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IPFW(8) BSD System Manager's Manual IPFW(8) NAME ipfw -- IP firewall and traffic shaper control program SYNOPSIS ipfw [-cq] add rule ipfw [-acdeftNS] {list | show} [number ...] ipfw [-f | -q] flush ipfw [-q] {delete | zero | resetlog} [set] [number ...] ipfw set [disable number ...] [enable number ...] ipfw set move [rule] number to number ipfw set swap number number ipfw set show ipfw {pipe | queue} number config config-options ipfw [-s [field]] {pipe | queue} {delete | list | show} [number ...] ipfw [-q] [-p preproc [-D macro[=value]] [-U macro]] pathname DESCRIPTION The ipfw utility is the user interface for controlling the ipfw(4) fire- wall and the dummynet(4) traffic shaper in FreeBSD. NOTE: this manual page refers to the newer version of ipfw introduced in July 2002, also known as ipfw2. The commands listed here are a superset of the old firewall, which we will call ipfw1 when it is necessary to distinguish between the two. ipfw2 is standard in FreeBSD CURRENT, whereas FreeBSD STABLE still uses ipfw1 unless the kernel is compiled with options IPFW2, and /sbin/ipfw and /usr/lib/libalias are recompiled with -DIPFW2 and reinstalled (the same effect can be achieved by adding IPFW2=TRUE to /etc/make.conf before a buildworld). See the IPFW2 ENHANCEMENTS Section for a list of features which are not present in ipfw1. This list can also be useful to revise your rules and write them more efficiently. An ipfw configuration, or ruleset, is made of a list of rules numbered from 1 to 65535. Packets are passed to ipfw in a number of different places in the protocol stack (depending on the source and destination of the packet, it is possible that ipfw is invoked multiple times on the same packet). The packet passed to the firewall is compared against each of the rules in the firewall ruleset. When a match is found, the action corresponding to the matching rule is performed. Depending on the action and certain system settings, packets can be rein- jected into the firewall at some rule after the matching one for further processing. An ipfw ruleset always includes a default rule (numbered 65535) which cannot be modified, and matches all packets. The action associated with the default rule can be either deny or allow depending on how the kernel is configured. If the ruleset includes one or more rules with the keep-state or limit option, then ipfw assumes a stateful behaviour, i.e. upon a match it will create dynamic rules matching the exact parameters (addresses and ports) of the matching packet. These dynamic rules, which have a limited lifetime, are checked at the first occurrence of a check-state, keep-state or limit rule, and are typ- ically used to open the firewall on-demand to legitimate traffic only. See the STATEFUL FIREWALL and EXAMPLES Sections below for more informa- tion on the stateful behaviour of ipfw. 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 ipfw commands. Rules can be added with the add command; deleted individually or in groups with the delete command, and globally with the flush command; dis- played, optionally with the content of the counters, using the show and list commands. Finally, counters can be reset with the zero and resetlog commands. Also, each rule belongs to one of 32 different sets , and there are ipfw 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 SETS OF RULES for more information on sets. The following options are available: -a While listing, show counter values. The show command just im- plies this option. -c When entering or showing rules, print them in compact form, i.e. without the optional "ip from any to any" string when this does not carry any additional information. -d While listing, show dynamic rules in addition to static ones. -e While listing, if the -d option was specified, also show expired dynamic rules. -f Don't ask for confirmation for commands that can cause problems if misused, i.e. flush. If there is no tty associated with the process, this is implied. -N Try to resolve addresses and service names in output. -q While adding, zeroing, resetlogging or flushing, be quiet about actions (implies -f). This is useful for adjusting rules by exe- cuting multiple ipfw commands in a script (e.g., `sh /etc/rc.firewall'), or by processing a file of many ipfw rules, across a remote login session. If a 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. -S While listing rules, show the set each rule belongs to. If this flag is not specified, disabled rules will not be listed. -s [field] While listing pipes, sort according to one of the four counters (total and current packets or bytes). -t While listing, show last match timestamp. To ease configuration, rules can be put into a file which is processed using ipfw as shown in the first synopsis line. An absolute pathname must be used. The file will be read line by line and applied as argu- ments to the ipfw utility. Optionally, a preprocessor can be specified using -p preproc where pathname is to be piped through. Useful preprocessors include cpp(1) and m4(1). If preproc doesn't start with a slash (`/') as its first charac- ter, the usual PATH name search is performed. Care should be taken