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my server is configured to have a public ip on one network card, and a private ip on the other network card... the server was running fine after some time... suddenly i'm unable to access my server from outside my isp network...

at first i thought it's because i recently changed my ip address, until the same thing happen to 3 other servers (that i'm in charged to) on totally different network... one is running with the same isp (same network as my previous server), one is running with different isp (not even in the same country), and the other one is running with different isp (same country)...

the conditions i checked :

from outside the server isp network:

  • able to ping to the server
  • traceroute to the server has no problem
  • unable to access the webserver
  • unable to access the ssh
  • unable to access the sftp

from inside the server isp network (using public ip):

  • able to ping to the server
  • traceroute to the server has no problem
  • able to access the webserver
  • able to access the ssh
  • able to access the sftp

there's no firewall set on the servers... is there anything i should check? i'm also asking to my isp as i put this question... this also happens only on occasion...

2 Answers 2

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  • able to ping to the server
  • traceroute to the server has no problem
  • unable to access the webserver
  • unable to access the ssh
  • unable to access the sftp

Is it possible that there's some kind of IP address conflict or routing issue, and that you're not actually pinging your server, but a different machine that is using your IP? Try do to a port scan + OS detection with nmap:

nmap -O <IP or hostname of your server>

and compare the results - how does the output change when the issue occurs?

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  • nothing changes... Sep 16, 2013 at 4:03
  • nothing changes... i took 4 test with nmap, 1. when occurred, from outside isp network 2. when occurred, from inside isp network 3. when normal, from outside isp network 4. when normal, from inside isp network the only change is when accessed from outside or inside isp network... from inside it shows the correct port opened with os detail showed as Linux 2.6.13 - 2.6.32... and when accessed from outside shows extra port (some are not opened at the server, but some are) with os detail showed as Linux 2.6.15 - 2.6.26 (likely embedded)... both are the same when occurred or not... Sep 16, 2013 at 4:13
  • Sounds like there is a router between you and your server that's causing these issues. Sep 16, 2013 at 7:43
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turns out there's a problem with the routing...

the server accepts the incoming packages, but when replying it uses a wrong address...

there are 2 default route on the server, one directly to the isp, and the other one to the local server...

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