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HTTP/2 to origin: bodyless response with a header block over 4 KB fails to parse and is sent to the client as 502/500

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評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
68/100
issue の種類
バグ
明瞭さ
明確に書かれている
活発さ
活発
技術スタック
cpp
領域
backend-api-design, networking

調査の方向性

Start with Http2Stream::send_headers and HttpSM::state_read_server_response_header, then follow HTTPHdr::parse_resp in HdrTSOnly.cc and MIMEScanner::get. Reproduce the bodyless HTTP/2 302 or 204 with an approximately 8.8 KB header block. Done means the large header parses across blocks and the client receives the origin response instead of a 502 or 500.

索引モデルが issue の本文から書いたものです。

説明

HTTP/2

Summary

If an HTTP/2 origin sends a bodyless response (HEADERS with END_STREAM) whose header block is larger than one IOBuffer block (about 4 KB), ATS fails to parse it. The client gets a 502 or 500 instead of the origin's response. It mostly shows up on redirects and 204s that carry a large Content-Security-Policy or Set-Cookie.

Cause

  1. Http2Stream::send_headers, which hands a received header to the HttpSM, prints the decoded response header into _receive_buffer one block at a time, adding a block whenever print() doesn't finish. A header of more than about 4 KB therefore spans several blocks, and a field can be split at a block boundary.
  2. With receive_end_stream set on an outbound stream, it then signals VC_EVENT_EOS, not READ_READY or READ_COMPLETE.
  3. HttpSM::state_read_server_response_header handles that EOS by parsing with eof = true. It logs Server closed connection while reading response header at this point, even though nothing closed.
  4. HTTPHdr::parse_resp (HdrTSOnly.cc) loops over the reader one block at a time and passes the same eof flag for every block. When a field is cut at the end of the first block, MIMEScanner::get treats it as the end of input and returns ParseResult::ERROR (unterminated field).
  5. The result is handle_server_setup_error, and the client gets an error. squid-style logs show the origin's real status (a 3xx or 204) next to the 5xx sent to the client, with o_bytes stopping just short of the field that crosses the 4 KB boundary.

Observed / reproduction

Reproduced on a 10.0.x build; HdrTSOnly.cc is identical on master.

  • An HTTP/2 origin returns 302 with no body and a header block of about 8.8 KB, including one long Content-Security-Policy field.
  • A GET through ATS: the origin logs a 302, and the client gets a 502 every time, with o_bytes=202 and the request retried once (two attempts).
  • The same responses had no errors while that origin was reached over HTTP/1.1.
  • In production traffic, one bodyless 3xx endpoint failed 150 of 150 requests.

Suggested fix

Either of these, or both:

  • In HTTPHdr::parse_resp, pass eof only for the last block, when there is no further block to read.
  • In Http2Stream, write the whole decoded header into one block sized to fit it, so a header from an HTTP/2 frame is never split.
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マージ済み PR（30日）
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