Squid can only bind to low numbered ports (such as port 80) if it is started as root. Squid is normally started by your system's rc scripts when the machine boots. Since these scripts run as root, Squid is started as root at bootup time.
Once Squid has been started, however, there is no need to run it as root. Good security practice is to run programs as root only when it's absolutely necessary, and for this reason Squid changes user and group ID's once it has bound to the incoming network port.
The cache_effective_user and cache_effective_group tags tell Squid what ID's to change to. The Unix security system would be useless if it allowed all users to change their ID's at will, so Squid only attempts to change ID's if the main program is started as root.
If you do not have root access to the machine, and are thus not starting Squid as root, you can simply leave this option commented out. Squid will then run with whatever user ID starts the actual Squid binary.
As discussed in chapter 2, this book assumes that you have created both a squid user and a squid group on your cache machine. The above tags should thus both be set to "squid".
Example 4-1. Effective User and Group IDs
cache_effective_user squid cache_effective_group squid
Squid can act as a proxy server for various Internet protocols. The most commonly used protocol is HTTP, but the File Transfer Protocol (FTP) is still alive and well.
FTP was written for authenticated file transfer (it requires a username and password). To provide public access, a special account is created: the anonymous user. When you log into an FTP server you use this as your username. As a password you generally use your email address. Most browsers these days automatically enter a useless email address.
It's polite to give an address that works, though. If one of your users abuses a site, it allows the site admin get hold of you easily.
Squid allows you to set the email address that is used with the ftp_user tag. You should probably create a squid@yourdomain.example email address specifically for people to contact you on.
There is another reason to enter a proper address here: some servers require a real email address. For your proxy to log into these ftp servers you will have to enter a real email address here.