Linux port not accessible from remote machine

linux

have two machines -windows and linux present locally.
I am trying to run remote debugging (using eclipse) on windows machine to a tomcat application on linux box(fedora).
I have enabled a debugging port 8800 on linux box and the startup log says “listening for transport dta_socket at 8800”
Now when I try to connect my eclipse to it, I get “Failed to connect to remote VM. Connection refused.”

Here is the port description on linux

lsof -i:8800

COMMAND PID USER FD TYPE DEVICE
SIZE/OFF NODE NAME

java 21930 root 4u IPv4 59436 0t0 TCP
*:sunwebadmin (LISTEN)

netstat -tulpn

Proto Recv-Q Send-Q Local Address
Foreign Address State PID/Program name

tcp 0 0 0.0.0.0:8800 0.0.0.0:* LISTEN
21930/java

when i hit the url http://{linux-ip}:8800 from either machine, Tomcat logs says
Debugger failed to attach : Handshake failed

Obviously the port is not accessible hence the debugger is not able to connect.
Please suggest, what steps to follow. Any pointers would be helpful too.

Best Answer

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:

  1. 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.

  2. 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.

  3. 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.

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