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Comment by cyanmagenta | Hacker News Reader

The cap for record size is 64KB, but you can have an arbitrary number of records, so larger images should be possible by combining multiple TXT record responses into one.

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A DNS message cannot exceed 64K, but multiple messages can be transmitted over TCP. That's the way AXFR (zone transfer) is done: one question, multiple responses.

https://github.com/m3047/rear_view_rpz/blob/b17cf943ccd7498d...

I put a link to[1] in the article, which was previously discussed here[2] with a correction to that article. Basically there's a 2 byte length header on TCP DNS responses, limiting the payload (i.e. the DNS message inside) to 64 KiB.

  [1]: https://www.netmeister.org/blog/dns-size.html#:~:text=65536%20bytes%20DNS%20payload%20%2B%202%20bytes%20size%20%3D%2065538%20bytes
  [2]: https://news.ycombinator.com/item?id=39257147

Also with EDNS0, you can send larger messages over UDP, too, because you aren't restricted to a single UDP payload, but can send multiple datagrams.

It's pretty cool how this tech has evolved. UDP has become so much more reliable that you can even do SSL over UDP, with a complex mechanism meant to account for dropped packets during the handshake process.

After many years of "4096 ought to be enough for anybody", the default max UDP size in BIND 9 was reduced to 1232. Frags are bad; jumbos are good, but frags are bad!

Right, but you aren’t limited to one DNS call. You could have the javascript resolve multiple text records, then combine them together.

You could even do this over UDP if you really wanted, just merging a bunch of 1200-byte records together

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