with this in environments where not all file systems are mounted (yet) by the time ipfw is being run (e.g. when they are mounted over NFS). Once -p has been specified, optional -D and -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. The ipfw pipe and queue commands are used to configure the traffic shaper, as shown in the TRAFFIC SHAPER CONFIGURATION Section below. PACKET FLOW ipfw 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 ipfw is invoked are listed below, together with the sysctl variables which control its invocation. ^ 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 | 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 on packet source and destination, and system configuration. In each of these places, the packet is passed to ipfw with all (and only) the fields that belong to that level. That is, incoming packets will include the MAC header when ipfw is invoked from ether_demux(), but the same packets will have the MAC header stripped off when ipfw is invoked from ip_input(). The complete ruleset is always used, irrespective of the place where ipfw 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 ipfw is called from ip_input() ) the match pattern will not match. However, a not operator in front of such patterns will cause the pattern to always match on those packets, which might cause undesired results. It is thus responsibility of the program- mer, if necessary, to write a suitable ruleset to differentiate among the possible places. skipto rules can be useful here, as an example: # 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 (yes, at the moment there is no way to differentiate between ether_demux and bdg_forward). RULE FORMAT The format of ipfw rules is the following: [rule_number] [set set_number] [prob match_probability] action [log [logamount number]] body where the body of the rule specifies which information is used for fil- tering packets, among the following: Layer-2 header fields When available IPv4 Protocol TCP, UDP, ICMP, etc. Source and dest. addresses and ports Direction See Section PACKET FLOW Transmit and receive interface By name or address Misc. IP header fields Version, type of service, data- gram length, identification, fragment flag (non-zero IP off- set), Time To Live IP options Misc. TCP header fields TCP flags (SYN, FIN, ACK, RST, etc.), sequence number, acknowl- edgment number, window TCP options ICMP types for ICMP packets User/group ID When the packet can be associate to a local socket. 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. rule_number Each rule is associated with a rule_number in the range 1..65535, with the latter reserved for the 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 default rule. Automatic rule numbers are assigned by incrementing the last non- default rule number by the value of the sysctl variable 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. set set_number Each rule is associated to a set_number in the range 0..31, with the latter reserved for the default rule. Sets can be individu- ally 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. prob 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 dummynet(4)) to simulate the effect of multiple paths lead- ing to out-of-order packet delivery. log [logamount number] When a packet matches a rule with the log keyword, a message will be logged to syslogd(8) with a LOG_SECURITY facility. The log- ging only occurs if the sysctl variable net.inet.ip.fw.verbose is set to 1 (which is the default when the kernel is compiled with IPFIREWALL_VERBOSE ) and the number of packets logged so far for that particular rule does not exceed ther 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 0 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 entry, see the resetlog command. 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. allow | accept | pass | permit Allow packets that match rule. The search terminates. check-state Checks the packet against the dynamic ruleset. If a match is found, execute the action associated with the rule which gener- ated this dynamic rule, otherwise move to the next rule. Check-state rules do not have a body. If no check-state rule is found, the dynamic ruleset is checked at the first keep-state or limit rule. count Update counters for all packets that match rule. The search con- tinues with the next rule. deny | drop Discard packets that match this rule. The search terminates. divert port Divert packets that match this rule to the divert(4) socket bound to port port. The search terminates. fwd | forward ipaddr[,port] Change the next-hop on matching packets to ipaddr, which can be an IP address in dotted quad or a host name. The search termi- nates if this rule matches. If ipaddr is a local address, then matching packets will be for- warded to port (or the port number in the packet if one is not specified in the rule) on the local machine. If ipaddr is not a local address, then the port number (if speci- fied) 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 fwd rule will not match layer-2 packets (those received on ether_input, ether_output, or bridged). The 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 netstat(1) entry look rather weird but is intended for use with transparent proxy servers. pipe pipe_nr Pass packet to a dummynet(4) "pipe" (for bandwidth limitation, delay, etc.). See the TRAFFIC SHAPER CONFIGURATION Section for further information. The search terminates; however, on exit from the pipe and if the sysctl(8) variable net.inet.ip.fw.one_pass is not set, the packet is passed again to the firewall code starting from the next rule. queue queue_nr Pass packet to a dummynet(4) "queue" (for bandwidth limitation using WF2Q). reject (Deprecated). Synonym for unreach host. 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 termi- nates. skipto number Skip all subsequent rules numbered less than number. The search continues with the first rule numbered number or higher. tee port Send a copy of packets matching this rule to the divert(4) socket bound to port port. The search terminates and the original packet is accepted (but see Section BUGS below). unreach code Discard packets that match this rule, and try to send an ICMP un- reachable notice with code code, where code is a number from 0 to 255, or one of these aliases: net, host, protocol, port, needfrag, srcfail, net-unknown, host-unknown, isolated, net-prohib, host-prohib, tosnet, toshost, filter-prohib, host-precedence or precedence-cutoff. The search terminates. 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) and op- erators -- i.e. all must match in order for the rule to match. Individ- ual patterns can be prefixed by the not operator to reverse the result of the match, as in ipfw add 100 allow ip from not 1.2.3.4 to any Additionally, sets of alternative match patterns ( or-blocks ) can be constructed by putting the patterns in lists enclosed between parentheses ( ) or braces { }, and using the or operator as follows: ipfw add 100 allow ip from { x or not y or z } to any 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 backslash \ in front of them to prevent such interpretations. The body of a rule must in general include a source and destination ad- dres specifier. The keyword any can be used in various places to specify that the content of a required field is irrelevant. The rule body has the following format: [proto from src to dst] [options] The first part (protocol from src to dst) is for backward compatibility with ipfw1. In ipfw2 any match pattern (including MAC headers, IPv4 pro- tocols, addresses and ports) can be specified in the options section. Rule fields have the following meaning: proto: protocol | { protocol or ... } An IPv4 protocol (or an or-block with multiple protocols) speci- fied by number or name (for a complete list see /etc/protocols). The ip or all keywords mean any protocol will match. src and dst: ip-address | { ip-address or ... } [ports] A single ip-address , or an or-block containing one or more of them, optionally followed by ports specifiers. ip-address: An address (or set of addresses) specified in one of the follow- ing ways, optionally preceded by a not operator: any matches any IP address. me matches any IP address configured on an interface in the system. The address list is evaluated at the time the packet is analysed. 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. addr/masklen Matches all addresses with base addr (specified as a dot- ted quad or a hostname) and mask width of 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 . addr/masklen{num,num,...} Matches all addresses with base address 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 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 ad- dresses: 1.2.3.128 1.2.3.35 1.2.3.55 1.2.3.89 . This format is particularly useful to handle sparse ad- dress sets within a single rule. Because the matching oc- curs using a bitmask, it takes constant time and dramati- cally reduces the complexity of rulesets. ports: [not] {port | port-port} [,...] For protocols which support port numbers (such as TCP and UDP), optional ports may be specified as one or more ports or port ranges, separated by commas but no spaces, and an optional not operator. The `-' notation specifies a range of ports (including boundaries). Service names (from /etc/services) may be used instead of numeric port values. The length of the port list is limited to 30 ports or ranges, though one can specify larger ranges by using an or-block in the options section of the rule. A backslash (`\') can be used to escape the dash (`-') character in a service name (from a shell, the backslash must be typed twice to avoid that the shell itself uses it as an escape charac- ter). ipfw add count tcp from any ftp\\-data-ftp to any 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 frag option for details on matching fragmented packets. RULE OPTIONS (MATCH PATTERNS) Additional match patterns can be used within rules. Zero or more of these so-called options can be present in a rule, optionally prefixed by the not operand, and possibly grouped into or-blocks. The following match patterns can be used (listed in alphabetical order): bridged Matches only bridged packets. dst-ip ip address Matches IP packets whose destination IP is one of the address(es) specified as argument. dst-port source ports Matches IP packets whose destination port is one