Zebra should store a persistent list of recent successful peers

[teor2345]

Motivation

Zebra currently fetches a new list of peers from DNS each time it launches.

This design has some benefits:

• Zebra behaviour is consistent and has minimal state

And some drawbacks:

• Zebra will fail if DNS is down or slow when it launches
  • see also Zebra should try harder to discover and distribute good peer addresses #1892 where Zebra regularly queries DNS for peers
• Zebra relies on peer addresses from DNS, which is not encrypted or authenticated by default
• Zebra leaks the exact time that it launches by making a DNS query

Solution

Zebra should keep a cached list of peer addresses on disk, and update it regularly. This list can be separate from the state cache.

To avoid privacy leaks:

• the list should only store peer addresses, and no other data
• it should be sorted to destroy any order in the peer list

Alternative 1: Only use DNS if cached peers don't work

zcashd tries both the cached list of peer addresses and new addresses from DNS. This design has the following tradeoffs:

• Zebra doesn't rely on DNS
• Every time Zebra starts, it can have bad peers added by its current DNS
• Launching is as fast as possible, regardless of the quality of the cached list or DNS
• Zebra leaks some information by making a DNS query every time it starts up
• Zebra can easily recover from a compromised or poorly performing peer list, but some bad peers will stay in the list

Alternative 2: Only use DNS if cached peers don't work

zcashd first tries the cached list of peer addresses, then makes a DNS query if none connect. This design has the following tradeoffs:

• Zebra doesn't rely on DNS unless it really needs to
• Launching might be slower if it takes a while to work out that all peers have failed
• Zebra leaks some information about the quality of its cached peer address list
• Zebra doesn't have a peer reputation system, so it's hard to work out if all peers are "bad"
• It's harder for Zebra to recover from a compromised or poorly performing peer list, because it might never use DNS again

[teor2345]

We can re-open this if it becomes a problem.

[teor2345]

@mpguerra this is the old ticket for the audit/stable release, I updated it with new designs.