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IPTABLES(8) IPTABLES(8) NNAAMMEE iptables - IP packet filter administration SSYYNNOOPPSSIISS iippttaabblleess --[[AADDCC]] chain rule-specification [options] iippttaabblleess --[[RRII]] chain rulenum rule-specification [options] iippttaabblleess --DD chain rulenum [options] iippttaabblleess --[[LLFFZZ]] [chain] [options] iippttaabblleess --[[NNXX]] chain iippttaabblleess --PP chain target [options] iippttaabblleess --EE old-chain-name new-chain-name DDEESSCCRRIIPPTTIIOONN IIppttaabblleess is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. There are several different tables which may be defined, and each table contains a number of built-in chains, and may contain user-defined chains. Each chain is a list of rules which can match a set of packets: each rule specifies what to do with a packet which matches. This is called a `target', which may be a jump to a user-defined chain in the same table. TTAARRGGEETTSS A firewall rule specifies criteria for a packet, and a target. If the packet does not match, the next rule in the chain is the examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain, or one of the special values _A_C_C_E_P_T, _D_R_O_P, _Q_U_E_U_E, or _R_E_T_U_R_N. _A_C_C_E_P_T means to let the packet through. _D_R_O_P means to drop the packet on the floor. _Q_U_E_U_E means to pass the packet to userspace (if supported by the kernel). _R_E_T_U_R_N means stop traversing this chain, and resume at the next rule in the previous (calling) chain. If the end of a built-in chain is reached, or a rule in a built-in chain with target _R_E_T_U_R_N is matched, the target specified by the chain policy determines the fate of the packet. TTAABBLLEESS There are current three independent tables (which tables are present at any time depends on the kernel configura� tion options and which modules are present). --tt,, ----ttaabbllee This option specifies the packet matching table which the command should operate on. If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there. The tables are as follows: ffiilltteerr This is the default table, and contains the built-in chains INPUT (for packets coming into the box itself), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets). nnaatt This table is consulted when a packet which is cre� ates a new connection is encountered. It consists of three built-ins: PREROUTING (for altering pack� ets as soon as they come in), OUTPUT (for altering locally-generated packets before routing), and POSTROUTING (for altering packets as they are about to go out). mmaannggllee This table is used for special� ized packet alteration. It has two built-in chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally- generated packets before routing). OOPPTTIIOONNSS The options that are recognized by iippttaabblleess can be divided into several different groups. CCOOMMMMAANNDDSS These options specify the specific action to perform; only one of them can be specified on the command line, unless otherwise specified below. For all the long versions of the command and option names, you only need to use enough letters to ensure that iippttaabblleess can differentiate it from all other options. --AA,, ----aappppeenndd Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination. --DD,, ----ddeelleettee Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a number in the chain (starting at 1 for the first rule) or a rule to match. --RR,, ----rreeppllaaccee Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command will fail. Rules are num� bered starting at 1. --II,, ----iinnsseerrtt Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1, the rule or rules are inserted at the head of the chain. This is also the default if no rule number is specified. --LL,, ----lliisstt List all rules in the selected chain. If no chain is selected, all chains are listed. It is legal to specify the --ZZ (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is effected by the other arguments given. --FF,, ----fflluusshh Flush the selected chain. This is equivalent to deleting all the rules one by one. --ZZ,, ----zzeerroo Zero the packet and byte counters in all chains. It is legal to specify the --LL,, ----lliisstt (list) option as well, to see the counters immediately before they are cleared; see above. --NN,, ----nneeww--cchhaaiinn Create a new user-defined chain of the given name. There must be no target of that name already. --XX,, ----ddeelleettee--cchhaaiinn Delete the specified user-defined chain. There must be no references to the chain (if there are you must delete or replace the referring rules before the chain can be deleted). If no argument is given, it will attempt to delete every non- builtin chain in the table. --PP,, ----ppoolliiccyy Set the policy for the chain to the given target. See the section TTAARRGGEETTSS for the legal