iptables(8) – Linux man page
iptables – administration tool for IPv4 packet filtering and NAT
iptables [-t table] -[AD] chain rule-specification [options]
iptables [-t table] -I chain [rulenum] rule-specification [options]
iptables [-t table] -R chain rulenum rule-specification [options]
iptables [-t table] -D chain rulenum [options]
iptables [-t table] -[LFZ] [chain] [options]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target [options]
iptables [-t table] -E old-chain-name new-chain-name
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also 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 that matches. This is called a ‘target’, which may be a jump to a user-defined chain in the same table.
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 ACCEPT, DROP, QUEUE, or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to userspace. (How the packet can be received by a userspace process differs by the particular queue handler. 2.4.x and 2.6.x kernels up to 2.6.13 include the ip_queue queue handler. Kernels 2.6.14 and later additionally include the nfnetlink_queue queue handler. Packets with a target of QUEUE will be sent to queue number ‘0’ in this case. Please also see the NFQUEUE target as described later in this man page.) RETURN 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 RETURN is matched, the target specified by the chain policy determines the fate of the packet.
There are currently three independent tables (which tables are present at any time depends on the kernel configuration options and which modules are present).-t, –tabletableThis 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:filter:This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for packets destined to local sockets), FORWARD (for packets being routed through the box), and OUTPUT (for locally-generated packets).nat:This table is consulted when a packet that creates a new connection is encountered. It consists of three built-ins: PREROUTING (for altering packets 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).mangle:This table is used for specialized packet alteration. Until kernel 2.4.17 it had two built-in chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally-generated packets before routing). Since kernel 2.4.18, three other built-in chains are also supported: INPUT (for packets coming into the box itself), FORWARD (for altering packets being routed through the box), and POSTROUTING (for altering packets as they are about to go out).raw:This table is used mainly for configuring exemptions from connection tracking in combination with the NOTRACK target. It registers at the netfilter hooks with higher priority and is thus called before ip_conntrack, or any other IP tables. It provides the following built-in chains: PREROUTING (for packets arriving via any network interface) OUTPUT (for packets generated by local processes)
The options that are recognized by iptables can be divided into several different groups.
COMMANDSThese 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 need to use only enough letters to ensure that iptables can differentiate it from all other options.-A, –appendchain rule-specificationAppend 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.-D, –deletechain rule-specification-D, –deletechain rulenumDelete 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.-I, –insertchain [rulenum] rule-specificationInsert 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.-R, –replacechain rulenum rule-specificationReplace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command will fail. Rules are numbered starting at 1.-L, –list [chain]List all rules in the selected chain. If no chain is selected, all chains are listed. As every other iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by
iptables -t nat -n -L
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is affected by the other arguments given. The exact rules are suppressed until you use
iptables -L -v
-F, –flush [chain]Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one by one.-Z, –zero [chain]Zero the packet and byte counters in all chains. It is legal to specify the -L, –list (list) option as well, to see the counters immediately before they are cleared. (See above.)-N, –new-chainchainCreate a new user-defined chain by the given name. There must be no target of that name already.-X, –delete-chain [chain]Delete the optional user-defined chain specified. 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. The chain must be empty, i.e. not contain any rules. If no argument is given, it will attempt to delete every non-builtin chain in the table.-P, –policychain targetSet the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.-E, –rename-chainold-chain new-chainRename the user specified chain to the user supplied name. This is cosmetic, and has no effect on the structure of the table.-h
Help. Give a (currently very brief) description of the command syntax.
PARAMETERSThe following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).-p, –protocol [!] protocolThe protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, icmp, or all, or it can be a numeric value, representing 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 all. Protocol all will match with all protocols and is taken as default when this option is omitted.-s, –source [!] address[/mask]Source specification. Address can be either a network name, a hostname (please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea), a network IP address (with /mask), or a plain IP address. The mask 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 ۲۴ is equivalent to ۲۵۵.۲۵۵.۲۵۵.۰. A “!” argument before the address specification inverts the sense of the address. The flag –src is an alias for this option.-d, –destination [!] address[/mask]Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The flag –dst is an alias for this option.-j, –jumptargetThis specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain (other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EXTENSIONS below). If this option is omitted in a rule (and -g is not used), then matching the rule will have no effect on the packet’s fate, but the counters on the rule will be incremented.-g, –gotochainThis specifies that the processing should continue in a user specified chain. Unlike the –jump option return will not continue processing in this chain but instead in the chain that called us via –jump.-i, –in-interface [!] nameName of an interface via which a packet was received (only for packets entering the INPUT, FORWARD and PREROUTING 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, any interface name will match.-o, –out-interface [!] nameName of an interface via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING 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, any interface name will match.[!] -f, –fragmentThis 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 unfragmented packets.-c, –set-countersPKTS BYTESThis enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).
OTHER OPTIONSThe following additional options can be specified:-v, –verboseVerbose output. This option makes the list command show the interface name, the rule options (if any), and the TOS masks. The packet and byte counters are also listed, with the suffix ‘K’, ‘M’ or ‘G’ for 1000, 1,000,000 and 1,000,000,000 multipliers respectively (but see the -x flag to change this). For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.-n, –numericNumeric output. IP addresses and port numbers will be printed in numeric format. By default, the program will try to display them as host names, network names, or services (whenever applicable).-x, –exactExpand 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 (multiples of 1000K) or G’s (multiples of 1000M). This option is only relevant for the -L command.–line-numbersWhen listing rules, add line numbers to the beginning of each rule, corresponding to that rule’s position in the chain.–modprobe=commandWhen adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).
iptables can use extended packet matching modules. These are loaded in two ways: implicitly, when -p or –protocol is specified, or with the -m or –match 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 -h or –help 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.
accountAccount traffic for all hosts in defined network/netmask.
