CentOS - Apr 2012 - 6.2 How to change hostname for each individual IP

Good day,

I was wondering if there was a way to change the hostname for each 
individual IP address allocated to a system. For example if only one NIC 
card is being used (single port) and the hostname of the machine is 
"example.example.org", this hostname is being displayed for all 
applications no matter what IP I set the application to BIND to. My 
server has multiple IP addresses setup as an ALIAS and ive tried setting 
up ptr's, A records, and even specifying the new hostname in /etc/hosts. 
No matter what I do when I connect to the application it read 
"example.example.org" which is the hostname of the machine rather then
the hostname I want it to read on a particular IP. Is this possible to 
change?

On 04/06/2012 08:18 AM, Jonathan Vomacka wrote:
> Good day,
>
> I was wondering if there was a way to change the hostname for each 
> individual IP address allocated to a system. For example if only one NIC 
> card is being used (single port) and the hostname of the machine is 
> "example.example.org", this hostname is being displayed for all 
> applications no matter what IP I set the application to BIND to. My 
> server has multiple IP addresses setup as an ALIAS and ive tried setting 
> up ptr's, A records, and even specifying the new hostname in
/etc/hosts.
> No matter what I do when I connect to the application it read 
> "example.example.org" which is the hostname of the machine rather
then
> the hostname I want it to read on a particular IP. Is this possible to 
> change?
 
If you are talking about when you ssh to the machine (or for anything
else that would use the hostname variable that it gets from the machine)
then I do not know of a way to make it be different based on the IP that
you connect to.

If you are using something that does DNS names, then if you setup the
proper forward and reverse records it will work.

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On 4/6/2012 11:18 AM, Johnny Hughes wrote:
> On 04/06/2012 08:18 AM, Jonathan Vomacka wrote:
>> Good day,
>>
>> I was wondering if there was a way to change the hostname for each
>> individual IP address allocated to a system. For example if only one
NIC
>> card is being used (single port) and the hostname of the machine is
>> "example.example.org", this hostname is being displayed for
all
>> applications no matter what IP I set the application to BIND to. My
>> server has multiple IP addresses setup as an ALIAS and ive tried
setting
>> up ptr's, A records, and even specifying the new hostname in
/etc/hosts.
>> No matter what I do when I connect to the application it read
>> "example.example.org" which is the hostname of the machine
rather then
>> the hostname I want it to read on a particular IP. Is this possible to
>> change?
>
> If you are talking about when you ssh to the machine (or for anything
> else that would use the hostname variable that it gets from the machine)
> then I do not know of a way to make it be different based on the IP that
> you connect to.
>
> If you are using something that does DNS names, then if you setup the
> proper forward and reverse records it will work.
>
I am referring to when binding one of the IP alias to an application. 
For example if I take any application installed on my server and tell it 
to connect to another source from a specific IP alias, or listen on a 
specific alias, there are issues with the hostname. When an external 
source does a lookup, it comes with the server's hostname instead of the 
hostname I want to show up for that specific IP. Do you know why this 
happens?

I've already edited /etc/hosts to point the IP to a specific hostname.
I've already added a proper PTR and A record for the IP address which 
match. Is there anything else I need to do at this point?

On Friday, April 06, 2012 09:18:27 AM Jonathan Vomacka
wrote:
> No matter what I do when I connect to the application it read 
> "example.example.org" which is the hostname of the machine rather
then
> the hostname I want it to read on a particular IP. Is this possible to 
> change?
Well, it probably isn't.  Hostnames should not be confused with FQDN's
that map to IP addresses; they are different (even though CentOS will by default
grab its hostname from reverse DNS if it gets its main interface's IP
address by DHCP (in the case of multiple interfaces, there is a preferred
interface that will assign the hostname)).

Hostnames belong to the host OS.  FQDN's belong to their respective IP
addresses; and IP addresses belong to their respective interfaces, not to the
host.  I know that's splitting hairs, but you're hitting the very corner
case that splits this particular hair.  The host itself does not have an IP
address.  Even in the case of a non-localhost-addressed loopback, the IP address
belongs to the loopback interface, not to the host.

This has been true for a very long time;  I've run multiple alias interfaces
since kernel 2.0 days on Red Hat Linux 4.1 (not RHEL, but old RHL), and this has
been the behavior for at least that long.

A host can only have one name (this is true for basically any IP host, whether
it's a Linux system, a BSD system, a Windows system, or a Cisco router. 
Especially on a router with a lot of interfaces (broadband aggregation routers,
for instance, can have thousands of interfaces with each one having a unique IP)
you don't want the name of the IP associated with the interface to override
the hostname.  And the hostname does not have to match what DNS says about the
FQDN that belongs to any interface on the system (I have a few of those, too).

Now, I can't quote RFC 'chapter and verse' on this, but I have never
seen a system where you could do what you're describing.  (that doesn't
mean they don't exist, just that I've not seen one in my limited
experience of AT&T Unix SVR2, Xenix V7 and SIII, Apollo DomainOS, and
Solaris).