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#+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
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