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Since few days, my dedicated server (Ubuntu 13.04) is no longer able to ping or connect to my NAS through DNS (hostname -> kr1.synology.me ) . The IP address ( 178.83.88.248 ) address against , is still reachable, I can ping to it, and also connect it via SSH.

Concretely I can do this :

sudo ssh root@178.83.88.248
sudo ping 178.83.88.248

But not this ( whereas before yes) :

sudo ssh root@example.com
sudo ping example.com

The problem is not from NAS side, because I can still reach it with root@kr1.synology.me from my local machine. The problem occurs only from my dedicated server.

Here are some options I did recently on my server:

  • I installed ufw and configured it to only allow some ports
  • I prevented the ability to connect as root in SSH
  • I prevented the ability to connect with SSH password (key required)
  • I installed and configured denyhosts and fail2ban
  • I installed rkhunter and chkrootkit  
  • I added no recursion & "Not Disclosed" to Bind9 configuration
  • I added nospoof to /etc/host.conf

Will you help me solve the problem? I spent several hours on it and can not clearly identify the cause.

  • ensure ufw is allowing port 53 udp (DNS), then fiddle with the bind9 configuration you added. You seem to know what you're doing, so I suggest you try reverting your changes one by one and see which one caused this. My hunch is it's one of those two since it's clearly a DNS issue. – roadmr Feb 2 '14 at 3:17
  • Add spoofalert to /etc/host.conf alerts are logged in syslog – user.dz Feb 2 '14 at 9:45
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Thanks to roadmr, I managed to solve the issue.

Indeed it was a problem related with bind9 configuration:

After I removed this two lines from the file "/etc/bind/named.conf.options", the issue was solved:

recursion no;
version "Not Disclosed";

I had added this two lines following an article which describe some tips to keep your server secured (point 6):

http://www.thefanclub.co.za/how-to/how-secure-ubuntu-1204-lts-server-part-1-basics

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