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Aritra Basu e78b1b3ecc
HTTP/3: add proxy CONNECT and MASQUE CONNECT-UDP support (ngtcp2 QUIC) 
This patch adds two major proxy capabilities to curl (ngtcp2 QUIC):
- HTTP/3 Proxy CONNECT: Tunnel HTTP/1.1 or HTTP/2 traffic through an
  HTTPS proxy that speaks HTTP/3 (QUIC) using the standard CONNECT
  method over an HTTP/3 connection.
- MASQUE CONNECT-UDP: Tunnel HTTP/3 (QUIC) traffic through an HTTP
  proxy (speaking HTTP/1.1, HTTP/2, or HTTP/3) using the extended
  CONNECT method with the CONNECT-UDP protocol (RFC9297 & RFC9298).

Public API additions:
- `CURLPROXY_HTTPS3`: new proxy type constant for HTTP/3 proxy
- `--proxy-http3`: new CLI flag to negotiate HTTP/3 with HTTPS proxy

The implementation adds two new filters:
- `H3-PROXY` - enables negotiating HTTP/3 (QUIC) to the proxy and
  running CONNECT/CONNECT-UDP through that proxy transport.
- `CAPSULE` - dedicated filter inserted between QUIC transport and
  HTTP-PROXY to handle datagram capsule encapsulation/decapsulation.

Here is how the curl filter chaining looks in different scenarios:
- HTTP/3 Proxy CONNECT (tunneling TCP protocols over QUIC proxy):
  conn -> HTTP/1.1 or HTTP/2  -> SSL -> HTTP-PROXY ->
                                 H3-PROXY -> HAPPY-EYEBALLS -> UDP
- MASQUE CONNECT-UDP (tunneling QUIC over any proxy):
  conn -> HTTP/3 -> CAPSULE -> HTTP-PROXY -> H3-PROXY ->
                               HAPPY-EYEBALLS -> UDP
  conn -> HTTP/3 -> CAPSULE -> HTTP-PROXY -> H1-PROXY or H2-PROXY ->
                               SSL -> HAPPY-EYEBALLS -> TCP

- Both features currently require the ngtcp2 QUIC backend.
- Both features are experimental (disabled by default). Enable with
  `--enable-proxy-http3`(autotools) or `-DUSE_PROXY_HTTP3=ON`(CMake).

Tests:
- tests/unit/unit3400.c: Unit tests for capsule protocol encode/decode
- tests/http/test_60_h3_proxy.py: Comprehensive pytest integration suite
- tests/http/testenv/h2o.py: Managing h2o instances with HTTP/1.1, HTTP/2,
  and HTTP/3 (QUIC) listeners, proxy.connect and proxy.connect-udp enabled.

References:
  RFC 9297 - HTTP Datagrams and the Capsule Protocol
  RFC 9298 - Proxying UDP in HTTP
  RFC 9000 §16 — Variable-Length Integer Encoding

Signed-off-by: Aritra Basu <aritrbas+gh@cisco.com>

Closes #21153
2026-05-27 08:49:53 +02:00
..
BUFQ.md BUFQ.md: re-sync with source code 2026-05-19 16:55:45 +02:00
BUFREF.md badwords: prefer 'null-terminated' 2026-03-18 11:40:13 +01:00
CHECKSRC.md checksrc: detect curlx_safefree() opportunities 2026-05-21 11:07:34 +02:00
CLIENT-READERS.md docs: avoid starting sentences with However, 2026-03-07 23:49:11 +01:00
CLIENT-WRITERS.md stop using the word 'just' 2026-03-03 15:30:22 +01:00
CODE_STYLE.md badwords: avoid 'simply' 2026-03-10 19:34:06 +01:00
CONNECTION-FILTERS.md HTTP/3: add proxy CONNECT and MASQUE CONNECT-UDP support (ngtcp2 QUIC) 2026-05-27 08:49:53 +02:00
CREDENTIALS.md creds: hold credentials 2026-05-12 16:05:15 +02:00
CURLX.md curlx: move the docs to docs/internals/ 2025-05-13 10:30:02 +02:00
DYNBUF.md curlx: move into to curlx/ 2025-05-07 11:01:15 +02:00
HASH.md lib: add meta_hash to connection, eliminate hash_offt 2025-04-22 15:57:18 +02:00
LLIST.md tidy-up: miscellaneous 2026-03-09 11:35:19 +01:00
MID.md docs: avoid starting sentences with However, 2026-03-07 23:49:11 +01:00
MQTT.md MQTT.md: remove no TLS (mqtts) support 2026-01-18 16:43:18 +01:00
MULTI-EV.md docs: fixup wording nits 2026-03-09 22:47:07 +01:00
NEW-PROTOCOL.md stop using the word 'just' 2026-03-03 15:30:22 +01:00
PEERS.md lib: introduce Curl_peer 2026-05-05 16:22:11 +02:00
PORTING.md docs/internals/PORTING.md: new document 2025-03-29 14:10:10 +01:00
RATELIMITS.md ratelimit: redesign 2025-11-24 23:34:05 +01:00
README.md docs/internals: new subdirectory 2024-08-16 08:59:25 +02:00
SCORECARD.md stop using the word 'just' 2026-03-03 15:30:22 +01:00
SPLAY.md internals/SPLAY.md: internal API documentation 2024-08-16 17:13:37 +02:00
STRPARSE.md stop using the word 'just' 2026-03-03 15:30:22 +01:00
THRDPOOL-AND-QUEUE.md spacecheck: verify filename lengths and characters 2026-03-25 18:56:40 +01:00
TIME-KEEPING.md docs: fixup wording nits 2026-03-09 22:47:07 +01:00
TLS-SESSIONS.md tidy-up: miscellaneous 2026-05-20 00:50:43 +02:00
UINT_SETS.md TIME-KEEPING.md: fold long lines 2025-12-18 23:00:06 +01:00
WEBSOCKET.md tidy-up: miscellaneous 2026-01-20 12:37:56 +01:00

README.md

Internals

This directory contains documentation covering libcurl internals; APIs and concepts that are useful for contributors and maintainers.

Public APIs are documented in the public documentation, not here.