CODEWITHSUNDEEP
rubyhttpsockets
Marius Marais
Marius Marais

Reputation: 532

How do you find a free TCP server port using Ruby?

I'm trying to create a use-once HTTP server to handle a single callback and need help with finding a free TCP port in Ruby.

This is the skeleton of what I'm doing:

require 'socket'
t = STDIN.read
port = 8081
while s = TCPServer.new('127.0.0.1', port).accept
  puts s.gets
  s.print "HTTP/1.1 200/OK\rContent-type: text/plain\r\n\r\n" + t
  s.close
  exit
end

(It echoes standard input to the first connection and then dies.)

How can I automatically find a free port to listen on?

This seems to be the only way to start a job on a remote server which then calls back with a unique job ID. This job ID can then be queried for status info. Why the original designers couldn't just return the job ID when scheduling the job I'll never know. A single port cannot be used because conflicts with multiple callbacks may occur; in this way the ports are only used for +- 5 seconds.

Upvotes: 15

Views: 9660

Answers (6)

yegor256
yegor256

Reputation: 105083

You can try random-port, a simple Ruby gem (I'm the author):

require 'random-port'
port = RandomPort::Pool.new.aquire

The best way, though, is to release it afterward:

RandomPort::Pool::SINGLETON.new.acquire do |port|
  # Use the TCP port, it will be returned back
  # to the pool afterward.
end

The pool is thread-safe and it guarantees that the port won't be used by another thread or anywhere else in the app, until it's released.

Upvotes: 1

zw963
zw963

Reputation: 1239

Maybe you want test weather one port is listening, following is worked for me system('(6<>/dev/tcp/127.0.0.1/9292) &>/dev/null'), return true if 9292 is listening, otherwise, return false.

Upvotes: 0

Aaron Hinni
Aaron Hinni

Reputation: 14716

Pass 0 in for the port number. This will cause the system to pick a port for you out of the ephemeral port range. Once you create the server, you can ask it for its addr, which will contain the port that the server is bound to.

server = TCPServer.new('127.0.0.1', 0)
port = server.addr[1]

Upvotes: 69

Marius Marais
Marius Marais

Reputation: 532

It is actually quite easy when you don't try to do everything in one line :-/

require 'socket'
t = STDIN.read

port = 8080 # preferred port
begin
  server = TCPServer.new('127.0.0.1', port)    
rescue Errno::EADDRINUSE
  port = rand(65000 - 1024) + 1024
  retry
end

# Start remote process with the value of port

socket = server.accept
puts socket.gets
socket.print "HTTP/1.1 200/OK\rContent-type: text/plain\r\n\r\n" + t
socket.close

This accomplishes (strong word) the same as the snippet in the question.

Upvotes: 3

Tometzky
Tometzky

Reputation: 23890

Don't communicate on random ports. Pick a default one and make it configurable. Random ports are incompatible with firewalls. FTP does this and firewall support for it is a nightmare - it has to deeply inspect packets.

Upvotes: -8

gnud
gnud

Reputation: 78538

I guess you could try all ports > 5000 (for example) in sequence. But how will you communicate to the client program what port you are listening to? It seems simpler to decide on a port, and then make it easily configurable, if you need to move your script between different enviroments.

For HTTP, the standard port is 80. Alternative ports i've seen used are 8080, 880 and 8000.

Upvotes: -9

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