Glenn English wrote: > What happens, apparently, is that nothing ever sets the > domain name at boot.
But a domain name isn't really important to the server kernel itself. A domain name is a piece of information that *others* need. But it isn't needed nor really used by the local server. The kernel doesn't care about it at all. > When the kernel wants an FQDN, This is a very important point and it is worth exploring. When do you think the kernel ever wants a FQDN? Please let's talk about it and I am sure we will find that this has nothing to do with the kernel. The number one piece of software on a system that needs to know the domain name is the mail transfer agent. Usually Postfix or Exim but could be Sendmail or others. They need to know this to generate a reply address, the "From:" address, so that others can reply to the email. > it does a machine-name lookup from /etc/hostname No. That file is only used at boot time to set the hostname. It is distro specific. Other systems use different files. > then looks in /etc/hosts for the machine-name. Whether a dns lookup checks /etc/hosts or not is dependent upon the configuration in /etc/nsswitch.conf (network services switch) which would normally list this: hosts: files dns But if avahi is installed will list some other problematic items that I am going to ignore for now. Also NIS/YP if installed will add other search locations. But given the above setting it will look in /etc/hosts first and then if not found will continue to do a dns lookup second. > And it expects to find the machine-name and the FQDN, on one > line. Maybe near the top -- I haven't looked into that. The format of the /etc/hosts file is a line by line linear search from top to bottom for the desired information. Reading of the file stops as soon as the data requested is found. As far as I know the kernel never needs to know the domain name and the kernel never does a dns lookup. The kernel only holds the value of the hostname. You can set the value. You can retrieve the value. And that is the entire involvement of the kernel in the issue. Random userland application programs want to know the domain name (such as the mail transfer agent) and these non-kernel userland programs will do various random things such as doing dns lookups trying to deduce a value for it. But they are not the kernel. Postfix for example gets the hostname and uses it to set its configuration for the myhostname and mydomainname variables that it uses when writing mail headers. If the hostname contains a FQDN then that string is used to set those variables. If not then it sets mydomain to "localdomain". Since these days it is encouraged to set the hostname to a short name without any FQDN since a host might be operating in multiple domains it means that most commonly postfix won't have a FQDN to get that information from and will default to "localdomain". Since that isn't desirable I usually go ahead and set the Postfix myhostname variable to the FQDN explicitly so that it can extract both myhostname and mydomainname as desired. (I didn't say "correctly" since there are many valid configuration possible and it is a judgement as to which is best. But as desired in my case means as I desire it. [smile]) > And if this doesn't work, it goes to DNS for a reverse look > up the of IP. If the DNS lookup returns something with a > machine-name that doesn't match /etc/hostname, it returns > an error. Yes. > I think this is how it works. From the futzing I've done, > that seems to at least be close to what happens... I just want to note that I wrote an explanation of this in my other reply but it appears that you did not read it? > This all strikes me as a little complex, but it works, and > there aren't several places where there an admin could put > a wrong domain name. And it doesn't happen too often, so I > guess it's OK. I could definitely see some improved documentation to help people understand all of the concepts. It seems perfect for a wiki page. > Sure would be nice, though, if this were clearly and simply > laid out in some documentation somewhere. I found lots of > places that said that the domain name is *not* to be stored > in /etc/hostname, but it was difficult to find where it *is* > to be stored... It is perfectly fine to put a FQDN in /etc/hostname. It is how a lot of machines are configured. However Debian discourages it. It is a different way of thinking. For example these days a machine might be serving web pages for a dozen different web sites all in different domains. The server system itself would be operating in several domains. Send it a request for an unconfigured domain and by default it will send back and error page that will often tell us the underlying hostname and domain. (For example sending a request to the IP address of www.debian.org but with a different hostname will tell you that it is running on "senfl" and nicely a pointer to more information about it since Debian is such an open project. Other sites do different things.) So forcing a single overriding domain upon everything running there is a little restrictive. A person might feel claustrophobic! Therefore thinking about things differently, not using a FQDN as the hostname, and configuring software to be more multidomain aware is a more general purpose way of thinking about multi-use systems. I think that is why Debian discourages using the FQDN as the hostname. But that doesn't mean that doing so anyway is incorrect. Also a lot of upstream software (such as Postfix) expects the hostname to be a FQDN. Bob
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