1. A FQDN
is of the form foo.bar.org. It has at least 2 parts, separated by a dot. Whether this must be "legal" depends on whether the machine with this name is going to be visible on the public Internet, where a scheme called the Domain Name System (DNS
) operates. If your machine needs to be accessible over the Internet, the way other machines will get there is by translating the FQDN into an IP address, using the DNS to do that translation. In that case, you need to be legal.
If, on the other hand, your server is attached to an isolated network which does not participate in the public DNS, you are free to come up with any FQDN you like. Beware, though, because if your isolated net eventually becomes attached to the public Internet, you will have to go back and change the FQDN for your server, and change any references to that name wherever they are (user's browser bookmarks, for example).
2. As with many things in this networked world, Samba is bound by the work of the Microsoft Empire. It needs a workgroup
to identify related machines that ought to be allowed free (or freer) access to each other's files, printers, and other services. This workgroup does not need to be related in any way the FQDN. It must, however, be the same for all the members who want to share info. Samba knows about a realm
only if it is involved in an Active Directory scheme for authenticating users. In that case, it uses the Kerberos realm as part of the user identification. More info is available on the implications of all this at http://www.samba.org.
3. Squid wants either the FQDN or the first part of it. Look at the documentation to decide which is wanted in a particular place.