– long (one counter per protocol TCP/UDP/IMCP/Other) and short statistics
– one iptables rule for all hosts in network/netmask
– loading/saving counters (by reading/writting to procfs entries)–aaddrnetwork/netmaskdefines network/netmask for which make statistics.–anamenamedefines name of list where statistics will be kept. If no is specified DEFAULT will be used.–ashorttable will colect only short statistics (only total counters without splitting it into protocols.Example usage:
account traffic for/to 192.168.0.0/24 network into table mynetwork:
# iptables -A FORWARD -m account –aname mynetwork –aaddr 192.168.0.0/24
account traffic for/to WWW serwer for 192.168.0.0/24 network into table mywwwserver:
# iptables -A INPUT -p tcp –dport 80 -m account –aname mywwwserver –aaddr 192.168.0.0/24 –ashort
# iptables -A OUTPUT -p tcp –sport 80 -m account –aname mywwwserver –aaddr 192.168.0.0/24 –ashort
# cat /proc/net/ipt_account/mynetwork # cat /proc/net/ipt_account/mywwwserver
# echo “ip = 192.168.0.1 packets_src = 0” > /proc/net/ipt_account/mywwserver
addrtypeThis module matches packets based on their address type. Address types are used within the kernel networking stack and categorize addresses into various groups. The exact definition of that group depends on the specific layer three protocol.The following address types are possible:UNSPEC
an unspecified address (i.e. 0.0.0.0) UNICAST an unicast address LOCAL a local address BROADCAST a broadcast address ANYCAST an anycast packet MULTICAST a multicast address BLACKHOLE a blackhole address UNREACHABLE an unreachable address PROHIBIT a prohibited address THROW FIXME NAT FIXME XRESOLVE FIXME–src-typetypeMatches if the source address is of given type–dst-typetypeMatches if the destination address is of given type
ahThis module matches the SPIs in Authentication header of IPsec packets.–ahspi [!] spi[:spi]
childlevelThis is an experimental module. It matches on whether the packet is part of a master connection or one of its children (or grandchildren, etc). For instance, most packets are level 0. FTP data transfer is level 1.–childlevel [!] level
commentAllows you to add comments (up to 256 characters) to any rule.–commentcommentExample:iptables -A INPUT -s 192.168.0.0/16 -m comment –comment “A privatized IP block”
conditionThis matches if a specific /proc filename is ‘0’ or ‘1’.–condition[!] filenameMatch on boolean value stored in /proc/net/ipt_condition/filename file
connbytesMatch by how many bytes or packets a connection (or one of the two flows constituting the connection) have tranferred so far, or by average bytes per packet.
The counters are 64bit and are thus not expected to overflow 😉
The primary use is to detect long-lived downloads and mark them to be scheduled using a lower priority band in traffic control.
The transfered bytes per connection can also be viewed through /proc/net/ip_conntrack and accessed via ctnetlink[!] –connbytesfrom:[to]match packets from a connection whose packets/bytes/average packet size is more than FROM and less than TO bytes/packets. if TO is omitted only FROM check is done. “!” is used to match packets not falling in the range.–connbytes-dir [original|reply|both]which packets to consider–connbytes-mode [packets|bytes|avgpkt]whether to check the amount of packets, number of bytes transferred or the average size (in bytes) of all packets received so far. Note that when “both” is used together with “avgpkt”, and data is going (mainly) only in one direction (for example HTTP), the average packet size will be about half of the actual data packets.Example:iptables .. -m connbytes –connbytes 10000:100000 –connbytes-dir both –connbytes-mode bytes …
connlimitAllows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).[!] –connlimit-abovenmatch if the number of existing tcp connections is (not) above n–connlimit-maskbitsgroup hosts using maskExamples:# allow 2 telnet connections per client hostiptables -p tcp –syn –dport 23 -m connlimit –connlimit-above 2 -j REJECT# you can also match the other way around:iptables -p tcp –syn –dport 23 -m connlimit ! –connlimit-above 2 -j ACCEPT# limit the nr of parallel http requests to 16 per class C sized network (24 bit netmask)iptables -p tcp –syn –dport 80 -m connlimit –connlimit-above 16 –connlimit-mask 24 -j REJECT
connmarkThis module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).–markvalue[/mask]Matches packets in connections with the given mark value (if a mask is specified, this is logically ANDed with the mark before the comparison).
connrateThis module matches the current transfer rate in a connection.–connrate[!] [from]:[to]Match against the current connection transfer rate being within ‘from’ and ‘to’ bytes per second. When the “!” argument is used before the range, the sense of the match is inverted.
