Linux – Unable to connect to Socket Server running on VPS!

javalinuxsocketvps

I Could need some help here…

I'm running a java socket server on a 64bit Linux VPS and listening on a port 5555 however my clients aren't able to connect to that port.
When I run a netstat -anp on the server it gives me local address of ::ffff:127.0.0.1:5555and foreign address of :::* and the state is LISTEN.

Is the ::ffff:in the local address part causing the problem?

Best Answer

That address is your localhost interface, not an external interface. I assume your clients are connecting from "outside" the server, correct? If so, you'll need to set your java app to listen on whatever IP address is assigned to an outward-facing interface.

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My guess is it's a firewall issue. To diagnose, either switch to root or use sudo to execute these commands described below:

Firstly, service iptables status will show you your current rules. Depending on how esoteric your firewall rules are, you may need to add them into the main question to debug but essentially you're looking for a line that accepts TCP port 8800 prior to a line that rejects all connections.

e.g.

Table: filter
Chain INPUT (policy ACCEPT)
num  target     prot opt source       destination
...
14   ACCEPT     tcp  --  0.0.0.0/0    0.0.0.0/0      state NEW tcp dpt:8800
15   REJECT     all  --  0.0.0.0/0    0.0.0.0/0      reject-with icmp-host-prohibited

If you don't have a REJECT line on your input chain, then the firewall is probably already turned off.

One way to get around this is obviously to turn off the firewall (service iptables stop) although this defeats the entire purpose of having a firewall.

If you need to add a new firewall rule, you have a couple of options:

Use the excellent Firewall Configuration tool (system-config-firewall) that ships with Fedora and can be found at the System->Administration->Firewall menu item in Gnome.
On the command line, insert a new rule before the REJECT rule by executing the following (uses numbers from my example above): iptables -I INPUT 15 -m state --state NEW -m tcp -p tcp --dport 8800 -j ACCEPT. NOTE that these changes are not permanent and will not persist beyond the next reboot.
Manually edit the /etc/sysconfig/iptables file and add a rule (-A INPUT -m state --state NEW -m tcp -p tcp --dport 8800 -j ACCEPT) to the firewall prior to the REJECT rule. Cycle the firewall service to pick up the changes via a service iptables restart. These changes will be applied every time the firewall service is started.

Ssh – Forward local port or socket file to remote socket file

Altough in the time, when the question was asked, it was really impossible, but it is possible nowadays.

You can to both: UNIX=>TCP and UNIX=>UNIX forwarding.

For example:

ssh \
  -R/var/run/mysql.sock:/var/run/mysql.sock \
  -R127.0.0.1:3306:/var/run/mysql.sock \
  somehost

It is possible since OpenSSH 6.7.

Best Answer

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Linux port not accessible from remote machine

Ssh – Forward local port or socket file to remote socket file

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