Set truncate bit for DNS responses#35
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kanashimia wants to merge 1 commit intoletsencrypt:mainfrom
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UDP buffer is only 512 bytes by default, it is pretty much never enough for the cases when there are multiple DNS records in a single response, so the responses are cut at arbitrary points which causes problems. This fixes that by truncating responses at record boundaries and setting TC bit, DNS clients should then read TC bit and retry over TCP which allows for a 64KB buffer. Truncate function also enables compression if message doesn't fit in the buffer uncompressed but fits if it is compressed. See docs: https://pkg.go.dev/github.com/miekg/dns#Msg.Truncate Was originally found in letsencrypt/pebble#536 (comment)
beautifulentropy
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jsha
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Feb 17, 2026
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UDP buffer is only 512 bytes by default, it is pretty much never enough for the cases when there are multiple DNS records in a single response, so the responses are cut at arbitrary points which causes problems.
This fixes that by truncating responses at record boundaries and setting TC bit, DNS clients should then read TC bit and retry over TCP which allows for a 64KB buffer.
Truncate function also enables compression if message doesn't fit in the buffer uncompressed but fits if it is compressed.
See docs: https://pkg.go.dev/github.com/miekg/dns#Msg.Truncate
Also here is a link for the source code of Truncate func, with regards to how it handles compression: https://github.com/miekg/dns/blob/ce76cb6c9b5f3b75ff44996597994ae6f13eae28/msg_truncate.go#L29-L51
Plus although I never used go before, Compress is set to false by default anyways from what I understand,
so I removed that
m.Compress = false, it is too confusing otherwise, Truncate overrides that value anyways.Was originally found in
letsencrypt/pebble#536 (comment)
@beautifulentropy
Although pebble now always queries over TCP, other clients still do a UDP->TCP fallback based on a TC bit set.