Command Options for IPTables

Structure of IPTables Command Options

Many iptables commands have the following structure:

iptables [-t <table-name>] <command> <chain-name> \
   <parameter-1> <option-1> \
   <parameter-n> <option-n>

<table-name> - Specifies which table the rule applies to. If omitted, the filter table is used.

<command> - Specifies the action to perform, such as appending or deleting a rule.

<chain-name> - Specifies the chain to edit, create, or delete.

<parameter>-<option> pairs - Parameters and associated options that specify how to process a packet that matches the rule.

The length and complexity of an iptables command can change significantly, based on its purpose.

For example, a command to remove a rule from a chain can be very short:

iptables -D <chain-name> <line-number>

In contrast, a command that adds a rule which filters packets from a particular subnet using a variety of specific parameters and options can be rather long. When constructing iptables commands, it is important to remember that some parameters and options require further parameters and options to construct a valid rule. This can produce a cascading effect, with the further parameters requiring yet more parameters. Until every parameter and option that requires another set of options is satisfied, the rule is not valid.

Type iptables -h to view a comprehensive list of iptables command structures.

Command Options

Command options instruct iptables to perform a specific action. Only one command option is allowed per iptables command. With the exception of the help command, all commands are written in upper-case characters.

The iptables commands are as follows:

• -A - Appends the rule to the end of the specified chain. Unlike the -I option described below, it does not take an integer argument. It always appends the rule to the end of the specified chain.

• -C - Checks a particular rule before adding it to the user-specified chain. This command can help you construct complex iptables rules by prompting you for additional parameters and options.
• -D <integer> | <rule> - Deletes a rule in a particular chain by number (such as 5 for the fifth rule in a chain), or by rule specification. The rule specification must exactly match an existing rule.
• -E - Renames a user-defined chain. A user-defined chain is any chain other than the default, pre-existing chains. (Refer to the -N option, below, for information on creating user-defined chains.) This is a cosmetic change and does not affect the structure of the table.

  If you attempt to rename one of the default chains, the system reports a Match not found error. You cannot rename the default chains.

• -F - Flushes the selected chain, which effectively deletes every rule in the chain. If no chain is specified, this command flushes every rule from every chain.
• -h - Provides a list of command structures, as well as a quick summary of command parameters and options.
• -I [<integer>] - Inserts the rule in the specified chain at a point specified by a user-defined integer argument. If no argument is specified, the rule is inserted at the top of the chain.

  As noted above, the order of rules in a chain determines which rules apply to which packets. This is important to remember when adding rules using either the -A or -I option.

  This is especially important when adding rules using the -I with an integer argument. If you specify an existing number when adding a rule to a chain, iptables adds the new rule before (or above) the existing rule.

• -L - Lists all of the rules in the chain specified after the command. To list all rules in all chains in the default filter table, do not specify a chain or table. Otherwise, the following syntax should be used to list the rules in a specific chain in a particular table:

  iptables -L <chain-name> -t <table-name>

  Additional options for the -L command option, which provide rule numbers and allow more verbose rule descriptions, are described in "Listing Options".

• -N - Creates a new chain with a user-specified name. The chain name must be unique, otherwise an error message is displayed.
• -P - Sets the default policy for the specified chain, so that when packets traverse an entire chain without matching a rule, they are sent to the specified target, such as ACCEPT or DROP.
• -R - Replaces a rule in the specified chain. The rule's number must be specified after the chain's name. The first rule in a chain corresponds to rule number one.
• -X - Deletes a user-specified chain. You cannot delete a built-in chain.
• -Z - Sets the byte and packet counters in all chains for a table to zero.

IPTables Parameter Options

Certain iptables commands, including those used to add, append, delete, insert, or replace rules within a particular chain, require various parameters to construct a packet filtering rule.

• -c - Resets the counters for a particular rule. This parameter accepts the PKTS and BYTES options to specify which counter to reset.