conntrackThis module, when combined with connection tracking, allows access to more connection tracking information than the “state” match. (this module is present only if iptables was compiled under a kernel supporting this feature)–ctstatestateWhere state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the packet is associated with no known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and RELATED meaning 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. SNAT A virtual state, matching if the original source address differs from the reply destination. DNAT A virtual state, matching if the original destination differs from the reply source.–ctprotoprotoProtocol to match (by number or name)–ctorigsrc[!] address[/mask]Match against original source address–ctorigdst[!] address[/mask]Match against original destination address–ctreplsrc[!] address[/mask]Match against reply source address–ctrepldst[!] address[/mask]Match against reply destination address–ctstatus[NONE|EXPECTED|SEEN_REPLY|ASSURED][,…]Match against internal conntrack states–ctexpiretime[:time]Match remaining lifetime in seconds against given value or range of values (inclusive)
dccp–source-port,–sport [!] port[:port]–destination-port,–dport [!] port[:port]–dccp-types [!] maskMatch when the DCCP packet type is one of ‘mask’. ‘mask’ is a comma-separated list of packet types. Packet types are: REQUEST RESPONSE DATA ACK DATAACK CLOSEREQ CLOSE RESET SYNC SYNCACK INVALID.–dccp-option [!] numberMatch if DCP option set.
dscpThis module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.–dscpvalueMatch against a numeric (decimal or hex) value [0-63].–dscp-classDiffServ ClassMatch the DiffServ class. This value may be any of the BE, EF, AFxx or CSx classes. It will then be converted into it’s according numeric value.
dstlimitThis module allows you to limit the packet per second (pps) rate on a per destination IP or per destination port base. As opposed to the ‘limit’ match, every destination ip / destination port has it’s own limit.THIS MODULE IS DEPRECATED AND HAS BEEN REPLACED BY ”hashlimit”–dstlimitavgMaximum average match rate (packets per second unless followed by /sec /minute /hour /day postfixes).–dstlimit-modemodeThe limiting hashmode. Is the specified limit per dstip, dstip-dstport tuple, srcip-dstip tuple, or per srcipdstip-dstport tuple.–dstlimit-namenameName for /proc/net/ipt_dstlimit/* file entry[–dstlimit-burstburst]Number of packets to match in a burst. Default: 5[–dstlimit-htable-sizesize]Number of buckets in the hashtable[–dstlimit-htable-maxmax]Maximum number of entries in the hashtable[–dstlimit-htable-gcintervalinterval]Interval between garbage collection runs of the hashtable (in miliseconds). Default is 1000 (1 second).[–dstlimit-htable-expiretimeAfter which time are idle entries expired from hashtable (in miliseconds)? Default is 10000 (10 seconds).
ecnThis allows you to match the ECN bits of the IPv4 and TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168–ecn-tcp-cwrThis matches if the TCP ECN CWR (Congestion Window Received) bit is set.–ecn-tcp-eceThis matches if the TCP ECN ECE (ECN Echo) bit is set.–ecn-ip-ectnumThis matches a particular IPv4 ECT (ECN-Capable Transport). You have to specify a number between ‘0’ and ‘3’.
espThis module matches the SPIs in ESP header of IPsec packets.–espspi [!] spi[:spi]
fuzzyThis module matches a rate limit based on a fuzzy logic controller [FLC]–lower-limitnumberSpecifies the lower limit (in packets per second).–upper-limitnumberSpecifies the upper limit (in packets per second).
hashlimitThis patch adds a new match called ‘hashlimit’. The idea is to have something like ‘limit’, but either per destination-ip or per (destip,destport) tuple.
It gives you the ability to express’1000 packets per second for every host in 192.168.0.0/16′
‘۱۰۰ packets per second for every service of 192.168.1.1’with a single iptables rule.–hashlimitrateA rate just like the limit match–hashlimit-burstnumBurst value, just like limit match–hashlimit-modedestip | destip-destportLimit per IP or per port–hashlimit-namefooThe name for the /proc/net/ipt_hashlimit/foo entry–hashlimit-htable-sizenumThe number of buckets of the hash table–hashlimit-htable-maxnumMaximum entries in the hash–hashlimit-htable-expirenumAfter how many miliseconds do hash entries expire–hashlimit-htable-gcintervalnumHow many miliseconds between garbage collection intervals
helperThis module matches packets related to a specific conntrack-helper.–helperstringMatches packets related to the specified conntrack-helper.string can be “ftp” for packets related to a ftp-session on default port. For other ports append -portnr to the value, ie. “ftp-2121”.
Same rules apply for other conntrack-helpers.
icmpThis extension is loaded if ‘–protocol icmp’ is specified. It provides the following option:–icmp-type [!] typenameThis 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
iprangeThis matches on a given arbitrary range of IPv4 addresses[!]–src-rangeip-ipMatch source IP in the specified range.[!]–dst-rangeip-ipMatch destination IP in the specified range.
ipv4optionsMatch on IPv4 header options like source routing, record route, timestamp and router-alert.–ssrr
To match packets with the flag strict source routing.
