating point numbers between 0 and 1 (0 not included), while .Ar min_th and .Ar max_th are integer numbers specifying thresholds for queue management (thresholds are computed in bytes if the queue has been defined in bytes, in slots otherwise). The .Xr dummynet 4 also supports the gentle RED variant (gred). Three .Xr sysctl 8 variables can be used to control the RED behaviour: l -tag -width indent t Em net.inet.ip.dummynet.red_lookup_depth specifies the accuracy in computing the average queue when the link is idle (defaults to 256, must be greater than zero) t Em net.inet.ip.dummynet.red_avg_pkt_size specifies the expected average packet size (defaults to 512, must be greater than zero) t Em net.inet.ip.dummynet.red_max_pkt_size specifies the expected maximum packet size, only used when queue thresholds are in bytes (defaults to 1500, must be greater than zero). .El .El .Sh CHECKLIST Here are some important points to consider when designing your rules: l -bullet t Remember that you filter both packets going .Cm in and .Cm out . Most connections need packets going in both directions. t Remember to test very carefully. It is a good idea to be near the console when doing this. If you cannot be near the console, use an auto-recovery script such as the one in

a /usr/share/examples/ipfw/change_rules.sh . t Don't forget the loopback interface. .El .Sh FINE POINTS l -bullet t There are circumstances where fragmented datagrams are unconditionally dropped. TCP packets are dropped if they do not contain at least 20 bytes of TCP header, UDP packets are dropped if they do not contain a full 8 byte UDP header, and ICMP packets are dropped if they do not contain 4 bytes of ICMP header, enough to specify the ICMP type, code, and checksum. These packets are simply logged as .Dq pullup failed since there may not be enough good data in the packet to produce a meaningful log entry. t Another type of packet is unconditionally dropped, a TCP packet with a fragment offset of one. This is a valid packet, but it only has one use, to try to circumvent firewalls. When logging is enabled, these packets are reported as being dropped by rule -1. t If you are logged in over a network, loading the .Xr kld 4 version of .Nm is probably not as straightforward as you would think. I recommend the following command line: d -literal -offset indent kldload /modules/ipfw.ko && \e ipfw add 32000 allow ip from any to any .Ed

p Along the same lines, doing an d -literal -offset indent ipfw flush .Ed

p in similar surroundings is also a bad idea. t The .Nm filter list may not be modified if the system security level is set to 3 or higher (see .Xr init 8 for information on system security levels). .El .Sh PACKET DIVERSION A .Xr divert 4 socket bound to the specified port will receive all packets diverted to that port. If no socket is bound to the destination port, or if the kernel wasn't compiled with divert socket support, the packets are dropped. .Sh SYSCTL VARIABLES A set of .Xr sysctl 8 variables controls the behaviour of the firewall and associated modules ( .Nm dummynet, bridge ). These are shown below together with their default value (but always check with the .Xr sysctl 8 command what value is actually in use) and meaning: l -tag -width indent t Em net.inet.ip.dummynet.expire : No 1 Lazily delete dynamic pipes/queue once they have no pending traffic. You can disable this by setting the variable to 0, in which case the pipes/queues will only be deleted when the threshold is reached. t Em net.inet.ip.dummynet.hash_size : No 64 Default size of the hash table used for dynamic pipes/queues. This value is used when no .Cm buckets option is specified when configuring a pipe/queue. t Em net.inet.ip.dummynet.max_chain_len : No 16 Target value for the maximum number of pipes/queues in a hash bucket. The product .Cm max_chain_len*hash_size is used to determine the threshold over which empty pipes/queues will be expired even when .Cm net.inet.ip.dummynet.expire=0 . t net.inet.ip.dummynet.red_lookup_depth : No 256 t net.inet.ip.dummynet.red_avg_pkt_size : No 512 t net.inet.ip.dummynet.red_max_pkt_size : No 1500 Parameters used in the computations of the drop probability for the RED algorithm. t Em net.inet.ip.fw.autoinc_step : No 100 Delta beween rule numbers when auto-generating them. The value must be in the range 1..1000. t Em net.inet.ip.fw.curr_dyn_buckets : Em net.inet.ip.fw.dyn_buckets The current number of buckets in the hash table for dynamic rules (readonly). t Em net.inet.ip.fw.debug : No 1 Controls debugging messages produced by .Nm . t Em net.inet.ip.fw.dyn_buckets : No 256 The number of buckets in the hash table for dynamic rules. Must be a power of 2, up to 65536. It only takes effect when all dynamic rules have expired, so you are advised to use a .Cm flush command to make sure that the hash table is resized. t Em net.inet.ip.fw.dyn_count : No 3 Current number of dynamic rules (read-only). t Em net.inet.ip.fw.dyn_keepalive : No 1 Enables generation of keepalive packets for .Cm keep-state rules on TCP sessions. A keepalive is generated to both sides of the connection every 5 seconds for the last 20 seconds of the lifetime of the rule. t Em net.inet.ip.fw.dyn_max : No 8192 Maximum number of dynamic rules. When you hit this limit, no more dynamic rules can be installed until old ones expire. t Em net.inet.ip.fw.dyn_ack_lifetime : No 300 t Em net.inet.ip.fw.dyn_syn_lifetime : No 20 t Em net.inet.ip.fw.dyn_fin_lifetime : No 1 t Em net.inet.ip.fw.dyn_rst_lifetime : No 1 t Em net.inet.ip.fw.dyn_udp_lifetime : No 5 t Em net.inet.ip.fw.dyn_short_lifetime : No 30 These variables control the lifetime, in seconds, of dynamic rules. Upon the initial SYN exchange the lifetime is kept short, then increased after both SYN have been seen, then decreased again during the final FIN exchange or when a RST is received. Both .Em dyn_fin_lifetime and .Em dyn_rst_lifetime must be strictly lower than 5 seconds, the period of repetition of keepalives. The firewall enforces that. t Em net.inet.ip.fw.enable : No 1 Enables the firewall. Setting this variable to 0 lets you run your machine without firewall even if compiled in. t Em net.inet.ip.fw.one_pass : No 1 When set, the packet exiting from the .Xr dummynet 4 pipe is not passed though the firewall again. Otherwise, after a pipe action, the packet is reinjected into the firewall at the next rule.

