Understanding Linux Network Internals

ICMP Types

Table 25-1 lists the ICMP types and the RFCs where they are defined, shows whether they are generally transmitted and processed by the kernel or in user space, and classifies each as an error or query message. The kernel symbols are listed in include/linux/icmp.h. The table lists only the ICMP message types the Linux kernel cares about (regardless of whether they are implemented). You can refer to the URL provided in the chapter's introduction for an updated list.

Table 25-1. ICMP types

Type	Name	TX by	RX by	RFC	Error/Query
aThis option was defined by the same author of RFC 792, but it is not defined in any RFC.
0 8	Echo Reply Echo Request	Kernel User	User Kernel	792 792	Query Query
1 2	Not assigned Not assigned
3	Destination Unreachable	Kernel	Kernel	792	Error
4	Source Quench (obsolete; see RFC 1812 section 4.3.3.3)
5	Redirect	Kernel	Kernel	792	Error
6	Alternate Host Address (obsoletea)
7	Not assigned
9 10	Router Advertisement Router Solicitation	User User	User User	1256	Query
11	Time Exceeded	Kernel	Kernel	792	Error
12	Parameter Problem	Kernel	Kernel	792	Error
13	Timestamp Request	User	Kernel	792	Query
14	Timestamp Reply	Kernel	User	792	Query
15 16	Information Request Information Reply (obsolete; see RFC 1122 section 3.2.2.7 and RFC 1812 section 4.3.3.7)
17 18	Address Mask Request Address Mask Reply	Kernel Kernel	Kernel Kernel	950	Query

ICMP types 1, 2, and 7 are simply ...