targets. Only non-user-defined chains can have policies, and neither built-in nor user-defined chains can be policy targets. --EE,, ----rreennaammee--cchhaaiinn Rename the user specified chain to the user sup� plied name; this is cosmetic, and has no effect on the structure of the table. --hh Help. Give a (currently very brief) description of the command syntax. PPAARRAAMMEETTEERRSS The following parameters make up a rule specification (as used in the add, delete, replace, append and check com� mands). --pp,, ----pprroottooccooll [!] _p_r_o_t_o_c_o_l The protocol of the rule or of the packet to check. The specified protocol can be one of _t_c_p, _u_d_p, _i_c_m_p, or _a_l_l, or it can be a numeric value, repre� senting one of these protocols or a different one. A protocol name from /etc/protocols is also allowed. A "!" argument before the protocol inverts the test. The number zero is equivalent to _a_l_l. Protocol _a_l_l will match with all protocols and is taken as default when this option is omit� ted. --ss,, ----ssoouurrccee [!] _a_d_d_r_e_s_s[/_m_a_s_k] Source specification. _A_d_d_r_e_s_s can be either a hostname, a network name, or a plain IP address. The _m_a_s_k can be either a network mask or a plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of _2_4 is equivalent to _2_5_5_._2_5_5_._2_5_5_._0. A "!" argument before the address specification inverts the sense of the address. The flag ----ssrrcc is a convenient alias for this option. --dd,, ----ddeessttiinnaattiioonn [!] _a_d_d_r_e_s_s[/_m_a_s_k] Destination specification. See the description of the --ss (source) flag for a detailed description of the syntax. The flag ----ddsstt is an alias for this option. --jj,, ----jjuummpp _t_a_r_g_e_t This specifies the target of the rule; ie. what to do if the packet matches it. The target can be a user-defined chain (not the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EEXXTTEENNSSIIOONNSS below). If this option is omitted in a rule, then matching the rule will have no effect on the packet's fate, but the counters on the rule will be incremented. --ii,, ----iinn--iinntteerrffaaccee [!] [_n_a_m_e] Optional name of an interface via which a packet is received (for packets entering the IINNPPUUTT, FFOORRWWAARRDD and PPRREERROOUUTTIINNGG chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then any interface which begins with this name will match. If this option is omitted, the string "+" is assumed, which will match with any interface name. --oo,, ----oouutt--iinntteerrffaaccee [!] [_n_a_m_e] Optional name of an interface via which a packet is going to be sent (for packets entering the FFOORRWWAARRDD, OOUUTTPPUUTT and PPOOSSTTRROOUUTTIINNGG chains). When the "!" argu� ment is used before the interface name, the sense is inverted. If the interface name ends in a "+", then any interface which begins with this name will match. If this option is omitted, the string "+" is assumed, which will match with any interface name. [[!!]] --ff,, ----ffrraaggmmeenntt This means that the rule only refers to second and further fragments of fragmented packets. Since there is no way to tell the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify them. When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfrag� mented packets. OOTTHHEERR OOPPTTIIOONNSS The following additional options can be specified: --vv,, ----vveerrbboossee Verbose output. This option makes the list command show the interface address, the rule options (if any), and the TOS masks. The packet and byte coun� ters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipli� ers respectively (but see the --xx flag to change this). For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed. --nn,, ----nnuummeerriicc Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the pro� gram will try to display them as host names, net� work names, or services (whenever applicable). --xx,, ----eexxaacctt Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (multiples of 1000) M's (mul� tiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the --LL command. ----lliinnee--nnuummbbeerrss When listing rules, add line numbers to the begin� ning of each rule, corresponding to that rule's position in the chain. MMAATTCCHH EEXXTTEENNSSIIOONNSS iptables can use extended packet matching modules. These are loaded in two ways: implicitly, when --pp or ----pprroottooccooll is specified, or with the --mm or ----mmaattcchh options, followed by the matching module name; after these, various extra command line options become available, depending on the specific module. You can specify multiple extended match modules in one line, and you can use the --hh or ----hheellpp options after the module has been specified to receive help specific to that module. The following are included in the base package, and most of these can be preceded by a !! to invert the sense of