• -d - Sets the destination hostname, IP address, or network of a packet that matches the rule. When matching a network, the following IP address/netmask formats are supported:
  • N.N.N.N/M.M.M.M - Where N.N.N.N is the IP address range and M.M.M.M is the netmask.
• N.N.N.N/M - Where N.N.N.N is the IP address range and M is the bitmask.

• -f - Applies this rule only to fragmented packets.

  You can use the exclamation point character (!) option after this parameter to specify that only unfragmented packets are matched.

  Distinguishing between fragmented and unfragmented packets is desirable, despite fragmented packets being a standard part of the IP protocol.

  Originally designed to allow IP packets to travel over networks with differing frame sizes, these days fragmentation is more commonly used to generate DoS attacks using mal-formed packets. It's also worth noting that IPv6 disallows fragmentation entirely.

• -i - Sets the incoming network interface, such as eth0 or ppp0. With iptables, this optional parameter may only be used with the INPUT and FORWARD chains when used with the filter table and the PREROUTING chain with the nat and mangle tables.

  This parameter also supports the following special options:

  • Exclamation point character (!) - Reverses the directive, meaning any specified interfaces are excluded from this rule.
• Plus character (+) - A wildcard character used to match all interfaces that match the specified string. For example, the parameter -i eth+ would apply this rule to any Ethernet interfaces but exclude any other interfaces, such as ppp0.

If the -i parameter is used but no interface is specified, then every interface is affected by the rule.

• -j - Jumps to the specified target when a packet matches a particular rule.

  The standard targets are ACCEPT, DROP, QUEUE, and RETURN.

  Extended options are also available through modules loaded by default with the Community Enterprise Linux iptables RPM package. Valid targets in these modules include LOG, MARK, and REJECT, among others. Refer to the iptables man page for more information about these and other targets.

  This option can also be used to direct a packet matching a particular rule to a user-defined chain outside of the current chain so that other rules can be applied to the packet.

  If no target is specified, the packet moves past the rule with no action taken. The counter for this rule, however, increases by one.

• -o - Sets the outgoing network interface for a rule. This option is only valid for the OUTPUT and FORWARD chains in the filter table, and the POSTROUTING chain in the nat and mangle tables. This parameter accepts the same options as the incoming network interface parameter (-i).
• -p <protocol> - Sets the IP protocol affected by the rule. This can be either icmp, tcp, udp, or all, or it can be a numeric value, representing one of these or a different protocol. You can also use any protocols listed in the /etc/protocols file.

  The "all" protocol means the rule applies to every supported protocol. If no protocol is listed with this rule, it defaults to "all".

• -s - Sets the source for a particular packet using the same syntax as the destination (-d) parameter.

IPTables Match Options

Different network protocols provide specialized matching options which can be configured to match a particular packet using that protocol. However, the protocol must first be specified in the iptables command. For example, -p <protocol-name> enables options for the specified protocol. Note that you can also use the protocol ID, instead of the protocol name. Refer to the following examples, each of which have the same effect:

iptables -A INPUT -p icmp --icmp-type any -j ACCEPT
iptables -A INPUT -p 5813 --icmp-type any -j ACCEPT

Service definitions are provided in the /etc/services file. For readability, it is recommended that you use the service names rather than the port numbers.

Secure the /etc/services file to prevent unauthorized editing. If this file is editable, crackers can use it to enable ports on your machine you have otherwise closed. To secure this file, type the following commands as root:

chown root.root /etc/services
chmod 0644 /etc/services
chattr +i /etc/services

This prevents the file from being renamed, deleted or having links made to it.

TCP Protocol

These match options are available for the TCP protocol (-p tcp):

• --dport - Sets the destination port for the packet.

To configure this option, use a network service name (such as www or smtp); a port number; or a range of port numbers.

To specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535.

Use an exclamation point character (!) after the --dport option to match all packets that do not use that network service or port.

To browse the names and aliases of network services and the port numbers they use, view the /etc/services file.

The --destination-port match option is synonymous with --dport.

