#+TITLE: Avahi/mDNS Configuration Fixes
#+DATE: 2026-01-23

* Problem Summary

On velox, mDNS hostname resolution was not working correctly from other machines on the LAN (e.g., ratio). Attempting to access =http://velox.local:8384= (Syncthing web UI) failed, while accessing via IP address worked.

* Issues Identified

** Issue 1: Hostname Conflict (velox-3.local)

*Symptom:* Avahi was running as =velox-3.local= instead of =velox.local=

*Cause:* Avahi was publishing on multiple network interfaces including virtual ones:
- =enp0s13f0u3= (physical LAN - correct)
- =docker0= (Docker bridge)
- =virbr0= (libvirt bridge)
- =vnet0= (VM virtual NIC)
- =tailscale0= (Tailscale VPN)

Each interface was effectively registering as a separate host, causing mDNS hostname conflicts with itself.

*Solution:* Restrict Avahi to only the physical LAN interface.

#+begin_src conf
# /etc/avahi/avahi-daemon.conf
[server]
allow-interfaces=enp0s13f0u3
#+end_src

** Issue 2: IPv6-Only Resolution

*Symptom:* =velox.local= resolved to IPv6 link-local address (=fe80::...=) only, but Syncthing was listening on IPv4 only (=0.0.0.0:8384=).

*Cause:* Default Avahi configuration does not publish A records (IPv4) in response to AAAA queries (IPv6).

*Solution:* Enable =publish-a-on-ipv6= to ensure IPv4 addresses are returned.

#+begin_src conf
# /etc/avahi/avahi-daemon.conf
[publish]
publish-a-on-ipv6=yes
#+end_src

** Issue 3: Conflicting mDNS Stacks

*Symptom:* Avahi logged warning: "Detected another IPv4 mDNS stack running on this host"

*Cause:* Both =avahi-daemon= and =systemd-resolved= were configured to handle mDNS:

#+begin_src conf
# /etc/systemd/resolved.conf (before fix)
[Resolve]
MulticastDNS=yes
#+end_src

*Solution:* Disable mDNS in systemd-resolved, let Avahi handle it exclusively.

#+begin_src conf
# /etc/systemd/resolved.conf
[Resolve]
Domains=~local
MulticastDNS=no
#+end_src

* Complete Fix Applied

** Files Modified

*** /etc/avahi/avahi-daemon.conf

Changes made:
#+begin_src diff
-#allow-interfaces=eth0
+allow-interfaces=enp0s13f0u3

-#publish-a-on-ipv6=no
+publish-a-on-ipv6=yes
#+end_src

*** /etc/systemd/resolved.conf

Changes made:
#+begin_src diff
-MulticastDNS=yes
+MulticastDNS=no
#+end_src

** Services Restarted

#+begin_src bash
sudo systemctl restart systemd-resolved
sudo systemctl restart avahi-daemon
#+end_src

* Verification

After fixes:
- Avahi runs as =velox.local= (not =velox-3.local=)
- No mDNS stack conflict warning
- From ratio: =avahi-resolve -n velox.local= returns =192.168.86.42=
- From ratio: =curl http://velox.local:8384/= returns HTTP 200

* Notes for archsetup

These configurations should be added to the Arch setup scripts:

1. Install avahi: =pacman -S avahi nss-mdns=

2. Configure =/etc/avahi/avahi-daemon.conf=:
   - Set =allow-interfaces= to physical LAN interface (determine dynamically or prompt user)
   - Set =publish-a-on-ipv6=yes=

3. Configure =/etc/systemd/resolved.conf=:
   - Set =MulticastDNS=no= to avoid conflict with Avahi

4. Enable and start avahi-daemon:
   #+begin_src bash
   systemctl enable --now avahi-daemon
   #+end_src

5. Ensure =/etc/nsswitch.conf= has mdns in hosts line:
   #+begin_src conf
   hosts: mymachines mdns_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] files dns
   #+end_src