Using degraded feature set UDP instead of UDP+EDNS0 for DNS server

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Which server is it, and what does resolved think of it?
resolvectl status
journalctl -u systemd-resolved --since "1 hour ago" | grep -i degraded

# Test EDNS0 against that server directly
dig +edns=0 example.com @192.168.1.1
dig +noedns example.com @192.168.1.1

# Point resolved at an upstream that handles EDNS0 properly
sudo mkdir -p /etc/systemd/resolved.conf.d
printf '[Resolve]\nDNS=1.1.1.1 9.9.9.9\nDNSSEC=allow-downgrade\n' | sudo tee /etc/systemd/resolved.conf.d/upstream.conf
sudo systemctl restart systemd-resolved
resolvectl flush-caches && resolvectl query example.com

How to diagnose DNS errors

DNS errors are best diagnosed by working down the delegation chain: root, TLD, authoritative nameserver, then the record itself. The most common mistake is testing only through your local resolver, which caches both good and bad answers and hides where the failure actually is. NXDOMAIN means the name definitively does not exist; SERVFAIL means the resolver could not get a valid answer, which is usually a DNSSEC or authoritative-server problem, not a typo.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Query the authoritative nameserver directly, bypassing all caches: dig @ns1.example.com example.com A. If that answers correctly, you have a caching or propagation issue, not a records issue.
2. Trace the full delegation with dig +trace example.com. This shows exactly which level of the hierarchy stops returning answers.
3. Distinguish NXDOMAIN from SERVFAIL from timeout. They have three entirely different causes. Check the status: line in dig output.
4. For SERVFAIL, test with DNSSEC validation disabled: dig +cd example.com. If that succeeds, the problem is a broken DNSSEC chain, often after a key rollover.
5. Check TTLs before declaring propagation broken. A record with a 24-hour TTL genuinely will not update for resolvers that cached it, and there is no way to force them.

Tools worth reaching for

• dig +trace
• dig @authoritative-ns
• dig +cd (DNSSEC bypass)
• kdig
• resolvectl status

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 freedesktop.org 📖 rfc-editor.org

Related DNS errors

• Bad Gateway - DNS resolutionReverse proxy cannot resolve the upstream server's hostname to an IP address.
• dig: ;; Truncated, retrying in TCP modeThe answer did not fit in the UDP message size the client offered, so the server set the TC…
• dig: status: SERVFAIL with EDE: 6 (DNSSEC Bogus)The resolver fetched the answer but its DNSSEC signatures did not validate, so it refused to…
• DNS cache poisoning detectedDNS resolver returned suspicious results, possibly indicating cache poisoning attack.
• DNS lookup timeoutDNS query took too long to complete, often due to slow or unresponsive DNS servers.
• DNS NXDOMAINDomain name does not exist in DNS, the queried domain is not registered or configured.
• DNS problem: NXDOMAIN looking up TXT for _acme-challenge.example.comLet's Encrypt queried the authoritative servers for the DNS-01 challenge record and was told…
• DNS resolution failedThe domain name could not be resolved to an IP address. DNS server may be unreachable or the…

See all 20 DNS errors →

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.