Hostname "Chicken and Egg"

[Rick Stevens]

Rick Stevens wrote:
> 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!

Actually, "." means "source", or "run the script IN THIS SHELL" so that
variables set will also be set for this process.  My second statement
about "sh" is wrong, as the variables set in the script would be set
only for the subshell and the current shell still won't know about them.

Now, I'm getting punchy!
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-   To err is human.  To forgive, a large sum of money is needed.    -
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