--- name: "icmp6" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "openbsd", "macos"] url: "https://new.carta.tech/icmp6/4" --- # icmp6(4) Internet Control Message Protocol for IPv6 ## NAME **icmp6** – Internet Control Message Protocol for IPv6 ## SYNOPSIS **#include \** **#include \** **#include \** *int*\ **socket**(*AF\_INET6*, *SOCK\_RAW*, *IPPROTO\_ICMPV6*); ## DESCRIPTION ICMPv6 is the error and control message protocol used by IPv6 and the IPv6 protocol family (see [ip6(4)](/ip6/4.md) and [inet6(4)](/inet6/4.md)). It may be accessed through a “raw socket” for network monitoring and diagnostic functions. The *proto* parameter to the [socket(2)](/socket/2.md) call to create an ICMPv6 socket may be obtained from [getprotobyname(3)](/getprotobyname/3.md). ICMPv6 sockets are connectionless, and are normally used with the [sendto(2)](/sendto/2.md) and [recvfrom(2)](/recvfrom/2.md) calls, though the [connect(2)](/connect/2.md) call may also be used to fix the destination for future packets (in which case [read(2)](/read/2.md) or [recv(2)](/recv/2.md) and [write(2)](/write/2.md) or [send(2)](/send/2.md) system calls may be used). Outgoing packets automatically have an IPv6 header prepended to them (based on the destination address). Incoming packets on the socket are received with the IPv6 header and any extension headers removed. ### Types ICMPv6 messages are classified according to the type and code fields present in the ICMPv6 header. The abbreviations for the types and codes may be used in rules in [pf.conf(5)](/pf.conf/5.md). The following types are defined: | | | | | --- | --- | --- | | **Num** | **Abbrev.** | **Description** | | 1 | unreach | Destination unreachable | | 2 | toobig | Packet too big | | 3 | timex | Time exceeded | | 4 | paramprob | Invalid IPv6 header | | 128 | echoreq | Echo service request | | 129 | echorep | Echo service reply | | 130 | groupqry | Group membership query | | 130 | listqry | Multicast listener query | | 131 | grouprep | Group membership report | | 131 | listenrep | Multicast listener report | | 132 | groupterm | Group membership termination | | 132 | listendone | Multicast listener done | | 133 | routersol | Router solicitation | | 134 | routeradv | Router advertisement | | 135 | neighbrsol | Neighbor solicitation | | 136 | neighbradv | Neighbor advertisement | | 137 | redir | Shorter route exists | | 138 | routrrenum | Route renumbering | | 139 | fqdnreq | FQDN query | | 139 | niqry | Node information query | | 139 | wrureq | Who-are-you request | | 140 | fqdnrep | FQDN reply | | 140 | nirep | Node information reply | | 140 | wrurep | Who-are-you reply | | 200 | mtraceresp | mtrace response | | 201 | mtrace | mtrace messages | The following codes are defined: | | | | | | --- | --- | --- | --- | | **Num** | **Abbrev.** | **Type** | | | 0 | noroute-unr | unreach | No route to destination | | 1 | admin-unr | unreach | Administratively prohibited | | 2 | beyond-unr | unreach | Beyond scope of source address | | 2 | notnbr-unr | unreach | Not a neighbor (obsolete) | | 3 | addr-unr | unreach | Address unreachable | | 4 | port-unr | unreach | Port unreachable | | 0 | transit | timex | Time exceeded in transit | | 1 | reassemb | timex | Time exceeded in reassembly | | 0 | badhead | paramprob | Erroneous header field | | 1 | nxthdr | paramprob | Unrecognized next header | | 2 | | redir | Unrecognized option | | 0 | redironlink | redir | Redirection to on-link node | | 1 | redirrouter | redir | Redirection to better router | ### Headers All ICMPv6 messages are prefixed with an ICMPv6 header. This header corresponds to the *icmp6\_hdr* structure and has the following definition: ``` struct icmp6_hdr { uint8_t