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.Dd December 2, 2023
.Dt NDP 4P
.Os
.Sh NAME
.Nm ndp ,
.Nm NDP
.Nd Neighbor Discovery Protocol
.Sh SYNOPSIS
.In sys/socket.h
.In sys/sockio.h
.In netinet/in.h
.In net/if.h
.Bd -literal
s = socket(PF_INET6, SOCK_DGRAM, 0);

struct lifreq lifr;
ioctl(s, SIOCLIFGETND, &lifr);
ioctl(s, SIOCLIFSETND, &lifr);
ioctl(s, SIOCLIFDELND, &lifr);
.Ed
.Sh DESCRIPTION
The Neighbor Discovery Protocol (NDP) is a protocol used to distribute and
request information about neighboring IPv6 systems on the local network, much
like
.Xr ARP 4P
for IPv4.
NDP is also responsible for spreading information about the network gateway and
how hosts should configure themselves
.Pq see Xr in.ndpd 8 for more on how this happens .
.Sh APPLICATION PROGRAMMING INTERFACE
The operating system provides several ioctls to help manipulate the mappings
obtained through NDP.
They are
.Sy SIOCLIFGETND ,
.Sy SIOCLIFSETND ,
and
.Sy SIOCLIFDELND ,
for getting, setting, and deleting respectively.
Each of these ioctls takes a
.Vt struct lifreq
.Pq see Xr if 4P for details ,
where the
.Fa lifr_lifru
field is of type
.Vt struct lif_nd_req :
.Bd -literal -offset 2m
typedef struct lif_nd_req {
        struct sockaddr_storage lnr_addr;
        uint8_t                 lnr_state_create;
        uint8_t                 lnr_state_same_lla;
        uint8_t                 lnr_state_diff_lla;
        int                     lnr_hdw_len;
        int                     lnr_flags;
        int                     lnr_pad0;
        char                    lnr_hdw_addr[ND_MAX_HDW_LEN];
} lif_nd_req_t;
.Ed
.Pp
The
.Fa lnr_addr
field should be filled in with an IPv6 address
.Pq see Xr sockaddr_in6 3SOCKET ,
and the
.Fa lnr_hdw_addr
is the link-layer address of length
.Fa lnr_hdw_len .
.Pp
State flags for
.Fa lnr_state_create ,
.Fa lnr_state_same_lla ,
and
.Fa lnr_state_diff_lla
can be set to one of the following values:
.Bl -tag -offset indent -width 16m
.It Sy ND_UNCHANGED
For ioctls that don't modify state
.It Sy ND_INCOMPLETE
Address resolution is currently in progress
.It Sy ND_REACHABLE
The link-layer address has recently been reachable
.It Sy ND_STALE
The link-layer address may be unreachable, and the system shouldn't do anything
.It Sy ND_DELAY
This entry hasn't yet started sending Neighbor Solicitations
.It Sy ND_PROBE
The operating system is currently sending out Neighbor Solicitations for the address
.It Sy ND_UNREACHABLE
The link-layer address is unreachable, and this entry is going to be deleted.
.El
.sp
When creating a new entry, the only valid values for
.Fa lnr_state_create
are
.Sy ND_REACHABLE
and
.Sy ND_STALE .
Any other value will return
.Sy EINVAL .
The
.Fa lnr_state_same_lla
and
.Fa lnr_state_diff_lla
fields are reserved for future use and can be safely set to
.Sy ND_UNCHANGED
and
.Sy ND_STALE
respectively.
.Pp
Flags that can be placed in
.Fa lnr_flags
are:
.Bl -tag -offset indent -width 16m
.It Sy NDF_ISROUTER_ON
Mark this entry as being a router.
This will cause Neighbor Advertisements for this address to be sent with the
R-bit (Router).
.It Sy NDF_ISROUTER_OFF
If this entry was flagged as being a router, remove the flag.
.It Sy NDF_ANYCAST_ON
Mark this entry as being for an anycast address.
This prevents sending Neighbor Advertisements with the O-bit (Override).
.It Sy NDF_ANYCAST_OFF
If this entry was flagged as an anycast address, remove the flag.
.It Sy NDF_STATIC
Prevent this entry from being deleted by the system.
.El
.sp
When using
.Sy SIOCLIFGETND ,
these flags represent the current state of the corresponding Neighbor Cache
Entry.
When using
.Sy SIOCLIFSETND ,
these flags represent what changes should be applied to the underlying entry.
.Pp
The only fields that need to be set for the
.Sy SIOCLIFGETND
or
.Sy SIOCLIFDELND
ioctls are
.Fa lifr_name
and
.Fa lnr_addr .
All other fields should be zeroed out.
After successfully getting an entry, the other fields will be filled in.
When using
.Sy SIOCLIFSETND ,
all fields should be set to an appropriate value, as described above, with the
exception of
.Fa lnr_pad0 ,
which is unused and only exists for padding purposes.
.Pp
After performing the ioctl, the following errors may be returned through the
global
.Sy errno
variable:
.Bl -tag -offset indent -width 16m
.It Sy EAFNOSUPPORT
A non-IPv6 socket was used to perform the ioctl.
.It Sy EINVAL
The request contents were bad.
This could be because conflicting flags were used, the specified interface
wasn't logical unit zero, or another reason.
.It Sy ENOMEM
The system ran out of memory for internal data structures.
.It Sy ENXIO
The specified interface does not exist.
.It Sy EPERM
The caller does not have permission to modify the Neighbor Cache Entries
associated with this interface.
They may be lacking the
.Sy PRIV_SYS_NET_CONFIG
privilege
.Po see Xr privileges 7 Pc ,
or the interface is managed by IPMP (IP Network Multipathing).
.It Sy ESRCH
There is no entry matching the specified address.
.El
.Sh EXAMPLES
The following examples demonstrate how to get and set NDP mappings using the
provided ioctls.
They can be compiled by using a C compiler and linking against the sockets
library.
.Ss Example 1: Getting a mapping
.Bd -literal -offset indent
$ gcc -Wall -lsocket -o get get.c
$ cat get.c
/*
 * Example of getting a mapping for a node name.
 */
#include <strings.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/sockio.h>
#include <unistd.h>
#include <netdb.h>
#include <net/if.h>

