Usage/Applicability of IP Alias for Single Node machines??

Is it possible to have IP aliases for Single node machines? I assume,
it is possible. But, I would like to know whether it is really useful
to have IP aliases for Single nodes.
Whenever there is a node failure, IP aliases will help for Transparent
Fail-over in Cluster systems. But, I am really not sure how exactly it
will help in case of Single node machines. Could anybody clarify?

Thanks!

qazmlp wrote:
> Is it possible to have IP aliases for Single node machines? I assume,
> it is possible. But, I would like to know whether it is really useful
> to have IP aliases for Single nodes.
> Whenever there is a node failure, IP aliases will help for
Transparent
> Fail-over in Cluster systems. But, I am really not sure how exactly
it
> will help in case of Single node machines. Could anybody clarify?
>
> Thanks!

Not sure what you mean by "single node" machine -- workstation?
dedicated server(s)? single as opposed to multi homed host?

Of course, an IP alias just means that the machine will listen for more
than one IP (per nic). Linux is very "flexible" this way -- does the
same thing when two nics on separte subnets are used. Anyway, this can
cause arp problems as you can imagine -- especially in failover usage.

There _were_ uses for an IP alias. The most common use in the past
related primarily to Apache virtual hosts and to receive e-mail on the
aliased IP #s -- both via rewrite rules. It can also be used to
provide for failover/balancing in a flakey fashion. None of these uses
are required these days (IMO) -- better behaved and more reliable means
exist to address these and similar problems.

The failover/balancing use is the "hack" I'm most familiar with these
days. It's usually part of a round robin dns load balancing effort.
It involves two machines prepared to listen for the same IP

Here's a simple example:
http://www.onlamp.com/pub/a/onlamp/2...inuxhacks.html

Not very pretty, as you can see.

I've also read where some people have resorted to ip aliases due to
co-location restraints -- must be really fun to administer and maintain


hth,
prg
email above disabled

On 5 Jan 2005 07:41:28 -0800, qazmlp wrote:

> Is it possible to have IP aliases for Single node machines? I assume,
> it is possible. But, I would like to know whether it is really useful
> to have IP aliases for Single nodes.
> Whenever there is a node failure, IP aliases will help for Transparent
> Fail-over in Cluster systems. But, I am really not sure how exactly it
> will help in case of Single node machines. Could anybody clarify?
>
> Thanks!

It is possible.
On many systems ifconfig will accept an 'alias' parameter to achieve this.
As for why you would want to do it, well think of all the reasons you may
want to look like two (or more) machines on the network but only have one
physical connection.
There aren't that many these days because NIC's and switch ports are cheap,
but it is still usefully if one is changing the IP address of a machine and
wants to accept connections on the old address for a while during the
changeover.
Followups set to comp.unix.admin as this is a general unix question a Linux
or Solaris one.
Adam

"qazmlp" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...

> Is it possible to have IP aliases for Single node machines? I assume,
> it is possible. But, I would like to know whether it is really useful
> to have IP aliases for Single nodes.

Definitely.

> Whenever there is a node failure, IP aliases will help for Transparent
> Fail-over in Cluster systems. But, I am really not sure how exactly it
> will help in case of Single node machines. Could anybody clarify?

Fault tolerance is not the only thing you might use an IP address for.
Here are some examples:

1) You run multiple secure web servers on the default port. The only way
the web server can know which certificate to send is by which IP the client
connected to.

2) You currently only have one server, but you might want to add a
second in the future. So you use two IP addresses and assign them both to
the same machine for now.

3) You want to protect against connection problems, and you have two
blocks of IP addresses assigned from two different IPs. You policy route
outbound based upon source address. You assign one IP from each block to
critical machines.

DS

> Is it possible to have IP aliases for Single node machines? I assume,
> it is possible. But, I would like to know whether it is really useful
> to have IP aliases for Single nodes.

Two main options:

1) Give a name for every application running on the host. In DNS
enter all of those names as A records pointing to the same IP
address. Then when you move an application you also need to
update DNS and everyone is happy.

2) Give a name for every application running on the host. In DNS
enter all of those names as A records and give them all their own
IP addresses. Give your interface multiple IP addresses. (On
Solaris this is done like hostname.hme0:2 and I leave it to you
to learn how on your version of UNIX). Then when you move an
application move its IP address also and everyone is happy and
no DNS changes are needed.

> But, I am really not sure how exactly it
> will help in case of Single node machines. Could anybody clarify?

If an idea is usefull on a cluster, it can be usefull on a single host.
Giving applications names adds modularity. One concept of
UNIX is "Everything modular everywhere at every layer".

In article <(E-Mail Removed) >
(E-Mail Removed) (qazmlp) wrote:
>
>Thanks!

Yes, it expires.

--
Lady Chatterly

"Hey sparky.......... Chatterley IS S BOT! Amazing that it is s k00ks
bot and YOU don't know it." -- Kenneth Pangborn