To match packets with the flag loose source routing.–no-srrTo match packets with no flag for source routing.[!] –rrTo match packets with the RR flag.[!] –tsTo match packets with the TS flag.[!] –raTo match packets with the router-alert option.[!] –any-optTo match a packet with at least one IP option, or no IP option at all if ! is chosen.Examples:$ iptables -A input -m ipv4options –rr -j DROPwill drop packets with the record-route flag.$ iptables -A input -m ipv4options –ts -j DROPwill drop packets with the timestamp flag.
lengthThis module matches the length of a packet against a specific value or range of values.–length [!] length[:length]
limitThis module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached (unless the ‘!’ flag is used). It can be used in combination with the LOG target to give limited logging, for example.–limitrateMaximum average matching rate: specified as a number, with an optional ‘/second’, ‘/minute’, ‘/hour’, or ‘/day’ suffix; the default is 3/hour.–limit-burstnumberMaximum 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.
mac–mac-source [!] addressMatch source MAC address. It must be of the form XX:XX:XX:XX:XX:XX. Note that this only makes sense for packets coming from an Ethernet device and entering the PREROUTING, FORWARD or INPUT chains.
markThis module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).–markvalue[/mask]Matches packets with the given unsigned mark value (if a mask is specified, this is logically ANDed with the mask before the comparison).
mportThis module matches a set of source or destination ports. Up to 15 ports can be specified. It can only be used in conjunction with -p tcp or -p udp.–source-portsport[,port[,port…]]Match if the source port is one of the given ports. The flag –sports is a convenient alias for this option.–destination-portsport[,port[,port…]]Match if the destination port is one of the given ports. The flag –dports is a convenient alias for this option.–portsport[,port[,port…]]Match if the both the source and destination ports are equal to each other and to one of the given ports.
multiportThis module matches a set of source or destination ports. Up to 15 ports can be specified. A port range (port:port) counts as two ports. It can only be used in conjunction with -p tcp or -p udp.–source-ports[!] port[,port[,port:port…]]Match if the source port is one of the given ports. The flag –sports is a convenient alias for this option.–destination-ports[!] port[,port[,port:port…]]Match if the destination port is one of the given ports. The flag –dports is a convenient alias for this option.–ports[!] port[,port[,port:port…]]Match if either the source or destination ports are equal to one of the given ports.
nthThis module matches every ‘n’th packet–everyvalueMatch every ‘value’ packet[–counternum]Use internal counter number ‘num’. Default is ‘0’.[–startnum]Initialize the counter at the number ‘num’ insetad of ‘0’. Most between ‘0’ and ‘value’-1.[–packetnum]Match on ‘num’ packet. Most be between ‘0’ and ‘value’-1.
osfThe idea of passive OS fingerprint matching exists for quite a long time, but was created as extension fo OpenBSD pf only some weeks ago. Original idea was lurked in some OpenBSD mailing list (thanks grange@open…) and than adopted for Linux netfilter in form of this code.
Original fingerprint table was created by Michal Zalewski <email@example.com>.
This module compares some data(WS, MSS, options and it’s order, ttl, df and others) from first SYN packet (actually from packets with SYN bit set) with dynamically loaded OS fingerprints.–log 1/0
If present, OSF will log determined genres even if they don’t match desired one.0 – log all determined entries, 1 – only first one.In syslog you find something like this:ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 22.214.171.124:4024 -> 126.96.36.199:139
ipt_osf: Unknown: 16384:106:1:48:020405B401010402 188.8.131.52:1239 -> 184.108.40.206:80–smartif present, OSF will use some smartness to determine remote OS. OSF will use initial TTL only if source of connection is in our local network.–netlinkIf present, OSF will log all events also through netlink NETLINK_NFLOG groupt 1.–genre[!] stringMatch a OS genre by passive fingerprintingExample:
#iptables -I INPUT -j ACCEPT -p tcp -m osf –genre Linux –log 1 –smart
NOTE: -p tcp is obviously required as it is a TCP match.
Fingerprints can be loaded and read through /proc/sys/net/ipv4/osf file. One can flush all fingerprints with following command:echo -en FLUSH > /proc/sys/net/ipv4/osfOnly one fingerprint per open/write/close.
Fingerprints can be downloaded from http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os
ownerThis module attempts to match various characteristics of the packet creator, for locally-generated packets. It is only valid in the OUTPUT chain, and even this some packets (such as ICMP ping responses) may have no owner, and hence never match.–uid-owneruseridMatches if the packet was created by a process with the given effective user id.–gid-ownergroupidMatches if the packet was created by a process with the given effective group id.–pid-ownerprocessidMatches if the packet was created by a process with the given process id.–sid-ownersessionidMatches if the packet was created by a process in the given session group.–cmd-ownernameMatches if the packet was created by a process with the given command name. (this option is present only if iptables was compiled under a kernel supporting this feature)NOTE: pid, sid and command matching are broken on SMP
physdevThis module matches on the bridge port input and output devices enslaved to a bridge device. This module is a part of the infrastructure that enables a transparent bridging IP firewall and is only useful for kernel versions above version 2.5.44.–physdev-in [!] nameName of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD and PREROUTING chains). If the interface name ends in a “+”, then any interface which begins with this name will match. If the packet didn’t arrive through a bridge device, this packet won’t match this option, unless ‘!’ is used.–physdev-out [!] nameName of a bridge port via which a packet is going to be sent (for packets entering the FORWARD, OUTPUT and POSTROUTING chains). If the interface name ends in a “+”, then any interface which begins with this name will match. Note that in the nat and mangle OUTPUT chains one cannot match on the bridge output port, however one can in the filter OUTPUT chain. If the packet won’t leave by a bridge device or it is yet unknown what the output device will be, then the packet won’t match this option, unless[!] –physdev-is-inMatches if the packet has entered through a bridge interface.[!] –physdev-is-outMatches if the packet will leave through a bridge interface.[!] –physdev-is-bridgedMatches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and POSTROUTING chains.
