feat: add client_headers field - ordered list of client header names

Add a new JSON field 'client_headers' containing all HTTP header names
received from the client (r->headers_in), in original order and with
original case preserved. Useful for browser/bot fingerprinting since
header order is client-specific.

Example: "client_headers":["Host","User-Agent","Accept"]

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

architecture.yml

@@ -148,6 +148,13 @@ module:
             If 0, it indicates a newly established TCP connection.
             If > 0, it confirms an active Keep-Alive session.
           example: 2
+        - name: client_headers
+          type: array of strings
+          description: >
+            Ordered list of all HTTP header names as received from the client
+            (r->headers_in), preserving original order and case.
+            Useful for browser/bot fingerprinting (header order is client-specific).
+          example: ["Host", "User-Agent", "Accept", "Accept-Language", "Accept-Encoding"]
         - name: content_length
           type: integer
           description: >
@@ -166,7 +173,7 @@ module:
             header_X-Request-Id: "abcd-1234"
             header_User-Agent: "curl/7.70.0"
       example_full: |
-        {"time":"2026-02-26T11:59:30Z","timestamp":1708948770000000000,"scheme":"https","src_ip":"192.0.2.10","src_port":45678,"dst_ip":"198.51.100.5","dst_port":443,"method":"GET","path":"/api/users","query":"id=1","host":"example.com","http_version":"HTTP/1.1","keepalives":0,"content_length":0,"header_X-Request-Id":"abcd-1234","header_User-Agent":"curl/7.70.0"}
+        {"time":"2026-02-26T11:59:30Z","timestamp":1708948770000000000,"scheme":"https","src_ip":"192.0.2.10","src_port":45678,"dst_ip":"198.51.100.5","dst_port":443,"method":"GET","path":"/api/users","query":"id=1","host":"example.com","http_version":"HTTP/1.1","keepalives":0,"client_headers":["Host","User-Agent","Accept","Accept-Language","Accept-Encoding","X-Request-Id"],"content_length":0,"header_X-Request-Id":"abcd-1234","header_User-Agent":"curl/7.70.0"}
 
 configuration:
   scope: global

mod_reqin_log.spec

@@ -1,4 +1,4 @@
-%global spec_version 1.0.18
+%global spec_version 1.0.19
 
 Name:           mod_reqin_log
 Version:        %{spec_version}
@@ -37,6 +37,11 @@ install -m 644 %{_pkgroot}/%{_sysconfdir}/httpd/conf.d/mod_reqin_log.conf %{buil
 %doc %{_docdir}/%{name}
 
 %changelog
+* Thu Mar 05 2026 Developer <dev@example.com> - 1.0.19
+- FEATURE: Add client_headers JSON field - ordered list of all header names
+  as received from the client, preserving original order and case
+- DOC: Update architecture.yml with client_headers field and example_full
+
 * Thu Mar 05 2026 Developer <dev@example.com> - 1.0.18
 - FIX: JsonSockLogMaxHeaders now counts configured headers (by position in list)
   regardless of their presence in the request, matching the documented behavior

src/mod_reqin_log.c

@@ -809,6 +809,28 @@ static void log_request(request_rec *r, reqin_log_config_t *cfg, reqin_log_child
         dynbuf_append(&buf, ka_buf, -1);
     }
 
+    /* client_headers - ordered list of all header names as received from the client,
+     * preserving original order and case */
+    {
+        const apr_array_header_t *arr = apr_table_elts(r->headers_in);
+        const apr_table_entry_t *elts = (const apr_table_entry_t *)arr->elts;
+        int first = 1;
+
+        dynbuf_append(&buf, ",\"client_headers\":[", 19);
+        for (int i = 0; i < arr->nelts; i++) {
+            if (elts[i].key != NULL) {
+                if (!first) {
+                    dynbuf_append(&buf, ",", 1);
+                }
+                dynbuf_append(&buf, "\"", 1);
+                append_json_string(&buf, elts[i].key);
+                dynbuf_append(&buf, "\"", 1);
+                first = 0;
+            }
+        }
+        dynbuf_append(&buf, "]", 1);
+    }
+
     /* Check buffer size before adding headers to prevent memory exhaustion */
     if (buf.len >= MAX_JSON_SIZE) {
         if (SHOULD_LOG(srv_conf, REQIN_LOG_LEVEL_DEBUG)) {