the match. ttccpp These extensions are loaded if `--protocol tcp' is speci� fied. It provides the following options: ----ssoouurrccee--ppoorrtt [!] [_p_o_r_t_[_:_p_o_r_t_]] Source port or port range specification. This can either be a service name or a port number. An inclusive range can also be specified, using the format _p_o_r_t:_p_o_r_t. If the first port is omitted, "0" is assumed; if the last is omitted, "65535" is assumed. If the second port greater then the first they will be swapped. The flag ----ssppoorrtt is an alias for this option. ----ddeessttiinnaattiioonn--ppoorrtt [!] [_p_o_r_t_[_:_p_o_r_t_]] Destination port or port range specification. The flag ----ddppoorrtt is an alias for this option. ----ttccpp--ffllaaggss [!] _m_a_s_k _c_o_m_p Match when the TCP flags are as specified. The first argument is the flags which we should exam� ine, written as a comma-separated list, and the second argument is a comma-separated list of flags which must be set. Flags are: SSYYNN AACCKK FFIINN RRSSTT UURRGG PPSSHH AALLLL NNOONNEE. Hence the command iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN will only match packets with the SYN flag set, and the ACK, FIN and RST flags unset. [[!!]] ----ssyynn Only match TCP packets with the SYN bit set and the ACK and FIN bits cleared. Such packets are used to request TCP connection initiation; for example, blocking such packets coming in an interface will prevent incoming TCP connections, but outgoing TCP connections will be unaffected. It is equivalent to ----ttccpp--ffllaaggss SSYYNN,,RRSSTT,,AACCKK SSYYNN. If the "!" flag precedes the "--syn", the sense of the option is inverted. ----ttccpp--ooppttiioonn [!] _n_u_m_b_e_r Match if TCP option set. uuddpp These extensions are loaded if `--protocol udp' is speci� fied. It provides the following options: ----ssoouurrccee--ppoorrtt [!] [_p_o_r_t_[_:_p_o_r_t_]] Source port or port range specification. See the description of the ----ssoouurrccee--ppoorrtt option of the TCP extension for details. ----ddeessttiinnaattiioonn--ppoorrtt [!] [_p_o_r_t_[_:_p_o_r_t_]] Destination port or port range specification. See the description of the ----ddeessttiinnaattiioonn--ppoorrtt option of the TCP extension for details. iiccmmpp This extension is loaded if `--protocol icmp' is speci� fied. It provides the following option: ----iiccmmpp--ttyyppee [!] _t_y_p_e_n_a_m_e This allows specification of the ICMP type, which can be a numeric ICMP type, or one of the ICMP type names shown by the command iptables -p icmp -h mmaacc ----mmaacc--ssoouurrccee [!] _a_d_d_r_e_s_s Match source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets entering the PPRREERROOUUTTIINNGG, FFOORRWWAARRDD or IINNPPUUTT chains for packets coming from an ethernet device. lliimmiitt This module matches at a limited rate using a token bucket filter: it can be used in combination with the LLOOGG target to give limited logging. A rule using this extension will match until this limit is reached (unless the `!' flag is used). ----lliimmiitt _r_a_t_e Maximum average matching rate: specified as a num� ber, with an optional `/second', `/minute', `/hour', or `/day' suffix; the default is 3/hour. ----lliimmiitt--bbuurrsstt _n_u_m_b_e_r The maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5. mmuullttiippoorrtt This module matches a set of source or destination ports. Up to 15 ports can be specified. It can only be used in conjunction with --pp ttccpp or --pp uuddpp. ----ssoouurrccee--ppoorrtt [_p_o_r_t_[_,_p_o_r_t_]] Match if the source port is one of the given ports. ----ddeessttiinnaattiioonn--ppoorrtt [_p_o_r_t_[_,_p_o_r_t_]] Match if the destination port is one of the given ports. ----ppoorrtt [_p_o_r_t_[_,_p_o_r_t_]] Match if the both the source and destination ports are equal to each other and to one of the given ports. mmaarrkk This module matches the netfilter mark field associated with a packet (which can be set using the MMAARRKK target below). ----mmaarrkk _v_a_l_u_e_[_/_m_a_s_k_] Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mark before the comparison). oowwnneerr This module attempts to match various characteristics of the packet creator, for locally-generated packets. It is only valid in the OOUUTTPPUUTT chain, and even this some packets (such as ICMP ping responses) may have no owner, and hence never match. ----uuiidd--oowwnneerr _u_s_e_r_i_d Matches if the packet was created by a process with the given effective user id. ----ggiidd--oowwnneerr _g_r_o_u_p_i_d Matches if the packet was created by a process with the given effective group id. ----ppiidd--oowwnneerr _p_r_o_c_e_s_s_i_d Matches if the packet was created by a process with the given process id. ----ssiidd--oowwnneerr _s_e_s_s_i_o_n_i_d Matches if the packet was created by a process in the given session group. ssttaattee This module, when combined with connection