Enable DNS¶

Disco includes an optional DNS server. When enabled, discovered hosts are resolvable as <hostname>.<domain> — for example node1.disco — using any standard DNS tool.

The DNS server only serves the disco domain; it does not forward other queries.

Port conflict with systemd-resolved¶

systemd-resolved binds its stub resolver on 127.0.0.53:53. If disco binds 0.0.0.0:53, the kernel rejects the bind with EADDRINUSE because the wildcard address overlaps with the existing 127.0.0.53 binding. The daemon continues running (the NSS module still works), but the DNS server silently fails to start.

The fix is to bind disco's DNS server on 127.0.0.1:53 instead of 0.0.0.0:53. The kernel treats 127.0.0.1 and 127.0.0.53 as distinct local addresses and allows both to hold port 53 simultaneously.

Configure¶

In /etc/disco/config.yaml:

dns:
  enabled: true
  port: 53
  domain: disco
  bind_addresses:
    - "127.0.0.1"    # use 127.0.0.1, not 0.0.0.0 — see port conflict note above
  ttl_healthy: 30
  ttl_stale: 10

If you are not running systemd-resolved, 0.0.0.0 works and binds on all interfaces.

Grant port permission¶

Port 53 is privileged. The Debian package unit already has AmbientCapabilities=CAP_NET_BIND_SERVICE. For source installs:

sudo setcap 'cap_net_bind_service=+ep' /usr/local/bin/disco-daemon

Alternatively, set dns.port to a value above 1023 and point resolvers at it explicitly.

Restart the daemon¶

sudo systemctl restart disco-daemon

Verify¶

Query a discovered host directly against disco's DNS server:

dig @127.0.0.1 node1.disco

The ANSWER SECTION should contain an A record for node1.

Integrate with systemd-resolved¶

With disco binding on 127.0.0.1:53, configure resolved to forward .disco queries there. Create /etc/systemd/resolved.conf.d/disco.conf:

[Resolve]
DNS=127.0.0.1
Domains=~disco

sudo systemctl restart systemd-resolved

The ~disco prefix marks this as a routing-only domain: resolved sends queries for names ending in .disco to 127.0.0.1:53 (disco), and everything else continues through its normal resolvers. Short names like node1 still need the NSS module or an explicit search disco line to resolve without the .disco suffix.

Verify resolved is forwarding correctly:

resolvectl status
# Look for: DNS Servers: 127.0.0.1
#            DNS Domain: ~disco

dig node1.disco    # resolves via resolved -> disco

Without systemd-resolved¶

Point /etc/resolv.conf directly at disco and set the search domain:

nameserver 127.0.0.1
search disco

With this, ping node1 resolves as node1.disco through disco's DNS server.