of the port(s) specified as argument. established Matches TCP packets that have the RST or ACK bits set. frag Matches packets that are fragments and not the first fragment of an IP datagram. Note that these packets will not have the next protocol header (e.g. TCP, UDP) so options that look into these headers cannot match. gid group Matches all TCP or UDP packets sent by or received for a group. A group may be specified by name or number. icmptypes types Matches ICMP packets whose ICMP type is in the list types. The list may be specified as any combination of ranges or individual types separated by commas. The supported ICMP types are: echo reply (0), destination unreachable (3), source quench (4), redirect (5), echo request (8), router advertisement (9), router solicitation (10), time-to-live exceeded (11), IP header bad (12), timestamp request (13), timestamp reply (14), information request (15), information reply (16), address mask request (17) and address mask reply (18). in | out Matches incoming or outgoing packets, respectively. in and out are mutually exclusive (in fact, out is implemented as not in). ipid id Matches IP packets whose ip_id field has value id. iplen len Matches IP packets whose total length, including header and data, is len bytes. ipoptions spec Matches packets whose IP header contains the comma separated list of options specified in spec. The supported IP options are: ssrr (strict source route), lsrr (loose source route), rr (record packet route) and ts (timestamp). The absence of a particular option may be denoted with a `!'. ipprecedence precedence Matches IP packets whose precedence field is equal to precedence. iptos spec Matches IP packets whose tos field contains the comma separated list of service types specified in spec. The supported IP types of service are: lowdelay (IPTOS_LOWDELAY), throughput (IPTOS_THROUGHPUT), reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST), congestion (IPTOS_CE). The absence of a particular type may be denoted with a `!'. ipttl ttl Matches IP packets whose time to live is ttl. ipversion ver Matches IP packets whose IP version field is ver. keep-state Upon a match, the firewall will create a dynamic rule, whose de- fault 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 sysctl(8) vari- ables), and the lifetime is refreshed every time a matching packet is found. layer2 Matches only layer2 packets, i.e. those passed to ipfw from ether_demux() and ether_output_frame(). limit {src-addr | src-port | dst-addr | dst-port} N 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. { MAC | mac } dst-mac src-mac Match packets with a given dst-mac and src-mac addresses, speci- fied as the any keyword (matching any MAC address), or six groups of hex digits separated by commas, and optionally followed by a mask indicating how many bits are significant, as in MAC 10:20:30:40:50:60/33 any 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. mac-type mac-type Matches packets whose Ethernet Type field corresponds to one of those specified as argument. mac-type is specified in the same way as port numbers (i.e. one or more comma-separated single val- ues or ranges). You can use symbolic names for known values such as vlan, ipv4, ipv6. Values can be enter as decimal or hexadeci- mal (if prefixed by 0x), and they are always printed as hexadeci- mal (unless the -N option is used, in which case symbolic resolu- tion will be attempted). proto protocol Matches packets with the corresponding IPv4 protocol. recv | xmit | via {ifX | if* | ipno | any} Matches packets received, transmitted or be going through, re- spectively, the interface specified by exact name (ifX), by de- vice name (if*), by IP address, or through some interface. The via keyword causes the interface to always be checked. If recv or xmit is used instead of 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.: ipfw add deny ip from any to any out recv ed0 xmit ed1 The recv interface can be tested on either incoming or outgoing packets, while the xmit interface can only be tested on outgoing packets. So out is required (and in is invalid) whenever xmit is used. 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. setup Matches TCP packets that have the SYN bit set but no ACK bit. This is the short form of "tcpflags syn,!ack". src-ip ip-address Matches IP packets whose source IP is one of the address(es) specified as argument. src-port ports Matches IP packets whose source port is one of the port(s) speci- fied as argument. tcpack ack TCP packets only. Match if the TCP header acknowledgment number field is set to ack. tcpflags spec TCP packets only. Match if the TCP header contains the comma separated list of flags specified in spec. The supported TCP flags are: fin, syn, rst, psh, ack and urg. The absence of a particular flag may be denoted with a `!'. A rule which contains a tcpflags specification can never match a fragmented packet which has a non-zero offset. See the frag option for details on matching fragmented packets. tcpseq seq TCP packets only. Match if the TCP header sequence number field is set to seq. tcpwin win TCP packets only. Match if the TCP header window field is set to win. tcpoptions spec TCP packets only. Match if the TCP header contains the comma separated list of options specified in spec. The supported TCP options are: mss (maximum segment size), window (tcp window advertisement), sack (selective ack), ts (rfc1323 timestamp) and cc (rfc1644 t/tcp connection count). The absence of a particular option may be denoted with a `!'