[erlang-questions] TCP Kernel bug/feature

Tom Samplonius <>
Fri May 18 04:39:56 CEST 2007


----- "Martin Carlson" <> wrote:
> Hi all,
> 
> Came across a strange Linux kernel bug/feature this morning while
> testing
> a TCP application we've written.
> 
> The following code captures the behavior.
> 
> server(Ip, Port) ->
>     {ok, Socket} = gen_tcp:listen(Port, [{ip, Ip}, {reuseaddr, true},
>                                          {packet, 4}, binary]),
>     {ok, Client} = gen_tcp:accept(Socket),
>     gen_tcp:close(Socket),
>     Client.
> 
> 
> client(Ip, Port) ->
>     case gen_tcp:connect(Ip, Port, [{packet, 4}, binary]) of
>         {ok, Client} ->
>             Client;
>         {error, econnrefused} ->
>             client(Ip, Port);
>         {error, Reason} ->
>             erlang:error(Reason)
>     end.
> 
> 
> node1() ->
>     server({127,0,0,1}, 12345),
>     halt().
> 
> 
> node2() ->
>     client("127.0.0.1", 12345),
>     io:format("Connection lost~n"),
>     client("127.0.0.1", 12345).
> 
> When running node1() on one Erlang node and node2 on another
> this creates a socket connected to itself. One can capture the socket
> and
> pass data forth and back.
> 
> netstat gives the following result:
> tcp        0      0 127.0.0.1:12345         127.0.0.1:12345        
> ESTABLISHED
> 
> This however only seem to work on Suse 9.3 with a stock kernel and
> has
> probably been fixed in newer versions of the kernel.
> 
> uname say:
> 2.6.11.4-20a-smp #1 SMP Wed Mar 23 21:52:37 UTC 2005 i686 i686 i386
> GNU/Linux


  I think this is way it is supposed to work.  A socket is a unique pair of source and destination ports.  You run a server on 12345, and then connect from 12345 to 12345.  So that is a legal socket.  This should work this way on most OSes.

  Normally when you establish a client connection, you don't special the port to establish the socket from.  You let the OS use the next available number from some magical pool of ports.  You should have a really good reason to establish a connection from a specific port.  


> 
> --
> Martin Carlson
> Erlang Training & Consulting
> http://www.erlang-consulting.com
> 


Tom



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