tracking, allows access to the connection tracking state for this packet. ----ssttaattee _s_t_a_t_e Where state is a comma separated list of the con� nection states to match. Possible states are IINNVVAALLIIDD meaning that the packet is associated with no known connection, EESSTTAABBLLIISSHHEEDD meaning that the packet is associated with a connection which has seen packets in both directions, NNEEWW meaning that the packet has started a new connection, or other� wise associated with a connection which has not seen packets in both directions, and RREELLAATTEEDD mean� ing that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error. uunncclleeaann This module takes no options, but attempts to match pack� ets which seem malformed or unusual. This is regarded as experimental. ttooss This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits). ----ttooss _t_o_s The argument is either a standard name, (use iptables -m tos -h to see the list), or a numeric value to match. TTAARRGGEETT EEXXTTEENNSSIIOONNSS iptables can use extended target modules: the following are included in the standard distribution. LLOOGG Turn on kernel logging of matching packets. When this option is set for a rule, the Linux kernel will print some information on all matching packets (like most IP header fields) via the kernel log (where it can be read with _d_m_e_s_g or _s_y_s_l_o_g_d(8)). ----lloogg--lleevveell _l_e_v_e_l Level of logging (numeric or see _s_y_s_l_o_g_._c_o_n_f(5)). ----lloogg--pprreeffiixx _p_r_e_f_i_x Prefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs. ----lloogg--ttccpp--sseeqquueennccee Log TCP sequence numbers. This is a security risk if the log is readable by users. ----lloogg--ttccpp--ooppttiioonnss Log options from the TCP packet header. ----lloogg--iipp--ooppttiioonnss Log options from the IP packet header. MMAARRKK This is used to set the netfilter mark value associated with the packet. It is only valid in the mmaannggllee table. ----sseett--mmaarrkk _m_a_r_k RREEJJEECCTT This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DDRROOPP. This target is only valid in the IINNPPUUTT, FFOORRWWAARRDD and OOUUTTPPUUTT chains, and user-defined chains which are only called from those chains. Several options control the nature of the error packet returned: ----rreejjeecctt--wwiitthh _t_y_p_e The type given can be iiccmmpp--nneett--uunnrreeaacchhaabbllee, iiccmmpp-- hhoosstt--uunnrreeaacchhaabbllee, iiccmmpp--ppoorrtt--uunnrreeaacchhaabbllee, iiccmmpp-- pprroottoo--uunnrreeaacchhaabbllee, iiccmmpp--nneett--pprroohhiibbiitteeddor iiccmmpp--hhoosstt-- pprroohhiibbiitteedd, which return the appropriate ICMP error message (port-unreachable is the default). The option eecchhoo--rreeppllyy is also allowed; it can only be used for rules which specify an ICMP ping packet, and generates a ping reply. Finally, the option ttccpp--rreesseett can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking _i_d_e_n_t probes which frequently occur when sending mail to broken mail hosts (which won't accept your mail otherwise). TTOOSS This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mmaannggllee table. ----sseett--ttooss _t_o_s You can use a numeric TOS values, or use iptables -j TOS -h to see the list of valid TOS names. MMIIRRRROORR This is an experimental demonstration target which inverts the source and destination fields in the IP header and retransmits the packet. It is only valid in the IINNPPUUTT, FFOORRWWAARRDD and PPRREERROOUUTTIINNGG chains, and user-defined chains which are only called from those chains. Note that the outgoing packets are NNOOTT seen by any packet filtering chains, connection tracking or NAT, to avoid loops and other problems. SSNNAATT This target is only valid in the nnaatt table, in the PPOOSSTTRROOUUTTIINNGG chain. It specifies that the source address of the packet should be modified (and all future packets in this connection will also be mangled), and rules should cease being examined. It takes one option: ----ttoo--ssoouurrccee _<_i_p_a_d_d_r_>_[_-_<_i_p_a_d_d_r_>_]_[_:_p_o_r_t_-_p_o_r_t_] which can specify a single new source IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies --pp ttccpp or --pp uuddpp). If no port range is specified, then source ports below 512 will be mapped to other ports below 512: those between 1024 will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration will occur. DDNNAATT This target is only valid in the nnaatt table, in the PPRREE�� RROOUUTTIINNGG and OOUUTTPPUUTT chains, and user-defined chains which are only called from those chains. It specifies that the destination address of the packet should be modified (and all future packets in this connection will also be man� gled), and rules should cease being examined. It takes one option: ----ttoo--ddeessttiinnaattiioonn _<_i_p_a_d_d_r_>_[_-_<_i_p_a_d_d_r_>_]_[_:_p_o_r_t_-_p_o_r_t_] which