Glenn English wrote:
> Let's see if I've got this straight...
>
> Debian squeeze gets the host's domain name from the first
> non-comment or non-empty line of /etc/hosts?? If it likes
> that line??
No. That is incorrect. Debian sets the hostname from /etc/hostname.
This is done at boot time in the /etc/init.d/hostname.sh script. This
is called during single user boot time from /etc/rcS.d/S??hostname.sh
symlink which will have a system dependent ordering number.
Once hostname is set using the sethostname(2) kernel system call
man 2 sethostname
then later the hostname may be queried using gethostname(2), most often
by programs but also by scripts using the 'hostname' program.
> Not from /etc/hostname
Yes from /etc/hostname. As described above.
> and not from the "kernel.domainname = "
> line in /etc/sysctl.conf? ("kernel.domainname = example.com" is
> that line, commented out, in my recently installed squeeze.)
Definitely not.
> And not from /etc/resolv.conf? Or does it rely on DNS?
DNS is used for the ill conceived 'hostname -f' option. Whoever wrote
that code should be stripped of any street cred. It is terrible.
Original Unix systems did not have that option. I can't tell you how
many times I have rescued systems that had their hostname *set* to
"-f" because someone ran "hostname -f" on them. Blech.
The way 'hostname -f' works is by looking for the first IP address it
can find associated with the first network interface that it can
find. Note that the system may have many of both. But with the first
one it finds it tries to do a DNS reverse lookup of the IP address.
If that name is found in DNS then it reports that name as the fully
qualified domain name. That may be okay for a very large number of
typical systems but it is also completely incorrect for many valid
systems with more IP addresses than just one and more network
interfaces than just one.
Note that a perfectly valid configuration may specify the hostname as
a fully qualified domain name. Many BSD systems are set that way.
And BSD is the progenitor of networking. So I have that a lot of good
weight. In which case if the hostname is set to a fully qualified
domain name then there is no need for a DNS lookup and none is done.
I always used to set hostnames to be FQDNs. It works great for single
domain systems. But these days I have so many multi-domain systems
that I don't do that anymore.
Hosts have network interfaces. Perhaps zero of them. Perhaps one.
Perhaps two. Perhaps many network interfaces.
Network interfaces have IP addresses. Perhaps zero of them. Perhaps
one of them. Perhaps many IP addresses.
IP addresses have reverse DNS names associated with them. PTR
records. They should have only one. But they might actually have
many PTR records for any given IP address. When looking up an IP
address it is possible that you will get multiple names returned.
As you can see the above tree of information might be larger than the
simple one-to-one mapping that the hacks that created 'hostname -f'
planned for when they added that feature. Sometimes much larger.
> I needed to change a domain name this morning on a computer
> I'm working on, and I was told to do those different things
> from several different websites. Setting it in /etc/hosts seems
> to have worked.
If all you changed was /etc/hosts then it is likely that your change
is incomplete.
Also, what mail transfer agent are you using? Postfix? Exim? Almost
certainly one of those will need a tweak too.
> Why is this trivial task so obscure?
Because it pulls together information from several different and
independent programs that are not related to each other except by all
running on the same machine.
> I must be missing something big time. hostname was coming up with
> the right answer, but hostname -f kept saying "Name or service not
> known". Can anyone tell me what I've got wrong? And the Debian way
> of setting the domain name? (It's working now, AFAIK, but I'd like
> to set it correctly...)
* /etc/hostname
Debian uses /etc/hostname to set the network and it is called from the
/etc/init.d/hostname.sh script. Red Hat uses /etc/sysconfig/network
to set the HOSTNAME=something.example.com variable called from
someplace different in their startup sequence. So at the very start
you need to change the appropriate file. Debian encourages using only
the short name but using the FQDN as the hostname is okay.
* /etc/hosts
Many modern systems associate the hostname with 127.0.1.1 instead of
either 127.0.0.1 or a public IP address such as 192.0.43.10. This is
to solve a problem. Mobile devices typically do not always have
networking enabled. Even desktops may have networking disabled for at
least some of the time. If a public address such as 192.0.43.10 is
used then programs such as mailer transfer agents and dns resolvers
and others have errors while the network is offline. So using the
public address in /etc/hosts is problematic. (Although perfectly fine
for servers and desktops that are always online.)
And alternatively using 127.0.0.1 is problematic because then instead
of the desired name 'hostname -f' would return "localhost" or possibly
"localhost.localdomain" depending upon what is in /etc/hosts. And if
the host is exchanging information such as hostnames with peers you
definitely want to avoid passing localhost as the hostname but want it
to use a name that will map through dns back to the public IP. So in
the end associating 127.0.1.1 for the local host name in
/etc/hosts really makes a lot of sense.
The localhost.localdomain is a hack / trick to make all of the system
configuration internally consistent with a private configuration in
isolation from a public network. That's great. But if you have a
public name and IP address then you would use it instead.
* /etc/mailname
This is a Debian specific file that is intended for configuring the
mail transport agent in a generic way. It can be used for either exim
or postfix.
* /etc/postfix/main.cf
I really only know Postfix so will only talk about it. Upstream
Postfix expects the hostname to be a FQDN. If it is then that fine
and no further configuration is needed. But if the hostname is a
short name then postfix needs to be told about it. Postfix uses it to
determine the domain name. Debian has patched Postfix so that it may,
if configured to do so, use /etc/mailname to set the myorigin
variable. This mostly works but I find it better to simply set
myhostname to the FQDN. So edit that file and set the myhostname
variable to the FQDN.
If you are using Exim then you will need to understand what needs to
be done for Exim.
* /etc/resolv.conf
This file controls setting the nameservers and the domain search
path. There are some other unnecessary things that can be set there
too. I recommend using the 'resolvconf' package and then letting it
configure /etc/resolv.conf. I also think you should run a local
caching nameserver too. Assuming that you would have two lines in
this file.
nameserver 127.0.0.1
search example.com subdomain.example.com
You might also see "0.0.0.0" used there as a alias for 127.0.0.1. The
reason is an obscure bug long fixed in older systems. Now it doesn't
matter.
The search line does two things. The first entry on the line is used
as the default domain name. And the entire list is used to search
when doing a dns lookup. So setting the default domain name as the
first field of the search line will set the system's current domain
name, if the hostname isn't a FQDN already, in which case the value
there is highest priority.
Some guides will have you set "domain example.com" too. That is
fine. But it is the older part of the configuration which has been
replaced by the search configuration. Since the search configuration
supersedes the domain configuration I always prefer to set search only
and nothing else. Setting both isn't DRY (don't repeat yourself) and
redundant. It is almost always better to set configuration in only
one place and not multiple places.
And that is pretty much all that is needed to rename a system to a new
hostname. I reboot to ensure that all daemons are restarted using the
new hostname. But strictly speaking that isn't needed if everything
is restarted.
If you have questions then ask. The above was simply an off the top
of the head description and I am sure I didn't do a great job of it.
Good luck!
Bob
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