I ran across a curious networking problem last week.
I have a few virtualizations running Ubuntu 12.04, that I keep that run a few development servers of various things. I recently gave one of them a static IP address so I could more easily access it in the near future, from wherever in my network I happen to be. This server was headless (no GUI), so I used my go-to method for setting a static IP address. That is, I opened up the network interfaces config file in
/etc/network/interfaces, and modified it to make it look something like this:
auto eth0 iface eth0 inet static address 10.0.0.100 netmask 255.255.255.0 gateway 10.0.0.1
This is exactly the protocol outlined in https://help.ubuntu.com/12.04/serverguide/network-configuration.html, in the section titled "Static IP Address Assignment," if you want to follow along. After a quick networking restart, I'm done! I've successfully got a brand new static IP. I ssh into my virtualization, and get to work!
Now for the twist!
Later that day, while running an innocuous command via ssh, I get a broken pipe! My ssh session is broken! I soon find out that my 12.04 box has reacquired an IP address dynamically.
For some background, you should know that the router I was using does have dhcp running, and will hand out dynamic IPs when requested, to a specified range. So for example, everything over .128, which is where the new IP address was located. I also happen to know that the IP address the 12.04 box had was not being used by anything else; it was free, so nothing was fighting over it.
I've ran into this problem before, so I knew what to do. Unintuitively, even if you set a static IP in this config file, which I would think would be authoritative, it won't stick in Ubuntu if you still have a dhcp client installed. I think I've ran into this in older versions of Ubuntu as well. I am unaware though, if later versions have changed this.
There are a few variants of the dhcp client. I don't need them, so my quick fix is simply to remove them. So to catch them and set the static IP once and for all:
change your config file sudo apt-get remove dhcp-client isc-dhcp-client dhcp3-client sudo /etc/init.d/networking restart
Another link that explains how to set up a static IP in a very similar way, and includes this important step as well as http://www.howtogeek.com/howto/ubuntu/change-ubuntu-server-from-dhcp-to-a-static-ip-address/.