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).
The tables are as follows:
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).
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).
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).
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.
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 need to use only enough letters to ensure that iptables can differentiate it from all other options.
The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).
[!] -f, --fragment
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 unfragmented packets.
The following additional options can be specified:
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.
Account traffic for all hosts in defined network/netmask.
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
This 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
This module matches the SPIs in Authentication header of IPsec packets.
This 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.
Allows you to add comments (up to 256 characters) to any rule.
iptables -A INPUT -s 192.168.0.0/16 -m comment --comment “A privatized IP block"
This matches if a specific /proc filename is ‘0’ or ‘1’.
Match 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
[!] --connbytes from:[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.
iptables .. -m connbytes --connbytes 10000:100000 --connbytesdir both --connbytes-mode bytes ...
Allows you to restrict the number of parallel TCP connections to a server per client IP address (or address block).
[!] --connlimit-above n
match if the number of existing tcp connections is (not) above n
# allow 2 telnet connections per client host iptables -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 ! --connlimitabove 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
This module matches the netfilter mark field associated with a connection (which can be set using the CONNMARK target below).
This module matches the current transfer rate in a connection.
This 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)
--source-port,--sport [!] port[:port]
This module matches the 6 bit DSCP field within the TOS field in the IP header. DSCP has superseded TOS within the IETF.
This 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’’
Number of packets to match in a burst. Default: 5
Number of buckets in the hashtable
Maximum number of entries in the hashtable
Interval between garbage collection runs of the hashtable (in miliseconds). Default is 1000 (1 second).
After which time are idle entries expired from hashtable (in miliseconds)? Default is 10000 (10 seconds).
This allows you to match the ECN bits of the IPv4 and TCP header. ECN is the Explicit Congestion Notification mechanism as specified in RFC3168
This module matches the SPIs in ESP header of IPsec packets.
This module matches a rate limit based on a fuzzy logic controller [FLC]
This 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’
‘100 packets per second for every service of 192.168.1.1’
with a single iptables rule.
This module matches packets related to a specific 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.
This extension is loaded if ‘--protocol icmp’ is specified. It provides the following option:
This matches on a given arbitrary range of IPv4 addresses
Match source IP in the specified range.
Match destination IP in the specified range.
Match on IPv4 header options like source routing, record route, timestamp and router-alert.
To match packets with the RR flag.
To match packets with the TS flag.
To match packets with the router-alert option.
To match a packet with at least one IP option, or no IP option at all if ! is chosen.
$ iptables -A input -m ipv4options --rr -j DROP will drop packets with the record-route flag.
$ iptables -A input -m ipv4options --ts -j DROP will drop packets with the timestamp flag.
This module matches the length of a packet against a specific value or range of values.
This 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.
--mac-source [!] address
Match 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.
This module matches the netfilter mark field associated with a packet (which can be set using the MARK target below).
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 -p tcp or -p udp.
This 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.
This module matches every ‘n’th packet
Use internal counter number ‘num’. Default is ‘0’.
Initialize the counter at the number ‘num’ insetad of ‘0’. Most between ‘0’ and ‘value’-1.
Match on ‘num’ packet. Most be between ‘0’ and ‘value’-1.
The 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.
In syslog you find something like this:
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 220.127.116.11:4024 -> 18.104.22.168:139
ipt_osf: Unknown: 16384:106:1:48:020405B401010402 22.214.171.124:1239 -> 126.96.36.199:80
#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/osf
Only one fingerprint per open/write/close.
Fingerprints can be downloaded from http://www.openbsd.org/cgibin/cvsweb/src/etc/pf.os
This 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.
NOTE: pid, sid and command matching are broken on SMP
This 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.
Matches if the packet has entered through a bridge interface.
Matches if the packet will leave through a bridge interface.
Matches if the packet is being bridged and therefore is not being routed. This is only useful in the FORWARD and POSTROUTING chains.
This module matches the link-layer packet type.
This modules matches the policy used by IPsec for handling a packet.
Attempt to detect TCP and UDP port scans. This match was derived from Solar Designer’s scanlogd.
