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It is possible to use the traditional AX.25 tooling in the Linux kernel to talk to a KISS modem, including Direwolf. The advantage of this is that you have access to 30 years worth of tools and documentation. The disadvantage is that you have tools and documentation which might not have been updated for 30 years.
Before you can use the kernel AX.25 system, the port(s) you will use needs configuring, in a file which might live at /etc/axports or /etc/ax25/axports depending on your distribution. You want to add a line like this, inserting your callsign:
kisstnc Y0URCALL 1200 255 2
The numbers set the baud rate and packet length. These are sensible defaults, read “man axports” if you want to know more.
If you have a hardware KISS TNC which presents to your Linux system as a serial device, ignore this next Direwolf-specific part. Direwolf listens for KISS connections on a TCP socket, but the Linux kernel can only connect to a device file. You can use socat to join the two together:
socat pty,raw,echo=0,link=/tmp/kisstnc TCP4:127.0.0.1:8001
This connects Direwolf's listening socket to a /dev/pts/XX file, and makes a symlink to it at /tmp/kisstnc.
Unfortunately you can't use /tmp/kisstnc directly in the kissattach command, so “ls /tmp/kisstnc” and see what the /dev/pts/XX file is, and use that. If you have a hardware TNC, use the /dev serial device your TNC is on.
sudo kissattach /dev/pts/10 kisstnc
This will attach the kernel AX.25 to the port you configured in axports earlier.
If you're using Direwolf, it will handle AX.25 CRCs for you, and we need to stop the kernel doing it as well, by doing:
kissparms -c 1 -p kisstnc
Finally, you can make an outbound call to a callsign or alias over this channel:
axcall kisstnc L0CALBBS
There are many other linux AX.25 commands available, you can run your own local personal mailbox, or a full-fledged node. Google for the “Linux Amateur Radio AX.25 HOWTO” - but be aware that some parts of that documentation are very old and not necessarily still relevant.