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At present, the Gateway performs DNS resolution for A & AAAA queries via `libc`. The `resolve` system call only provides us with the resolved IPs but not any of the metadata around the query such as TTL. As a result, we can only cache DNS queries for a static amount of time, currently 30s. It would be more correct to cache them for their TTL instead. To do so, we re-introduce `hickory-resolver` to our codebase. Deliberately, we only use it for resolving A and AAAA records on the Gateway for now. DNS resolution for SRV & TXT records happens one layer below and uses the same infrastructure as DNS resolution on the Client. Merging this is difficult however because the Gateway still supports the control protocol of 1.3.x clients. That one requires DNS resolution prior to setting up the connection of DNS resources which means it needs to happen in the event-loop of the Gateway binary and cannot be moved into the `Tunnel` where DNS resolution for Client and SRV/TXT records happen. Once we can drop support for 1.3.x clients, this Gateway's event-loop will simplify drastically which will allow us to refactor this to a more unified approach of DNS resolution. Until then, we can at least fix the hardcoded TTL by using `hickory-resolver` in the event-loop. The functionality is guarded behind a feature-flag which - as usual - is off by default (i.e. for as long as we haven't fetched the flags). The feature flag is already configured to `true` for staging and production so we can test the new behaviour. Resolves: #8232 Related: #10385