Fix TCP fallback for truncated DNS responses#11
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retlehs merged 2 commits intoretlehs:mainfrom Apr 12, 2026
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The query() functions in both internal/dns and internal/mail use UDP by default but never retry over TCP when the response is truncated. Domains with many TXT records (e.g. staffbase.com with 9 TXT records, ~603 bytes) exceed the 512-byte UDP limit, causing the server to set the TC flag and return a partial answer. Records near the end of the response — often SPF — are silently dropped. Check resp.Truncated after the initial UDP exchange and retry with a TCP client, matching the behavior of dig and other resolvers. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
query()in bothinternal/dnsandinternal/mailuses UDP only. Domains with many TXT records (e.g. 9+ records, >512 bytes) cause the DNS server to set the TC (truncated) flag. Records near the end of the response — often SPF — are silently dropped.resp.Truncatedafter the UDP exchange and retry over TCP, matching the behavior ofdigand other standard resolvers.mail_test.goanddns_test.gowith local mock DNS servers that simulate truncation, verifying the TCP fallback path and the no-truncation fast path.Reproduction
Test plan
go test ./internal/mail/ -run TestQuery -v— TCP fallback and UDP-only paths passgo test ./internal/dns/ -run TestQuery -v— samego test ./...— full suite (147 tests), zero failures🤖 Generated with Claude Code