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I've seen various 'options' that the different DNS seeders support, for example x1, x5, x9, and xd:

vSeeds.emplace_back("seed.bitcoin.sipa.be", true); // Pieter Wuille, only supports x1, x5, x9, and xd
vSeeds.emplace_back("dnsseed.bluematt.me", true); // Matt Corallo, only supports x9
vSeeds.emplace_back("dnsseed.bitcoin.dashjr.org", false); // Luke Dashjr
vSeeds.emplace_back("seed.bitcoinstats.com", true); // Christian Decker, supports x1 - xf
vSeeds.emplace_back("seed.bitcoin.jonasschnelli.ch", true); // Jonas Schnelli, only supports x1, x5, x9, and xd
vSeeds.emplace_back("seed.btc.petertodd.org", true); // Peter Todd, only supports x1, x5, x9, and xd

(from chainparams.cpp)

But I can't find anything which actually explains what these options are. What do these options do and why do only some seeders offer some of the options?

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4

This is 'service bit filtering.'

You can run nslookup seed.bitcoin.sipa.be, and get a list of bitcoin nodes.

You can also run nslookup x1.seed.bitcoin.sipa.be, and get a list of nodes with NODE_NETWORK set.

(source 1. source 2.)

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2
  • So the number following the X is like a bitmask for the service bits? Makes sense, thanks :)
    – MeshCollider ♦
    Oct 7 '17 at 6:50
  • 1
    Exactly, it sets what bits are required in the result.
    – Pieter Wuille
    Oct 7 '17 at 9:57

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