icmp6_type; /* type field */ uint8_t icmp6_code; /* code field */ uint16_t icmp6_cksum; /* checksum field */ union { uint32_t icmp6_un_data32[1]; /* type-specific */ uint16_t icmp6_un_data16[2]; /* type-specific */ uint8_t icmp6_un_data8[4]; /* type-specific */ } icmp6_dataun; } __packed; #define icmp6_data32 icmp6_dataun.icmp6_un_data32 #define icmp6_data16 icmp6_dataun.icmp6_un_data16 #define icmp6_data8 icmp6_dataun.icmp6_un_data8 #define icmp6_pptr icmp6_data32[0] /* parameter prob */ #define icmp6_mtu icmp6_data32[0] /* packet too big */ #define icmp6_id icmp6_data16[0] /* echo request/reply */ #define icmp6_seq icmp6_data16[1] /* echo request/reply */ #define icmp6_maxdelay icmp6_data16[0] /* mcast group membership*/ ``` *icmp6\_type* describes the type of the message. Suitable values are defined in ⟨*netinet/icmp6.h*⟩. *icmp6\_code* describes the sub-type of the message and depends on *icmp6\_type*. *icmp6\_cksum* contains the checksum for the message and is filled in by the kernel on outgoing messages. The other fields are used for type-specific purposes. ### Filters Because of the extra functionality of ICMPv6 in comparison to ICMPv4, a larger number of messages may be potentially received on an ICMPv6 socket. Input filters may therefore be used to restrict input to a subset of the incoming ICMPv6 messages so only interesting messages are returned by the [recv(2)](/recv/2.md) family of calls to an application. The *icmp6\_filter* structure may be used to refine the input message set according to the ICMPv6 type. By default, all messages types are allowed on newly created raw ICMPv6 sockets. The following macros may be used to refine the input set: - *void* **ICMP6\_FILTER\_SETPASSALL**(*struct icmp6\_filter \*filterp*) Allow all incoming messages. *filterp* is modified to allow all message types. - *void* **ICMP6\_FILTER\_SETBLOCKALL**(*struct icmp6\_filter \*filterp*) Ignore all incoming messages. *filterp* is modified to ignore all message types. - *void* **ICMP6\_FILTER\_SETPASS**(*int type*, *struct icmp6\_filter \*filterp*) Allow ICMPv6 messages with the given *type*. *filterp* is modified to allow such messages. - *void* **ICMP6\_FILTER\_SETBLOCK**(*int type*, *struct icmp6\_filter \*filterp*) Ignore ICMPv6 messages with the given *type*. *filterp* is modified to ignore such messages. - *int* **ICMP6\_FILTER\_WILLPASS**(*int type*, *const struct icmp6\_filter \*filterp*) Determine if the given filter will allow an ICMPv6 message of the given type. - *int* **ICMP6\_FILTER\_WILLBLOCK**(*int type*, *const struct icmp6\_filter \*filterp*) Determine if the given filter will ignore an ICMPv6 message of the given type. The [getsockopt(2)](/getsockopt/2.md) and [setsockopt(2)](/setsockopt/2.md) calls may be used to obtain and install the filter on ICMPv6 sockets at option level IPPROTO\_ICMPV6 and name ICMP6\_FILTER with a pointer to the *icmp6\_filter* structure as the option value. ## SEE ALSO [getsockopt(2)](/getsockopt/2.md), [recv(2)](/recv/2.md), [send(2)](/send/2.md), [setsockopt(2)](/setsockopt/2.md), [socket(2)](/socket/2.md), [getprotobyname(3)](/getprotobyname/3.md), [dtrace\_mib(4)](/dtrace_mib/4.md), [inet6(4)](/inet6/4.md), [ip6(4)](/ip6/4.md), [netintro(4)](/netintro/4.md) W. Stevens and M. Thomas, *Advanced Sockets API for IPv6*, RFC 2292, February 1998. A. Conta and S. Deering, *Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification*, RFC 2463, December 1998. W. Stevens, M. Thomas, E. Nordmark, and T. Jinmei, *Advanced Sockets Application Program Interface (API) for IPv6*, RFC 3542, May 2003. A. Conta, S. Deering, and M. Gupta, *Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification*, RFC 4443, March 2006.