Add flag to choose advertising local IP to Prysm node

[cortze]

This PR addresses #31

Changes work locally:

• with the flag on the command:

level=INFO msg="Adding ourselves as a trusted peer to Prysm" peer_id=16Uiu2HAmBpAB8GPWRTC77i8ZUqJSoqTatiaPqqcJDrPGMYwhXM1M "on local maddr"=/ip4/127.0.0.1/tcp/9020

• without the flag on the command, and the default behaviour:

level=INFO msg="Adding ourselves as a trusted peer to Prysm" peer_id=16Uiu2HAmTTJTsbhNvWAwjEBck21g3Ap4FFBUnDxnTFPzavzdWHRQ "on priv maddr"=/ip4/192.168.1.109/tcp/9020

[guillaumemichel]

Does it need to be specifically ip4 or ip6 or it doesn't matter and both addresses are acceptable?

[cortze]

That is a good question. I've been checking at the Prysm code, and it doesn't seem to check anything, it just parses the maddr info using the go-libp2p primitives:

func (s *Server) AddTrustedPeer(w http.ResponseWriter, r *http.Request) {
     ...
     info, err := peer.AddrInfoFromString(addrRequest.Addr)
     ....

Do you think that could be a problem? We don't seem to be checking anything like that for the private maddr neither:

// PrivateListenMaddr returns the first multiaddress in a private IP range that
// this host is listening on.
func (h *Host) PrivateListenMaddr() (ma.Multiaddr, error) {
	for _, maddr := range h.Addrs() {
		if manet.IsIPLoopback(maddr) {
			continue
		}
		if !manet.IsPrivateAddr(maddr) {
			continue
		}
		return maddr, nil
	}
	return nil, fmt.Errorf("no private multi address found in %s", h.Addrs())
}

[guillaumemichel]

As long as both Hermes and Prysm have both an ip4 and ip6 and can reach each other on both stacks it is fine (or they are both using only ip4 XOR ip6). Otherwise, we could also restrict both to use ip4 (or ip6) only

[cortze]

Coming back to this since it got hanging for a few weeks.
We generally run both the Prysm node and Hermes on ip4, is that a blocker to merge the PR @guillaumemichel ?
Should I make it explicit on the Readme, or add a sanity check when spawning Hermes?

[guillaumemichel]

LGTM if everything runs on the ip4 stack