Liquidsoap dev : Interaction with the server

Interaction with the server

Liquidsoap starts with one or several scripts as its configuration, and then streams forever if everything goes well. Once started, you can still interact with it by means of the server. The server allows you to run commands. Some are general and always available, some belong to a specific operator. For example the request.queue() instances register commands to enqueue new requests, the outputs register commands to start or stop the outputting, display the last ten metadata chunks, etc.

The protocol of the server is a simple human-readable one. Currently it does not have any kind of authentication and permissions. It is currently available via two media: TCP and Unix sockets. The TCP socket provides a simple telnet-like interface, available only on the local host by default. The Unix socket interface (cf. the server.socket setting) is through some sort of virtual file. This is more constraining, which allows one to restrict the use of the socket to some priviledged users.

You can find more details on how to configure the server in the documentation of the settings key server, in particular server.telnet for the TCP interface and server.socket for the Unix interface. Liquidsoap also embeds some documentation about the available server commands.

Now, we shall simply enable the Telnet interface to the server, by setting set("server.telnet",true) or simply passing the -t option on the command-line. In a complete case analysis we set up a request.queue() instance to play user requests. It had the identifier "queue". We are now going to interact via the server to push requests into that queue:

dbaelde@selassie:~$ telnet localhost 1234
Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
queue.push /path/to/some/file.ogg
5
END
request.metadata 5
[...]
END
queue.push http://remote/audio.ogg
6
END
request.trace 6
[...see if the download started/succeeded...]
END
exit

Of course, the server isn't very user-friendly. But it is easy to write scripts to interact with Liquidsoap in that way, to implement a website or an IRC interface to your radio. However, this sort of tool is often bound to a specific usage, so we have not released any of ours. Feel free to ask the community about code that you could re-use.

Interactive variables

Sometimes it is useful to control a variable using telnet. A simple way to achive this is to use the interactive.float function. For instance, in order to dynamically the volume of a source:

# Register a telnet variable named volume with 1 as initial value
v = interactive.float("volume", 1.)

# Change the volume accordingly
source = amplify(v, source)

The first line registers the variable volume on the telnet. Its value can be changed using the telnet command

var.set volume = 0.5

and it can be retrieved using

var.get volume

Similarly, we can switch between two tracks using interactive.bool and switch as follows:

# Activate the telnet server
set("server.telnet",true)

# The two sources
s1 = playlist("~/Music")
s2 = sine()

# Create an interactive boolean
b = interactive.bool("button", true)

# Switch between the tracks depending on the boolean
s = switch(track_sensitive=false,[(b,s1), ({true},s2)])

# Output the result
output.pulseaudio(s)

By default the source s1 is played. To switch to s2, you can connect on the telnet server and type var.set button = false.

Interactive commands

Starting with liquidsoap version 1.3.4, you can register custom server commands to interact with the client with applications such as implementing a pub/sub mechanism.

There main commands are:

• server.write, server.read, server.readchars and server.readline to read and write interactively
• server.condition, server.wait, server.signal and server.broadcast to control the execution of the command

Read/Write

Writing a partial response is done using the following syntactic sugar:

server.write "string to write" then
  log("string done writting!")
  # Do more stuff then send the final response:
  "Done!"
end

Read a value can be done 3 different ways. Most simple one is server.readline:

server.readline ret then
  log("Read line: #{ret}")
  # Do more stuff then send the final response:
  "Done!"
end

Then you can read a fixed number of characters:

server.readchars 15 : ret then
  log("Read 15 characters: #{ret}")
  # Do more stuff then send the final response:
  "Done!"
end

Finally, you can read until reaching a marker, which can be any string or regular expression:

server.read "OVER[\r\n]+" :  ret then
  log("Read until OVER: #{ret}")
  # Do more stuff then send the final response:
  "Done!"
end

Control flow

You can pause and resume server commands using an API similar to Unix conditions:

• server.condition() creates a condition variable
• server.wait pauses a server command. See below for details
• server.signal(c) resumes one waiting command
• server.broadcast(c) resumes all waiting commands

server.wait is used through a syntactic sugar:

server.wait c then
  log("Command has resumed!")
  # Do more stuff then send the final response:
  "Done!"
end

