FRAME(4)

✨ NAME ✨

frame — frame protocol family. Hii bestie, welcome to FRAME(4) from the Kernel Interfaces Manual — we're about to make low-level frame stuff make sense 💅

🛠️ SYNOPSIS 🛠️

pseudo-device af_frame

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

💖 DESCRIPTION 💖

The frame protocol family is literally your interface for sending and receiving low level network interface frames through the normal socket(2) mechanisms. Slay.

It supports the SOCK_DGRAM socket type. And hear me out: only root may create frame protocol family sockets. That's the vibe check, bestie 👑

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. Not me trying to catch those packets — they're already gone 💀.

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. Periodt.

Sockets bound to the frame family use the following address structure — copy it exactly, no freestyling:

#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. Iykyk.

🌐 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. Cute.

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. It's giving "catch everything" energy 🦋.

An interface and address must be specified when sending Ethernet frames. No cap.

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. 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. The struct is exactly:

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

• FRAME_DEL_MEMBERSHIP struct frame_mreq — Configure an Ethernet interface to disable reception of frames destined to the specified multicast Ethernet address. Uses the same struct frame_mreq as above, bestie.
• 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. Iconic.

💌 AUTHORS 💌

David Gwynne <dlg@openbsd.org>. 🫶

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Okay bestie, this page came from the Debian May 16, 2025 build of FRAME(4) — mwah 💋