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18.2 Datalink Socket Address Structure

We will encounter datalink socket address structures as return values contained in some of the messages returned on a routing socket. Figure 18.1 shows the definition of the structure, which is defined by including <net/if_dl.h>.

Figure 18.1 Datalink socket address structure.
struct sockaddr_dl {
  uint8_t      sdl_len;
  sa_family_t  sdl_family;   /* AF_LINK */
  uint16_t     sdl_index;    /* system assigned index, if > 0 */
  uint8_t      sdl_type;     /* IFT_ETHER, etc. from <net/if_types.h> */
  uint8_t      sdl_nlen;     /* name length, starting in sdl_data[0] */
  uint8_t      sdl_alen;     /* link-layer address length */
  uint8_t      sdl_slen;     /* link-layer selector length */
  char         sdl_data[12]; /* minimum work area, can be larger;
                                contains i/f name and link-layer address */

Each interface has a unique positive index, and we will see this returned by the if_nametoindex and if_nameindex functions later in this chapter, along with the IPv6 multicasting socket options in Chapter 21 and some advanced IPv4 and IPv6 socket options in Chapter 27.

The sdl_data member contains both the name and link-layer address (e.g., the 48-bit MAC address for an Ethernet interface). The name begins at sdl_data[0] and is not null-terminated. The link-layer address begins sdl_nlen bytes after the name. This header defines the following macro to return the pointer to the link-layer address:

#define LLADDR(s)     ((caddr_t)((s)->sdl_data + (s)->sdl_nlen))

These socket address structures are variable-length (p. 89 of TCPv2). If the link-layer address and name exceed 12 bytes, the structure will be larger than 20 bytes. The size is normally rounded up to the next multiple of 4 bytes on 32-bit systems. We will also see in Figure 22.3 that when one of these structures is returned by the IP_RECVIF socket option, all three lengths are 0 and there is no sdl_data member at all.

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