p Note: bridged and layer 2 packets coming out of a pipe are never reinjected in the firewall irrespective of the value of this variable. t Em net.inet.ip.fw.verbose : No 1 Enables verbose messages. t Em net.inet.ip.fw.verbose_limit : No 0 Limits the number of messages produced by a verbose firewall. t Em net.link.ether.ipfw : No 0 Controls whether layer-2 packets are passed to .Nm . Default is no. t Em net.link.ether.bridge_ipfw : No 0 Controls whether bridged packets are passed to .Nm . Default is no. .El .Sh IPFW2 ENHANCEMENTS This Section lists the features that have been introduced in .Nm ipfw2 and were not present in .Nm ipfw1 . We list them in order of the potential impact that they can have in writing your rulesets. You might want to consider using these features in order to write your rulesets in a more efficient way. l -tag -width indent t Address sets .Nm ipfw1 does not supports address sets (those in the form .Ar addr/masklen{num,num,...} ).

p A minor difference between .Nm ipfw1 and .Nm ipfw2 is that the former allows addresses to be specified as .Ar ipno:mask where the mask can be an arbitrary bitmask instead of a countiguous set of bits. .Nm ipfw2 no longer supports this syntax though it would be trivial to reintroduce it as it is supported on the kernel side. t Port specifications .Nm ipfw1 only allows one port range when specifying TCP and UDP ports, and is limited to 10 entries instead of the 15 allowed by .Nm ipfw2 . Also, in .Nm ipfw1 you can only specify ports when the rule is requesting .Cm tcp or .Cm udp packets. With .Nm ipfw2 you can put port specifications in rules matching all packets, and the match will be attempted only on those packets carrying protocols which include port identifiers.

p Finally, .Nm ipfw1 allowed the first port entry to be specified as .Ar port:mask where .Ar mask can be an arbitrary 16-bit mask. This syntax is of questionable usefulness and it is not supported anymore in .Nm ipfw2 . t Or-blocks .Nm ipfw1 does not support Or-blocks. t keepalives .Nm ipfw1 does not generate keepalives for stateful sessions. As a consequence, it might cause idle sessions to drop because the lifetime of the dynamic rules expires. t Sets of rules .Nm ipfw1 does not implement sets of rules. t MAC header filtering and Layer-2 firewalling. .Nm ipfw1 does not implement filtering on MAC header fields, nor it is invoked on packets from .Cm ether_demux() and .Cm ether_output_frame(). The sysctl variable .Em net.link.ether.ipfw has no effect there. t Options The following options are not supported in .Nm ipfw1 (RELENG_4) rules:

p .Cm layer2, ipid, iplen, ipprecedence, iptos, ipttl, .Cm ipversion, tcpack, tcpseq, tcpwin . t Dummynet options The following option for .Nm dummynet pipes/queues is not supported: .Cm noerror . .El .Sh EXAMPLES There are far too many possible uses of .Nm so this Section will only give a small set of examples.