can specify a single new destination IP address, an inclusive range of IP addresses, and optionally, a port range (which is only valid if the rule also specifies --pp ttccpp or --pp uuddpp). If no port range is specified, then the destination port will never be modified. MMAASSQQUUEERRAADDEE This target is only valid in the nnaatt table, in the PPOOSSTTRROOUUTTIINNGG chain. It should only be used with dynami� cally assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target. Mas� querading is equivalent to specifying a mapping to the IP address of the interface the packet is going out, but also has the effect that connections are _f_o_r_g_o_t_t_e_n when the interface goes down. This is the correct behavior when the next dialup is unlikely to have the same interface address (and hence any established connections are lost anyway). It takes one option: ----ttoo--ppoorrttss _<_p_o_r_t_>_[_-_<_p_o_r_t_>_] This specifies a range of source ports to use, overriding the default SSNNAATT source port-selection heuristics (see above). This is only valid with if the rule also specifies --pp ttccpp or --pp uuddpp). RREEDDIIRREECCTT This target is only valid in the nnaatt table, in the PPRREE�� RROOUUTTIINNGG and OOUUTTPPUUTT chains, and user-defined chains which are only called from those chains. It alters the destina� tion IP address to send the packet to the machine itself (locally-generated packets are mapped to the 127.0.0.1 address). It takes one option: ----ttoo--ppoorrttss _<_p_o_r_t_>_[_-_<_p_o_r_t_>_] This specifies a destination port or range or ports to use: without this, the destination port is never altered. This is only valid with if the rule also specifies --pp ttccpp or --pp uuddpp). EEXXTTRRAA EEXXTTEENNSSIIOONNSS The following extensions are not included by default in the standard distribution. ttttll This module matches the time to live field in the IP header. ----ttttll _t_t_l Matches the given TTL value. TTTTLL This target is used to modify the time to live field in the IP header. It is only valid in the mmaannggllee table. ----ttttll--sseett _t_t_l Set the TTL to the given value. ----ttttll--ddeecc _t_t_l Decrement the TTL by the given value. ----ttttll--iinncc _t_t_l Increment the TTL by the given value. UULLOOGG This target provides userspace logging of matching pack� ets. When this target is set for a rule, the Linux kernel will multicast this packet through a _n_e_t_l_i_n_k socket. One or more userspace processes may then subscribe to various multicast groups and receive the packets. ----uulloogg--nnllggrroouupp_<_n_l_g_r_o_u_p_> This specifies the netlink group (1-32) to which the packet is sent. ----uulloogg--pprreeffiixx_<_p_r_e_f_i_x_> Prefix log messages with the specified prefix; up to 32 characters long, and useful fro distinguish� ing messages in the logs. ----uulloogg--ccpprraannggee_<_s_i_z_e_> Number of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. DDIIAAGGNNOOSSTTIICCSS Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which appear to be caused by invalid or abused command line parameters cause an exit code of 2, and other errors cause an exit code of 1. BBUUGGSS Check is not implemented (yet). CCOOMMPPAATTIIBBIILLIITTYY WWIITTHH IIPPCCHHAAIINNSS This iippttaabblleess is very similar to ipchains by Rusty Rus� sell. The main difference is that the chains IINNPPUUTT and OOUUTTPPUUTT are only traversed for packets coming into the local host and originating from the local host respec� tively. Hence every packet only passes through one of the three chains; previously a forwarded packet would pass through all three. The other main difference is that --ii refers to the input interface; --oo refers to the output interface, and both are available for packets entering the FFOORRWWAARRDD chain. iippttaabblleess is a pure packet filter when using the default `filter' table, with optional extension modules. This should simplify much of the previous confusion over the combination of IP masquerading and packet filtering seen previously. So the following options are handled differ� ently: -j MASQ -M -S -M -L There are several other changes in iptables. SSEEEE AALLSSOO The iptables-HOWTO, which details more iptables usage, the NAT-HOWTO, which details NAT, and the netfilter-hacking- HOWTO which details the internals. AAUUTTHHOORRSS Rusty Russell wrote iptables, in early consultation with Michael Neuling. Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in iptables, then wrote the mangle table, the owner match, the mark stuff, and ran around doing cool stuff everywhere. James Morris wrote the TOS target, and tos match. Jozsef Kadlecsik wrote the REJECT target. Harald Welte wrote the ULOG target, TTL match+target and libipulog. The Netfilter Core Team is: Marc Boucher, James Morris and Rusty Russell. Aug 11, 2000 1
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