FastCGI: ensure HTTP_HOST is set to the requested target host.

Previously, the HTTP_HOST environment variable was constructed from the
Host request header field, which doesn't work well with HTTP/2 and
HTTP/3 where Host may be supplanted by the ":authority" pseudo-header
field per RFC 9110, section 7.2. Also, it might give an incorrect
HTTP_HOST value from HTTP/1.x requests given in the absolute form, in
which case the Host header must be ignored by the server, per RFC 9112,
section 3.2.2.

The fix is to redefine the HTTP_HOST default from a protocol-specific
value given in the $host variable. This will now use the Host request
header field, ":authority" pseudo-header field, or request line target
URI depending on request HTTP version.

Also the CGI specification (RFC 3875, 4.1.18) notes

  The server SHOULD set meta-variables specific to the protocol and
  scheme for the request. Interpretation of protocol-specific
  variables depends on the protocol version in SERVER_PROTOCOL.

Closes: https://github.com/nginx/nginx/issues/256
Closes: https://github.com/nginx/nginx/issues/455
Closes: https://github.com/nginx/nginx/issues/912

1 files changed, 11 insertions, 1 deletions

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index 6b1977340..f7f0696a8 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -656,9 +656,18 @@ static ngx_str_t  ngx_http_fastcgi_hide_headers[] = {
 };
 
 
+static ngx_keyval_t  ngx_http_fastcgi_headers[] = {
+    { ngx_string("HTTP_HOST"),
+      ngx_string("$host$is_request_port$request_port") },
+    { ngx_null_string, ngx_null_string }
+};
+
+
 #if (NGX_HTTP_CACHE)
 
 static ngx_keyval_t  ngx_http_fastcgi_cache_headers[] = {
+    { ngx_string("HTTP_HOST"),
+      ngx_string("$host$is_request_port$request_port") },
     { ngx_string("HTTP_IF_MODIFIED_SINCE"),
       ngx_string("$upstream_cache_last_modified") },
     { ngx_string("HTTP_IF_UNMODIFIED_SINCE"), ngx_string("") },
@@ -3276,7 +3285,8 @@ ngx_http_fastcgi_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
         conf->params_source = prev->params_source;
     }
 
-    rc = ngx_http_fastcgi_init_params(cf, conf, &conf->params, NULL);
+    rc = ngx_http_fastcgi_init_params(cf, conf, &conf->params,
+                                      ngx_http_fastcgi_headers);
     if (rc != NGX_OK) {
         return NGX_CONF_ERROR;
     }