Make connection "read only" for use in daemon

[mh166]

Hi,

before anything, I want to say thanks for this amazing tool! It really helped me accomplish so much with it! 😊

TL;DR: How can I use websockat as a "read only" service for use with systemd, that does not need an active STDIN?

---

At the moment I'm having a slight problem, but maybe I'm just overlooking something: I have a working script, that connects to a wss:// endpoint and reads log messages from there. I don't need or want to send anything towards that direction, it should only listen!

For now, I'm using a shellscript to work with the messages and let it sit in a screen instance, so I can safely detach from the terminal and have it running in the background. websocat's command line in this script is nothing but standard:

/usr/local/bin/websocat wss://example.net/websocket -H 'Cookie: X-Authorization=MyVeryLongAndSecureToken'

Ultimately, I would like to have it running as a daemon. Therefore I created the following little systemd unit:

cortex:~# systemctl cat jwc-monitor.service 
# /etc/systemd/system/jwc-monitor.service
[Unit]
Description=JWC Monitor

[Service]
ExecStart=/usr/local/bin/jwc-monitor
Restart=on-failure

[Install]
WantedBy=multi-user.target 
cortex:~#

But instead of enjoying a nice little service running in the background, it always fails on me and results in the following state:

cortex:~# systemctl status jwc-monitor.service ● jwc-monitor.service - JWC Monitor
   Loaded: loaded (/etc/systemd/system/jwc-monitor.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Mon 2021-08-23 22:35:50 CEST; 45min ago
  Process: 13753 ExecStart=/usr/local/bin/jwc-monitor (code=exited, status=0/SUCCESS)
 Main PID: 13753 (code=exited, status=0/SUCCESS)
cortex:~#

It soon occured to me, that the problem is probably with websocat not having a valid STDIN available. After a bit of debugging, I was able to reproduce the same error also in a normal shell by closing fd 0 like so (note the 0>&- at the end of the command line):

cortex:~# /usr/local/bin/websocat wss://example.net/websocket -H 'Cookie: X-Authorization=MyVeryLongAndSecureToken' -vv 0>&-
[INFO  websocat::lints] Auto-inserting the line mode
[DEBUG websocat] Done third phase of interpreting options.
[DEBUG websocat] Done fourth phase of interpreting options.
[DEBUG websocat] Preparation done. Now actually starting.
[DEBUG websocat::sessionserve] Serving Line2Message(ThreadedStdio) to Message2Line(WsClient("wss://example.net/websocket")) with Options { websocket_text_mode: true, websocket_protocol: None, websocket_reply_protocol: None, udp_oneshot_mode: false, udp_broadcast: false, udp_multicast_loop: false, udp_ttl: None, udp_join_multicast_addr: [], udp_join_multicast_iface_v4: [], udp_join_multicast_iface_v6: [], udp_reuseaddr: false, unidirectional: false, unidirectional_reverse: false, max_messages: None, max_messages_rev: None, exit_on_eof: false, oneshot: false, unlink_unix_socket: false, unix_socket_accept_from_fd: false, exec_args: [], ws_c_uri: "ws://0.0.0.0/", linemode_strip_newlines: false, linemode_strict: false, origin: None, custom_headers: [("Cookie", [88, 45, 65, 117, 116, 104, 111, 114, 105, 122, 97, 116, 105, 111, 110, 61, 77, 121, 86, 101, 114, 121, 76, 111, 110, 103, 65, 110, 100, 83, 101, 99, 117, 114, 101, 84, 111, 107, 101, 110])], custom_reply_headers: [], websocket_version: None, websocket_dont_close: false, websocket_ignore_zeromsg: false, one_message: false, no_auto_linemode: false, buffer_size: 65536, broadcast_queue_len: 16, read_debt_handling: Warn, linemode_zero_terminated: false, restrict_uri: None, serve_static_files: [], exec_set_env: false, no_exit_on_zeromsg: false, reuser_send_zero_msg_on_disconnect: false, process_zero_sighup: false, process_exit_sighup: false, socks_destination: None, auto_socks5: None, socks5_bind_script: None, tls_domain: None, tls_insecure: false, headers_to_env: [], max_parallel_conns: None, ws_ping_interval: None, ws_ping_timeout: None, request_uri: None, request_method: None, request_headers: [], autoreconnect_delay_millis: 20, ws_text_prefix: None, ws_binary_prefix: None, ws_binary_base64: false, ws_text_base64: false, asyncstdio: false, foreachmsg_wait_reads: false }[INFO  websocat::stdio_threaded_peer] get_stdio_peer (threaded)
[DEBUG websocat::sessionserve] Underlying connection established
[INFO  websocat::ws_client_peer] get_ws_client_peer
[INFO  websocat::ws_client_peer] Connected to ws
[DEBUG websocat::my_copy] BrokenPipe: read_done
[DEBUG websocat::my_copy] done
[INFO  websocat::sessionserve] Forward finished
[DEBUG websocat::sessionserve] Forward shutdown finished
[DEBUG websocat::ws_peer] drop WsWriteWrapper
[INFO  websocat::ws_peer] Received WebSocket close message
[DEBUG websocat::ws_peer] The close message is None
[DEBUG websocat::my_copy] BrokenPipe: read_done
[DEBUG websocat::my_copy] done
[INFO  websocat::sessionserve] Reverse finished
[DEBUG websocat::sessionserve] Reverse shutdown finished
[INFO  websocat::sessionserve] Both directions finished
cortex:~#

As you can see, the "BrokenPipe" leads to the termination of the session. If I run it like that without closing STDIN, everything works just fine (in the shell).

How could I make this work as a read only service?

[mh166]

Apparently, I just found a workaround. Kind of...

When running websocat with the following command line options, it even works as a systemd service:

/usr/local/bin/websocat -t - autoreconnect:wss://example.net/websocket -H 'Cookie: X-Authorization=MyVeryLongAndSecureToken'

Notice that the important part is -t - autoreconnect: as a whole. When using it like that, the service keeps running in the background as it should. As soon as I leave out the autoreconnect: part and just use -t - wss://..., it fails just as before.

While this seems like a happy accident (and indeed, it kind of is), I would prefer to handle reconnects on my own, as I might have to renew the auth token.

Therefore, I'll leave this issue open, as any input or suggestions are still very much welcome to get this sorted out properly.

[vi]

Sorry, I have missed the issue when if was originally posted.

I don't know why usage or non-usage of stdin is mapped to read-write or read-only modes, but all mentioned command lines (including the adjusted one) do attempt to read from stdin.

If shell closes stdin, I except Rust standard library to re-open it as /dev/null, so reading it would immediately produce EOF, which sends typically WebSocket close message (maybe autoreconnect: blocks it), which causes server to also close the connection.

Maybe you want -u/-U (unidirectional, in one direction or the other) mode and/or -n (do not send WebSocket close message) options. autoreconnect: may or may not be appropriate. I'd left it out, making Websocat exit in case of broken connections, but also making Systemd restart it more controllably.

[mh166]

Sorry, completely missed to respond: using the -u/-U worked perfectly. Thanks so much for the help! :)