doc: update helps for addnode rpc and -addnode/-maxconnections config options #21710

Since #9319 proposed by Gregory Maxwell and released in v0.14, peers manually added through the -addnode config option or using the addnode RPC have their own separate limit of 8 connections that does not compete with other inbound or outbound connection usage and is not subject to the limitation imposed by the -maxconnections option.

This PR updates the -addnode and -maxconnections config options and the addnode RPC help docs with this information.

-addnode config option help

$ bitcoind -h | grep -A5 addnode=
  -addnode=<ip>
       Add a node to connect to and attempt to keep the connection open (see
       the addnode RPC help for more info). This option can be specified
       multiple times to add multiple nodes; connections are limited to
       8 at a time and are counted separately from the -maxconnections
       limit.

$ bitcoind -h | grep -A3 maxconnections=
  -maxconnections=<n>
       Maintain at most <n> connections to peers (default: 125). This limit
       does not apply to connections manually added via -addnode or the
       addnode RPC, which have a separate limit of 8.

addnode rpc help

$ bitcoin-cli help addnode
addnode "node" "command"

Attempts to add or remove a node from the addnode list.
Or try a connection to a node once.
Nodes added using addnode (or -connect) are protected from DoS disconnection and are not required to be
full nodes/support SegWit as other outbound peers are (though such peers will not be synced from).
Addnode connections are limited to 8 at a time and are counted separately from the -maxconnections limit.

Interesting. I didn't know -addnode had any separate limits. Concept ACK.

Will have to test today and maybe ask related questions here or SE or Reddit.

Interesting. I didn't know -addnode had any limits. Concept ACK.

Thanks @prayank23. Indeed, as it's not documented AFAIK except in the 0.14 release notes a few years back, I imagine that many users won't ever know unless they look into the codebase, e.g. in net.h

static const int MAX_ADDNODE_CONNECTIONS = 8;

and then verifying how the constant is used, or they infer it by using -netinfo with more than 8 addnode peers configured and seeing that the number of manual connections never exceeds 8 (or by using getpeerinfo with jq).

I imagine that many users won't ever know unless they look into the codebase, e.g. in net.h

Maybe I missed it when I was experimenting with outgoing connections few days back: https://bitcoin.stackexchange.com/questions/103753/increase-maximum-number-of-outgoing-connections-for-full-node

I should have tried addnode to get more outgoing connections.

Not sure what's the best way to test this. I tried adding random nodes from https://bitnodes.io/nodes/?q=United%20States using addnode RPC in Bitcoin Core (Mainnet). Looking at the total number of outgoing connections: 13 which is more than 10 I can verify addnode has separate limits that do not compete with other inbound or outbound connection usage.

Added maxconnections=4 in bitcoin.conf, relaunched Bitcoin Core, tried adding random nodes from above link and it worked as expected (I could see more than 4 peers connected) and mentioned in https://github.com/bitcoin/bitcoin/pull/21710/commits/b4fcbcfb49461b96bc72fb64d6152de7c5ce00de

@prayank23 thanks for having a look. If you run a node with more than 8 -addnode peers (you can get a list of, say, 20 peers from your addrman if needed with bitcoin-cli getnodeaddresses 20), you'll never see more than 8 connections in the "manual" column of cli -netinfo:

        ipv4    ipv6   onion     i2p   total   block  manual
in         0       0       1       0       1
out        4       0      10       4      18       2       8
total      4       0      11       4      19

Codewise, I suggest looking at the code returned by git grep nMaxAddnode -- there are only six lines.

ACK b4fcbcfb49461b96bc72fb64d6152de7c5ce00de

Looks good to me. Documenting in the help that addnode connections are counted separately from maxconnections is useful context to add.

Thanks @jarolrod!

If anyone is curious, the separate connections limit for addnode peers was added by @gmaxwell in #9319. I've added the PR link to the PR description.

I was experimenting with outgoing connections few days back: https://bitcoin.stackexchange.com/questions/103753/increase-maximum-number-of-outgoing-connections-for-full-node

I should have tried addnode to get more outgoing connections.

FWIW, I found #6014 to be an interesting discussion about connection counts.

ACK https://github.com/bitcoin/bitcoin/commit/b4fcbcfb49461b96bc72fb64d6152de7c5ce00de

FWIW, I found #6014 to be an interesting discussion about connection counts.

Thanks for sharing the link. I have 3 follow up questions after reading few things in that PR:

1. If more than 8 outgoing connections are bad for the network, can we at least have one option in bitcoin.conf to reduce this to something below 8 if the user wants to? Maybe keep Default=8, MIN=2, MAX=8 and OUTBOUND_FULL_RELAY_CONNECTIONS can be anything between 2 and 8.
2. If the user can add more outgoing connections using addnode which has a separate limit so total number of full relay outgoing connections can be more than 8, is this okay and does not the involve the risks mentioned in #6014 ?
3. How did we decide this number 8? Why is it not 7 or 9?

If more than 8 outgoing connections are bad for the network, can we at least have one option in bitcoin.conf to reduce this to something below 8 if the user wants to?

See #4687.

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Conflicts

No conflicts as of last run.