Digital Ocean – DNS Issues & Kernel Issues

Here’s a short story, partially still in progress. With all the security breaches going on I thought for a few moments that I might have been caught up in one of them. This past Thursday night I noticed that both my virtual servers, which are hosted by Digital Ocean, were very slow to login via SSH, I mean extremely slow on the order of 8 to 10 seconds. Accessing the WordPress installation on one of those virtual servers was also extremely slow. I began to fear that one of my servers had fallen victim to some vulnerability or exploit and because I have shared ssh keys that might mean that both servers could have possibly been accessed without my knowledge or permission.

I checked that both DNS servers were responding and resolving queries;

/etc/resolv.conf:
 nameserver 4.2.2.2
 nameserver 8.8.8.8

I was able to resolve www.google.com from both 4.2.2.2 as well as 8.8.8.8 so I initially thought the problem wasn’t with DNS. However, when I was unable to figure out where the problem was I decided to disable DNS resolution within SSH by placing the following statement in /etc/ssh/sshd_config;

UseDNS no

A quick restart of sshd (service sshd restart) and the problem seemed to be resolved.. but what had fixed it?

I went back to the /etc/resolv.conf file and decided to change the order of the DNS servers, placing Google’s DNS server ahead of Level3 and adding a second Google DNS server into the mix. Digital Ocean doesn’t maintain their own DNS infrastructure which is somewhat surprising for such a large provider.

/etc/resolv.conf
 nameserver 8.8.4.4
 nameserver 8.8.8.8
 nameserver 4.2.2.2

And to my surprise when I went back to test the problem was gone? So was there some problem with 4.2.2.2?

With that problem partially explained I decided to apply the latest and greatest patches and security updates for CentOS 6.5. And there I also ran into any problem… upon rebooting I found that the OS was unable load the driver for the network interface eth0, returning the error;

FATAL: Could not load /lib/modules/2.6.32-358.6.2.el6.x86_64/modules.dep

I was able to quickly change my kernel version via the Digital Ocean control panel to 2.6.32-431.20.3.el6.x86_64 and reboot.

With all that done I turned my attention toward my WordPress administration which was still extremely slow.  I had been guessing that the problem was probably related to some plugin that I recently updated so I went back and disabled all the plugins to see if I could possibly find the one that was causing the slow down. That search proved fruitless so I turn my attention to the performance of Nginx, PHP-FPM and MySQL, After optimizing the tables and some of the configurations within MySQL I found the response of the WordPress Admin portal better (3-5 seconds) but there’s still a problem somewhere there that I need to track down.

Here are a few resources if you are struggling with tunning your MySQL instance;

• MySQL Tuning Primer Script by Matthew Montgomery
• MySQLTuner-perl by Major Hayden
• mysqlfragfinder by Phil Dufault

It’s never a boring day blogging on the Internet.

Cheers!