Understanding DNS TXT Record Size Limits: UDP vs TCP Protocol Differences
By
dgl
Hand-rolled, kettle-boiled, baked to perfection. Worth every minute at the bakery.
Summary
This technical article explores the practical limits of DNS TXT records, explaining that while many sources incorrectly state a 255-byte limit, the actual constraint depends on the DNS protocol used. With UDP, the limit is around 1232 bytes due to DNS payload size, but using TCP allows for up to 64KB of data in TXT records. The author appears to be investigating how to leverage these extended limits for practical applications like serving images over DNS.
Key quotes
· 3 pulledWithin a TXT record there are multiple character-strings and those are limited in length (because a single byte is used for their length), however there can be many of them.
The actual limit is limited by the size of the DNS payload, which for UDP is these days around 1232 bytes.
However if we use TCP, which doesn't require anything special, other than the normal fallback to TCP that DNS does, then we can serve up to 64KB.
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