icmp6(4) Internet Control Message Protocol for IPv6


In sys/socket.h In netinet/in.h In netinet/icmp6.h Ft int Fn socket AF_INET6 SOCK_RAW IPPROTO_ICMPV6


ICMPv6 is the error and control message protocol used by IPv6 and the IPv6 protocol family (see ip6(4) and inet6(4)). It may be accessed through a ``raw socket'' for network monitoring and diagnostic functions.

The Fa proto parameter to the socket(2) call to create an ICMPv6 socket may be obtained from getprotobyname(3). ICMPv6 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 read(2) or recv(2) and write(2) or send(2) 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.


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.conf5. The following types are defined:

Num Ta Abbrev. Ta Description
1 Ta unreach Ta Destination unreachable
2 Ta toobig Ta Packet too big
3 Ta timex Ta Time exceeded
4 Ta paramprob Ta Invalid IPv6 header
128 Ta echoreq Ta Echo service request
129 Ta echorep Ta Echo service reply
130 Ta groupqry Ta Group membership query
130 Ta listqry Ta Multicast listener query
131 Ta grouprep Ta Group membership report
131 Ta listenrep Ta Multicast listener report
132 Ta groupterm Ta Group membership termination
132 Ta listendone Ta Multicast listener done
133 Ta routersol Ta Router solicitation
134 Ta routeradv Ta Router advertisement
135 Ta neighbrsol Ta Neighbor solicitation
136 Ta neighbradv Ta Neighbor advertisement
137 Ta redir Ta Shorter route exists
138 Ta routrrenum Ta Route renumbering
139 Ta fqdnreq Ta FQDN query
139 Ta niqry Ta Node information query
139 Ta wrureq Ta Who-are-you request
140 Ta fqdnrep Ta FQDN reply
140 Ta nirep Ta Node information reply
140 Ta wrurep Ta Who-are-you reply
200 Ta mtraceresp Ta mtrace response
201 Ta mtrace Ta mtrace messages

The following codes are defined:

Num Ta Abbrev. Ta Type Ta Description
0 Ta noroute-unr Ta unreach Ta route to destination
1 Ta admin-unr Ta unreach Ta Administratively prohibited
2 Ta beyond-unr Ta unreach Ta Beyond scope of source address
2 Ta notnbr-unr Ta unreach Ta Not a neighbor (obsolete)
3 Ta addr-unr Ta unreach Ta Address unreachable
4 Ta port-unr Ta unreach Ta Port unreachable
0 Ta transit Ta timex Ta Time exceeded in transit
1 Ta reassemb Ta timex Ta Time exceeded in reassembly
0 Ta badhead Ta paramprob Ta Erroneous header field
1 Ta nxthdr Ta paramprob Ta Unrecognized next header
2 Ta Ta redir Ta Unrecognized option
0 Ta redironlink Ta redir Ta Redirection to on-link node
1 Ta redirrouter Ta redir Ta Redirection to better router


All ICMPv6 messages are prefixed with an ICMPv6 header. This header corresponds to the Vt 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 Aq Pa 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.


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) family of calls to an application.

The Vt 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:

Fn void ICMP6_FILTER_SETPASSALL struct icmp6_filter *filterp
Allow all incoming messages. filterp is modified to allow all message types.
Fn void ICMP6_FILTER_SETBLOCKALL struct icmp6_filter *filterp
Ignore all incoming messages. filterp is modified to ignore all message types.
Ft void Fn ICMP6_FILTER_SETPASS int type struct icmp6_filter *filterp
Allow ICMPv6 messages with the given Fa type . filterp is modified to allow such messages.
Ft void Fn ICMP6_FILTER_SETBLOCK int type struct icmp6_filter *filterp
Ignore ICMPv6 messages with the given Fa type . filterp is modified to ignore such messages.
Ft int Fn ICMP6_FILTER_WILLPASS int type const struct icmp6_filter *filterp
Determine if the given filter will allow an ICMPv6 message of the given type.
Ft int Fn 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) and setsockopt(2) 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 Vt icmp6_filter structure as the option value.