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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:

The following codes are defined:

MIB Variables
The ICMP protocol implements a number of variables in the net.inet.icmp branch of the sysctl(3) MIB.

maskrepl

(boolean) Enable/disable replies to ICMP Address Mask Request 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.

icmplim

(integer) Bandwidth limit for ICMP replies in packets/second. If set to zero, no limiting will occur. Defaults to 200.

icmplim_output

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

drop_redirect

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

log_redirect

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

bmcastecho

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

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.

reply_from_interface

(boolean) Use the IP address of the interface the packet came through in 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.

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.

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), inet(4), intro(4), ip(4), pf.conf(5)

HISTORY

The icmp protocol appeared in 4.3BSD.

BSD March 26, 2015 BSD