Weird network problem

[Tim]

Allegedly, on or about 29 May 2013, Timothy Murphy sent:
> You all seem to be finding it difficult to follow my meaning.
> I'm saying that the term "dynamic IP" is normally used
> to refer to an ISP giving the same client different IP addresses
> at different times, in order to to limit the number of address
> required.
> Whether or not that is done using dhcp is irrelevant to this point. 

Well, you did ask *about* "DHCP," at least twice.  That wasn't the
question you did ask earlier.  And I was beginning to wonder whether you
wanted to know how to configure one, or just how it worked.

You asked:
>>> As a matter of interest, how do you configure DHCP
>>> to work with a dynamic IP?
>>>
>>> I use ddclient with dyndns, 
>>> but are you saying one doesn't need to do something like that?

Then asked about servers rather than clients.  Going back to that first
message, you did ask two wildly different things (DHCP and ddclient).
Firstly, you don't need to configure DHCP to handle dynamic IPs, it
already does that.  The second question is unrelated to the first, if
there's anything that might need to be configured to handle dynamic IPs,
it would be ddclient.  But I'm not familiar with it, it may already
manage that without needing you to configure anything further, unless
you'd already modified it with a fixed IP.

Dynamic IP merely means *not* static IP (and it's not just for ISPs,
it's the same for LANs).  In that there's no expectation that you'll get
the same IP, one is not specifically assigned to you.  You may get the
same one, you may not, the server gets to choose.  And my message
explained how DHCP went about giving you one.  While it's not the only
way to assign dynamic addresses, it'd be the most common.  And other
protocols would use similar techniques.  Other uses of the term, such as
you described (where an ISP may try to share 1,000 IPs between 2,000
customers), are just how or why it may be used.

So, regardless of how you get assigned a dynamic address, if you're
having to tell something like dyndyns about any changes to it, that's a
separate issue.

If you felt like a lot of hard work, you could probably write something
that was triggered by your DHCP client to talk to dyndns, if DHCP was
responsible for your address changes.  Less work would be to use
NetworkManager's despatch scripts to update your dyndns data whenever
NetworkManager detected a change in your network status.  Or, especially
if you don't use NetworkManager, you could use something (e.g. ddclient)
that's already been designed to handle dynamic address changes, that may
use some other technique to determine your external address and notice
when it changes.

It's been years since I've done something like this, and I didn't use
the dyndns service, nor do I recall whether it used ddclient or
something else.  But the latter was the technique that mine used (it
looked at something outside of my LAN, figures out my external IP
address, then updated the external "dynamic IP to a domain name"
service, if it noticed that my IP had changed).

-- 
[tim at localhost ~]$ uname -rsvp
Linux 3.8.12-100.fc17.x86_64 #1 SMP Wed May 8 15:36:14 UTC 2013 x86_64

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