pkttypeThis module matches the link-layer packet type.–pkt-type[unicast|broadcast|multicast]
policyThis modules matches the policy used by IPsec for handling a packet.–dirin|outUsed to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation. in is valid in the PREROUTING, INPUT and FORWARD chains, out is valid in the POSTROUTING, OUTPUT and FORWARD chains.–polnone|ipsecMatches if the packet is subject to IPsec processing.–strictSelects whether to match the exact policy or match if any rule of the policy matches the given policy.–reqididMatches the reqid of the policy rule. The reqid can be specified with setkey(8) using unique:id as level.–spispiMatches the SPI of the SA.–protoah|esp|ipcompMatches the encapsulation protocol.–modetunnel|transportMatches the encapsulation mode.–tunnel-srcaddr[/mask]Matches the source end-point address of a tunnel mode SA. Only valid with –mode tunnel.–tunnel-dstaddr[/mask]Matches the destination end-point address of a tunnel mode SA. Only valid with –mode tunnel.–next
Start the next element in the policy specification. Can only be used with –strict
psdAttempt to detect TCP and UDP port scans. This match was derived from Solar Designer’s scanlogd.–psd-weight-thresholdthresholdTotal weight of the latest TCP/UDP packets with different destination ports coming from the same host to be treated as port scan sequence.–psd-delay-thresholddelayDelay (in hundredths of second) for the packets with different destination ports coming from the same host to be treated as possible port scan subsequence.–psd-lo-ports-weightweightWeight of the packet with privileged (<=1024) destination port.–psd-hi-ports-weightweightWeight of the packet with non-priviliged destination port.
quotaImplements network quotas by decrementing a byte counter with each packet.–quotabytesThe quota in bytes.KNOWN BUGS: this does not work on SMP systems.
randomThis module randomly matches a certain percentage of all packets.–averagepercentMatches the given percentage. If omitted, a probability of 50% is set.
realmThis matches the routing realm. Routing realms are used in complex routing setups involving dynamic routing protocols like BGP.–realm[!]value[/mask]Matches a given realm number (and optionally mask).
recentAllows you to dynamically create a list of IP addresses and then match against that list in a few different ways.
For example, you can create a ‘badguy’ list out of people attempting to connect to port 139 on your firewall and then DROP all future packets from them without considering them.–namenameSpecify the list to use for the commands. If no name is given then ‘DEFAULT’ will be used.[!] –setThis will add the source address of the packet to the list. If the source address is already in the list, this will update the existing entry. This will always return success (or failure if ‘!’ is passed in).[!] –rcheckCheck if the source address of the packet is currently in the list.[!] –updateLike –rcheck, except it will update the “last seen” timestamp if it matches.[!] –removeCheck if the source address of the packet is currently in the list and if so that address will be removed from the list and the rule will return true. If the address is not found, false is returned.[!] –secondssecondsThis option must be used in conjunction with one of –rcheck or –update. When used, this will narrow the match to only happen when the address is in the list and was seen within the last given number of seconds.[!] –hitcounthitsThis option must be used in conjunction with one of –rcheck or –update. When used, this will narrow the match to only happen when the address is in the list and packets had been received greater than or equal to the given value. This option may be used along with –seconds to create an even narrower match requiring a certain number of hits within a specific time frame.–rttl
This option must be used in conjunction with one of –rcheck or –update. When used, this will narrow the match to only happen when the address is in the list and the TTL of the current packet matches that of the packet which hit the –set rule. This may be useful if you have problems with people faking their source address in order to DoS you via this module by disallowing others access to your site by sending bogus packets to you.Examples:# iptables -A FORWARD -m recent –name badguy –rcheck –seconds 60 -j DROP
# iptables -A FORWARD -p tcp -i eth0 –dport 139 -m recent –name badguy –set -j DROPOfficial website (http://snowman.net/projects/ipt_recent/) also has some examples of usage.
/proc/net/ipt_recent/* are the current lists of addresses and information about each entry of each list.
Each file in /proc/net/ipt_recent/ can be read from to see the current list or written two using the following commands to modify the list:echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULTto Add to the DEFAULT listecho -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULTto Remove from the DEFAULT listecho clear > /proc/net/ipt_recent/DEFAULTto empty the DEFAULT list.The module itself accepts parameters, defaults shown:ip_list_tot=۱۰۰Number of addresses remembered per tableip_pkt_list_tot=۲۰Number of packets per address rememberedip_list_hash_size=۰Hash table size. 0 means to calculate it based on ip_list_tot, default: 512ip_list_perms=۰۶۴۴Permissions for /proc/net/ipt_recent/* filesdebug=۰Set to 1 to get lots of debugging info
sctp–source-port,–sport [!] port[:port]–destination-port,–dport [!] port[:port]–chunk-types [!] all|any|onlychunktype[:flags] […]The flag letter in upper case indicates that the flag is to match if set, in the lower case indicates to match if unset.
Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF ASCONF_ACK
chunk type available flags
DATA U B E u b e
ABORT T t
SHUTDOWN_COMPLETE T t
(lowercase means flag should be “off”, uppercase means “on”)Examples:
iptables -A INPUT -p sctp –dport 80 -j DROP
iptables -A INPUT -p sctp –chunk-types any DATA,INIT -j DROP
iptables -A INPUT -p sctp –chunk-types any DATA:Be -j ACCEPT
setThis modules macthes IP sets which can be defined by ipset(۸).–set setname flag[,flag…]where flags are src and/or dst and there can be no more than six of them. Hence the command
iptables -A FORWARD -m set --set test src,dst
will match packets, for which (depending on the type of the set) the source address or port number of the packet can be found in the specified set. If there is a binding belonging to the mached set element or there is a default binding for the given set, then the rule will match the packet only if additionally (depending on the type of the set) the destination address or port number of the packet can be found in the set according to the binding.
stateThis module, when combined with connection tracking, allows access to the connection tracking state for this packet.–statestateWhere state is a comma separated list of the connection states to match. Possible states are INVALID meaning that the packet could not be identified for some reason which includes running out of memory and ICMP errors which don’t correspond to any known connection, ESTABLISHED meaning that the packet is associated with a connection which has seen packets in both directions, NEW meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and RELATED meaning 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.
stringThis modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.–algobm|kmpSelect the pattern matching strategy. (bm = Boyer-Moore, kmp = Knuth-Pratt-Morris)–fromoffsetSet the offset from which it starts looking for any matching. If not passed, default is 0.–tooffsetSet the offset from which it starts looking for any matching. If not passed, default is the packet size.–stringpatternMatches the given pattern. –hex-stringpattern Matches the given pattern in hex notation.
tcpThese extensions are loaded if ‘–protocol tcp’ is specified. It provides the following options:–source-port [!] port[:port]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 port:port. 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 –sport is a convenient alias for this option.–destination-port [!] port[:port]Destination port or port range specification. The flag –dport is a convenient alias for this option.–tcp-flags [!] mask compMatch when the TCP flags are as specified. The first argument is the flags which we should examine, written as a comma-separated list, and the second argument is a comma-separated list of flags which must be set. Flags are: SYN ACK FIN RST URG PSH ALL NONE. 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.[!] –synOnly match TCP packets with the SYN bit set and the ACK,RST 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 –tcp-flags SYN,RST,ACK,FIN SYN. If the “!” flag precedes the “–syn”, the sense of the option is inverted.–tcp-option [!] numberMatch if TCP option set.–mssvalue[:value]Match TCP SYN or SYN/ACK packets with the specified MSS value (or range), which control the maximum packet size for that connection.
tcpmssThis matches the TCP MSS (maximum segment size) field of the TCP header. You can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.[!]–mss value[:value]”Match a given TCP MSS value or range.
timeThis matches if the packet arrival time/date is within a given range. All options are facultative.–timestartvalueMatch only if it is after ‘value’ (Inclusive, format: HH:MM ; default 00:00).–timestopvalueMatch only if it is before ‘value’ (Inclusive, format: HH:MM ; default 23:59).–dayslistofdaysMatch only if today is one of the given days. (format: Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)–datestartdateMatch only if it is after ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 1970)–datestopdateMatch only if it is before ‘date’ (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 2037)
tosThis module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).–tostosThe argument is either a standard name, (use
iptables -m tos -h
to see the list), or a numeric value to match.
ttlThis module matches the time to live field in the IP header.–ttl-eqttlMatches the given TTL value.–ttl-gtttlMatches if TTL is greater than the given TTL value.–ttl-ltttlMatches if TTL is less than the given TTL value.
u32U32 allows you to extract quantities of up to 4 bytes from a packet, AND them with specified masks, shift them by specified amounts and test whether the results are in any of a set of specified ranges. The specification of what to extract is general enough to skip over headers with lengths stored in the packet, as in IP or TCP header lengths.
Details and examples are in the kernel module source.
udpThese extensions are loaded if ‘–protocol udp’ is specified. It provides the following options:–source-port [!] port[:port]Source port or port range specification. See the description of the –source-port option of the TCP extension for details.–destination-port [!] port[:port]Destination port or port range specification. See the description of the –destination-port option of the TCP extension for details.
uncleanThis module takes no options, but attempts to match packets which seem malformed or unusual. This is regarded as experimental.
iptables can use extended target modules: the following are included in the standard distribution.
BALANCEThis allows you to DNAT connections in a round-robin way over a given range of destination addresses.–to-destinationipaddr-ipaddrAddress range to round-robin over.
CLASSIFYThis module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).–set-classMAJOR:MINORSet the major and minor class value.
CLUSTERIPThis module allows you to configure a simple cluster of nodes that share a certain IP and MAC address without an explicit load balancer in front of them. Connections are statically distributed between the nodes in this cluster.–new
Create a new ClusterIP. You always have to set this on the first rule for a given ClusterIP.–hashmodemodeSpecify the hashing mode. Has to be one of sourceip, sourceip-sourceport, sourceip-sourceport-destport–clustermacmacSpecify the ClusterIP MAC address. Has to be a link-layer multicast address–total-nodesnumNumber of total nodes within this cluster.–local-nodenumLocal node number within this cluster.–hash-initrndSpecify the random seed used for hash initialization.
CONNMARKThis module sets the netfilter mark value associated with a connection–set-mark mark[/mask]Set connection mark. If a mask is specified then only those bits set in the mask is modified.–save-mark [–mask mask]Copy the netfilter packet mark value to the connection mark. If a mask is specified then only those bits are copied.–restore-mark [–mask mask]Copy the connection mark value to the packet. If a mask is specified then only those bits are copied. This is only valid in the mangle table.
DNATThis target is only valid in the nat table, in the PREROUTING and OUTPUT 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 mangled), and rules should cease being examined. It takes one type of option:–to-destinationipaddr[-ipaddr][:port–port]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 -p tcp or -p udp). If no port range is specified, then the destination port will never be modified.In Kernels up to 2.6.10 you can add several –to-destination options. For those kernels, if you specify more than one destination address, either via an address range or multiple –to-destination options, a simple round-robin (one after another in cycle) load balancing takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to multiple ranges anymore.
