A half-baked quickstart for AX.25 networking
This file only documents the way that one of the Xastir developers
uses AX.25 to share a TNC across his home network from a Raspberry Pi
with a KISS TNC attached via a USB/Serial adapter. As of 19 Dec 2025
this is working perfectly for me.
A lot of documentation out there is ancient
If you Google "Linux AX.25" you'll find lots of documentation,
including something called "Linux AX.25 HOWTO". It's old, but more
thorough than what I can provide here.
The primary Xastir wiki has pages on setting up
AX.25 networking, but they are very old and are documenting use of old
versions of kissattach that required you to set up an IP address for
AX.25 devices. This is no longer true and is in fact undesirable for
APRS, because if you do it then other programs on your computer might
try to send TCP/IP data out over your radio.
The instructions here are for a completely bare-bones AX.25 setup that
is just enough to use for APRS and nothing else.
- Install libax25, libax25-dev, ax25-tools, and ax25-apps
- Make sure you built Xastir after installing the libax25 stuff and
that configure told you AX.25 kernel I/O drivers were enabled.
- Set up the file /etc/ax25/axports:
# /etc/ax25/axports
#
# The format of this file is:
#
# name callsign speed paclen window description
aprs XX4XXX-1 4800 255 7 144.39 APRS (1200 bps)
In the last line, replace "XX4XXX-1" with your own callsign, and give
it an SSID that's different from the one you want to use for Xastir.
The first field is just the name you want to call the interface.
Since I use it only for APRS, I called the interface "aprs". The baud
rate in the third field is the baud rate of the serial port to which
the TNC is connected, not the on-air baud rate of the TNC (which is
usually 1200 on VHF).
/usr/sbin/kissattach /dev/MyTNC aprs
/usr/sbin/kissparms -p aprs -t 250 -s 200 -r 32 -l 100 -f n
Here, "/dev/MyTNC" is the actual device name that my USB-serial
adapter shows up as. I set it up to show up that way every time the
device is plugged in (instead of randomly changing port names depending on
which order things were plugged in) by setting udev rules. Sorry,
can't document that. Google it. Otherwise, just look
at what devices show up in /dev when you plug in your USB-serial
adapter and use that device name here.
"aprs" on the kissattach line is the interface I declared in
/etc/ax25/axports. It will become the network device name.
What are those kiss parameters
These kissparams arguments set a transmit delay of 250ms, a slot time
of 2 seconds (?), a persistence of 32, a transmit tail of 100ms, and
half-duplex operation. These parameters were taken right out of other
guidance I found on the xastir.org web site.
-
That's it. Once kissattach is running, we have an AX.25 network
device called "aprs" that we can use with AX.25-aware programs like
Xastir. There are others.
-
Making this stuff start up when you boot your computer is left as an
exercise for the reader. Good luck with that.
-
If you can't get it working, open a Discussion (not an Issue)on
Xastir's GitHub repo and/or post a question to the Xastir mailing
list.
