1.\" Copyright (c) 1986, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)icmp.4 8.1 (Berkeley) 6/5/93 29.\" 30.Dd December 11, 2024 31.Dt ICMP 4 32.Os 33.Sh NAME 34.Nm icmp 35.Nd Internet Control Message Protocol 36.Sh SYNOPSIS 37.In sys/types.h 38.In sys/socket.h 39.In netinet/in.h 40.Ft int 41.Fn socket AF_INET SOCK_RAW proto 42.Sh DESCRIPTION 43.Tn ICMP 44is the error and control message protocol used 45by 46.Tn IP 47and the Internet protocol family. 48It may be accessed 49through a 50.Dq raw socket 51for network monitoring 52and diagnostic functions. 53The 54.Fa proto 55parameter to the socket call to create an 56.Tn ICMP 57socket 58is obtained from 59.Xr getprotobyname 3 . 60.Tn ICMP 61sockets are connectionless, 62and are normally used with the 63.Xr sendto 2 64and 65.Xr recvfrom 2 66calls, though the 67.Xr connect 2 68call may also be used to fix the destination for future 69packets (in which case the 70.Xr read 2 71or 72.Xr recv 2 73and 74.Xr write 2 75or 76.Xr send 2 77system calls may be used). 78.Pp 79Outgoing packets automatically have an 80.Tn IP 81header prepended to 82them (based on the destination address). 83Incoming packets are received with the 84.Tn IP 85header and options intact. 86.Ss Types 87ICMP messages are classified according to the type and code fields 88present in the ICMP header. 89The abbreviations for the types and codes may be used in rules in 90.Xr pf.conf 5 . 91The following types are defined: 92.Bl -column x xxxxxxxxxxxx -offset indent 93.It Sy Num Ta Sy Abbrev. Ta Sy Description 94.It 0 Ta echorep Ta "Echo reply" 95.It 3 Ta unreach Ta "Destination unreachable" 96.It 4 Ta squench Ta "Packet loss, slow down" 97.It 5 Ta redir Ta "Shorter route exists" 98.It 6 Ta althost Ta "Alternate host address" 99.It 8 Ta echoreq Ta "Echo request" 100.It 9 Ta routeradv Ta "Router advertisement" 101.It 10 Ta routersol Ta "Router solicitation" 102.It 11 Ta timex Ta "Time exceeded" 103.It 12 Ta paramprob Ta "Invalid IP header" 104.It 13 Ta timereq Ta "Timestamp request" 105.It 14 Ta timerep Ta "Timestamp reply" 106.It 15 Ta inforeq Ta "Information request" 107.It 16 Ta inforep Ta "Information reply" 108.It 17 Ta maskreq Ta "Address mask request" 109.It 18 Ta maskrep Ta "Address mask reply" 110.It 30 Ta trace Ta Traceroute 111.It 31 Ta dataconv Ta "Data conversion problem" 112.It 32 Ta mobredir Ta "Mobile host redirection" 113.It 33 Ta ipv6-where Ta "IPv6 where-are-you" 114.It 34 Ta ipv6-here Ta "IPv6 i-am-here" 115.It 35 Ta mobregreq Ta "Mobile registration request" 116.It 36 Ta mobregrep Ta "Mobile registration reply" 117.It 39 Ta skip Ta SKIP 118.It 40 Ta photuris Ta Photuris 119.El 120.Pp 121The following codes are defined: 122.Bl -column x xxxxxxxxxxxx xxxxxxxx -offset indent 123.It Sy Num Ta Sy Abbrev. Ta Sy Type Ta Sy Description 124.It 0 Ta net-unr Ta unreach Ta "Network unreachable" 125.It 1 Ta host-unr Ta unreach Ta "Host unreachable" 126.It 2 Ta proto-unr Ta unreach Ta "Protocol unreachable" 127.It 3 Ta port-unr Ta unreach Ta "Port unreachable" 128.It 4 Ta needfrag Ta unreach Ta "Fragmentation needed but DF bit set" 129.It 5 Ta srcfail Ta unreach Ta "Source routing failed" 130.It 6 Ta net-unk Ta unreach Ta "Network unknown" 131.It 7 Ta host-unk Ta unreach Ta "Host unknown" 132.It 8 Ta isolate Ta unreach Ta "Host isolated" 133.It 9 Ta net-prohib Ta unreach Ta "Network administratively prohibited" 134.It 10 Ta host-prohib Ta unreach Ta "Host administratively prohibited" 135.It 11 Ta net-tos Ta unreach Ta "Invalid TOS for network" 136.It 12 Ta host-tos Ta unreach Ta "Invalid TOS for host" 137.It 13 Ta filter-prohib Ta unreach Ta "Prohibited access" 138.It 14 Ta host-preced Ta unreach Ta "Precedence violation" 139.It 15 Ta cutoff-preced Ta unreach Ta "Precedence cutoff" 140.It 0 Ta redir-net Ta redir Ta "Shorter route for network" 141.It 1 Ta redir-host Ta redir Ta "Shorter route for host" 142.It 2 Ta redir-tos-net Ta redir Ta "Shorter route for TOS and network" 143.It 3 Ta redir-tos-host Ta redir Ta "Shorter route for TOS and host" 144.It 0 Ta normal-adv Ta routeradv Ta "Normal advertisement" 145.It 16 Ta common-adv Ta routeradv