Allow loading a list of relays from an API

[metachris]

mev-boost should have the ability to load a list of relays from an URL. This allows updating the used relays without needing to update the CL clients / mev-boost service.

• This list should be continuously updated by mev-boost
• It should be possible to supply multiple URLs
• If the URLs fail to load on startup, hardcoded relays are used as a fallback and in the background mev-boost retries loading relays from URLs

[ralexstokes]

just pointing out that this is a classic "security vs convenience" trade-off.

if we assume a "battle-hardened" validation setup then not only does the operator have to open IP access to the relays but also to this "relay provider" and the relay provider must take a lot of care to not be compromised.

the "low-hanging fruit" class of damage that could be done by compromise of an endpoint like this issue suggests is pretty minimal although I can see how it could be exploited for a liveness fault (esp. if I found a zero-day in the local builder fallback pipeline) -- im sure we can think of increasingly damaging attacks if we spend more time on it

[ralexstokes]

its also worth considering other discovery mechanisms beyond a registry provided by a direct API.

I know at least ethereum and bitcoin have DNS-based discovery modes (if someone goes down this road, look into authenticating the DNS records a la DNSSEC) and in the limit we could even bake our own discovery layer on top of discv5.

[metachris]

Thanks for the input Alex 🙏 Let's discuss and evaluate this further.

This feature is particularly relevant for parties that manage a large amount of validators, where reconfiguring and restarting mev-boost manually across a large number of nodes is cumbersome.

The best setup to use this feature seems a statically hosted JSON on multiple providers (rather than a dynamic API/service).

[ralexstokes]

yeah I think re-loading a local file is much easier to secure than making arbitrary network requests :)

[metachris]

There has not been much interest in loading a remote list, so let's start with the local config and see if the need arises to allow loading it from an API as well.

[metachris]

For reference, this could also be solved by a local script that queries the API, or a smart contract, and writes the local config file.