First of all, the syntax of this configuration file is far from being perfect. If you've got some better ideas just drop me a line... is the default configuration file for This file may contain several sections and comments. Each section begins with the section name and the left curly brace and ends with the right curly brace in a single line. A comment starts with a hash mark at the beginning of a line. Blank lines are silently ignored. The following sections are valid: and The sections and have all a very similar syntax. Each line starts with an identifier followed by one or more blanks and one or more section specific entries or defined identifiers separated by blanks. A valid identifier is case sensitive and consists of letters, digits, underscores and hyphens. If two or more identifiers in one section are equal, the corresponding entries are merged to the first identifier. Hence, it's not possible to overwrite perviously defined identifiers. As a result the order of the section entries is irrelevant and it's possible to define a section more than once. Include other configuration files. Each line in this section, enclosed in quotation marks ("), must be a valid filename. The contents of this file are added to the actual configuration file and each file should contain at least one section (a comment only file is not really useful...). Set some global settings. Each line in this section starts with one of the following identifiers followed by one or more blanks and the desired value: and If there are multiple definitions of one entry the last definition is stored. A valid default log priority (see The default log prefix. Each iptables logmessage starts with this prefix. The default limit value for logmessages (see The default burst value for logmessages (see The default limit value (see The default burst value (see The default prefix for accounting chains. This section defines all needed services. A service description starts with the protocol (see followed by parameters in parenthesis. Most protocols don't need any parameters. The only exceptions are tcp, udp and icmp. The tcp and udp parameter defines the source and destionation port(-range). The source and destination ports are separated by a slash (/) and portranges are separated by a colon (eg. tcp(123:333/99): tcp protocol, source-portrange 123-333, destination port 99). Empty source or destination ports are expanded to 1:65535. The icmp protocol parameter must be a valid icmp type (see iptables -p icmp --help). This section defines all needed networks and hosts. A network description starts with a valid IPv4 address (dotted quad), an optional netmask in cidr notation (number of bits) or an optional MAC-address (with a prefixed equal sign (=). Some valid entries are: 127.0.0.1 127.0.0.0/8 192.168.0.1=00:00:00:00:00:FF. This section defines all needed (physical and bridged) interfaces (eg. eth0, lo, ppp0). This section defines all needed numerical (decimal) values for packet marking purposes. Due to better partitioning of the packetfilter, rules can be split into these sections. Internally they are equivalent and contain all rules. As an exception to all other sections the order of entries in these sections is important. The default policy for the chains INPUT, OUTPUT and FORWARD is DROP (see and it's not possible to change this. Each line in in this section begins with or followed by '+', '-' or a mark identifier enclosed in curly braces (or, in case of fw followed by '>'). The identifiers and define rules for incoming, outgoing and forwarded IP-packets. Each packet with an INVALID state (see is matched by and The lines starting with and define rules to modify the source or destination address or the destination port. The plus and minus signs specify the type of the rule: '+' accepts matching packets and '-' drops them. As a special case the identifier out and fw accept the greater than (>) sign to modify the MSS depending on the PMTU (see A very basic ruleset may look like this: This allows every outgoing traffic and rejects all incoming connections (because of the default policy). To be more specific, each line may contain several parameters. Each parameter starts with a single character followed by an equal sign (=) and one or more previously defined identifiers (in the corresponding sections) separated by commas. The following parameters are valid: The source address or network. The destination address or network. The input interface. The output interface. The physical input interface (only useful when used with bridged interfaces). The physical output interface (only useful when used with bridged interfaces). The service description (protocol). The mark field associated with a packet. The the new source address in nat rules. The the new destination address in nat rules. The the new service description in nat rules. This is only valid with tcp or udp packets. This parameter sets some 'flags'. A flag definition starts with the flag identifier and optional parameters in parenthesis. Valid flags are: - Logs matching packages to syslog. The given parameter is included in the log entry. The number of logged packets and the loglevel can be set in the sysconfig section. - Only valid in DROP rules. This is used to send back an error packet in response to the matched packet. The default behaviour is a packet with set RST flag on tcp connections and a destination-unreachable icmp packet in every other case. Valid parameters are listed in in the REJECT section. - Create an accounting chain for all matching packages and possible responses. The optional parameter is a part of the name of the chain. - Limits the number of matching packets. The default values are set in the sysconfig section. Other values can be defined with the optional parameter. The first entry sets a new limit and the second parameter (separated by a comma (,)) sets the burst value (see Limit and Burst in sysconfig section). It's possible to invert the identifier of one of following parameters - if it expands to ecactly one object - by prepending a exclamation mark (!): (eg.: s=!local p=!http). Configuration files are located in /etc/uif. There is a sample configuration in iptables(8) uif(8) This manual page was written by Jörg Platte <joerg.platte@gmx.de> and Cajus Pollmeier <pollmeier@gonicus.de>, for the Debian GNU/Linux system (but may be used by others).