Книга: Iptables Tutorial 1.2.2

ICMP matches

ICMP matches

These are the ICMP matches. These packets are even more ephemeral, that is to say short lived, than UDP packets, in the sense that they are connectionless. The ICMP protocol is mainly used for error reporting and for connection controlling and suchlike. ICMP is not a protocol subordinated to the IP protocol, but more of a protocol that augments the IP protocol and helps in handling errors. The headers of ICMP packets are very similar to those of the IP headers, but differ in a number of ways. The main feature of this protocol is the type header, that tells us what the packet is for. One example is, if we try to access an unaccessible IP address, we would normally get an ICMP host unreachable in return. For a complete listing of ICMP types, see the ICMP types appendix. There is only one ICMP specific match available for ICMP packets, and hopefully this should suffice. This match is implicitly loaded when we use the --protocol ICMP match and we get access to it automatically. Note that all the generic matches can also be used, so that among other things we can match on the source and destination addresses.

Table 10-4. ICMP matches

Match --icmp-type
Kernel 2.3, 2.4, 2.5 and 2.6
Example iptables -A INPUT -p icmp --icmp-type 8
Explanation This match is used to specify the ICMP type to match. ICMP types can be specified either by their numeric values or by their names. Numerical values are specified in RFC 792. To find a complete listing of the ICMP name values, do an iptables --protocol icmp --help, or check the ICMP types appendix. This match can also be inverted with the ! sign in this, --icmp-type ! 8, fashion. Note that some ICMP types are obsolete, and others again may be "dangerous" for an unprotected host since they may, among other things, redirect packets to the wrong places. The type and code may also be specified by their typename, numeric type, and type/code as well. For example --icmp-type network-redirect, --icmp-type 8 or --icmp-type 8/0. For a complete listing of the names, type iptables -p icmp --help.

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