Full example

In the following, we define two commands:

• wait: when executing the command, the client waits for a message. Message can be one of:
  • "exit": terminate command
  • "read": read one line from the client and print it back
  • Otherwise, the client prints the received value
• send <value>: when executing this command, the client sends <value> to all waiting clients.

c = server.condition()

value = ref ""

def wait(_) =
  def rec fn () =
    server.write ">> " then
      server.wait c then
        value = !value
        if value == "exit" then
          "All done!"
        elsif value == "read" then
          server.write "Write me sumething mister..\n" then
            server.readline ret then
              server.write "Read: #{ret}\n" then
                fn()
              end
            end
          end
        else
          server.write "Received value: #{value}\n" then
            fn()
          end
        end
      end
    end
  end

  fn ()
end

def send(v) =
  value := v
  server.signal(c)
  "Ok!"
end

Example of use:

send:

Connected to localhost.
Escape character is '^]'.
send foo
Ok!
END
send read
Ok!
END
send exit
Ok!
END

wait:

Connected to localhost.
Escape character is '^]'.
wait
>> Received value: foo
>> Write me sumething mister..
Here's to you mon ami!
Read: Here's to you mon ami!
>> All done!
END
exit
Bye!

Securing the server

The command server provided by liquidsoap is very convenient for manipulating a running instance of Liquidsoap. However, no authentication mechanism is provided. The telnet server has no authentication and listens by default on the localhost (127.0.0.1) network interface, which means that it is accessible to any logged user on the machine.

Many users have expressed interest into setting up a secured access to the command server, using for instance user and password information. While we understand and share this need, we do not believe this is a task that lies into Liquidsoap's scope. An authentication mechanism is not something that should be implemented naively. Being SSH, HTTP login or any other mechanism, all these methods have been, at some point, exposed to security issues. Thus, implementing our own secure access would require a constant care about possible security issues.

Rather than doing our own home-made secure acces, we believe that our users should be able to define their own secure access to the command server, taking advantage of a mainstream authentication mechanism, for instance HTTP or SSH login. In order to give an example of this approach, we show here how to create a SSH access to the command server: we create a SSH user that, when logging through SSH, has only access to the command server.

First, we enable the unix socket for the command server in Liquidsoap:

set("server.socket",true)
set("server.socket.path","/path/to/socket")

When started, liquidsoap will create a socket file /path/to/socket that can be used to interact with the command server. For instance, if your user has read and write rights on the socket file, you can do

socat /path/to/socket -

The interface is then exactly the same has for the telnet server.

We define now a new “shell”. This shell is in fact the invokation of the socat command. Thus, we create a /usr/local/bin/liq_shell file with the following content:

#!/bin/sh
# We test if the file is a socket, readable and writable.
if [ -S /path/to/socket ] && [ -w /path/to/socket ] && \
   [ -r /path/to/socket ]; then
  socat /path/to/socket -
else
# If not, we exit..
  exit 1
fi

We set this file as executable, and we add it in the list of shells in /etc/shells.

Now, we create a user with the liq_shell as its shell:

adduser --shell /usr/local/bin/liq_shell liq-user

You also need to make sure that liq-user has read and write rights on the socket file.

Finally, when logging through ssh with liq-user, we get:

11:27 toots@leonard % ssh liq-user@localhost
liq-user@localhost's password:
Linux leonard 2.6.32-4-amd64 #1 SMP Mon Apr 5 21:14:10 UTC 2010 x86_64

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue Oct  5 11:26:52 2010 from localhost
help
Available commands:
(...)
| exit
| help [<command>]
| list
| quit
| request.alive
| request.all
| request.metadata <rid>
| request.on_air
| request.resolving
| request.trace <rid>
| uptime
| var.get <variable>
| var.list
| var.set <variable> = <value>
| version

Type "help <command>" for more information.
END
exit
Bye!
END
Connection to localhost closed.

This is an example of how you can use an existing secure access to secure the access to liquidsoap's command server. This way, you make sure that you are using a mainstream secure application, here SSH.

This example may be adapted similarly to use an online HTTP login mechanism, which is probably the most comment type of mechanism intented for the command line server.