SERVFAIL

SERVFAIL is DNS response code 2. It tells the client that the server failed to process the query, without saying exactly why. Unlike NXDOMAIN, it does not claim the name is missing; it means the resolver could not obtain a trustworthy answer.

Why it matters

It is one of the most common reasons a site becomes unreachable at the DNS layer while the web server is fine. Typical causes are authoritative nameservers that are unreachable or return errors, lame delegation (the listed servers do not host the zone), a broken or mismatched DNSSEC setup and misconfigured zone data. DNSSEC failures are especially subtle: validating resolvers return SERVFAIL, while non-validating ones answer normally.

Example

$ dig example.com A @8.8.8.8
;; status: SERVFAIL
$ dig example.com A +cd @8.8.8.8
;; status: NOERROR

The +cd flag (checking disabled) skips DNSSEC validation. If it succeeds, the problem is a signature or DS record mismatch.

Common mistakes

• Changing nameservers without updating the DS record at the registrar.
• Looking only at the web server when DNS is failing.
• Not querying the authoritative servers directly to isolate the fault.

Related

• NXDOMAIN
• DS record
• DNSSEC
• टूल: DNSSEC checker

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