Hostname "Chicken and Egg"

[Rick Stevens]

Bob Kinney wrote:
> Happy Holidays all--
> 
> I'm running RH9, and recently switched from being directly connected to
> the internet (dhcp giving me a publicly routed address) to being behind
> a router.
> 
> I reconfigured my IP address to be a static, private one now, and my 
> hostname now comes up as "localhost.localdomain".  I'm trying to name the
> host "vectra".
> 
> Looking at the man page for hostname, it says that:
> " The  host  name  is   usually   set   once   at   system   startup   in
>        /etc/rc.d/rc.inet1  or  /etc/init.d/boot  (normally by reading the con-
>        tents of a file which contains the host name, e.g.  /etc/hostname)."
> 
> Although the files mentioned aren't there, I found in /etc/rc.sysinit
> a section that seems to set the hostname by *calling* /bin/hostname.  
>  >>
> HOSTNAME=`/bin/hostname`
> if [ -f /etc/sysconfig/network ]; then
>     . /etc/sysconfig/network
> else
>     NETWORKING=no
> fi
> if [ -z "$HOSTNAME" -o "$HOSTNAME" = "(none)" ]; then
>     HOSTNAME=localhost
> fi

Fine.  Yes, it runs /bin/hostname and /bin/hostname will return null if
the hostname hasn't been set yet.  The next bit tests to see if the file
"/etc/sysconfig/network" exists.  If it does, then it is executed.  If
not, the variable NETWORKING is set to "no".

As you can see by this stuff you quoted:

[root at localhost init.d]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=localhost.localdomain
[root at localhost init.d]#

running /etc/sysconfig/network will set NETWORKING to yes and set the
hostname to "localhost.localdomain".

The next test says "if the variable HOSTNAME is null (zero length) OR
if HOSTNAME contains the string "(none)", then set HOSTNAME to
localhost".

> It would seem that the hostname is being set either by the
> /etc/sysconfig/network file, or by the fallout case listed in
> /etc/rc.sysinit (if -z $HOSTNAME ...).

That's exactly right.

> I don't know why the hostname command would fail:
> 
> [root at localhost init.d]# cat /etc/hostname
> vectra
> [root at localhost init.d]#

Because that's now how things are set up.  If you follow the rest of the
startup sequence, in the /etc/rc.d/init.d/network bit, it checks to see
if /etc/sysconfig/network exists, and if it does, runs it.  It then
checks to see if NETWORKING is set to "yes".  If so, it launches the
necessary /etc/sysconfig/network-scripts/ifcfg-* scripts to bring the
network interfaces up.  It then calls "/sbin/hostname $HOSTNAME" to
set the host name.

Wow!  Lots of network questions this week!

Now, as for setting things up, your hostname IS set by the "HOSTNAME="
bit of /etc/sysconfig/network.  If, however, you use DHCP, your DHCP
server can replace your hostname when it hands you your IP address,
route and DNS servers.

If you DON'T use DHCP, then whatever you put in /etc/sysconfig/network
sticks.  The problem is, you may not have your IP address associated to
your hostname and the odds are that it won't be in a DNS server
anywhere, either, since you're on a private network.  So things like "X"
won't work anymore.

How do you fix that?  For a fixed IP, go ahead and put your FQDN in
the /etc/sysconfig/network's "HOSTNAME=" section (by FQDN, I mean
your hostname.yourdomainname.tld,, in your case "vectra.mydomain.com").
THEN edit /etc/hosts and add a line that contains your fixed IP, a tab,
your FQDN, a tab, then your hostname, e.g.

192.168.0.10	vectra.mydomain.com	vectra

By putting this in /etc/hosts, the system never asks DNS for the IP
associated with your hostname--it has it in its local tables.  X will
start normally and life will be good again.  ;-)

> Also, what does the line ". /etc/sysconfig/network" do?
> Is "." (dot) a command?

Well, yes.  It actually means "run the shell on this script".  The
line:

	. /etc/sysconfig/network

is the same as saying

	sh /etc/sysconfig/network

Necessary because /etc/sysconfig/network doesn't have the execute bit
set, nor does it have "#!/bin/bash" or "#!/bin/sh" as the first line.

Whew!  I've been busy today!
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-   "I was remembering the immortal words of Socrates when he said,  -
-   'I drank what?'"                 -- Val Kilmer in "Real Genius"  -
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