Response headers from a proxied request can be thrown away after reading a large header. If a header exceeds the the buffer size by a small amount -- say the buffer size is 8 KB and the header is 10 KB -- the next header that is read will be thrown away.

This looks to be due to ap_rgetline_core throwing away the extra bit of data when it detects that the buffer would be overrun. ap_proxy_read_headers() in mod_proxy_http.c then tries to soak up the 'extra data' which was already thrown away by ap_rgetline_core and causes the next header to be thrown away:

    while ((len = ap_getline(field, MAX_STRING_LEN, rr, 1))
            >= MAX_STRING_LEN - 1) {
        /* soak up the extra data */
    }

Removing the while loop does not fix the problem because extra data will be left from a large header that is at least 3x the proxy response buffer size. The extra data is then seen as an invalid header and results in a HTTP 502 response.


In the example below, X-Test-Header-2 will be thrown away.

Sample config:
<VirtualHost *:80>
  ProxyPass "/test" "http://localhost:8080"
  ProxyPassReverse "/test" "http://localhost:8080"
</VirtualHost>

<VirtualHost *:8080>
  Header set X-Test-Header-1 "AAAAAAAAAAAAAAA..." # large header > 8KB
  Header set X-Test-Header-2 "Testing 1 2 3"
</VirtualHost>

Request/response:
$ curl -sD - http://localhost/test -o /dev/null
HTTP/1.1 200 OK
Date: Tue, 20 Mar 2018 15:46:37 GMT
Server: Apache/2.5.1-dev (Unix)
Last-Modified: Sun, 06 Nov 2016 05:34:43 GMT
ETag: "2d-5409b43abe2c0"
Accept-Ranges: bytes
Content-Length: 45
X-Test-Header-1: AAAA... [truncated]
Content-Type: text/html

Backend trace:
[Tue Mar 20 15:46:37.801645 2018] [headers:trace2] [pid 86969:tid 123145425690624] mod_headers.c(880): AH01502: headers: ap_headers_output_filter()
[Tue Mar 20 15:46:37.801714 2018] [http:trace3] [pid 86969:tid 123145425690624] http_filters.c(1070): [client ::1:59315] Response sent with status 200, headers:
[Tue Mar 20 15:46:37.801767 2018] [http:trace5] [pid 86969:tid 123145425690624] http_filters.c(1079): [client ::1:59315]   Date: Tue, 20 Mar 2018 15:46:37 GMT
[Tue Mar 20 15:46:37.801790 2018] [http:trace5] [pid 86969:tid 123145425690624] http_filters.c(1082): [client ::1:59315]   Server: Apache/2.5.1-dev (Unix)
[Tue Mar 20 15:46:37.801889 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   Last-Modified: Sun, 06 Nov 2016 05:34:43 GMT
[Tue Mar 20 15:46:37.801915 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   ETag: \\"2d-5409b43abe2c0\\"
[Tue Mar 20 15:46:37.801931 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   Accept-Ranges: bytes
[Tue Mar 20 15:46:37.801944 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   Content-Length: 45
[Tue Mar 20 15:46:37.801972 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   X-Test-Header-1: AAAA...
[Tue Mar 20 15:46:37.802032 2018] [http:trace4] [pid 86969:tid 123145425690624] http_filters.c(900): [client ::1:59315]   X-Test-Header-2: Testing 1 2 3

Proxy trace:
[Tue Mar 20 15:46:37.805911 2018] [http:trace3] [pid 86970:tid 123145344696320] http_filters.c(1070): [client ::1:59314] Response sent with status 200, headers:
[Tue Mar 20 15:46:37.805926 2018] [http:trace5] [pid 86970:tid 123145344696320] http_filters.c(1079): [client ::1:59314]   Date: Tue, 20 Mar 2018 15:46:37 GMT
[Tue Mar 20 15:46:37.805938 2018] [http:trace5] [pid 86970:tid 123145344696320] http_filters.c(1082): [client ::1:59314]   Server: Apache/2.5.1-dev (Unix)
[Tue Mar 20 15:46:37.805952 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   Last-Modified: Sun, 06 Nov 2016 05:34:43 GMT
[Tue Mar 20 15:46:37.805964 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   ETag: \\"2d-5409b43abe2c0\\"
[Tue Mar 20 15:46:37.805976 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   Accept-Ranges: bytes
[Tue Mar 20 15:46:37.805987 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   Content-Length: 45
[Tue Mar 20 15:46:37.806009 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   X-Test-Header-1: AAAA...
[Tue Mar 20 15:46:37.806057 2018] [http:trace4] [pid 86970:tid 123145344696320] http_filters.c(900): [client ::1:59314]   Content-Type: text/html
[Tue Mar 20 15:46:37.806090 2018] [core:trace6] [pid 86970:tid 123145344696320] util_filter.c(797): [client ::1:59314] reinstate empty brigade to full brigade in 'req_core' output filter