fix(transfer): batch AXFR records by message size instead of count#8002
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The transfer plugin batched zone transfer records using a fixed count of 500 records per message. When a zone has many large records (e.g. long TXT or DNSSEC RRSIG), the resulting message can exceed the 64KB TCP limit, causing truncation or silent record loss. Replace the record count threshold with a byte size check using dns.Len per record, flushing before the batch exceeds 63000 bytes. Also fix a potential deadlock in the final batch send when the client disconnects mid-transfer by using a select on errCh. Fixes coredns#7859 Signed-off-by: umut-polat <52835619+umut-polat@users.noreply.github.com>
yongtang
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Apr 4, 2026
Filippo125
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Apr 10, 2026
…oredns#8002) Signed-off-by: Filippo <filippo.ferrazini@gmail.com>
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The transfer plugin batches zone transfer records using a fixed count of 500 records per message. When a zone contains many large records (long TXT strings, DNSSEC RRSIG, etc.), the combined size can exceed the 64KB TCP message limit, causing truncation or silent record loss during AXFR.
Changes
plugin/transfer/transfer.go: replace thelen(rrs) > 500threshold with a byte-size check usingdns.Len()per record, flushing the batch before it exceeds 63000 bytes (leaving headroom for message header and question section)ch <- rrscould block forever sincetr.Outalready exited. Wrap it in aselectonerrChplugin/transfer/transfer_test.go: addTestTransferLargeRecordBatchingwith 300 TXT records (~250 bytes each, ~75KB total) asserting that every packed message stays withindns.MaxMsgSizeand no records are lostRepro (before fix)
Create 500 TXT records with 250-byte payloads, serve with the file plugin + transfer, then:
Fixes #7859