FRAME(4) | Device Drivers Manual | FRAME(4)

# NAME

`frame` — frame protocol family

# SYNOPSIS

`pseudo-device af_frame`

`#include <sys/types.h>`
`#include <net/frame.h>`

# DESCRIPTION

The `frame` protocol family provides an interface for sending and receiving low level network interface frames through the normal socket(2) mechanisms.

The `frame` protocol family supports the `SOCK_DGRAM` socket type. Only root may create `frame` protocol family sockets.

`frame` protocol family sockets are designed as an alternative to bpf(4) for handling low data and packet rate communication protocols. Rather than filtering every frame entering the system before the network stack, like bpf(4), processing of the `frame` protocol family runs after the built in protocol handlers in the kernel, thus avoiding the overhead. For this reason, it is not possible to handle IPv4 or IPv6 packets with `frame` protocol sockets because the kernel network stack consumes them before the receive handling for `frame` sockets is run.

Sockets can be created in the `frame` protocol family by using `AF_FRAME` as the domain argument to socket(2). The type of interface, as per `<net/if_types.h>`, is specified as the socket protocol. Currently only Ethernet interfaces are supported.

Sockets bound to the `frame` family use the following address structure:

    #define FRAME_ADDRLEN	8
    #define FRAME_DATALEN	32

    struct sockaddr_frame {
    	uint8_t		sfrm_len;
    	uint8_t		sfrm_family;
    	uint16_t	sfrm_proto;
    	unsigned int	sfrm_ifindex;
    	uint8_t		sfrm_addr[FRAME_ADDRLEN];
    	char		sfrm_ifname[IFNAMSIZ];
    	uint8_t		sfrm_data[FRAME_DATALEN];
    };

The interface used for transmitting or receiving frames with a `frame` domain socket may be specified by using an interface index with sfrm_ifindex, or by name with sfrm_ifname. The use of other struct sockaddr_frame fields depends on the type of interface.

## Ethernet frame sockets

A `frame` socket for use with Ethernet interfaces can be created using `IFT_ETHER` as the protocol argument to socket(2):

    int sock = socket(AF_FRAME, SOCK_DGRAM, IFT_ETHER);

The Ethernet protocol is specified with sfrm_proto in network byte order. Ethernet addresses are specified using the first 6 bytes of sfrm_addr.

Ethernet `frame` sockets may receive frames on all interfaces by specifying 0 for sfrm_ifindex when using bind(2). Similarly, a “wildcard” local address of all zeros can be specified in sfrm_addr.

An interface and address must be specified when sending Ethernet frames.

Ethernet sockets support the following `frame` socket options using `IFT_ETHER` as the level argument with setsockopt(2) and getsockopt(2):

`FRAME_RECVDSTADDR` int
  Enable or disable the reception of the Ethernet destination address as a struct ether_addr control message for frames received with recvmsg(2).
`FRAME_RECVPRIO` int
  Enable or disable the reception of the mbuf packet priority field as an int sized control message for frames received with recvmsg(2).
`FRAME_ADD_MEMBERSHIP` struct frame_mreq
  Configure an Ethernet interface to enable reception of frames destined to the specified multicast Ethernet address.

      struct frame_mreq {
      	unsigned int	fmr_ifindex;
      	uint8_t		fmr_addr[FRAME_ADDRLEN];
      	char		fmr_ifname[IFNAMSIZ];
      };

  An interface must be specified using either fmr_ifindex or fmr_ifname. The multicast Ethernet address is specified in the first 6 bytes of fmr_addr.

`FRAME_DEL_MEMBERSHIP` struct frame_mreq
  Configure an Ethernet interface to disable reception of frames destined to the specified multicast Ethernet address.
`FRAME_SENDPRIO` int
  Specify an mbuf priority value between `IF_HDRPRIO_MIN` (0) and `IF_HDRPRIO_MAX` (7) for frames sent with the Ethernet `frame` socket, or `IF_HDRPRIO_PACKET` to use the existing mbuf priority value. The default is `IF_HDRPRIO_PACKET`.

# EXAMPLES

To receive LLDP frames on the em0 Ethernet interface:

    struct sockaddr_frame sfrm = {
    	.sfrm_family = AF_FRAME,
    	.sfrm_ifname = "em0",
    	.sfrm_proto = htons(ETHERTYPE_LLDP),
    };
    struct frame_mreq fmr = {
    	.fmr_ifname = "em0",
    	.fmr_addr = { 0x01, 0x80, 0xc2, 0x00, 0x00, 0x0e },
    };
    int sock;

    sock = socket(AF_FRAME, SOCK_DGRAM, IFT_ETHER);
    if (sock == -1)
    	err(1, "ethernet frame socket");
    if (bind(sock, (struct sockaddr *)&sfrm, sizeof(sfrm)) == -1)
    	err(1, "bind");
    if (setsockopt(sock, IFT_ETHER, FRAME_ADD_MEMBERSHIP,
        &fmr, sizeof(fmr)) == -1)
    	err(1, "join lldp multicast group");

    for (;;) {
    	socklen_t sfrmlen = sizeof(sfrm);
    	uint8_t frame[2048];
    	ssize_t rv;

    	rv = recvfrom(sock, frame, sizeof(frame), 0,
    	    (struct sockaddr *)&sfrm, &sfrmlen);
    	if (rv == -1)
    		err(1, "lldp recv");
    	printf("received %zd bytes from %s", rv,
    	    ether_ntoa((struct ether_addr *)sfrm.sfrm_addr));
    }

# SEE ALSO

socket(2), netintro(4)

# HISTORY

`frame` domain sockets appeared in OpenBSD 7.7.

# AUTHORS

David Gwynne <dlg@openbsd.org>.

May 16, 2025 | Debian