When a request arrives with TSIG, attach a TSIG record to the response
so dns.ResponseWriter computes the MAC at write time using the secret
in TsigSecret. Without this, BIND nsupdate complains "expected a TSIG
or SIG(0)" on every UPDATE, even when the update applies successfully.
Two response paths fixed:
- handleUpdate success/per-rcode replies (update.go)
- ServeDNS rejection when TSIG verification fails (plugin.go)
The new helper in tsig.go is a no-op for unsigned requests. Unknown
keys still silently skip signing — we can't authenticate to a peer we
don't share a key with.
Tests verify both branches: signed request → response carries matching
TSIG (key name + algorithm); unsigned request → response stays plain.
Replaces the Phase-1.3 refuseUpdate() stub with a real RFC 2136 handler.
Caddy via caddy-dns/rfc2136 can now inject and remove records.
UPDATE message handling (update.go):
- Zone section validation: must be exactly one SOA-typed record naming
a zone we're authoritative for. Returns FORMERR/NOTAUTH otherwise.
- Prerequisites (§3.2): name-exists, RRset-exists, name-NOT-exists,
RRset-NOT-exists semantics implemented. First failure short-circuits
with the spec's rcode (NXDOMAIN/NXRRSET/YXDOMAIN/YXRRSET).
- Updates (§3.4.2): add RR, delete RRset (CLASS=ANY+RDLEN=0), delete
all RRsets at name (CLASS=ANY+TYPE=ANY), delete specific RR (CLASS=
NONE).
- Apex SOA/NS protected: synthetic and cannot be added or removed via
UPDATE. Apex wipe (TYPE=ANY at apex) also refused.
- Default TTL applied to incoming records with TTL=0.
TSIG (tsig.go + setup.go):
- setup() now populates dnsserver.Config.TsigSecret so the underlying
dns.Server auto-verifies signatures via miekg/dns.
- checkTSIG() in ServeDNS gates UPDATEs: rejects if no TSIG, unknown
key name, algorithm-downgrade attempt, or w.TsigStatus() != nil.
- No TSIG keys configured → all UPDATEs refused (safety default).
- Algorithm pinning prevents downgrade attacks (e.g. forced HMAC-MD5).
Tests (update_test.go): 11 new cases covering happy paths and every
error rcode. Total: 35 top-level test passes, 0 failures.
ServeDNS dispatch now calls handleUpdate after auth gate. The
refuseUpdate() stub is gone. UPDATE end-to-end via nsupdate requires
the custom CoreDNS image (Phase 2) to verify TSIG plumbing on the
dns.Server side.