Ta "Selective advertisement" 146.It 0 Ta transit Ta timex Ta "Time exceeded in transit" 147.It 1 Ta reassemb Ta timex Ta "Time exceeded in reassembly" 148.It 0 Ta badhead Ta paramprob Ta "Invalid option pointer" 149.It 1 Ta optmiss Ta paramprob Ta "Missing option" 150.It 2 Ta badlen Ta paramprob Ta "Invalid length" 151.It 1 Ta unknown-ind Ta photuris Ta "Unknown security index" 152.It 2 Ta auth-fail Ta photuris Ta "Authentication failed" 153.It 3 Ta decrypt-fail Ta photuris Ta "Decryption failed" 154.El 155.Ss MIB Variables 156The 157.Tn ICMP 158protocol implements a number of variables in the 159.Va net.inet.icmp 160branch of the 161.Xr sysctl 3 162MIB. 163.Bl -tag -width ".Va icmplim_output" 164.It Va maskrepl 165.Pq Vt boolean 166Enable/disable replies to ICMP Address Mask Request packets. 167Defaults to false. 168.It Va maskfake 169.Pq Vt "unsigned integer" 170When 171.Va maskrepl 172is set and this value is non-zero, 173it will be used instead of the real address mask when 174the system replies to an ICMP Address Mask Request packet. 175Defaults to 0. 176.It Va icmplim 177.Pq Vt unsigned integer 178Mean rate limit for replies in packets/second. 179The actual limit is 180.Va icmplim 181plus a random jitter limited by 182.Va icmplim_jitter . 183If set to zero, no limiting will occur. 184Defaults to 200. 185.It Va icmplim_jitter 186.Pq Vt unsigned integer 187A random jitter between the negative of 188.Va icmplim_jitter 189and 190.Va icmplim_jitter 191is applied to 192.Va icmplim 193for limiting the sending rate of replies. 194.Va icmplim_jitter 195must be smaller than 196.Va icmplim , 197if 198.Va icmplim 199is not zero. 200If set to zero, no jitter will be applied. 201Defaults to 16. 202.It Va icmplim_output 203.Pq Vt boolean 204Enable/disable logging of ICMP replies bandwidth limiting. 205Defaults to true. 206.It Va drop_redirect 207.Pq Vt boolean 208Enable/disable dropping of ICMP Redirect packets. 209Defaults to false. 210.It Va log_redirect 211.Pq Vt boolean 212Enable/disable logging of ICMP Redirect packets. 213Defaults to false. 214.It Va bmcastecho 215.Pq Vt boolean 216Enable/disable ICMP replies received via broadcast or multicast. 217Defaults to false. 218.It Va reply_src 219.Pq Vt str 220An interface name used for the ICMP reply source in response to packets 221which are not directly addressed to us. 222By default continue with normal source selection. 223.It Va reply_from_interface 224.Pq Vt boolean 225Use the IP address of the interface the packet came through in for 226responses to packets which are not directly addressed to us. 227If enabled, this rule is processed before all others. 228By default, continue with normal source selection. 229Enabling this option is particularly useful on routers because it 230makes external traceroutes show the actual path a packet has taken 231instead of the possibly different return path. 232.It Va quotelen 233.Pq Vt integer 234Number of bytes from original packet to quote in ICMP reply. 235This number is internally enforced to be at least 8 bytes (per RFC792) 236and at most the maximal space left in the ICMP reply mbuf. 237.It Va tstamprepl 238.Pq Vt boolean 239Enable/disable replies to ICMP Timestamp packets. 240Defaults to true. 241.El 242.Sh ERRORS 243A socket operation may fail with one of the following errors returned: 244.Bl -tag -width Er 245.It Bq Er EISCONN 246when trying to establish a connection on a socket which 247already has one, or when trying to send a datagram with the destination 248address specified and the socket is already connected; 249.It Bq Er ENOTCONN 250when trying to send a datagram, but 251no destination address is specified, and the socket has not been 252connected; 253.It Bq Er ENOBUFS 254when the system runs out of memory for 255an internal data structure; 256.It Bq Er EADDRNOTAVAIL 257when an attempt is made to create a 258socket with a network address for which no network interface 259exists. 260.El 261.Sh SEE ALSO 262.Xr recv 2 , 263.Xr send 2 , 264.Xr inet 4 , 265.Xr intro 4 , 266.Xr ip 4 , 267.Xr pf.conf 5 268.Sh HISTORY 269The 270.Nm 271protocol appeared in 272.Bx 4.3 . 273