feat: add configurable RTSP User-Agent header

Add --rtsp-user-agent CLI flag and rtsp-user-agent config option to
customize the User-Agent sent in upstream RTSP requests. Some RTSP
servers validate or require a specific User-Agent for compatibility.

Also refactor e2e config tests into a parametrized framework covering
config file, CLI, and CLI-over-config priority for multiple options.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stackia

docs/en/reference/configuration.md

@@ -32,7 +32,6 @@ rtp2httpd [options]
 
 > [!TIP]
 > In addition to global configuration, you can specify upstream interfaces per request using the `r2h-ifname` and `r2h-ifname-fcc` URL parameters. See [URL Formats](/en/guide/url-formats) for details.
-
 > [!TIP]
 > On FreeBSD, specifying upstream interfaces is not supported except for multicast.
 
@@ -79,7 +78,12 @@ rtp2httpd [options]
 
 - `-U, --noudpxy` - Disable UDPxy compatibility mode (when disabled, only services defined in `[services]` or `external-m3u` can be used)
 
-### RTSP NAT Traversal
+### RTSP Options
+
+- `-u, --rtsp-user-agent <value>` - User-Agent header for upstream RTSP requests (default: `rtp2httpd/<version>`)
+  - Applies to upstream RTSP requests such as OPTIONS, DESCRIBE, SETUP, PLAY, TEARDOWN, and GET_PARAMETER
+  - Some upstream RTSP servers validate or require a specific User-Agent, so this option can be used for compatibility
+  - When left empty, the built-in default value is used
 
 - `-N, --rtsp-stun-server <host:port>` - STUN server address (default: disabled)
   - When an RTSP server only supports UDP transport and the client is behind NAT, STUN can be used to attempt NAT traversal (not guaranteed to succeed)
@@ -192,6 +196,10 @@ udp-rcvbuf-size = 524288
 # Not recommended if rtp2httpd is behind a reverse proxy (nginx/caddy/lucky, etc.)
 zerocopy-on-send = no
 
+# User-Agent for upstream RTSP requests (default: rtp2httpd/<version>)
+# Configure this when an upstream RTSP server requires a specific User-Agent for compatibility
+rtsp-user-agent = rtp2httpd/custom
+
 # STUN server for RTSP NAT traversal (default: disabled)
 # When an RTSP server only supports UDP transport and the client is behind NAT, STUN can be used to attempt NAT traversal (not guaranteed to succeed)
 # Format: host:port or host (default port: 3478)

docs/reference/configuration.md

@@ -32,7 +32,6 @@ rtp2httpd [选项]
 
 > [!TIP]
 > 除了全局配置外，还可以在每个请求的 URL 中通过 `r2h-ifname` 和 `r2h-ifname-fcc` 参数指定上游接口，详见 [URL 格式说明](../guide/url-formats.md)。
-
 > [!TIP]
 > FreeBSD 系统下不支持指定除组播外的接口。
 
@@ -79,7 +78,12 @@ rtp2httpd [选项]
 
 - `-U, --noudpxy` - 禁用 UDPxy 兼容模式 (禁用后只能使用 `[services]` 或 `external-m3u` 中定义的服务)
 
-### RTSP NAT 穿透
+### RTSP 相关
+
+- `-u, --rtsp-user-agent <值>` - 上游 RTSP 请求的 User-Agent 头 (默认: `rtp2httpd/<version>`)
+  - 作用于 OPTIONS、DESCRIBE、SETUP、PLAY、TEARDOWN、GET_PARAMETER 等上游 RTSP 请求
+  - 某些上游 RTSP 服务器会校验或依赖特定 User-Agent，可通过此参数进行兼容
+  - 留空时使用内置默认值
 
 - `-N, --rtsp-stun-server <host:port>` - STUN 服务器地址 (默认: 禁用)
   - 当 RTSP 服务器仅支持 UDP 传输且客户端位于 NAT 后时，可尝试使用 STUN 进行 NAT 穿透（不保证成功）
@@ -192,6 +196,10 @@ udp-rcvbuf-size = 524288
 # 如果你的 rtp2httpd 位于反向代理之后 (nginx/caddy/lucky 等)，不建议开启这个选项
 zerocopy-on-send = no
 
+# 上游 RTSP 请求的 User-Agent（默认: rtp2httpd/<version>）
+# 当上游 RTSP 服务器要求特定 User-Agent 时可配置此项
+rtsp-user-agent = rtp2httpd/custom
+
 # STUN 服务器用于 RTSP NAT 穿透（默认: 禁用）
 # 当 RTSP 服务器仅支持 UDP 传输且客户端位于 NAT 后时，可尝试使用 STUN 进行 NAT 穿透（不保证成功）
 # 格式: host:port 或 host（默认端口 3478）