nxdns/docs/how-to/troubleshoot.md

Troubleshoot nxdns

Symptoms an nxdns install actually produces, what to run to identify each one, and what to change. Every symptom on this page was reproduced on the machine that wrote it, and every diagnosis command was run there. Two details differ from a real install and cannot be otherwise on that machine: it has no installed service, so the log lines were read from a foreground run instead of journalctl -u nxdns, and ports 53 and 8080 were occupied, so DNS and the API were exercised on unprivileged ports. Fixes that need root are marked.

The exit codes themselves are listed in the CLI reference.

The service exits with code 2

Symptom. The process stops immediately. The last two lines are the error and a pointer:

nxdns run failed: NoUsableUpstreams
run `nxdns check` to see the configuration in full

Exit 2 is reserved for a small set of faults run raises itself: NoUsableUpstreams, BadBindAddress, BadRateLimit and BadCertificate.

Diagnosis.

nxdns check

check prints every problem it finds, not the first, and names the source it checked on its first line.

Fixes by cause.

• NoUsableUpstreams — the database has no enabled upstream. On a fresh install this means the seed file was missing or in the wrong place; the start log says no configuration file at '/etc/nxdns/config.zon'; using the database as it is. Write the seed file and start again against the still empty database, or nxdns import <file> --force.

• BadCertificate — a DoH or DoT listener is enabled and its certificate or key is unreadable, too large, unparseable, or the key does not belong to the certificate. run names both paths before it exits: doh_server: '<cert>' + '<key>': certificate file is not readable.

  check does not catch most of this. It tests only that each file is readable, and warns when the key is readable beyond its owner; it never opens the PEM. Parsing and the key/certificate pairing happen when run builds the TLS context, so check can print OK: no problems found on a configuration run then refuses. Reproduced here with a self-signed pair and the key from a second, unrelated pair:

  $ nxdns check --config config.zon
  checking configuration file config.zon
  OK https://cloudflare-dns.com/dns-query
  OK: no problems found                                             # exit 0
  
  $ nxdns run --config config.zon --data-dir ./data
  warning(tls_server): mbedtls_pk_check_pair failed: RSA - Key failed to pass the validity check of the library (-16896)
  doh_server: 'cert.pem' + 'mismatched-key.pem': private key does not belong to the certificate
  nxdns run failed: BadCertificate                                  # exit 2
  

  A cert file containing not a certificate behaves the same way — check exits 0, run exits 2 with certificate PEM could not be parsed. So a successful check means the paths and permissions are right, not that the certificate is usable; the only test of that is starting the service. Fix the path, the ownership, or the pair; see Enable DoH and DoT.

• BadRateLimit — a rate limit or window is zero. import refuses such a configuration, so this only reaches a database that was edited by hand.

• BadBindAddress — dns.bind_ipv4 or dns.bind_ipv6 is not an address of that family.

The service exits with code 1 on a seed file you just wrote

Symptom. A first start against an empty database prints the validation problem and stops, but with exit code 1, not 2:

groups: no group named 'default'; every unknown client is assigned to it
nxdns run failed: MissingDefaultGroup

A syntax error behaves the same way:

config: 2:42: error: expected ',' after initializer
nxdns run failed: ParseZon

So does a seed file whose upstream list is empty or all disabled:

upstreams: at least one upstream must be enabled
nxdns run failed: NoUpstreams

NoUpstreams from a seed file is not the same fault as NoUsableUpstreams above: the first is a file run refused, the second is a database run accepted and found empty. Only the second is exit 2.

Diagnosis. Run the same file through check, which reports it as a configuration problem and exits 2:

nxdns check --config /etc/nxdns/config.zon

Fix. Correct the file the diagnostics name and start again. The database is still empty after a failed seed, so the next start re-reads the file. Note that nxdns check and nxdns import of the same bad file exit 2 while nxdns run exits 1 — the exit code differs by command, the diagnostics do not. All three commands were run here against a file missing its default group, one with a syntax error and one with no enabled upstream, and every pair came out that way.

Port 53 is already taken

Symptom. The process exits 1, having named the socket it could not have:

cannot bind udp [::1]:53: AddressInUse
nxdns run failed: AddressInUse

A bind conflict is a runtime failure, not a configuration fault, so this is exit 1 and nxdns check will not find it.

Diagnosis.

ss -lnup 'sport = :53'
ss -lntp 'sport = :53'
systemctl is-active systemd-resolved

On most systemd distributions the holder is systemd-resolved, which runs a stub listener on 127.0.0.53:53 and on some setups binds 0.0.0.0:53.

