Guides Packet Winlink

Packet and Winlink

Email by radio, for the day the internet is not there. This guide builds the chain in the order that lets you test each link before adding the next: the modem, then the mail client over the internet, then the mail client over the air.

Link	Program	What it does
Modem, VHF/UHF	Direwolf	Turns the sound card into a 1200-baud packet TNC
Modem, HF	ardopcf	ARDOP, the open HF Winlink modem
Modem, HF	Mercury	An open HF modem Pat drives through its VARA transport
Mail client	Pat	Winlink email, with a web interface
Gateways	Winlink RMS stations	Other hams' stations that move your mail to and from the internet

Install

hammunition install station packet --dry-run
hammunition install station packet

The packet profile carries Direwolf, Pat, ARDOP, Mercury, FreeDATA, the AX.25 tools, LinBPQ, the packet terminals and the APRS clients. If you have no mail client, an interactive install offers one; it never installs one silently.

No kernel AX.25? That is fine.

Linux 7.1 removed the kernel's AX.25 stack, and several distributions already ship without it. This guide never needs it: Pat talks to Direwolf directly over Direwolf's AGW port. The kernel stack is only for the older tools that need an ax0 interface; see Kernel AX.25.

1. Direwolf: the modem

Set up rig control and radio audio first. Then find the radio's sound card by its ALSA name:

sudo apt install alsa-utils   # if arecord is missing
arecord -l

A line like card 1: CODEC [USB Audio CODEC], device 0 gives the card's name, CODEC. Use the name, not the number: plughw:CARD=CODEC,DEV=0 keeps working when the card numbers change.

Create ~/direwolf.conf. Direwolf reads that file when started from your home directory, and searches your home directory if it is not in the current one:

# The radio's sound card, by name. Both directions.
ADEVICE plughw:CARD=CODEC,DEV=0
ACHANNELS 1
CHANNEL 0
# Your callsign, in place of the example's N0CALL placeholder.
MYCALL N0CALL
MODEM 1200
# Key the radio through rigctld (model 2 = NET rigctl), see the rig-control guide.
PTT RIG 2 localhost:4532
# The ports Pat and the other clients connect to.
AGWPORT 8000
KISSPORT 8001

The PTT line depends on your interface. The shipped example lists the others: PTT /dev/ttyUSB0 RTS for an interface that keys on RTS (a Digirig does), PTT CM108 for a USB sound card with a PTT pin. Use the /dev/serial/by-id/ path, not ttyUSB0.

Start it:

direwolf

It prints the audio device it opened and ends with the AGW and KISS ports it is listening on. Leave it running. Tune the radio to your local packet frequency (a Winlink gateway's, found in step 4), and every packet heard is printed with an audio level, like audio level = 50(25/24). Aim for roughly 50; adjust the radio's USB audio output level to get there.

Device busy?

Direwolf and ardopcf open the sound card directly through ALSA. If PipeWire already has it open, they fail with device busy. In pavucontrol, Configuration tab, set the radio's card profile to Off: PipeWire lets go of it, and the desktop keeps its own sound. Unmeasured on the field laptop.

As a service

The Direwolf package also installs a system service, direwolf.service, which runs Direwolf as its own direwolf user from /etc/direwolf.conf and only starts if that file exists (read from Ubuntu 24.04's package, 2026-09-30). Use it for an unattended node. For a station you sit at, running it in your session as above is simpler.

2. Pat: configure

Debian and Ubuntu install Pat's command as pat-winlink, not pat.

pat-winlink configure

That opens the configuration in your editor. Set these, leaving everything else as it is:

"mycall": "N0CALL",
"locator": "FN31pr",
"ax25": {
  "engine": "agwpe"
},
"agwpe": {
  "addr": "localhost:8000",
  "radio_port": 0
},

engine is the change that matters: Pat 0.15.1's default is "linux", the kernel stack. agwpe points it at Direwolf's port 8000 instead (defaults read from Pat 0.15.1, 2026-09-30). Keep the rest of the ax25 block as it was when you edit it.

3. Pat: send a message over the internet first

This proves Pat, your account and your mailbox before any radio is involved.

pat-winlink http

Open http://localhost:8080. Compose a message to yourself, then use Action → Connect with the telnet alias, which reaches Winlink's servers over the internet. Your first connection registers your callsign with Winlink; follow the instructions in the message Winlink sends back, which include setting a password. Put that password in Pat's secure_login_password, and keep it private: it is the only thing between your callsign and anyone else sending mail as you.

The web interface only listens on localhost. Leave it that way.

4. Find a gateway

pat-winlink rmslist --mode packet --sort-distance

Pat downloads Winlink's list of gateways and prints them nearest first, with callsign, frequency and distance. Pat needs your locator for the distances. Pick one you can reach; a packet gateway on VHF needs line of sight or a digipeater.

5. Send it over the air

Tune the radio to the gateway's frequency, with Direwolf running. Then, in the web interface's Connect dialog, choose transport ax25, enter the gateway's callsign (with its SSID, like N0CALL-10), and connect. From the command line, the same thing:

pat-winlink connect ax25+agwpe:///N0CALL-10

Direwolf's window shows the packets going both ways. Pat's shows the session, and mail in the outbox leaves.