• --sport - Sets the source port of the packet using the same options as --dport. The --source-port match option is synonymous with --sport.
• --syn - Applies to all TCP packets designed to initiate communication, commonly called SYN packets. Any packets that carry a data payload are not touched.

  Use an exclamation point character (!) after the --syn option to match all non-SYN packets.

• --tcp-flags <tested flag list> <set flag list> - Allows TCP packets that have specific bits (flags) set, to match a rule.

  The --tcp-flags match option accepts two parameters. The first parameter is the mask; a comma-separated list of flags to be examined in the packet. The second parameter is a comma-separated list of flags that must be set for the rule to match.

  The possible flags are:

  • ACK
• FIN
• PSH
• RST
• SYN
• URG
• ALL
• NONE

For example, an iptables rule that contains the following specification only matches TCP packets that have the SYN flag set and the ACK and FIN flags not set:

--tcp-flags ACK,FIN,SYN SYN

Use the exclamation point character (!) after the --tcp-flags to reverse the effect of the match option.

• --tcp-option - Attempts to match with TCP-specific options that can be set within a particular packet. This match option can also be reversed with the exclamation point character (!).

UDP Protocol

These match options are available for the UDP protocol (-p udp):

• --dport - Specifies the destination port of the UDP packet, using the service name, port number, or range of port numbers. The --destination-port match option is synonymous with --dport.

• --sport - Specifies the source port of the UDP packet, using the service name, port number, or range of port numbers. The --source-port match option is synonymous with --sport.

For the --dport and --sport options, to specify a range of port numbers, separate the two numbers with a colon (:). For example: -p tcp --dport 3000:3200. The largest acceptable valid range is 0:65535.

ICMP Protocol

The following match options are available for the Internet Control Message Protocol (ICMP) (-p icmp):

• --icmp-type - Sets the name or number of the ICMP type to match with the rule. A list of valid ICMP names can be retrieved by typing the iptables -p icmp -h command.

Additional Match Option Modules

Additional match options are available through modules loaded by the iptables command.

To use a match option module, load the module by name using the -m <module-name>, where <module-name> is the name of the module.

Many modules are available by default. You can also create modules to provide additional functionality.

The following is a partial list of the most commonly used modules:

• limit module - Places limits on how many packets are matched to a particular rule.

When used in conjunction with the LOG target, the limit module can prevent a flood of matching packets from filling up the system log with repetitive messages or using up system resources.

Refer to "Target Options" for more information about the LOG target.

The limit module enables the following options:

• --limit - Sets the maximum number of matches for a particular time period, specified as a <value>/<period> pair. For example, using --limit 5/hour allows five rule matches per hour.

Periods can be specified in seconds, minutes, hours, or days.

If a number and time modifier are not used, the default value of 3/hour is assumed.

• --limit-burst - Sets a limit on the number of packets able to match a rule at one time.

  This option is specified as an integer and should be used in conjunction with the --limit option.

  If no value is specified, the default value of five (5) is assumed.

• state module - Enables state matching.

  The state module enables the following options:

  • --state - match a packet with the following connection states:
  • ESTABLISHED - The matching packet is associated with other packets in an established connection. You need to accept this state if you want to maintain a connection between a client and a server.
• INVALID - The matching packet cannot be tied to a known connection.
• NEW - The matching packet is either creating a new connection or is part of a two-way connection not previously seen. You need to accept this state if you want to allow new connections to a service.
• RELATED - The matching packet is starting a new connection related in some way to an existing connection. An example of this is FTP, which uses one connection for control traffic (port 21), and a separate connection for data transfer (port 20).

These connection states can be used in combination with one another by separating them with commas, such as -m state --state INVALID,NEW.

• mac module - Enables hardware MAC address matching.

  The mac module enables the following option:

  • --mac-source - Matches a MAC address of the network interface card that sent the packet. To exclude a MAC address from a rule, place an exclamation point character (!) after the --mac-source match option.

