p2p: Assume v2transport for addresses from seeds #35766

1. 
   mzumsande commented at 5:01 PM on July 21, 2026: contributor

   gmaxwell noted in #30951 (comment) that addresses loaded from dns seeds and fixed seeds are still assumed to be v1, so a new node won't use v2transport for the first few connections it makes. By now, the vast majority of reachable nodes (~80% according to https://bitnod.es/) in the network supports BIP324, and even if the optimistic guess would turn out to be wrong for a given node, we would just reconnect with v1. This would also be necessary for a v2-only option (#30951), but I think it makes sense to change the default regardless of that PR. Note that -seednode and addr-fetch connections already use v2transport by default.

2. p2p: Assume v2transport for addresses from seeds

   By now, the vast majority of nodes in the network supports BIP324.
   Even if the optimistic guess would turn out to be wrong for a given
   node, we would just reconnect with v1.
   This is better than making v1 connections with peers when both nodes support v2.

   cf0f2aeae0
3. DrahtBot added the label P2P on Jul 21, 2026
4. 
   DrahtBot commented at 5:01 PM on July 21, 2026: contributor

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   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35766.

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5. 
   sedited commented at 5:17 PM on July 21, 2026: contributor

   Nice, Concept ACK

6. 
   darosior commented at 5:50 PM on July 21, 2026: member

   Concept ACK.

7. in src/protocol.h:374 in cf0f2aeae0

    369 | + * Service flags we assume for addresses obtained from the DNS seeds and the
    370 | + * fixed seeds, which don't come with service flags attached.
    371 | + * BIP324 support can be safely assumed because the vast majority of listening nodes signals NODE_P2P_V2, and if the
    372 | + * assumption is wrong for a given peer we simply reconnect using v1 transport.
    373 | + */
    374 | +constexpr ServiceFlags SeedsAssumedServiceFlags() { return ServiceFlags(SeedsServiceFlags() | NODE_P2P_V2); }
   

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   darosior commented at 6:15 PM on July 21, 2026:

   What's the reason for keeping SeedsServiceFlags() around now that it's only called by this function?

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   mzumsande commented at 6:42 PM on July 21, 2026:

   It determines the required flags that we ask the DNS seeds for, see https://github.com/mzumsande/bitcoin/blob/cf0f2aeae009c98615bffd4dafd74e46f20a0ae6/src/net.cpp#L2389. We don't require NODE_P2P_V2 from the seeds because it is not necessary - v1 nodes can continue to be returned from seeds because we can downgrade.

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   darosior commented at 7:02 PM on July 21, 2026:

   Right, should have actually read the code before commenting.

8. 
   gmaxwell commented at 8:38 PM on July 21, 2026: contributor

   utACK

9. 
   w0xlt commented at 10:45 PM on July 21, 2026: contributor

   ACK cf0f2aeae009c98615bffd4dafd74e46f20a0ae6

10. DrahtBot requested review from darosior on Jul 21, 2026
11. DrahtBot requested review from sedited on Jul 21, 2026

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