What's the limit of what can be in a TXT record?
Some places say 255 bytes. They are wrong. Within a TXT record there are multiple character-strings (RFC 1035 section 3.3.14) 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. That is obviously quite low. 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.
I set out to demonstrate exactly that, by using Google Public DNS's JSON API and then serving large TXT responses over TCP, from a custom server.
This mostly just works, the main issue is not with the length, but with binary data, because JSON isn't really designed to handle binary data. Therefore there is some slightly custom JSON parsing. Using raw binary data in a TXT record avoids the overhead of Base64 or another encoding, meaning more data can be packed in.
👉 See it in action. For more read the comments in image.html.
Non-browser
It is possible to query this via dig. Although turning it back into binary output is a bit tricky, as the presentation form of DNS responses is escaped for output.
You can retrieve the data with dig and a little Perl to unescape and combine the character sequences:
$ dig +short dog.log.battery.st TXT | perl -pe'chomp; s/" "//g; s/^"//; s/"$//; s/\\(\d{3})/chr $1/eg; s/\\([\\"])/$1/g' > dog.avif $ sha256sum dog.avif 7058fbd20ef2af84d5efb0ae7d91f87ce7a912380636c468b32f2c759cbb9130 dog.avif
... continue reading