Refer to the iptables man page for more match options available through modules.

Target Options

When a packet has matched a particular rule, the rule can direct the packet to a number of different targets which determine the appropriate action. Each chain has a default target, which is used if none of the rules on that chain match a packet or if none of the rules which match the packet specify a target.

The following are the standard targets:

• <user-defined-chain> - A user-defined chain within the table. User-defined chain names must be unique. This target passes the packet to the specified chain.

• ACCEPT - Allows the packet through to its destination or to another chain.
• DROP - Drops the packet without responding to the requester. The system that sent the packet is not notified of the failure.
• QUEUE - The packet is queued for handling by a user-space application.
• RETURN - Stops checking the packet against rules in the current chain. If the packet with a RETURN target matches a rule in a chain called from another chain, the packet is returned to the first chain to resume rule checking where it left off. If the RETURN rule is used on a built-in chain and the packet cannot move up to its previous chain, the default target for the current chain is used.

In addition, extensions are available which allow other targets to be specified. These extensions are called target modules or match option modules and most only apply to specific tables and situations. Refer to "Additional Match Option Modules" for more information about match option modules.

Many extended target modules exist, most of which only apply to specific tables or situations. Some of the most popular target modules included by default in Community Enterprise Linux are:

• LOG - Logs all packets that match this rule. Because the packets are logged by the kernel, the /etc/syslog.conf file determines where these log entries are written. By default, they are placed in the /var/log/messages file.

Additional options can be used after the LOG target to specify the way in which logging occurs:

• --log-level - Sets the priority level of a logging event. Refer to the syslog.conf man page for a list of priority levels.

• --log-ip-options - Logs any options set in the header of an IP packet.
• --log-prefix - Places a string of up to 29 characters before the log line when it is written. This is useful for writing syslog filters for use in conjunction with packet logging.

  Due to an issue with this option, you should add a trailing space to the log-prefix value.

• --log-tcp-options - Logs any options set in the header of a TCP packet.
• --log-tcp-sequence - Writes the TCP sequence number for the packet in the log.

• REJECT - Sends an error packet back to the remote system and drops the packet.

  The REJECT target accepts --reject-with <type> (where <type> is the rejection type) allowing more detailed information to be returned with the error packet. The message port-unreachable is the default error type given if no other option is used. Refer to the iptables man page for a full list of <type> options.

Other target extensions, including several that are useful for IP masquerading using the nat table, or with packet alteration using the mangle table, can be found in the iptables man page.

Listing Options

The default list command, iptables -L [<chain-name>], provides a very basic overview of the default filter table's current chains. Additional options provide more information:

• -v - Displays verbose output, such as the number of packets and bytes each chain has processed, the number of packets and bytes each rule has matched, and which interfaces apply to a particular rule.

• -x - Expands numbers into their exact values. On a busy system, the number of packets and bytes processed by a particular chain or rule may be abbreviated to Kilobytes, Megabytes (Megabytes) or Gigabytes. This option forces the full number to be displayed.
• -n - Displays IP addresses and port numbers in numeric format, rather than the default hostname and network service format.
• --line-numbers - Lists rules in each chain next to their numeric order in the chain. This option is useful when attempting to delete the specific rule in a chain or to locate where to insert a rule within a chain.
• -t <table-name> - Specifies a table name. If omitted, defaults to the filter table.

The following examples illustrate the use of several of these options. Note the difference in the byte display by including the -x option.

~]# iptables -L OUTPUT -v -n -x
Chain OUTPUT (policy ACCEPT 64005 packets, 6445791 bytes)
    pkts      bytes target     prot opt in     out     source               destination
    1593   133812 ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0
~]# iptables -L OUTPUT -v -n
Chain OUTPUT (policy ACCEPT 64783 packets, 6492K bytes)
    pkts bytes target     prot opt in     out     source               destination
    1819  153K ACCEPT     icmp --  *      *       0.0.0.0/0            0.0.0.0/0
~]#