[BBLISA] A question on DHCP "shoulds".

[Dean Anderson]

On Mon, 31 Aug 2009, John Hanks wrote:
> I'm going to take the opposing viewpoint, if only to make this a more
> lively discussion.

:-)  

> My opinion is that the only machines in an environment that should be
> set statically are the DHCP and DNS servers and, if these are
> virtualized, the hosts which make up the virtualization
> infrastructure. My view of a network infrastructure places DHCP and
> DNS at the foundation. If I find myself layering complexity later,
> like making many static IP address assignments, then I prefer to step back
> and fix the underlying foundational issues in a way that preserves the
> centralized control of IP address assignment.


I've always setup networks so that servers have static ip addresses 
and desktops/laptops have dynamic IP addresses. As some have pointed 
out, there are several means of giving servers static IP addresses, 
either via configuration or via fixed dhcp assignments.

The downside of static configuration is the effort to change ip
addresses when the server is relocated. One usually has to do the change
before shutting the server down. Of course, the principle case where
that doesn't work is after a serious power failure and one has 
physically relocated the server somewhere else.

The upside of static configuration is that the server will boot while 
the DHCP and DNS is still down. Otherwise, you need to delay booting 
everything until DHCP and DNS servers come up.

The downside of dhcp fixed configuration is that DHCP needs to know 
about the MAC address of the server.  In large corporations, DHCP and 
departmental servers are often run by different groups.

The upside of dhcp fixed, is that the server can be easily moved to a 
new ip address, physically relocated etc;  It will always have an IP 
address that will work on the network its plugged into.

> > By creating the reservation, I ensure it can never be assigned, even
> > by accident, to any other system.
>
> What you have found comfort in is the exact reason I prefer the
> opposite solution. Enforcing this level of documentation upkeep, 

A problem with referring to servers by name, when the name has to 
dynamically change IP addresses is that many clients lookup the name 
once, and then continue to use that IP address. If the IP address 
changes, some clients will lose. One doesn't want the web server to be 
unable to contact the database server because the dhcp lease expired.

Other systems (e.g.  domain controllers) can't change IP address very
easilly, and must know the IP addresses of other servers in their group 
of servers.

 

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