AX.25 and Direwolf under Linux
I initially was interested in getting Winlink working, so I could send and
receive email from anywhere in the world, even thousands of miles from a phone tower, or Internet
As the name suggests, it's heavily focussed around Windows. I don't have any Windows computers in my house, and I don't want to use Wine, so that looked like a dead-end.
So I changed my approach a little. I wondered if it was possible to get an IP connection over an HF radio link.
And that's what AX.25 is. I remember seeing AX.25 stuff in the
kernel options when compiling my own kernels many years ago.
The good thing about the OSI networking model is that you can replace any layer, and the other layers don't care.
Indeed - people have used pigeons as their physical layer before.
So, as the other layers won't notice the difference, all your existing software will work as normal.
Just really really slowly. 300bps is incredibly slow. If you thought a 56k modem was slow, you ain't seen nothing yet. Still - a slow connection is better than no connection.
One other thing - encryption is banned in amateur radio, so don't use HTTPS, or SSH, or anything else like that over the link.
To get two Linux computers talking over AX.25 and Direwolf
Perform the following steps on both computers. Remember to use a different IP address (in the same subnet!) on
Also, make sure you use different callsigns
This assumes that you are 22.214.171.124 and the other station is 126.96.36.199. One of you will need to reverse the IP addresses in these instructions for this to work.
Install the necessary stuff
sudo apt-get install ax25-tools direwolf
Add the following to /etc/ax25/axports
YOURCALLSIGN YOURCALLSIGN 0 255 7 HF
Save the following config. Adjust ADEVICE, MYCALL, and the PTT lines
ADEVICE plughw:1,0 ACHANNELS 1 CHANNEL 0 MYCALL YOURCALLSIGN MODEM 300 1600:1800 7@30 /4 PTT RIG 237 /dev/ttyUSB0 AGWPORT 8000 KISSPORT 8001
Pick a frequency
Pick a clear frequency on a band that is open between both stations. Try and keep to the narrowband parts of the bandplan.
In terminal one, run
/path/to/direwolf -t 0 -p -B 300 -c /path/to/direwolf-ax25.conf
It should show something like this:
Dire Wolf version 1.5 Includes optional support for: hamlib Reading config file /path/to/direwolf-ax25.conf Audio device for both receive and transmit: plughw:2,0 (channel 0) Channel 0: 300 baud, AFSK 1600 & 1800 Hz, D, 44100 sample rate / 3. 0.0: D 1510 & 1710 0.1: D 1540 & 1740 0.2: D 1570 & 1770 0.3: D 1600 & 1800 0.4: D 1630 & 1830 0.5: D 1660 & 1860 0.6: D 1690 & 1890 Ready to accept AGW client application 0 on port 8000 ... Ready to accept KISS TCP client application 0 on port 8001 ... Virtual KISS TNC is available on /dev/pts/1 Created symlink /tmp/kisstnc -> /dev/pts/1
If you have any problems, build it from source.
Set up the networking
In terminal two, run:
sudo modprobe -v ax25 # The pts device has to match the one output by direwolf in the previous step sudo kissattach /dev/pts/1 YOURCALLSIGN sudo kissparms -p YOURCALLSIGN -t 300 -l 30 -s 20 -r 80 -f n sudo ifconfig ax0 188.8.131.52/30 sudo arp -H ax25 -s 184.108.40.206 OTHERCALLSIGN
Allow packets to and from your machine
# The following commands will allow anything in and out of ax0. sudo iptables -I INPUT -i ax0 -j ACCEPT sudo iptables -I FORWARD -i ax0 -j ACCEPT sudo iptables -I FORWARD -o ax0 -j ACCEPT sudo iptables -I OUTPUT -o ax0 -j ACCEPT sudo iptables -L -n -v
Try the network
Ping is the easiest way to try it. The following command sends 2 pings with 1 byte of payload, and some long timeouts
ping -s 1 -i 20 -W 20 -c 2 220.127.116.11
When you hit enter, your radio should transmit, and the other end should reply
PING 18.104.22.168 (22.214.171.124) 1(29) bytes of data. 9 bytes from 126.96.36.199: icmp_seq=1 ttl=64 9 bytes from 188.8.131.52: icmp_seq=2 ttl=64 --- 184.108.40.206 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 37ms
I'm not quite sure where the 37ms comes from. Each ping took about 7 seconds to send and receive a reply.
As a further test, you can try telneting to the other machine on a tcp port that is open. Maybe 22, 25, 80, etc
telnet 220.127.116.11 22
If that works, you should eventually see the SSH banner.
$ telnet 18.104.22.168 22 Trying 22.214.171.124... Connected to 126.96.36.199. Escape character is '^]'. SSH-2.0-OpenSSH_7.4p1 Raspbian-10+deb9u6 BOO! Protocol mismatch. Connection closed by foreign host.
Remember - encryption is not allowed on amateur radio (for good reason), so don't SSH to it, or visit any HTTPS websites.
Use tshark to see what packets are going in and out of the ax0 interface:
tshark -npi ax0
Other things to check are ifconfig ax0, route -n | grep ax0, arp -H ax25
AX.25 seems to use a 16-bit CRC, and if
there is a single error in the packet, the checksum is invalid, and the packet is discarded. On a noisy HF
circuit, this could make it very inefficient, retrying each packet, over and over, until it gets through
It appears that AX.25 doesn't employ any forward error correction. The fact that FX.25 exists seems to imply that there isn't any. (I can't find any reference to FX.25 in my Linux distro though, so I'm assuming it's not supported).
I had a problem with a version of Direwolf, so I downloaded and compiled my own version from https://github.com/wb2osz/direwolf, which was pretty painless
Yes, I'm aware that 188.8.131.52 has been assigned. Yes, I'm aware I won't be able to reach the "real" 184.108.40.206 while I'm playing with this. Yes, I'm aware of RFC1918. Yes, I'm aware of AMPRNet. No, it *really* doesn't matter for a closed network between two nodes.