p .Ss BASIC PACKET FILTERING This command adds an entry which denies all tcp packets from .Em cracker.evil.org to the telnet port of .Em wolf.tambov.su from being forwarded by the host:

p .Dl "ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet"

p This one disallows any connection from the entire crackers network to my host:

p .Dl "ipfw add deny ip from 123.45.67.0/24 to my.host.org"

p A first and efficient way to limit access (not using dynamic rules) is the use of the following rules:

p .Dl "ipfw add allow tcp from any to any established" .Dl "ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup" .Dl "ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup" .Dl "..." .Dl "ipfw add deny tcp from any to any"

p The first rule will be a quick match for normal TCP packets, but it will not match the initial SYN packet, which will be matched by the .Cm setup rules only for selected source/destination pairs. All other SYN packets will be rejected by the final .Cm deny rule.

p If you administer one or more subnets, you can take advantage of the .Nm ipfw2 syntax to specify address sets and or-blocks and write extremely compact rulesets which selectively enable services to blocks of clients, as below:

p .Dl "goodguys=\*q{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }\*q" .Dl "badguys=\*q10.1.2.0/24{8,38,60}\*q" .Dl "" .Dl "ipfw add allow ip from ${goodguys} to any" .Dl "ipfw add deny ip from ${badguys} to any" .Dl "... normal policies ..."

p The .Nm ipfw1 syntax would require a separate rule for each IP in the above example. .Ss DYNAMIC RULES In order to protect a site from flood attacks involving fake TCP packets, it is safer to use dynamic rules:

p .Dl "ipfw add check-state" .Dl "ipfw add deny tcp from any to any established" .Dl "ipfw add allow tcp from my-net to any setup keep-state"

p This will let the firewall install dynamic rules only for those connection which start with a regular SYN packet coming from the inside of our network. Dynamic rules are checked when encountering the first .Cm check-state or .Cm keep-state rule. A .Cm check-state rule should be usually placed near the beginning of the ruleset to minimize the amount of work scanning the ruleset. Your mileage may vary.

p To limit the number of connections a user can open you can use the following type of rules:

p .Dl "ipfw add allow tcp from my-net/24 to any setup limit src-addr 10" .Dl "ipfw add allow tcp from any to me setup limit src-addr 4"

p The former (assuming it runs on a gateway) will allow each host on a /24 network to open at most 10 TCP connections. The latter can be placed on a server to make sure that a single client does not use more than 4 simultaneous connections.

p .Em BEWARE : stateful rules can be subject to denial-of-service attacks by a SYN-flood which opens a huge number of dynamic rules. The effects of such attacks can be partially limited by acting on a set of .Xr sysctl 8 variables which control the operation of the firewall.

p Here is a good usage of the .Cm list command to see accounting records and timestamp information:

p .Dl ipfw -at list

p or in short form without timestamps:

p .Dl ipfw -a list

p which is equivalent to:

p .Dl ipfw show

p Next rule diverts all incoming packets from 192.168.2.0/24 to divert port 5000:

p .Dl ipfw divert 5000 ip from 192.168.2.0/24 to any in

p .Ss TRAFFIC SHAPING The following rules show some of the applications of .Nm and .Xr dummynet 4 for simulations and the like.

p This rule drops random incoming packets with a probability of 5%:

p .Dl "ipfw add prob 0.05 deny ip from any to any in"

p A similar effect can be achieved making use of dummynet pipes:

p .Dl "ipfw add pipe 10 ip from any to any" .Dl "ipfw pipe 10 config plr 0.05"

p We can use pipes to artificially limit bandwidth, e.g. on a machine acting as a router, if we want to limit traffic from local clients on 192.168.2.0/24 we do:

p .Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out" .Dl "ipfw pipe 1 config bw 300Kbit/s queue 50KBytes"

p note that we use the .Cm out modifier so that the rule is not used twice. Remember in fact that .Nm rules are checked both on incoming and outgoing packets.

p Should we like to simulate a bidirectional link with bandwidth limitations, the correct way is the following:

p .Dl "ipfw add pipe 1 ip from any to any out" .Dl "ipfw add pipe 2 ip from any to any in" .Dl "ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes" .Dl "ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes"

p The above can be very useful, e.g. if you want to see how your fancy Web page will look for a residential user which is connected only through a slow link. You should not use only one pipe for both directions, unless you want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet, IRDA). It is not necessary that both pipes have the same configuration, so we can also simulate asymmetric links.