int get(char *host) {
	struct lifreq lifr;
	struct addrinfo hints, *serverinfo, *p;
	int err, s;

	bzero(&hints, sizeof (struct addrinfo));
	hints.ai_family = PF_INET6;
	hints.ai_protocol = IPPROTO_IPV6;

	if ((err = getaddrinfo(host, NULL, &hints, &serverinfo)) != 0) {
		(void) fprintf(stderr, "Unable to lookup %s: %s\en", host,
		    gai_strerror(err));
		return (1);
	}

	s = socket(AF_INET6, SOCK_DGRAM, 0);
	if (s < 0) {
		perror("Failed to open IPv6 socket");
		return (1);
	}

	for (p = serverinfo; p != NULL; p = p->ai_next) {
		/* Zero out structure */
		bzero(&lifr, sizeof (struct lifreq));
		(void) strlcpy(lifr.lifr_name, "net0",
		    sizeof (lifr.lifr_name));
		(void) memcpy(&lifr.lifr_nd.lnr_addr, p->ai_addr,
		    sizeof (struct sockaddr_storage));

		/* Get mapping */
		if (ioctl(s, SIOCLIFGETND, &lifr) < 0) {
			perror("Unable to get NDP mapping");
			continue;
		}

		/*
		 * lifr.lifr_nd.lnr_hdw_addr now contains the MAC address,
		 * and can be used as desired.
		 */
	}

	/*
	 * Clean up linked list.
	 */
	freeaddrinfo(serverinfo);
	return (0);
}

int main(int argc, char *argv[]) {
	if (argc < 2)
		exit(1);
	return (get(argv[1]));
}
.Ed
.sp
Deleting a mapping would work similarly, except that instead of using
.Sy SIOCLIFGETND ,
you would instead use the
.Sy SIOCLIFDELND
ioctl.
.Ss Example 2: Adding a mapping
.Bd -literal -offset indent
$ gcc -Wall -lsocket -o set set.c
$ cat set.c
/*
 * Example of setting a mapping to an all-zero Ethernet address.
 */
#include <strings.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/sockio.h>
#include <unistd.h>
#include <netdb.h>
#include <net/if.h>

int set(char *host) {
	struct lifreq lifr;
	struct addrinfo hints, *serverinfo, *p;
	int err, s;

	bzero(&hints, sizeof (struct addrinfo));
	hints.ai_family = PF_INET6;
	hints.ai_protocol = IPPROTO_IPV6;

	if ((err = getaddrinfo(host, NULL, &hints, &serverinfo)) != 0) {
		(void) fprintf(stderr, "Unable to lookup %s: %s\en", host,
		    gai_strerror(err));
		return (1);
	}

	s = socket(AF_INET6, SOCK_DGRAM, 0);
	if (s < 0) {
		perror("Failed to open IPv6 socket");
		return (1);
	}

	for (p = serverinfo; p != NULL; p = p->ai_next) {
		/* Zero out structure */
		bzero(&lifr, sizeof (struct lifreq));
		(void) strlcpy(lifr.lifr_name, "net0",
		    sizeof (lifr.lifr_name));
		(void) memcpy(&lifr.lifr_nd.lnr_addr, p->ai_addr,
		    sizeof (struct sockaddr_storage));

		lifr.lifr_nd.lnr_state_create = ND_REACHABLE;
		lifr.lifr_nd.lnr_flags = NDF_STATIC;

		/* Get mapping */
		if (ioctl(s, SIOCLIFSETND, &lifr) < 0) {
			perror("Unable to set NDP mapping");
			continue;
		}
	}

	/*
	 * Clean up linked list.
	 */
	freeaddrinfo(serverinfo);
	return (0);
}

int main(int argc, char *argv[]) {
	if (argc < 2)
		exit(1);
	return (set(argv[1]));
}
.Ed
.Sh SEE ALSO
.Xr sockaddr_in6 3SOCKET ,
.Xr privileges 7 ,
.Xr ifconfig 8 ,
.Xr in.ndpd 8 ,
.Xr ndp 8
.Rs
.%A Narten, T.
.%A Nordmark, E.
.%A Simpson, W.
.%A Soliman, H.
.%R Neighbor Discovery for IP version 6
.%T RFC 4861
.%D September 2007
.Re