Linux X25 Stack¶

The Linux kernel has had an (experimental?) X.25 implementation for ages. Surprisingly, this is still present even in 2023.

The kernel code appears to support

• X.25 over LAPB/Ethernet (DTE role; DCE requires kernel patch)
• X.25 over generic HDLC
  • this would work e.g. over DAHDI_NET devices, and hence over BRI/PRI cards supported by DAHDI
• X.25 over X.21 using a FarSite FarSync card

There also apparently was an async_x25 driver for running X.25 over asynchronous serial ports, but that appears to be gone from present-day Linux.

x25-utils¶

The original code can be found at https://www.baty.hanse.de/linux-x25/

The ancient userspace utility code has been imported into a git repo at https://gitea.osmocom.org/retronetworking/x25-utils

X.25 over HDLC / DAHDI¶

roox reported in April 2023 that he sucessfully tested AF_X25 socket connections between 2 Linux machines over the hdlcX interfaces you get with DAHDI/nethdlc (Linux generic HDLC layer)

• https://pmtu.de/.dump/af_x25_sockets_server.c
• https://pmtu.de/.dump/af_x25_sockets_client.c
• https://pmtu.de/.dump/x25-hacks.tar.gz (contains the above, plus also af_packet server/client)

Example x25trace output¶

# x25trace
---------------------------------------------------------------------
Port: hdlc0
X.25: LCI 001 : CALL ACCEPTED
Packet Size:     07 07
Window Size:      2  2

Port: hdlc0
X.25: LCI 001 : CLEAR REQUEST - Cause DTE Originated - Diag 0

Port: hdlc0
X.25: LCI 001 : CLEAR CONFIRMATION

---------------------------------------------------------------------