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I have a static IP provided by the administrator of my Linux CentOS 6.5. The problem is that while I can ping this machine, connect to it and use it to access the internet from within my network, I can neither ping it nor otherwise access this IP from outside the network.

What is the problem? I have disabled the Firewall of CentOS and still can only use it from internal to external and not vice versa.

What should I do?

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    The reason might be there's another network firewall (being administered by your "administrator") that is blocking all pings to your network/subnet. Or, maybe there is no route from your "outside" machine to your CentOS. To further troubleshoot both of these, what happens if you ping from "outside" to another machine on the same "inside" network of your CentOS?
    – Alaa Ali
    Nov 14, 2014 at 2:02
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    Does the IP starts with 10.x.x.x, 172.16-31.x.x or 192.168.x.x? Dec 9, 2015 at 13:41
  • From the description you've given it is likely that your ip even if static is private, so it is normal that is not reachable from the outside the network. If you want to reach it from outside your administrator must configure the firewall/router to implemente appropriate routing rules.
    – LilloX
    Dec 9, 2015 at 13:42
  • Please show the output of "ip route" and "ip addr" on your CentOS server.
    – Liczyrzepa
    Feb 3, 2017 at 15:32
  • I proposed to close the question because while the user has been present, even recently, the question was never updated with additional informations requested and those informations are probably needed for a solution.
    – A.B
    Jan 5, 2019 at 16:54

3 Answers 3

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There can be a lot of causes to that issue:

  • NAT (ask your netadmin if there is NAT exemption for your IP address in the Firewalls)
  • ACL : ask your netadmin to check his access-lists to verify if traffic to your internal host is allowed.
  • Service filtering (again ask your netadmin to check if he/she allowed the services you need (icmp for ping, tcp/80 for http, tcp/443 for https, etc.)
  • System firewall : disable your firewall completely.

For further diagnostics, try to scan your host from the internal and the external networks with nmap, it can help by comparing the output..

I guess this covers 99% of the problems I've seen out there.

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If you tried above methods proposed and they are unsuccessful, this means that you're behind a firewall (smart switch/router or server) that are after your PC. The reason you're unable to ping your PC is that the ICMP traffic is blocked.

Do you have access to other services as Apache (port 80) for example?

In case your PC is still unreachable, there are two things that you can do:

1) Call the system administrator and ask to remove the filters applied to your IP (note that they'll probably ask you for specific ports so think that you need before write/call);

2) You can always use VPN to be visible to the world. Good solutions are openVPN for example or you can by one.

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In your firewall :

itptables --A INPUT -p icmp -j ACCEPT

This should be before any rejecting rules, in case you were using the ACCEPT policy in your firewall

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