AF_AX25(4) AX.25 amateur packet radio protocol family


AX.25 is a protocol used extensively by radio amateurs. The Linux AX.25 protocol family permits access to these protocols via the standard networking socket metaphor.

The AX.25 protocol layer supports both connected mode and datagram (UI) frame modes. IP traffic may be stacked on top of AX.25 frames for IP transmission over the AX.25 medium.

The primary mode of operation is connected mode which is the mode used for a socket of type SOCK_SEQPACKET (stream sockets are not available in AX.25). This requires that the user ensures output data is suitably packetised, and that input data is read a packet at a time into a buffer of suitable size. The Linux AX.25 protocol layer can operate in standard AX.25 mode with three bit sequence numbers or in PE1CHL extended AX.25 mode which uses seven bit sequence numbers. The protocol passed to the socket is used for all outgoing frames. Passing 0 causes the normal AX.25 Text PID to be used.

SOCK_DGRAM gives access to AX.25 UI frames. For access to special frames (of any form) SOCK_RAW can be used. There is no SOCK_PACKET support under AX.25. Instead an AF_INET socket of type SOCK_PACKET should be used.

AX.25 addresses consist of 6 ascii characters and a number called the SSID. These are encoded into a sockaddr_ax25 structure which is provided to the relevant system calls. When digipeaters are included a callsign path can be much more complex. When this is the case a struct full_sockaddr_ax25 should be passed to the system calls.

AX.25 has some unusual properties. Notably in a multi-user system an AX.25 address is often associated with a user, and some users may not have such an association. a set of ioctl calls are provided to manage an association table, and in addition the superuser may use an arbitrary callsign by binding to the callsign desired and specifying the port to use as a first digipeated hop.

AX.25 supports the following socket options for SOL_AX25. AX25_T1 is the T1 timer in 1/10ths of a second, AX25_T2 is the T2 timer in 1/10ths of a second, AX25_T3 is the T3 timer. The window is settable with AX25_WINDOW. AX25_N2, the retry counter is also configurable. There is no 'infinite retry' option supported however. The method of backoff for retries is configurable via the socket option AX25_BACKOFF, a value of true indicates the use of exponential backoff and false simple linear backoff. The mode of a connection made be altered to be either standard AX.25 or extended AX.25 via AX25_EXTSEQ. It is possible to have the complete AX.25 header returned to the application by setting AX25_HDRINCL to true, programs must be aware of the internal structure of AX.25 frames to use this option. Note that if AX.25 fragmentation is encountered, only the control information of the first frame is returned along with the defragmented data.


Too numerous to list in full currently.

  • Minor protocol violations exist.


Alan Cox GW4PTS <[email protected]>

Jonathan Naylor G4KLX <[email protected]>