HF: ARDOP

Winlink on HF runs through ardopcf. It is a modem like Direwolf, but for HF, and it includes its own web interface.

ardopcf -G 8514 8515 plughw:CARD=CODEC,DEV=0 plughw:CARD=CODEC,DEV=0

8515 is the port Pat expects (its default ardop.addr), the two device names are capture and playback, and -G 8514 serves ardopcf's own web interface at http://localhost:8514, where you can watch levels and traffic. That page is the ARDOP GUI: 73Linux installs a separate ARDOPGUI program for it, and the catalog does not, because ardopcf ships the interface itself. Both ports listen on every interface, so on a shared network a host firewall decides who else can open it. For PTT, either let Pat key the radio through hamlib (set "ptt_ctrl": true and a rig in Pat's ardop section, pointing at a hamlib_rigs entry for localhost:4532), or give ardopcf -p with the radio's serial port for RTS keying. Not both: ardopcf's own usage notes warn against two programs keying through the same port.

pat-winlink rmslist --mode ardop --band 40m --sort-distance
pat-winlink connect 'ardop:///N0CALL?freq=7101.5'

freq is the dial frequency in kHz, set through rig control before the call. The gateway list gives each station's.

ardopcf and CM108 keying

The catalog builds ardopcf with one compiler warning relaxed, because GCC 14 turned it into an error. That warning is a real bug in its CM108 PTT path. Test CM108 keying before relying on it; RTS and CAT keying are unaffected. ardopcf's page has the detail.

Choosing an HF modem

Three free HF modems come with the profile, and the one to use is decided by the station at the other end, because none of them understands the others on the air:

• ardopcf for a gateway or station that runs ARDOP. Pat's ardop:// transport, above.
• Mercury for a station that runs Mercury. Mercury speaks VARA HF's TCP interface to your software, so Pat reaches it through its varahf:// transport with no change. It does not speak VARA's waveform, so a gateway that runs only VARA is out of reach.
• FreeDATA for messages and files straight to another FreeDATA station, in a browser. It is its own system, not Winlink, and Pat does not use it.

VARA itself, the proprietary modem many Winlink gateways also run, is not installed by Hammunition in 1.0; see not carried. Its Wine prefix is post-1.0 and optional now that Mercury covers the free end of Pat's VARA transport.

All three transmit on HF when a client keys them. A licence and the band plan apply, and nothing here keys the rig without your rig-control setup.

HF: Mercury

mercury -i plughw:CARD=CODEC,DEV=0 -o plughw:CARD=CODEC,DEV=0 -P serial -A /dev/ttyUSB0

Mercury listens for Pat on port 8300, which is Pat's default varahf.addr (localhost:8300), so Pat's varahf section can stay as it is. -i and -o are capture and playback, and mercury -z lists the sound devices. -P serial -A keys the radio on the serial port's RTS line, DigiRig style; -R <model> -A <device> keys through hamlib instead, and with no -P Mercury leaves keying to Pat ("ptt_ctrl": true and a rig in Pat's varahf section, as for ARDOP above). Kali's packaged Mercury is 1.9.13, which has no -P: use hamlib or Pat's keying there.

pat-winlink connect 'varahf:///N0CALL'
pat-winlink connect 'varahf:///N0CALL?p2p=true'

The second form is a peer-to-peer session with another operator rather than a gateway. Pat logs "got a vara command I wasn't expecting" for Mercury's SN and BITRATE status lines; that is harmless. Mercury's channel-busy detector is off by default, so listen before you connect.

Station to station: FreeDATA

freedata

That starts FreeDATA's server and opens http://127.0.0.1:5000/gui in your browser. Before anything else, open Settings and replace the example callsign AA1AAA and grid JN48ea with yours, then choose the sound devices and the keying (rigctld, flrig, serial PTT or VOX). Stop the server when you are done: it has no login, and its page says why that matters in a browser you also use for the web.

Keyboard packet and BBSes

For a live connection to a packet BBS or node rather than mail, the terminals in the profile connect to the same Direwolf port: Paracon and QtTermTCP to the AGW port 8000, anything speaking KISS to 8001. LinBPQ is a whole node: a BBS, a chat server and a Winlink gateway of your own, and a larger project than this guide.

What was measured

Pat's command name, its subcommands, its default configuration and its transport list were read from Pat 0.15.1 (Ubuntu 24.04) on 2026-09-30. Direwolf's example configuration was read from its 1.7 release, its service file and its hamlib linking from Ubuntu 24.04's package. ardopcf's options are from its own usage notes at the release the catalog builds. The packet VM campaign covers the install on Debian 13; a message sent over the air from the field laptop is not yet in the bench record.

Mercury's options are from its 1.9.15 -h and README. On 2026-09-30, Debian's Pat 0.16.0 carried a message peer to peer (varahf:///…?p2p=true) between two Mercury 1.9.15 instances on the field laptop, their audio crossed through named pipes at real time and PTT set to none: no radio and no sound card were involved, and the session took about two minutes for one line of text. Mercury over the air, and FreeDATA's server, window and audio path, have not been exercised here.