. uid user Match all TCP or UDP packets sent by or received for a user. A user may be matched by name or identification number. SETS OF RULES Each rule belongs to one of 32 different sets , numbered 0 to 31. Set 31 is reserved for the default rule. By default, rules are put in set 0, unless you use the set N attribute when entering a new rule. Sets can be individually and atomically en- abled or disabled, so this mechanism permits an easy way to store multi- ple configurations of the firewall and quickly (and atomically) switch between them. The command to enable/disable sets is ipfw set disable number ... [enable number ...] where multiple enable or disable sections can be specified. Command exe- cution is atomic on all the sets specified in the command. By default, all sets are enabled. When you disable a set, its rules behave as if they were not existing in the firewall configuration, with only one exception: o 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; The set number of rules can be changed with the command ipfw set move {rule rule-number | old-set} to new-set Also, you can atomically swap two rulesets with the command ipfw set swap first-set second-set See the EXAMPLES Section on some possible uses of sets of rules. 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 check-state, keep-state and limit options of rules. Dynamic rules are created when a packet matches a keep-state or limit rule, causing the creation of a dynamic rule which will match all and only packets with a given protocol between a src-ip/src-port dst-ip/dst-port pair of addresses ( src and dst are used here only to de- note the initial match addresses, but they are completely equivalent af- terwards). Dynamic rules will be checked at the first check-state, keep-state or limit occurrence, and the action performed upon a match will be the same as in the parent rule. Note that no additional attributes other than protocol and IP addresses and ports are checked on dynamic rules. The typical use of dynamic rules is to keep a closed firewall configura- tion, 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: ipfw add check-state ipfw add allow tcp from my-subnet to any setup ipfw add deny tcp from any to any 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: ipfw add check-state ipfw add allow udp from my-subnet to any ipfw add deny udp from any to any Dynamic rules expire after some time, which depends on the status of the flow and the setting of some sysctl variables. See Section SYSCTL VARIABLES for more details. For TCP sessions, dynamic rules can be in- structed to periodically send keepalive packets to refresh the state of the rule when it is about to expire. See Section EXAMPLES for more examples on how to use dynamic rules. TRAFFIC SHAPER CONFIGURATION ipfw is also the user interface for the dummynet(4) traffic shaper. The shaper operates by dividing packets into flows according to a user-speci- fied mask on different fields of the IP header. Packets belonging to the same flow are then passed to two different objects, named pipe or queue. A pipe emulates a link with given bandwidth, propagation delay, queue size and packet loss rate. Packets transit through the pipe according to its parameters. A 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. The ipfw pipe configuration format is the following: pipe number config pipe-configuration The ipfw queue configuration format is the following: queue number config queue-configuration The following parameters can be configured for a pipe: bw bandwidth | device Bandwidth, measured in [K|M]{bit/s|Byte/s}. A value of 0 (default) means unlimited bandwidth. The unit must follow immediately the number, as in ipfw pipe 1 config bw 300Kbit/s 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 tun(4) device supports this functionality, for use in conjunction with ppp(8). delay 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 "options HZ=1000" to reduce the granularity to 1ms or less). Default value is 0, meaning no delay. The following parameters can be configured for a queue: pipe 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. weight 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. Finally, the following parameters can be configured for both pipes and queues: buckets hash-table-size Specifies the size of the hash table used for storing the various queues. Default value is 64 controlled by the sysctl(8) variable net.inet.ip.dummynet.hash_size, allowed range is 16 to 1024. mask mask-specifier The dummynet(4) lets you to create per-flow queues. A flow iden- tifier is constructed by masking the IP addresses, ports and pro- tocol 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: dst-ip mask, src-ip mask, dst-port mask, src-port mask, proto mask or all, where the latter means all bits in all fields are significant. When used within a pipe configuration, each flow is assigned a rate equal to the rate of the pipe. When used within a 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. 