I'm wondering what are the drawbacks, from a security point of view, of having more than one service (let's say the web server, the mail server and the DNS server) on the same machine, rather than having those services split on more machines?
We can also assume I have a backup DNS and mail server, so if the machine goes down, I can switch to the backup services.
EDIT:
What are the pros and cons of having these services on different machines? Are the advantages of the second solution (services split) whorthwhile for spending more money and time (properly configuring those machines) on splitting those services on different machines?