p Should we like to verify network performance with the RED queue management algorithm:

p .Dl "ipfw add pipe 1 ip from any to any" .Dl "ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1"

p Another typical application of the traffic shaper is to introduce some delay in the communication. This can affect a lot applications which do a lot of Remote Procedure Calls, and where the round-trip-time of the connection often becomes a limiting factor much more than bandwidth:

p .Dl "ipfw add pipe 1 ip from any to any out" .Dl "ipfw add pipe 2 ip from any to any in" .Dl "ipfw pipe 1 config delay 250ms bw 1Mbit/s" .Dl "ipfw pipe 2 config delay 250ms bw 1Mbit/s"

p Per-flow queueing can be useful for a variety of purposes. A very simple one is counting traffic:

p .Dl "ipfw add pipe 1 tcp from any to any" .Dl "ipfw add pipe 1 udp from any to any" .Dl "ipfw add pipe 1 ip from any to any" .Dl "ipfw pipe 1 config mask all"

p The above set of rules will create queues (and collect statistics) for all traffic. Because the pipes have no limitations, the only effect is collecting statistics. Note that we need 3 rules, not just the last one, because when .Nm tries to match IP packets it will not consider ports, so we would not see connections on separate ports as different ones.

p A more sophisticated example is limiting the outbound traffic on a net with per-host limits, rather than per-network limits:

p .Dl "ipfw add pipe 1 ip from 192.168.2.0/24 to any out" .Dl "ipfw add pipe 2 ip from any to 192.168.2.0/24 in" .Dl "ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes" .Dl "ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes" .Ss SETS OF RULES To add a set of rules atomically, e.g. set 18:

p .Dl "ipfw disable set 18" .Dl "ipfw add NN set 18 ... # repeat as needed" .Dl "ipfw enable set 18"

p To delete a set of rules atomically the command is simply:

p .Dl "ipfw delete set 18"

p To test a ruleset and disable it and regain control if something goes wrong:

p .Dl "ipfw disable set 18" .Dl "ipfw add NN set 18 ... # repeat as needed" .Dl "ipfw enable set 18 ; echo done; sleep 30 && ipfw disable set 18"

p Here if everything goes well, you press control-C before the "sleep" terminates, and your ruleset will be left active. Otherwise, e.g. if you cannot access your box, the ruleset will be disabled after the sleep terminates thus restoring the previous situation. .Sh SEE ALSO .Xr cpp 1 , .Xr m4 1 , .Xr bridge 4 , .Xr divert 4 , .Xr dummynet 4 , .Xr ip 4 , .Xr ipfirewall 4 , .Xr protocols 5 , .Xr services 5 , .Xr init 8 , .Xr kldload 8 , .Xr reboot 8 , .Xr sysctl 8 , .Xr syslogd 8 .Rs .%A "S. Floyd" .%A "V. Jacobson" .%T "Random Early Detection gateways for Congestion Avoidance" .%D "August 1993" .Re .Rs .%A "B. Braden" .%A "D. Clark" .%A "J. Crowcroft" .%A "B. Davie" .%A "S. Deering" .%A "D. Estrin" .%A "S. Floyd" .%A "V. Jacobson" .%A "G. Minshall" .%A "C. Partridge" .%A "L. Peterson" .%A "K. Ramakrishnan" .%A "S. Shenker" .%A "J. Wroclawski" .%A "L. Zhang" .%T "Recommendations on Queue Management and Congestion Avoidance in the Internet" .%D "April 1998" .%O "RFC 2309" .Re .Sh BUGS The syntax has grown over the years and sometimes it might be confusing. Unfortunately, backward compatibility prevents cleaning up mistakes done in the definition of the syntax.

p .Em !!! WARNING !!!

p Misconfiguring the firewall can put your computer in an unusable state, possibly shutting down network services and requiring console access to regain control to it.

p Incoming packet fragments diverted by .Cm divert or .Cm tee are reassembled before delivery to the socket. The action used on those packet is the one from the rule which matches the first fragment of the packet.

p Packets that match a .Cm tee rule should not be immediately accepted, but should continue going through the rule list. This may be fixed in a later version.

p Packets diverted to userland, and then reinserted by a userland process (such as .Xr natd 8 ) will lose various packet attributes, including their source interface. If a packet is reinserted in this manner, later rules may be incorrectly applied, making the order of .Cm divert rules in the rule sequence very important. .Sh AUTHORS .An Ugen J. S. Antsilevich , .An Poul-Henning Kamp , .An Alex Nash , .An Archie Cobbs , .An Luigi Rizzo .

p .An -nosplit API based upon code written by .An Daniel Boulet for BSDI.

p Work on .Xr dummynet 4 traffic shaper supported by Akamba Corp. .Sh HISTORY The .Nm utility first appeared in .Fx 2.0 . .Xr dummynet 4 was introduced in .Fx 2.2.8 . Stateful extensions were introduced in .Fx 4.0 . .Nm ipfw2 was introduced in Summer 2002.