https://bz.apache.org/bugzilla/show_bug.cgi?id=70176
Bug ID: 70176
Summary: mod_proxy drops backend Content-Length: 0 on HEAD
responses
Product: Apache httpd-2
Version: 2.4-HEAD
Hardware: All
OS: Linux
Status: NEW
Severity: minor
Priority: P2
Component: mod_proxy
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Apache httpd 2.4.x removes a valid backend-provided Content-Length: 0
from reverse-proxied HEAD responses.
Reproducer:
- Apache httpd 2.4.58 (Ubuntu 24.04)
- mod_proxy + mod_proxy_http
- Backend: Ceph RGW 17.2.7
- A signed S3 HeadObject request for a zero-byte object
Backend response:
HTTP/1.1 200 OK
Content-Length: 0
...
Apache reverse-proxy response:
HTTP/1.1 200 OK
...
# Content-Length is absent
A non-zero Content-Length is preserved through the same ProxyPass route.
The behavior comes from modules/http/http_filters.c,
merge_response_headers():
https://github.com/apache/httpd/blob/2.4.68/modules/http/http_filters.c#L140
if (r->header_only
&& (clheader = apr_table_get(r->headers_out, "Content-Length"))
&& !strcmp(clheader, "0")) {
apr_table_unset(r->headers_out, "Content-Length");
}
This historical workaround cannot distinguish a core-generated zero
length from an authoritative Content-Length: 0 received by mod_proxy.
Expected behavior:
Preserve backend Content-Length: 0 for reverse-proxied HEAD responses.
Suggested minimal change by removing behavior only for proxy requests:
if (r->proxyreq == PROXYREQ_NONE
&& r->header_only
&& (clheader = apr_table_get(r->headers_out, "Content-Length"))
&& !strcmp(clheader, "0")) {
apr_table_unset(r->headers_out, "Content-Length");
}
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