ICMP(4)

NAME
icmp - Internet Control Message Protocol

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
socket(AF_INET, SOCK_RAW, proto);

DESCRIPTION
ICMP is the error and control message protocol used by IP and the Internet protocol family. It may be accessed through a “raw socket” for network monitoring and diagnostic functions. The proto parameter to the socket call to create an ICMP socket is obtained from getprotobyname(3). ICMP sockets are connectionless, and are normally used with the sendto(2) and recvfrom(2) calls, though the connect(2) call may also be used to fix the destination for future packets (in which case the read(2) or recv(2) and write(2) or send(2) system calls may be used).

Outgoing packets automatically have an IP header prepended to them (based on the destination address). Incoming packets are received with the IP header and options intact.

  Types
ICMP messages are classified according to the type and code fields present in the ICMP header. The abbreviations for the types and codes may be used in rules in pf.conf(5). The following types are defined:
Num | Abbrev. | Description
0 | echorep | Echo reply
3 | unreach | Destination unreachable
4 | squench | Packet loss, slow down
5 | redir | Shorter route exists
6 | althost | Alternate host address
8 | echoreq | Echo request
9 | routeradv | Router advertisement
10 | routersol | Router solicitation
11 | timex | Time exceeded
12 | paramprob | Invalid IP header
13 | timereq | Timestamp request
14 | timerep | Timestamp reply
15 | inforeq | Information request
16 | inforep | Information reply
17 | maskreq | Address mask request
18 | maskrep | Address mask reply
30 | trace | Traceroute
31 | dataconv | Data conversion problem
32 | mobredir | Mobile host redirection
33 | ipv6-where | IPv6 where-are-you
34 | ipv6-here | IPv6 i-am-here
35 | mobregreq | Mobile registration request
36 | mobregrep | Mobile registration reply
39 | skip | SKIP
40 | photuris | Photuris

The following codes are defined:
Num | Abbrev. | Type | Description
0 | net-unr | unreach | Network unreachable
1 | host-unr | unreach | Host unreachable
2 | proto-unr | unreach | Protocol unreachable
3 | port-unr | unreach | Port unreachable
4 | needfrag | unreach | Fragmentation needed but DF bit set
5 | srcfail | unreach | Source routing failed
6 | net-unk | unreach | Network unknown
7 | host-unk | unreach | Host unknown
8 | isolate | unreach | Host isolated
9 | net-prohib | unreach | Network administratively prohibited
10 | host-prohib | unreach | Host administratively prohibited
11 | net-tos | unreach | Invalid TOS for network
12 | host-tos | unreach | Invalid TOS for host
13 | filter-prohib | unreach | Prohibited access
14 | host-preced | unreach | Precedence violation
15 | cutoff-preced | unreach | Precedence cutoff
0 | redir-net | redir | Shorter route for network
1 | redir-host | redir | Shorter route for host
2 | redir-tos-net | redir | Shorter route for TOS and network
3 | redir-tos-host | redir | Shorter route for TOS and host
0 | normal-adv | routeradv | Normal advertisement
16 | common-adv | routeradv | Selective advertisement
0 | transit | timex | Time exceeded in transit
1 | reassemb | timex | Time exceeded in reassembly
0 | badhead | paramprob | Invalid option pointer
1 | optmiss | paramprob | Missing option
2 | badlen | paramprob | Invalid length
1 | unknown-ind | photuris | Unknown security index
2 | auth-fail | photuris | Authentication failed
3 | decrypt-fail | photuris | Decryption failed

  MIB (sysctl) Variables
The ICMP protocol implements a number of variables in the net.inet.icmp branch of the sysctl(3) MIB, which can also be read or modified with sysctl(8).

bmcastecho
    (boolean) Enable/disable ICMP replies received via broadcast or multicast. Defaults to false.

drop_redirect
    (boolean) Enable/disable dropping of ICMP Redirect packets. Defaults to false.

icmplim
    (unsigned integer) Mean rate limit for replies in packets/second. The actual limit is icmplim plus a random jitter limited by icmplim_jitter. If set to zero, no limiting will occur. Defaults to 200.

icmplim_jitter
    (unsigned integer) A random jitter between the negative of icmplim_jitter and icmplim_jitter is applied to icmplim for limiting the sending rate of replies. icmplim_jitter must be smaller than icmplim, if icmplim is not zero. If set to zero, no jitter will be applied. Defaults to 16.

icmplim_output
    (boolean) Enable/disable logging of ICMP replies bandwidth limiting. Defaults to true.

log_redirect
    (boolean) Enable/disable logging of ICMP Redirect packets. Defaults to false.

maskfake
    (unsigned integer) When maskrepl is set and this value is non-zero, it will be used instead of the real address mask when the system replies to an ICMP Address Mask Request packet. Defaults to 0.

maskrepl
    (boolean) Enable/disable replies to ICMP Address Mask Request packets. Defaults to false.

quotelen
    (integer) Number of bytes from original packet to quote in ICMP reply. This number is internally enforced to be at least 8 bytes (per RFC792) and at most the maximal space left in the ICMP reply mbuf.

redirtimeout
    (integer) Delay in seconds before expiring route created by ICMP redirect.

reply_from_interface
    (boolean) Use the IP address of the interface the packet came in through for responses to packets which are not directly addressed to us. If enabled, this rule is processed before all others. By default, continue with normal source selection. Enabling this option is particularly useful on routers because it makes external traceroutes show the actual path a packet has taken instead of the possibly different return path.

reply_src
    (str) An interface name used for the ICMP reply source in response to packets which are not directly addressed to us. By default continue with normal source selection.

tstamprepl
    (boolean) Enable/disable replies to ICMP Timestamp packets. Defaults to true.

ERRORS
A socket operation may fail with one of the following errors returned:

[EISCONN]
    when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected;

[ENOTCONN]
    when trying to send a datagram, but no destination address is specified, and the socket has not been connected;

[ENOBUFS]
    when the system runs out of memory for an internal data structure;

[EADDRNOTAVAIL]
    when an attempt is made to create a socket with a network address for which no network interface exists.

SEE ALSO
recv(2), send(2), sysctl(3), inet(4), intro(4), ip(4), pf.conf(5), sysctl(8)

HISTORY
The icmp protocol implementation appeared in 4.2BSD.