Implements network quotas by decrementing a byte counter with each packet.
KNOWN BUGS: this does not work on SMP systems.
This module randomly matches a certain percentage of all packets.
This matches the routing realm. Routing realms are used in complex routing setups involving dynamic routing protocols like BGP.
Allows 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.
This 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).
Check if the source address of the packet is currently in the list.
Like --rcheck, except it will update the “last seen” timestamp if it matches.
Check 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.
[!] --seconds seconds
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 was seen within the last given number of seconds.
[!] --hitcount hits
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 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.
# 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 DROP
Official 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/DEFAULT to Add to the DEFAULT list
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT to Remove from the DEFAULT list
echo clear > /proc/net/ipt_recent/DEFAULT to empty the DEFAULT list.
The module itself accepts parameters, defaults shown:
Number of addresses remembered per table
Number of packets per address remembered
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512
Permissions for /proc/net/ipt_recent/* files
Set to 1 to get lots of debugging info
--source-port,--sport [!] port[:port]
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
(lowercase means flag should be “off", uppercase means “on")
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
This modules macthes IP sets which can be defined by ipset(8) .
This module, when combined with connection tracking, allows access to the connection tracking state for this packet.
This modules matches a given string by using some pattern matching strategy. It requires a linux kernel >= 2.6.14.
These extensions are loaded if ‘--protocol tcp’ is specified. It provides the following options:
Only 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 --tcpflags SYN,RST,ACK,FIN SYN. If the “!” flag precedes the “--syn", the sense of the option is inverted.
This 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.
This matches if the packet arrival time/date is within a given range. All options are facultative.
This module matches the 8 bits of Type of Service field in the IP header (ie. including the precedence bits).
This module matches the time to live field in the IP header.
U32 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.
These extensions are loaded if ‘--protocol udp’ is specified. It provides the following options:
This 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.
This allows you to DNAT connections in a round-robin way over a given range of destination addresses.
This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).
This 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.
This module sets the netfilter mark value associated with a connection
This 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:
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 --todestination 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.
This 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.
This target allows to selectively work around known ECN blackholes. It can only be used in the mangle table.
Allows 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.
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 fw
Earlier 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 0x10503
Using 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=0x10000
On the routers with hundreds of users there should be significant load decrease (e.g. twice).
Strip 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
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 dmesg or syslogd(8) ). 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).
This 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.
This 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:
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 INPUT, FORWARD and PREROUTING chains, and userdefined 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.
This 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.
This 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.
It can only be used with Kernel versions 2.6.14 or later, since it
the nfnetlink_queue kernel support.
This target disables connection tracking for all packets matching that rule.
It can only be used in the
This 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:
This 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:
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
This is used to explicitly override the core network stack’s routing decision. mangle table.
Similar to SNAT/DNAT depending on chain: it takes a range of addresses (’--to 188.8.131.52-184.108.40.206’) and gives a client the same source-/destination-address for each connection.
This modules adds and/or deletes entries from IP sets which can be defined by ipset(8) .
The bindings to follow must previously be defined in order to use multilevel adding/deleting by the SET target.
This 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:
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.
Captures 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 TARPIT
To 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 DROP
NOTE: 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
This 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. 2) Small mail works fine, but large emails hang. 3) 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
These options are mutually exclusive.
This is used to set the 8-bit Type of Service field in the IP header. It is only valid in the mangle table.
This target has no options. It just turns on packet tracing for all packets that match this rule.
This 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
This 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.
Encrypt TCP and UDP traffic using a simple XOR encryption
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/
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(8) , iptables-restore(8) , ip6tables(8) , ip6tables-save(8) , ip6tables-restore(8) , libipq(3) .
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.
Rusty Russell originally 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 and NFQUEUE target, the new libiptc, as well as the TTL, DSCP, ECN matches and targets.
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik, Patrick McHardy, James Morris, Harald Welte and Rusty Russell.
Man page originally written by Herve Eychenne <firstname.lastname@example.org>.
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