clustering services on linux

I would use a distribution that has clustering services as part of the package, such as CentOS. It is a rebuild of Red Hat Enterprise Linux, so there is plenty of documentation available, including the red hat site. To use HA clustering, you will need some sort of shared storage, so when one head fails the service can still see its files. For example, your user's home directories. Rather than replicate the data from one server to the other, shared SCSI/iSCSI/Fiber Channel disks are available to both servers. You can either use a standard format (ext3, reiser, etc) and ensure it is only mounted on one head at a time (beware file corruption if both heads have simultaneous access) or use GFS, Red Hat's Global File System, which is also available in the CentOS build. GFS handles the locking to ensure file integrity, so both heads can access the same partition at the same time. Check out Red Hat's documentation on their site to get an idea of what is involved.

If you speak about SMTP/DNS/HTTP, in the most cases you *don't need* HA cluster - you can use several MX records for mail servers and so on.

RHEL (and CentOS) provides Red Hat Cluster Suite, which provides you with HA framework. Read for more : http://www.redhat.com/docs/manuals/csgfs/.