Good question. Without looking into it, one theory I might have is that the client does the DNS resolution and then just proxies the HTTP request through Signal

Good question. Without looking into it, one theory I might have is that the client does the DNS resolution and then just proxies the HTTP request through Signal's servers. In general, I find the privacy implications of either solution a bit hard to reason about. Having Signal proxy the requests leaks to the website that someone is sending a link to the site via Signal, but not who is sending the link. If the link itself is sufficiently unique, though, that could be an issue. On the other hand, if someone sends me a link and I in turn send it on (without clicking) via Signal, and doing so causes my client IP to be revealed, that seems sorta bad. So the proxy makes sense here. /shrug I don't think any of the risks here are huge, and the options all have tradeoffs, which I guess is why you can disable the feature.