Question

How does multichain discover peers ?

Answer 1

Peers share information with other peers about the peers that they're aware of, similarly to any peer-to-peer gossip network. You can see the specific messages addr and getaddr here:

https://en.bitcoin.it/wiki/Protocol_documentation#addr

All this happens automatically within MultiChain for you, so you don't really need to worry about it.

Comments

Does multichain have any built in facility for dealing with peers that are located behind firewalls and/or NATs?

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Not explicitly - if two peers are behind a firewall/NAT and you want them to be able to connect to each other, you'll need to use port forwarding at the router level on at least one side, to enable it to receive incoming peer connections.