discv5: topic index should check ENR endpoint is equal to source IP of reg traffic

[fjl]

In the base layer DHT, we allow nodes to add ENRs into the node table even when the IP on the ENR doesn't match the endpoint they are sending from. This is to make NAT discovery work through reflected packets.

This policy is not appropriate for the topic index, however, since it would open an attack vector where nodes can try to submit multiple ENRs with different IPs to work around the address restrictions in the ad cache. So it is better to strictly enforce the IP and UDP source port being equal to the ENR information in order to place an ad.

[jm-clius]

Why not simply make the admission control based on the endpoint IP rather than the encoded IP (of which there can be more than one, in any case)? This seems to me to have been the intention behind the IP tree. Doing this potentially does increase the possibility of attempted redirect-type attacks (encoding a target IP in a fake ENR and advertising that), but this is an attack vector in any case and mitigated by the signature, the fact that attackers would still be subject to admission control, and the low attack utility (you couldn't eclipse the target IP).

[fjl]

We generally want to uphold the property in the topic index that nodes can only register themselves. The ad cache entry is an ENR, so we have to ensure what is being placed there is owned by the endpoint that is performing the registration.