DSCPThis target allows to alter the value of the DSCP bits within the TOS header of the IPv4 packet. As this manipulates a packet, it can only be used in the mangle table.–set-dscpvalueSet the DSCP field to a numerical value (can be decimal or hex)–set-dscp-classclassSet the DSCP field to a DiffServ class.
ECNThis target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.–ecn-tcp-removeRemove all ECN bits from the TCP header. Of course, it can only be used in conjunction with -p tcp.
IPMARKAllows you to mark a received packet basing on its IP address. This can replace many mangle/mark entries with only one, if you use firewall based classifier.
This target is to be used inside the mangle table, in the PREROUTING, POSTROUTING or FORWARD hooks.–addrsrc/dstUse source or destination IP address.–and-maskmaskPerform bitwise ‘and’ on the IP address and this mask.–or-maskmaskPerform bitwise ‘or’ on the IP address and this mask.The order of IP address bytes is reversed to meet “human order of bytes”: 192.168.0.1 is 0xc0a80001. At first the ‘and’ operation is performed, then ‘or’.
We create a queue for each user, the queue number is adequate to the IP address of the user, e.g.: all packets going to/from 192.168.5.2 are directed to 1:0502 queue, 192.168.5.12 -> 1:050c etc.
We have one classifier rule:tc filter add dev eth3 parent 1:0 protocol ip fwEarlier we had many rules just like below:iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.2 -j MARK –set-mark 0x10502
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.3 -j MARK –set-mark 0x10503Using IPMARK target we can replace all the mangle/mark rules with only one:iptables -t mangle -A POSTROUTING -o eth3 -j IPMARK –addr=dst –and-mask=0xffff –or-mask=0x10000On the routers with hundreds of users there should be significant load decrease (e.g. twice).
IPV4OPTSSTRIPStrip all the IP options from a packet.
The target doesn’t take any option, and therefore is extremly easy to use :
# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP
LOGTurn 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 dmesg or syslogd(۸)). This is a “non-terminating target”, i.e. rule traversal continues at the next rule. So if you want to LOG the packets you refuse, use two separate rules with the same matching criteria, first using target LOG then DROP (or REJECT).–log-levellevelLevel of logging (numeric or see syslog.conf(۵)).–log-prefixprefixPrefix log messages with the specified prefix; up to 29 letters long, and useful for distinguishing messages in the logs.–log-tcp-sequenceLog TCP sequence numbers. This is a security risk if the log is readable by users.–log-tcp-optionsLog options from the TCP packet header.–log-ip-optionsLog options from the IP packet header.–log-uidLog the userid of the process which generated the packet.
MARKThis is used to set the netfilter mark value associated with the packet. It is only valid in the mangle table. It can for example be used in conjunction with iproute2.–set-markmark
MASQUERADEThis target is only valid in the nat table, in the POSTROUTING chain. It should only be used with dynamically assigned IP (dialup) connections: if you have a static IP address, you should use the SNAT target. Masquerading 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 forgotten 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:–to-portsport[-port]This specifies a range of source ports to use, overriding the default SNAT source port-selection heuristics (see above). This is only valid if the rule also specifies -p tcp or -p udp.
MIRRORThis 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 INPUT, FORWARD and PREROUTING chains, and user-defined chains which are only called from those chains. Note that the outgoing packets are NOT seen by any packet filtering chains, connection tracking or NAT, to avoid loops and other problems.
NETMAPThis target allows you to statically map a whole network of addresses onto another network of addresses. It can only be used from rules in the nat table.–toaddress[/mask]Network address to map to. The resulting address will be constructed in the following way: All ‘one’ bits in the mask are filled in from the new ‘address’. All bits that are zero in the mask are filled in from the original address.
NFQUEUEThis target is an extension of the QUEUE target. As opposed to QUEUE, it allows you to put a packet into any specific queue, identified by its 16-bit queue number.–queue-numvalueThis specifies the QUEUE number to use. Valud queue numbers are 0 to 65535. The default value is 0.It can only be used with Kernel versions 2.6.14 or later, since it requiresthe nfnetlink_queue kernel support.
NOTRACKThis target disables connection tracking for all packets matching that rule.It can only be used in theraw table.
REDIRECTThis target is only valid in the nat table, in the PREROUTING and OUTPUT chains, and user-defined chains which are only called from those chains. It redirects the packet to the machine itself by changing the destination IP to the primary address of the incoming interface (locally-generated packets are mapped to the 127.0.0.1 address). It takes one option:–to-portsport[-port]This specifies a destination port or range of ports to use: without this, the destination port is never altered. This is only valid if the rule also specifies -p tcp or -p udp.
REJECTThis is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:–reject-withtypeThe type given can be
which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset 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 ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won’t accept your mail otherwise).(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
SAMESimilar to SNAT/DNAT depending on chain: it takes a range of addresses (‘–to 220.127.116.11-18.104.22.168’) and gives a client the same source-/destination-address for each connection.–to<ipaddr>-<ipaddr>Addresses to map source to. May be specified more than once for multiple ranges.–nodstDon’t use the destination-ip in the calculations when selecting the new source-ip
SETThis modules adds and/or deletes entries from IP sets which can be defined by ipset(۸).–add-set setname flag[,flag…]add the address(es)/port(s) of the packet to the sets–del-set setname flag[,flag…]delete the address(es)/port(s) of the packet from the sets, where flags are src and/or dst and there can be no more than six of them.The bindings to follow must previously be defined in order to usemultilevel adding/deleting by the SET target.
