iMac forgets IP address

[Ben]

Dear All,

To be breif: my iMacs forget their IP address and choose one at
random whenever the DHCP server disapears. How can I stop this from
happening?

The DHCP server is on the other side of a wireless link for
reasons beyond my control. Whenever the link goes down (normally one
of the relay stations looses power - I live in Africa) we loose the
internet, and the iMacs can no longer see the DHCP server. Now I can
live with an intermitant internet connection, but what really annoys
me is the way the iMacs respond to a missing DHCP server.
They seem to pick an IP address at random! It may not even be
in the same subnet mask as the original! This is throwing my local
network : file sharing (even through appletalk) and ethernet printing
..

All I want it to do, is to carry on using the last address it
recieved through DHCP until I can get them to run a generator at the
base station and the server returns. Any ideas? Anyone?

My thanks in advance,

Ben.

[Heath Raftery]

In comp.sys.mac.system Ben <ambenyhathaway@hotmail.com> wrote:

> To be breif: my iMacs forget their IP address and choose one at
> random whenever the DHCP server disapears. How can I stop this from
> happening?

The iMac is acting as it should. The DHCP spec (RFC 2131) specifies that a
DHCP server give a client (your iMac) an IP address and an associated lease.
The lease stipulates how long the iMac is allowed to claim that IP as its
own. When the lease is up, the iMac must give up the IP address and request
a new one.

When it can't find the DHCP server, there are a few things it can do. What
is most common, and what you are probably seeing, is that the NIC (Network
Interface Card, your ethernet interface) will 'self-assign' an IP address,
usually in the 192.168.x.x subnet. This self assignment is designed to
automatically set up private networking, since several NICs on the same
network, without a DHCP server, should all choose one of these IPs, set
their subnet mask appropriately and thus be able to communicate over IP.

I don't think there is a lot you can do about this. It is up to the
administrator of the DHCP server to specify the length of the lease (usually
range from several hours to a few weeks), and if you can't get this changed
you might be stuck! You could skip the DHCP entirely and manually assign
your own settings, based on those you've seen from the server. This could
break if someone else uses the same IP, the network changes subnets
(unlikely?), and may well go against your DHCP administrator's wishes.

--
*--------------------------------------------------------*
| ^Nothing is foolproof to a sufficiently talented fool^ |
| Heath Raftery, HRSoftWorks _\|/_ |
*______________________________________m_('.')_m__ _______*

[Tom Harrington]

In article <bk8u9l$80l$1@seagoon.newcastle.edu.au>,
Heath Raftery <hraftery@myrealbox.com> wrote:

> In comp.sys.mac.system Ben <ambenyhathaway@hotmail.com> wrote:

> > To be breif: my iMacs forget their IP address and choose one at
> > random whenever the DHCP server disapears. How can I stop this from
> > happening?

>
> The iMac is acting as it should. The DHCP spec (RFC 2131) specifies that a
> DHCP server give a client (your iMac) an IP address and an associated lease.
> The lease stipulates how long the iMac is allowed to claim that IP as its
> own. When the lease is up, the iMac must give up the IP address and request
> a new one.
>
> When it can't find the DHCP server, there are a few things it can do. What
> is most common, and what you are probably seeing, is that the NIC (Network
> Interface Card, your ethernet interface) will 'self-assign' an IP address,
> usually in the 192.168.x.x subnet. This self assignment is designed to
> automatically set up private networking, since several NICs on the same
> network, without a DHCP server, should all choose one of these IPs, set
> their subnet mask appropriately and thus be able to communicate over IP.

Self-assigned addresses are actually in the 169.254.x.x range. But it's
true that this is how DHCP is supposed to work. If the addresses didn't
change when the server disappeared, they'd still probably have to change
later when it returned.

From the description, it's apparent that each of the iMacs in question
is getting its IP address from the DHCP server. If it were me I'd set
it up so that only one of them did this. Then on that iMac, turn on
internet sharing and let the other iMacs get their addresses from it.
That is, an arrangement something like this:

[Remote DHCP server] ---- [iMac as router] ---- [multiple other iMacs]

In this case the first iMac acts as a local DHCP server for all of the
others. All of the iMacs would be configured to use DHCP, but only one
of them would actually be talking to the remote DHCP server. In the
event that the remote DHCP server is unavailable, local DHCP service is
still active (so long as the router iMac is still running) and the local
network runs the same as always. It'd just be internet access that was
down.

It would also be possible to use a generic router, such as one of those
made by Linksys or Netgear or someone, in the role of the router iMac
above.

This is more or less how my home network runs.

--
Tom "Tom" Harrington
Macaroni, Automated System Maintenance for Mac OS X.
Version 1.4: Best cleanup yet, gets files other tools miss.
See [url]http://www.atomicbird.com/[/url]

[Jerry Kindall]

In article <6bf96b6c.0309162153.2a1353da@posting.google.com >, Ben
<ambenyhathaway@hotmail.com> wrote:

> Dear All,
>
> To be breif: my iMacs forget their IP address and choose one at
> random whenever the DHCP server disapears. How can I stop this from
> happening?
>
> The DHCP server is on the other side of a wireless link for
> reasons beyond my control. Whenever the link goes down (normally one
> of the relay stations looses power - I live in Africa) we loose the
> internet, and the iMacs can no longer see the DHCP server. Now I can
> live with an intermitant internet connection, but what really annoys
> me is the way the iMacs respond to a missing DHCP server.
> They seem to pick an IP address at random! It may not even be
> in the same subnet mask as the original! This is throwing my local
> network : file sharing (even through appletalk) and ethernet printing

This is according to spec. When there's no DHCP server, TCP/IP
automatically assigns addresses from the 169.x.x.x block.

