Options

The following ipchains options are more general in nature. Some of them control rather esoteric features of the IP chains software:

- b

Causes the command to generate two rules. One rule matches the parameters supplied, and the other rule added matches the corresponding parameters in the reverse direction.

- v

Causes ipchains to be verbose in its output. It will supply more information.

- n

Causes ipchains to display IP address and ports as numbers without attempting to resolve them to their corresponding names.

- l

Enables kernel logging of matching datagrams. Any datagram that matches the rule will be logged by the kernel using its printk() function, which is usually handled by the sysklogd program and written to a log file. This is useful for making unusual datagrams visible.

- o[maxsize]

Causes the IP chains software to copy any datagrams matching the rule to the userspace "netlink" device. The maxsize argument limits the number of bytes from each datagram that are passed to the netlink device. This option is of most use to software developers, but may be exploited by software packages in the future.

- m markvalue

Causes matching datagrams to be marked with a value. Mark values are unsigned 32-bit numbers. In existing implementations this does nothing, but at some point in the future, it may determine how the datagram is handled by other software such as the routing code. If a markvalue begins with a + or -, the value is added or subtracted from the existing markvalue.

- t andmask xormask

Enables you to manipulate the "type of service" bits in the IP header of any datagram that matches this rule. The type of service bits are used by intelligent routers to prioritize datagrams before forwarding them. The Linux routing software is capable of this sort prioritization. The andmask and xormask represent bit masks that will be logically ANDed and ORed with the type of service bits of the datagram respectively. This is an advanced feature that is discussed in more detail in the IPCHAINS-HOWTO.

- x

Causes any numbers in the ipchains output to be expanded to their exact values with no rounding.

- y

Causes the rule to match any TCP datagram with the SYN bit set and the ACK and FIN bits clear. This is used to filter TCP connection requests.