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. plr packet-loss-rate Packet loss rate. Argument 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. queue {slots | sizeKbytes} Queue size, in slots or 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. red | gred w_q/min_th/max_th/max_p Make use of the RED (Random Early Detection) queue management al- gorithm. w_q and max_p are floating point numbers between 0 and 1 (0 not included), while min_th and max_th are integer numbers specifying thresholds for queue management (thresholds are com- puted in bytes if the queue has been defined in bytes, in slots otherwise). The dummynet(4) also supports the gentle RED variant (gred). Three sysctl(8) variables can be used to control the RED behaviour: 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) net.inet.ip.dummynet.red_avg_pkt_size specifies the expected average packet size (defaults to 512, must be greater than zero) 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). CHECKLIST Here are some important points to consider when designing your rules: o Remember that you filter both packets going in and out. Most connec- tions need packets going in both directions. o 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 /usr/share/examples/ipfw/change_rules.sh. o Don't forget the loopback interface. FINE POINTS o There are circumstances where fragmented datagrams are uncondition- ally 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 "pullup failed" since there may not be enough good data in the packet to produce a meaningful log entry. o 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. o If you are logged in over a network, loading the kld(4) version of ipfw is probably not as straightforward as you would think. I recom- mend the following command line: kldload /modules/ipfw.ko && \ ipfw add 32000 allow ip from any to any Along the same lines, doing an ipfw flush in similar surroundings is also a bad idea. o The ipfw filter list may not be modified if the system security level is set to 3 or higher (see init(8) for information on system security levels). PACKET DIVERSION A 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. SYSCTL VARIABLES A set of sysctl(8) variables controls the behaviour of the firewall and associated modules ( dummynet, bridge ). These are shown below together with their default value (but always check with the sysctl(8) command what value is actually in use) and meaning: net.inet.ip.dummynet.expire: 1 Lazily delete dynamic pipes/queue once they have no pending traf- fic. 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. net.inet.ip.dummynet.hash_size: 64 Default size of the hash table used for dynamic pipes/queues. This value is used when no buckets option is specified when con- figuring a pipe/queue. net.inet.ip.dummynet.max_chain_len: 16 Target value for the maximum number of pipes/queues in a hash bucket. The product max_chain_len*hash_size is used to determine the threshold over which empty pipes/queues will be expired even when net.inet.ip.dummynet.expire=0. net.inet.ip.dummynet.red_lookup_depth: 256 net.inet.ip.dummynet.red_avg_pkt_size: 512 net.inet.ip.dummynet.red_max_pkt_size: 1500 Parameters used in the computations of the drop probability for the RED algorithm. net.inet.ip.fw.autoinc_step: 100 Delta beween rule numbers when auto-generating them. The value must be in the range 1..1000. net.inet.ip.fw.curr_dyn_buckets: net.inet.ip.fw.dyn_buckets The current number of buckets in the hash table for dynamic rules (readonly). net.inet.ip.fw.debug: 1 Controls debugging messages produced by ipfw. net.inet.ip.fw.dyn_buckets: 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 dy- namic rules have expired, so you are advised to use a flush com- mand to make sure that the hash table is resized. net.inet.ip.fw.dyn_count: 3 Current number of dynamic rules (read-only). net.inet.ip.fw.dyn_keepalive: 1 Enables generation of keepalive packets for keep-state rules on TCP sessions. A keepalive is generated to both sides of the con- nection every 5 seconds for the last 20 seconds of the lifetime of the rule. net.inet.ip.fw.dyn_max: 8192 Maximum number of dynamic rules. When you hit this limit, no more dynamic rules can be installed until old ones expire. net.inet.ip.fw.dyn_ack_lifetime: 300 net.inet.ip.fw.dyn_syn_lifetime: 20 net.inet.ip.fw.dyn_fin_lifetime: 1 net.inet.ip.fw.dyn_rst_lifetime: 1 net.inet.ip.fw.dyn_udp_lifetime: 5 net.inet.ip.fw.dyn_short_lifetime: 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 dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower than 5 seconds, the period of repetition of keepalives. The fire- wall enforces that. net.inet.ip.fw.enable: 1 Enables the firewall. Setting this variable to 0 lets you run your machine without firewall even if compiled in. net.inet.ip.fw.one_pass: 1 When set, the packet exiting from the dummynet(4) pipe is not passed though the firewall again. Otherwise, after a pipe ac- tion, the packet is reinjected into the firewall at the next rule. Note: bridged and layer 2 packets coming out of a pipe are