SNATThis target is only valid in the nat table, in the POSTROUTING 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 type of option:–to-sourceipaddr[-ipaddr][:port–port]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 -p tcp or -p udp). If no port range is specified, then source ports below 512 will be mapped to other ports below 512: those between 512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be mapped to 1024 or above. Where possible, no port alteration will occur.In Kernels up to 2.6.10, you can add several –to-source options. For those kernels, if you specify more than one source address, either via an address range or multiple –to-source options, a simple round-robin (one after another in cycle) takes place between these addresses. Later Kernels (>= 2.6.11-rc1) don’t have the ability to NAT to multiple ranges anymore.
TARPITCaptures and holds incoming TCP connections using no local per-connection resources. Connections are accepted, but immediately switched to the persist state (0 byte window), in which the remote side stops sending data and asks to continue every 60-240 seconds. Attempts to close the connection are ignored, forcing the remote side to time out the connection in 12-24 minutes.
This offers similar functionality to LaBrea <http://www.hackbusters.net/LaBrea/> but doesn’t require dedicated hardware or IPs. Any TCP port that you would normally DROP or REJECT can instead become a tarpit.
To tarpit connections to TCP port 80 destined for the current machine:iptables -A INPUT -p tcp -m tcp –dport 80 -j TARPITTo significantly slow down Code Red/Nimda-style scans of unused address space, forward unused ip addresses to a Linux box not acting as a router (e.g. “ip route 10.0.0.0 255.0.0.0 ip.of.linux.box” on a Cisco), enable IP forwarding on the Linux box, and add:iptables -A FORWARD -p tcp -j TARPIT
iptables -A FORWARD -j DROPNOTE:
If you use the conntrack module while you are using TARPIT, you should also use the NOTRACK target, or the kernel will unnecessarily allocate resources for each TARPITted connection. To TARPIT incoming connections to the standard IRC port while using conntrack, you could:iptables -t raw -A PREROUTING -p tcp –dport 6667 -j NOTRACK
iptables -A INPUT -p tcp –dport 6667 -j TARPIT
TCPMSSThis target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface’s MTU minus 40). Of course, it can only be used in conjunction with -p tcp. It is only valid in the mangle table.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:1)
Web browsers connect, then hang with no data received.
Small mail works fine, but large emails hang.
ssh works fine, but scp hangs after initial handshaking.Workaround: activate this option and add a rule to your firewall configuration like:
iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
-j TCPMSS --clamp-mss-to-pmtu
–set-mssvalueExplicitly set MSS option to specified value.–clamp-mss-to-pmtuAutomatically clamp MSS value to (path_MTU – 40).These options are mutually exclusive.
TOSThis is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.–set-tostosYou can use a numeric TOS values, or use
iptables -j TOS -h
to see the list of valid TOS names.
TRACEThis target has no options. It just turns on packet tracing for all packets that match this rule.
TTLThis is used to modify the IPv4 TTL header field. The TTL field determines how many hops (routers) a packet can traverse until it’s time to live is exceeded.Setting or incrementing the TTL field can potentially be very dangerous,so it should be avoided at any cost.Don’t ever set or increment the value on packets that leave your local network!mangle table.–ttl-setvalueSet the TTL value to ‘value’.–ttl-decvalueDecrement the TTL value ‘value’ times.–ttl-incvalueIncrement the TTL value ‘value’ times.
ULOGThis target provides userspace logging of matching packets. When this target is set for a rule, the Linux kernel will multicast this packet through a netlink socket. One or more userspace processes may then subscribe to various multicast groups and receive the packets. Like LOG, this is a “non-terminating target”, i.e. rule traversal continues at the next rule.–ulog-nlgroupnlgroupThis specifies the netlink group (1-32) to which the packet is sent. Default value is 1.–ulog-prefixprefixPrefix log messages with the specified prefix; up to 32 characters long, and useful for distinguishing messages in the logs.–ulog-cprangesizeNumber of bytes to be copied to userspace. A value of 0 always copies the entire packet, regardless of its size. Default is 0.–ulog-qthresholdsizeNumber of packet to queue inside kernel. Setting this value to, e.g. 10 accumulates ten packets inside the kernel and transmits them as one netlink multipart message to userspace. Default is 1 (for backwards compatibility).
XOREncrypt TCP and UDP traffic using a simple XOR encryption–keystringSet key to “string”–block-sizeSet block size
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.
Bugs? What’s this? 😉 Well, you might want to have a look at http://bugzilla.netfilter.org/
Compatibility With Ipchains
This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT and OUTPUT are only traversed for packets coming into the local host and originating from the local host respectively. Hence every packet only passes through one of the three chains (except loopback traffic, which involves both INPUT and OUTPUT chains); previously a forwarded packet would pass through all three.
The other main difference is that -i refers to the input interface; -o refers to the output interface, and both are available for packets entering the FORWARD chain.
iptables 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 differently:
There are several other changes in iptables.
iptables-save(۸), iptables-restore(۸), ip6tables(۸), ip6tables-save(۸), ip6tables-restore(۸), libipq(۳).
The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter internals.