Fix. Turn off the stub listener and keep resolved for the host's own lookups:

mkdir -p /etc/systemd/resolved.conf.d
printf '[Resolve]\nDNSStubListener=no\n' > /etc/systemd/resolved.conf.d/nxdns.conf
systemctl restart systemd-resolved

If /etc/resolv.conf is a symlink to /run/systemd/resolve/stub-resolv.conf, repoint it at /run/systemd/resolve/resolv.conf so the host still resolves.

Not verified on this host: this needs root, and systemd-resolved is inactive here with port 53 free, so the conflict could not be reproduced against it. The bind failure itself was reproduced by starting a second nxdns on a port the first already held, which is the same error path.

Do not fix this by pointing the host's /etc/resolv.conf at nxdns when that host is where nxdns resolves its own upstream DoH and DoT hostnames. That is a startup cycle, not a fix.

The container restarts in a loop

Symptom. docker compose ps shows the container restarting, and the log is one line repeated:

nxdns run failed: AccessDenied

Diagnosis.

docker inspect -f '{{.State.Status}} exit={{.State.ExitCode}} restarts={{.RestartCount}}' docker-nxdns-1
stat -c '%a %u:%g %n' deploy/docker/etc-nxdns/config.zon

Exit 1 with AccessDenied means the container could not read the seed file. The container runs as uid 65532 and /etc/nxdns is mounted read-only, so a file at mode 0600 owned by your own uid is unreadable to it and the container cannot repair it.

Fix. Either make the file world-readable, when it holds no secret:

chmod 0644 deploy/docker/etc-nxdns/config.zon

or give it to the container's uid:

chown 65532:65532 deploy/docker/etc-nxdns/config.zon
chmod 0600 deploy/docker/etc-nxdns/config.zon

The 0644 path was verified here, including the recovery: after the chmod the container started and answered queries. The chown needs root and was not run here.

A container that exits 2 instead — nxdns run failed: NoUsableUpstreams after no configuration file at '/etc/nxdns/config.zon' — has no seed file at all on a fresh volume. Create deploy/docker/etc-nxdns/config.zon and bring it up again; see Install with Docker.

The container cannot reach its upstreams

Symptom. The container starts, but every query fails and nxdns check inside it reports each upstream as unreachable.

Diagnosis. Look at what the host resolves with:

cat /etc/resolv.conf

Fix. If it points at the nxdns container, repoint it at a real resolver. The container resolves its upstream DoH and DoT hostnames through the host's DNS configuration, so pointing that at nxdns makes nxdns depend on itself to start. LAN clients point at nxdns; the container's own host does not.

The disk is filling up

Symptom. Writes stop but DNS keeps answering. The journal shows the transition:

warning(disk_monitor): disk state ok -> critical: 33349095424 bytes free on /var/lib/nxdns

Diagnosis.

curl -s http://127.0.0.1:8080/api/health

/api/health needs no login and reports the state and what has been gated:

{"status":"degraded","disk":{"state":"critical","free_bytes":33349079040,"db_bytes":180224,"log_bytes":0,"sample_failures":0},"upstreams":{"available":1,"total":1},"queries_dropped":0,"writer_failed":false,"refreshes_gated":1,"snapshot_generation":2}

/metrics carries the same free, database and log byte gauges as nxdns_disk_free_bytes, nxdns_disk_db_bytes and nxdns_disk_log_bytes; the state itself is on /api/health, not in the metrics output.

What the state means. The monitor samples free space and database sizes once a minute. Below disk.warn_free_mb it logs the transition. Below disk.min_free_mb it gates every non-essential write: the query logger holds its batches, the client tracker stops persisting, and blocklist refreshes are skipped and counted in refreshes_gated. Resolution never degrades because the disk is full — this was verified by setting the thresholds above the free space on the volume: the state went critical, a refresh was gated, and queries kept being answered.

Fix. Recover space — lower logging.retention_days, or stop the service and delete querylog.db — and writes resume on the next sample.

Blocklists are not filtering

Symptom. Domains that should be blocked resolve normally.

Diagnosis. Read the startup line:

journalctl -u nxdns | grep 'serving on'

It ends in either blocklist generation N or unfiltered (no blocklist snapshot).

Fix. unfiltered means no snapshot loaded at all; the download or compile warning that explains it is earlier in the same start. nxdns serves anyway on purpose — a household loses more from DNS that refuses to start than from a window of unfiltered answers.

A generation number with nothing being blocked is a different problem: the snapshot loaded but has no sources in it. The line blocklist snapshot generation 1: 0 of 0 sources loaded says exactly that. Add a source in the admin interface, or in the seed file before the first start.

A database stamped by a newer binary

Symptom. After putting an older binary back, it will not start:

warning(migrations): config.db is at schema version 99; this nxdns binary supports 2
nxdns run failed: SchemaTooNew

Fix. There is no downgrade. Import the export you took before upgrading into a fresh data directory with the older binary; see Upgrade nxdns.