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docs: add warning about HTTP/3 and proxy incompatibility - #373
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Proxies are not supported when HTTP/3 is enabled. This documents the limitation in both Python and Node.js client options. Closes #364 Generated with [Claude Code](https://claude.ai/code) via [Happy](https://happy.engineering) Co-Authored-By: Claude <noreply@anthropic.com> Co-Authored-By: Happy <yesreply@happy.engineering>
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My agent got freaked by the weird http3 + proxy behavior. This is an attempt to clean it up so it returns an error and doesn't silently change behavior. ## AI Summary HTTP/3 through a proxy isn't supported, and the docs say so. That was decided in #246 and #364, documented in #373, and reqwest tracks it upstream as seanmonstar/reqwest#2833. This PR doesn't try to add that support. It makes the unsupported combination fail safe instead of quietly sending traffic from the machine's own IP address. The change only matters when **HTTP/3 is enabled and a proxy is set**, whether through `with_proxy` / `proxyUrl` or through `HTTPS_PROXY` / `ALL_PROXY`. Everything else behaves as before, including the default, where HTTP/3 is off. ## What `master` does with HTTP/3 and a proxy | Request | On `master` | |---|---| | `http3_prior_knowledge` | Every request goes straight to the target over QUIC from the real IP. The proxy is never used. | | Not forced, and the host advertises h3 in its DNS HTTPS record | `H3Engine` looks the record up directly at `8.8.8.8`, which already exposes the hostname from the real IP. The request then goes out over QUIC from the real IP too. | | Not forced, and the host advertises h3 only through `Alt-Svc` | The first request goes through the proxy. impit then caches the host as h3-capable, and every later request to it leaks. A quick one-request test looks fine. | None of these cases logs anything or returns an error. I reproduced this on `master` with a local logging CONNECT proxy and `https://www.google.com/`. The forced request never reached the proxy. strace showed QUIC packets going directly to `142.251.153.119:443`, and the DNS lookups going to `8.8.8.8` / `8.8.4.4`. ## With this PR - When the client is built with a proxy, HTTP/3 is disabled and a warning is logged. All traffic goes through the proxy over TCP (HTTP/2 or HTTP/1.1), and no lookups go directly to `8.8.8.8`. - A request with `http3_prior_knowledge` fails with `Http3Disabled`, whose message now mentions the proxy. It no longer goes out directly. - A proxy that comes only from `HTTPS_PROXY` / `ALL_PROXY` is covered too. On `master` it leaks the same way, and the docs don't mention it. The PR also removes `.with_http3()` from the `ImpitBuilder` doc example, which combined it with a proxy. ## Not changed - HTTP/3 still can't go through a proxy. That's blocked in reqwest. - `NO_PROXY` isn't considered. If any proxy is set, HTTP/3 is off for all hosts. That's a deliberately conservative choice. - I disable HTTP/3 rather than fail at build time, so existing `http3: true` + proxy setups keep working over HTTP/2. That's easy to flip to an error if you'd prefer. - The unused QUIC endpoint that reqwest sets up whenever the `http3` feature is compiled in is handled separately in #595. 🤖 Generated with [Claude Code](https://claude.com/claude-code) https://claude.ai/code/session_0148RYR4BTGyCYHENSRZjC3S Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Jindřich Bär <jindrichbar@gmail.com>
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Added warning to Python and Node.js documentation that proxies are not supported when HTTP/3 is enabled.
Closes #364