never reinjected in the firewall irrespective of the value of this variable. net.inet.ip.fw.verbose: 1 Enables verbose messages. net.inet.ip.fw.verbose_limit: 0 Limits the number of messages produced by a verbose firewall. net.link.ether.ipfw: 0 Controls whether layer-2 packets are passed to ipfw. Default is no. net.link.ether.bridge_ipfw: 0 Controls whether bridged packets are passed to ipfw. Default is no. IPFW2 ENHANCEMENTS This Section lists the features that have been introduced in ipfw2 and were not present in 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. Handling of non-IPv4 packets ipfw1 will silently accept all non-IPv4 packets (which ipfw1 will only see when net.link.ether.bridge_ipfw=1). ipfw2 will filter all packets (including non-IPv4 ones) according to the ruleset. To achieve the same behaviour as ipfw1 you can use the following as the very first rule in your ruleset: ipfw add 1 allow layer2 not mac-type ip The layer2 options might seem redundant, but it is necessary -- packets passed to the firewall from layer3 will not have a MAC header, so the mac-type ip pattern will always fail on them, and the not operator will make this rule into a pass-all. Address sets ipfw1 does not supports address sets (those in the form addr/masklen{num,num,...} ). A minor difference between ipfw1 and ipfw2 is that the former al- lows addresses to be specified as ipno:mask where the mask can be an arbitrary bitmask instead of a countiguous set of bits. ipfw2 no longer supports this syntax though it would be trivial to reintroduce it as it is supported on the kernel side. Port specifications ipfw1 only allows one port range when specifying TCP and UDP ports, and is limited to 10 entries instead of the 15 allowed by ipfw2. Also, in ipfw1 you can only specify ports when the rule is requesting tcp or udp packets. With ipfw2 you can put port specifications in rules matching all packets, and the match will be attempted only on those packets carrying protocols which in- clude port identifiers. Finally, ipfw1 allowed the first port entry to be specified as port:mask where mask can be an arbitrary 16-bit mask. This syn- tax is of questionable usefulness and it is not supported anymore in ipfw2. Or-blocks ipfw1 does not support Or-blocks. keepalives 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. Sets of rules ipfw1 does not implement sets of rules. MAC header filtering and Layer-2 firewalling. ipfw1 does not implement filtering on MAC header fields, nor it is invoked on packets from ether_demux() and ether_output_frame(). The sysctl variable net.link.ether.ipfw has no effect there. Options The following options are not supported in ipfw1 dst-ip, dst-port, layer2, mac, mac-type, src-ip, src-port. Additionally, the following options are not supported in ipfw1 (RELENG_4) rules: ipid, iplen, ipprecedence, iptos, ipttl, ipversion, .Cm tcpack, tcpseq, tcpwin. Dummynet options The following option for dummynet pipes/queues is not supported: noerror. EXAMPLES There are far too many possible uses of ipfw so this Section will only give a small set of examples. BASIC PACKET FILTERING This command adds an entry which denies all tcp packets from cracker.evil.org to the telnet port of wolf.tambov.su from being for- warded by the host: ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet This one disallows any connection from the entire crackers network to my host: ipfw add deny ip from 123.45.67.0/24 to my.host.org A first and efficient way to limit access (not using dynamic rules) is the use of the following rules: ipfw add allow tcp from any to any established ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup ... ipfw add deny tcp from any to any 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 setup rules only for selected source/destination pairs. All other SYN packets will be rejected by the final deny rule. If you administer one or more subnets, you can take advantage of the ipfw2 syntax to specify address sets and or-blocks and write extremely compact rulesets which selectively enable services to blocks of clients, as below: goodguys="{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }" badguys="10.1.2.0/24{8,38,60}" ipfw add allow ip from ${goodguys} to any ipfw add deny ip from ${badguys} to any ... normal policies ... The ipfw1 syntax would require a separate rule for each IP in the above example. DYNAMIC RULES In order to protect a site from flood attacks involving fake TCP packets, it is safer to use dynamic rules: ipfw add check-state ipfw add deny tcp from any to any established ipfw add allow tcp from my-net to any setup keep-state This will let the firewall install dynamic rules only for those connec- tion which start with a regular SYN packet coming from the inside of our network. Dynamic rules are checked when encountering the first check-state or keep-state rule. A 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. To limit the number of connections a user can open you can use the fol- lowing type of rules: ipfw add allow tcp from my-net/24 to any setup limit src-addr 10 ipfw add allow tcp from any to me setup limit src-addr 4 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 si- multaneous connections. 