I have recently upgraded my cluster from 11.10 to 12.04. After the upgrade, I am having trouble running apt-get on the cluster nodes. I can ssh between the nodes (client-to-client; client-to-head; client-to-external etc.). However, sudo apt-get update produces the following errors:

Ign http://us.archive.ubuntu.com precise InRelease
Ign http://security.ubuntu.com precise-security InRelease
Ign http://www.openfoam.org maverick InRelease
Ign http://us.archive.ubuntu.com precise-updates InRelease
Err http://security.ubuntu.com precise-security Release.gpg
Something wicked happened resolving 'security.ubuntu.com:http' (-5 - No address associated with hostname) Err http://www.openfoam.org maverick Release.gpg
Something wicked happened resolving 'www.openfoam.org:http' (-5 - No address associated with hostname) Ign http://us.archive.ubuntu.com precise-backports InRelease
Ign http://www.openfoam.org maverick Release
Ign http://security.ubuntu.com precise-security Release
Err http://us.archive.ubuntu.com precise Release.gpg
Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname) Ign http://security.ubuntu.com precise-security/main Sources/DiffIndex
Err http://us.archive.ubuntu.com precise-updates Release.gpg
Something wicked happened resolving 'us.archive.ubuntu.com:http' (-5 - No address associated with hostname) Ign http://security.ubuntu.com precise-security/restricted Sources/DiffIndex
15% [Connecting to us.archive.ubuntu.com] [Connecting to security.ubuntu.com] [Connecting to www.openfoam.org]

On the headnode, I have in /etc/network/iterfaces:

auto eth0 iface eth0 inet static address 192.168.0.1/24

On the client nodes, I have /etc/network/iterfaces: auto eth0 iface eth0 inet static address 192.168.0.101 netmask 255.255.255.0 gateway 192.168.0.1

Please advise.