Netcat "Connection refused" on localhost

Question

I am trying to get a value from a netcat connection started at a php file, but it dies with:

localhost [127.0.0.1] 2000 (?) : Connection refused

I don't know why, but it works well if I ssh it as apache user (www-data). This is what I've done:

1. Start an endless loop serving a date with a little delay:

   $ (while true; do nc -l -p 2000 -c "sleep 5; date"; done)&
   

2. Check if is working:

   $ su www-data
   $ nc localhost 2000
   Fri Oct 16 21:33:20 COT 2009
   

3. Create /var/www/test.php as follows:

   <?php
   exec('nc localhost 2000>>/var/www/dates.txt 2>>/var/www/errors.txt &');
   ?>
   

4. Run it on a browser:

   http://myserver.com/test.php
   

5. Finally take a look at both txt's, dates is empty (nothing like the response in #2) and errors has the "Connection refused" error.

The server is a LAMP cluster running Ubuntu Server 9.04 with DRBD and Heartbeat.

What is driving me crazy is that this test.php works well in my laptop (LAMP on Ubuntu Desktop 9.04) and the server seems to have the ports already open and listening:

$ netstat -ntpl
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:4743            0.0.0.0:*               LISTEN      2326/openhpid   
tcp        0      0 0.0.0.0:3306            0.0.0.0:*               LISTEN      3364/mysqld     
tcp        0      0 0.0.0.0:2000            0.0.0.0:*               LISTEN      9510/nc         
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      3470/apache2    
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      2320/sshd       
tcp        0      0 127.0.0.1:3551          0.0.0.0:*               LISTEN      2354/apcupsd    
tcp6       0      0 :::22                   :::*                    LISTEN      2320/sshd

---

This is what I really want to archive: diagram (I don't have enogh points to insert images yet, lol)

• send the weight value from the truck scale to the server when the server requests it
• send raw text (epson escaped text format) to the printer's serial port

So in the client pc are constantly running two listening netcat connections, one for getting the weight and the other to print raw text.

Solution

Well, it was a permission problem after all... fixed editing /etc/sudoers with visudo to add:

www-data ALL = NOPASSWD: /bin/nc

Comments

[Istvan]

What happens if you are using the 127.0.0.1 as an ip instead of the localhost?

[coma]

it's not about the ip address, even a foreign ip give me the same problem.

[Admin]

Related: my Q&A on Unix & Linux Stack Exchange: "Connection refused" when I try to send a UDP packet with netcat on an embedded-linux board

[coma]

the local interface is up, in fact, "nc localhost 2000" and "nc 127.0.0.1" work in the console but they give me the error when runned in a php file with "exec" or "shell_exec".

[coma]

I'm going to give it the space between them but that works fine in the console, the problem is making that work from a php. If you know a better way to archive this please tell me!!!

[Dan Carley]

Could you explain the purpose of what you're doing?

[coma]

Of course!, let me edit the question to clarify the purpose of this.

[Bort]

This is horribly insecure. You just allowed anyone who breaks into your web application to listen on any port and worse yet run any application AS ROOT.

[coma]

Well, this was for an intranet app (in the middle of the colombian jungle btw, running by ethernet cables between only two pc's without internet) that we drop about one year ago, in fact this is from 2009 and we are in 2013, so thanks for the downvote dude.

[Bort]

None of that was in the question. The rest of the users reading this might try the same tactic and fall victim because they aren't in the middle of a jungle.