Hopefully, you won't need to worry about the visible_hostname directive. However, you'll need to set it if Squid can't figure out the hostname of the machine on which it is running. When this happens, Squid complains and refuses to run:
% squid -Nd1 FATAL: Could not determine fully qualified hostname. Please set 'visible_hostname'
Squid wants to be sure about its hostname for a number of reasons:
The hostname appears in Squid's error messages. This helps users identify the source of potential problems.
The hostname appears in the HTTP Via header of cache misses that Squid forwards. When the request arrives at the origin server, the Via header contains a list of all proxies involved in the transaction. Squid also uses the Via header to detect forwarding loops. I'll talk about forwarding loops in Chapter 10.
Squid uses internal URLs for certain things, such as the icons for FTP directory listings. When Squid generates an HTML page for an FTP directory, it inserts embedded images for little icons that indicate the type of each file in the directory. The icon URLs contain the cache's hostname so that web browsers request them directly from Squid.
Each HTTP reply from Squid includes an X-Cache header. This isn't an official HTTP header. Rather, it is an extension header that indicates if the response was a cache hit or a cache miss. Since requests and responses may flow through more than one cache, each X-Cache header includes the name of the cache reporting hit or miss. Here's a sample response that passed through two caches:
HTTP/1.0 200 OK Date: Mon, 29 Sep 2003 22:57:23 GMT Content-type: text/html Content-length: 733 X-Cache: HIT from bo2.us.ircache.net X-Cache: MISS from bo1.us.ircache.net
Squid tries to figure out the hostname automatically at startup. First it calls the gethostname( ) function, which usually returns the correct hostname. Next, Squid attempts a DNS lookup on the hostname with gethostbyname( ). This function typically returns both IP addresses and the canonical name for the system. If gethostbyname( ) succeeds, Squid uses the canonical name in error messages, Via headers, etc.
Squid may be unable to determine its fully qualified hostname for a number of reasons, including:
The hostname may not be set.
The hostname may be missing from the DNS zone or /etc/hosts files.
The Squid system's DNS client configuration may be incorrect or missing. On Unix, you should check the /etc/resolv.conf and /etc/host.conf files.
If you see the fatal message mentioned previously, you need either to fix the hostname and DNS information or explicitly configure the hostname for Squid. In most cases, it is sufficient to ensure the hostname command returns a fully qualified hostname and add an entry to /etc/hosts. If that doesn't work, just set the visible hostname in squid.conf: