ICMP6(4) FreeBSD Kernel Interfaces Manual ICMP6(4)


icmp6Internet Control Message Protocol for IPv6


#include < sys/socket.h>
#include < netinet/in.h>
#include < netinet/icmp6.h>



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 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.conf(5). 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 Description
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


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.


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 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) 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 icmp6_filter structure as the option value.


getsockopt(2), recv(2), send(2), setsockopt(2), socket(2), getprotobyname(3), inet6(4), ip6(4), netintro(4)

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.

November 7, 2012 FreeBSD