Testnet fixed seeds don't work about bitcoin

[achow101]

I'm currently working on a new DNS seeder and crawler that we'd be able to use for generating the fixed seeds for all networks. This will also include onion, i2p, and cjdns nodes so they don't have to be done manually as we are doing now.

[mzumsande]

I'm not sure why we'd only want to ship onion seeds for testnet, as that only makes it, if anything, slightly less usable.

I think the reason is that we have working dns seeds for testnet, but since these don't provide onion addresses, we needed some mechanism to find peers to make it possible to bootstrap with -onlynet=onion. If we added fixed IPv4 and IPv6 seeds for testnet, they would just have the role of a backup mechanism anyway. This is obviously important for mainnet, but maybe not really necessary for testnet?

[willcl-ark]

This is because all the seeds we have for testnet are NET_ONION: https://paste.256k1.dev/xosamodiga.log and so get filtered by the predicate in ThreadOpenConnections:

seed_addrs.erase(std::remove_if(seed_addrs.begin(), seed_addrs.end(),
                                [&fixed_seed_networks](const CAddress& addr) { return fixed_seed_networks.count(addr.GetNetwork()) == 0; }),
                 seed_addrs.end());

>>> p fixed_seed_networks
$3 = std::unordered_set with 2 elements = {
  [0] = NET_IPV6,
  [1] = NET_IPV4
}

In fact, as far as I can tell it doesn't appear that we have ever had ipv4 seeds for testnet? So perhaps this is a (historical and) purposeful decision. It appears this has been the case since 2014 when this file was introduced. I roughly checked a few version previous and only found references to mainnet seeds. This would seem like an odd decision to me though, so perhaps I'm missing something here.

For mainnet we have nodes_main.txt which includes auto-fetched addresses from a dns seeder, and nodes_main_manual.txt which adds onion and I2P seeds.
By contrast for testnet nodes_test.txt is not in fact ipv4 addresses, but manually-added tor v3 addresses.

If we want testnet nodes to be able to sync without relying on dns seeds then we should consider adding some testnet IPV4 addresses.

[fanquake]

This would seem like an odd decision to me though, so perhaps I'm missing something here.

I agree. I'm not sure why we'd only want to ship onion seeds for testnet, as that only makes it, if anything, slightly less usable. Maybe they can be added, and the test in feature_config_args extended to cover testnet.

[fanquake]

This is obviously important for mainnet, but maybe not really necessary for testnet?

Possibly, but it seems to just slightly reduce usability for no real benefit? I guess we were also inconsistently not shipping fixed i2p or cjdns seeds for testnet either, but with the current update i2p will now be shipped.

[willcl-ark]

See #30008