ICMP6(4)

NAME
icmp6 - Internet Control Message Protocol for IPv6

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

  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). 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 listerner 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 (obselete)
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 {
    	u_int8_t	icmp6_type;	/* type field */
    	u_int8_t	icmp6_code;	/* code field */
    	u_int16_t	icmp6_cksum;	/* checksum field */
    	union {
    		u_int32_t icmp6_un_data32[1]; /* type-specific */
    		u_int16_t icmp6_un_data16[2]; /* type-specific */
    		u_int8_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) 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 ICMPV6_FILTER with a pointer to the icmp6_filter structure as the option value.

  Non-privileged ICMPv6

ICMPv6 sockets can be opened with the SOCK_DGRAM socket type without requiring root privileges. The synopsis is the following:

socket(AF_INET6, SOCK_DGRAM, IPPROTO_ICMPV6)

This can only be used to send ICMPv6 echo requests to gauge the quality of the connectivity to a host, to receive destination unreachable message for path MTU discovery, or to receveive time exceeded message for traceroute.

A socket opened with superuser privileges can send any kind of ICMPv6 message and can also use the ip6(4) options that require superuser privileges.

SEE ALSO
getsockopt(2), recv(2), send(2), setsockopt(2), socket(2), getprotobyname(3), inet6(4), ip6(4), netintro(4) W. Stevens, M. Thomas, Advanced Sockets API for IPv6, RFC 2292, February 1998. A. Conta, S. Deering, Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification, RFC 2463, December 1998.