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 sysctl(8) variables which control the operation of the firewall. Here is a good usage of the list command to see accounting records and timestamp information: ipfw -at list or in short form without timestamps: ipfw -a list which is equivalent to: ipfw show Next rule diverts all incoming packets from 192.168.2.0/24 to divert port 5000: ipfw divert 5000 ip from 192.168.2.0/24 to any in TRAFFIC SHAPING The following rules show some of the applications of ipfw and dummynet(4) for simulations and the like. This rule drops random incoming packets with a probability of 5%: ipfw add prob 0.05 deny ip from any to any in A similar effect can be achieved making use of dummynet pipes: ipfw add pipe 10 ip from any to any ipfw pipe 10 config plr 0.05 We can use pipes to artificially limit bandwidth, e.g. on a machine act- ing as a router, if we want to limit traffic from local clients on 192.168.2.0/24 we do: ipfw add pipe 1 ip from 192.168.2.0/24 to any out ipfw pipe 1 config bw 300Kbit/s queue 50KBytes note that we use the out modifier so that the rule is not used twice. Remember in fact that ipfw rules are checked both on incoming and outgo- ing packets. Should we like to simulate a bidirectional link with bandwidth limita- tions, the correct way is the following: ipfw add pipe 1 ip from any to any out ipfw add pipe 2 ip from any to any in ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes 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. Should we like to verify network performance with the RED queue manage- ment algorithm: ipfw add pipe 1 ip from any to any ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1 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: ipfw add pipe 1 ip from any to any out ipfw add pipe 2 ip from any to any in ipfw pipe 1 config delay 250ms bw 1Mbit/s ipfw pipe 2 config delay 250ms bw 1Mbit/s Per-flow queueing can be useful for a variety of purposes. A very simple one is counting traffic: ipfw add pipe 1 tcp from any to any ipfw add pipe 1 udp from any to any ipfw add pipe 1 ip from any to any ipfw pipe 1 config mask all 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 ipfw tries to match IP packets it will not consider ports, so we would not see connections on separate ports as different ones. A more sophisticated example is limiting the outbound traffic on a net with per-host limits, rather than per-network limits: ipfw add pipe 1 ip from 192.168.2.0/24 to any out ipfw add pipe 2 ip from any to 192.168.2.0/24 in ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes SETS OF RULES To add a set of rules atomically, e.g. set 18: ipfw disable set 18 ipfw add NN set 18 ... # repeat as needed ipfw enable set 18 To delete a set of rules atomically the command is simply: ipfw delete set 18 To test a ruleset and disable it and regain control if something goes wrong: ipfw disable set 18 ipfw add NN set 18 ... # repeat as needed ipfw enable set 18 ; echo done; sleep 30 && ipfw disable set 18 Here if everything goes well, you press control-C before the "sleep" ter- minates, and your ruleset will be left active. Otherwise, e.g. if you cannot access your box, the ruleset will be disabled after the sleep ter- minates thus restoring the previous situation. SEE ALSO cpp(1), m4(1), bridge(4), divert(4), dummynet(4), ip(4), ipfirewall(4), protocols(5), services(5), init(8), kldload(8), reboot(8), sysctl(8), syslogd(8) S. Floyd and V. Jacobson, Random Early Detection gateways for Congestion Avoidance, August 1993. B. Braden, D. Clark, J. Crowcroft, B. Davie, S. Deering, D. Estrin, S. Floyd, V. Jacobson, G. Minshall, C. Partridge, L. Peterson, K. Ramakrishnan, S. Shenker, J. Wroclawski, and L. Zhang, Recommendations on Queue Management and Congestion Avoidance in the Internet, April 1998, RFC 2309. 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. !!! WARNING !!! 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. Incoming packet fragments diverted by divert or tee are reassembled be- fore delivery to the socket. The action used on those packet is the one from the rule which matches the first fragment of the packet. Packets that match a tee rule should not be immediately accepted, but should continue going through the rule list. This may be fixed in a later version. Packets diverted to userland, and then reinserted by a userland process (such as 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 divert rules in the rule sequence very important. AUTHORS Ugen J. S. Antsilevich, Poul-Henning Kamp, Alex Nash, Archie Cobbs, Luigi Rizzo. API based upon code written by Daniel Boulet for BSDI. Work on dummynet(4) traffic shaper supported by Akamba Corp. HISTORY The ipfw utility first appeared in FreeBSD 2.0. dummynet(4) was intro- duced in FreeBSD 2.2.8. Stateful extensions were introduced in FreeBSD 4.0. ipfw2 was introduced in Summer 2002. BSD August 13, 2002 BSD
NAME | SYNOPSIS | DESCRIPTION | PACKET FLOW | RULE FORMAT | SETS OF RULES | STATEFUL FIREWALL | TRAFFIC SHAPER CONFIGURATION | CHECKLIST | FINE POINTS | PACKET DIVERSION | SYSCTL VARIABLES | IPFW2 ENHANCEMENTS | EXAMPLES | SEE ALSO | BUGS | AUTHORS | HISTORY
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