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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 August 1, 2022 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 (sysctl) 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, which can also be read or modified with 163.Xr sysctl 8 . 164.Bl -tag -width ".Va icmplim_output" 165.It Va bmcastecho 166.Pq Vt boolean 167Enable/disable ICMP replies received via broadcast or multicast. 168Defaults to false. 169.It Va drop_redirect 170.Pq Vt boolean 171Enable/disable dropping of ICMP Redirect packets. 172Defaults to false. 173.It Va icmplim 174.Pq Vt integer 175Bandwidth limit for ICMP replies in packets/second. 176If set to zero, no limiting will occur. 177Defaults to 200. 178.It Va icmplim_output 179.Pq Vt boolean 180Enable/disable logging of ICMP replies bandwidth limiting. 181Defaults to true. 182.It Va log_redirect 183.Pq Vt boolean 184Enable/disable logging of ICMP Redirect packets. 185Defaults to false. 186.It Va maskfake 187.Pq Vt "unsigned integer" 188When 189.Va maskrepl 190is set and this value is non-zero, 191it will be used instead of the real address mask when 192the system replies to an ICMP Address Mask Request packet. 193Defaults to 0. 194.It Va maskrepl 195.Pq Vt boolean 196Enable/disable replies to ICMP Address Mask Request packets. 197Defaults to false. 198.It Va quotelen 199.Pq Vt integer 200Number of bytes from original packet to quote in ICMP reply. 201This number is internally enforced to be at least 8 bytes (per RFC792) 202and at most the maximal space left in the ICMP reply mbuf. 203.It Va redirtimeout 204.Pq Vt integer 205Delay in seconds before expiring route created by ICMP redirect. 206.It Va reply_from_interface 207.Pq Vt boolean 208Use the IP address of the interface the packet came in through for 209responses to packets which are not directly addressed to us. 210If enabled, this rule is processed before all others. 211By default, continue with normal source selection. 212Enabling this option is particularly useful on routers because it 213makes external traceroutes show the actual path a packet has taken 214instead of the possibly different return path. 215.It Va reply_src 216.Pq Vt str 217An interface name used for the ICMP reply source in response to packets 218which are not directly addressed to us. 219By default continue with normal source selection. 220.It Va tstamprepl 221.Pq Vt boolean 222Enable/disable replies to ICMP Timestamp packets. 223Defaults to true. 224.El 225.Sh ERRORS 226A socket operation may fail with one of the following errors returned: 227.Bl -tag -width Er 228.It Bq Er EISCONN 229when trying to establish a connection on a socket which 230already has one, or when trying to send a datagram with the destination 231address specified and the socket is already connected; 232.It Bq Er ENOTCONN 233when trying to send a datagram, but 234no destination address is specified, and the socket has not been 235connected; 236.It Bq Er ENOBUFS 237when the system runs out of memory for 238an internal data structure; 239.It Bq Er EADDRNOTAVAIL 240when an attempt is made to create a 241socket with a network address for which no network interface 242exists. 243.El 244.Sh SEE ALSO 245.Xr recv 2 , 246.Xr send 2 , 247.Xr sysctl 3 , 248.Xr inet 4 , 249.Xr intro 4 , 250.Xr ip 4 , 251.Xr pf.conf 5 , 252.Xr sysctl 8 253.Sh HISTORY 254The 255.Nm 256protocol implementation appeared in 257.Bx 4.2 . 258