> All I want it to do, is to carry on using the last address it
> recieved through DHCP until I can get them to run a generator at the
> base station and the server returns. Any ideas? Anyone?

Use static IP addresses, or have the DHCP server give out really long
leases on addresses. Or run your own local wireless gateway and have
it give out the IP addresses.

--
Jerry Kindall, Seattle, WA <http://www.jerrykindall.com/>

When replying by e-mail, use plain text ONLY to make sure I read it.
Due to spam and viruses, I filter all mail with HTML or attachments.

[George Williams]

Ben wrote:

> All I want it to do, is to carry on using the last address it
> recieved through DHCP until I can get them to run a generator at the
> base station and the server returns. Any ideas? Anyone?

It can't do that, but they may be able to connect it to an UPS.
If you use a router at home, then at least your local network
will still operate.

[Heath Raftery]

Tom Harrington <tph@pcisys.no.spam.dammit.net> wrote:

> In article <bk8u9l$80l$1@seagoon.newcastle.edu.au>,
> Heath Raftery <hraftery@myrealbox.com> wrote:

>> In comp.sys.mac.system Ben <ambenyhathaway@hotmail.com> wrote:

>> > To be breif: my iMacs forget their IP address and choose one at
>> > random whenever the DHCP server disapears. How can I stop this from
>> > happening?

>>
>> The iMac is acting as it should. The DHCP spec (RFC 2131) specifies that a
>> DHCP server give a client (your iMac) an IP address and an associated lease.
>> The lease stipulates how long the iMac is allowed to claim that IP as its
>> own. When the lease is up, the iMac must give up the IP address and request
>> a new one.
>>
>> When it can't find the DHCP server, there are a few things it can do. What
>> is most common, and what you are probably seeing, is that the NIC (Network
>> Interface Card, your ethernet interface) will 'self-assign' an IP address,
>> usually in the 192.168.x.x subnet. This self assignment is designed to
>> automatically set up private networking, since several NICs on the same
>> network, without a DHCP server, should all choose one of these IPs, set
>> their subnet mask appropriately and thus be able to communicate over IP.

> Self-assigned addresses are actually in the 169.254.x.x range. But it's

Ah yes, of course. Sorry, I had 192.168 on the brain since that is the
private subnet here at work. 192.168/16 is like 172.16/12 and 10/8 ranges
that are reserved for private networks (like the subnet Tom is suggesting).
Tom's suggestion makes sense.

In case anyone is interested, the 169.254/16 stuff is called various names
(surprisingly, Microsoft has changed its name many times) but most recently
APIPA (Automatic Private IP Addressing) and is supported by Win98, Win2000
and above and MacOS 8.5 and above.

--
*--------------------------------------------------------*
| ^Nothing is foolproof to a sufficiently talented fool^ |
| Heath Raftery, HRSoftWorks _\|/_ |
*______________________________________m_('.')_m__ _______*

[Gordon B. Alley]

In article <6bf96b6c.0309162153.2a1353da@posting.google.com >,
[email]ambenyhathaway@hotmail.com[/email] (Ben) wrote:

> Dear All,
>
> To be breif: my iMacs forget their IP address and choose one at
> random whenever the DHCP server disapears. How can I stop this from
> happening?
>
> The DHCP server is on the other side of a wireless link for
> reasons beyond my control. Whenever the link goes down (normally one
> of the relay stations looses power - I live in Africa) we loose the
> internet, and the iMacs can no longer see the DHCP server. Now I can
> live with an intermitant internet connection, but what really annoys
> me is the way the iMacs respond to a missing DHCP server.
> They seem to pick an IP address at random! It may not even be
> in the same subnet mask as the original! This is throwing my local
> network : file sharing (even through appletalk) and ethernet printing
> .
>
> All I want it to do, is to carry on using the last address it
> recieved through DHCP until I can get them to run a generator at the
> base station and the server returns. Any ideas? Anyone?

Put a router between your local network and the wireless link. Then the
router will act as a DHCP server for your local network. The router
will, in turn, receive its IP address from the original DHCP server.
Even if the router loses its connection to the Internet, it would
continue to act as a DHCP server for your network.

The side benefit of using a router is that it will act as a firewall for
your local network.

This is exactly they way my home network is configured (two Macs and a
network printer, connected via an XRouter Pro to my cable modem). When
my cable modem connection goes down, I can still share between my Macs
and use my printer.

--
Gordon Alley <*>
<mailto:galley@texas.net>
<http://galley.home.texas.net>

[Clark Martin]

In article <6bf96b6c.0309162153.2a1353da@posting.google.com >,
[email]ambenyhathaway@hotmail.com[/email] (Ben) wrote:

> Dear All,
>
> To be breif: my iMacs forget their IP address and choose one at
> random whenever the DHCP server disapears. How can I stop this from
> happening?
>
> The DHCP server is on the other side of a wireless link for
> reasons beyond my control. Whenever the link goes down (normally one
> of the relay stations looses power - I live in Africa) we loose the
> internet, and the iMacs can no longer see the DHCP server. Now I can
> live with an intermitant internet connection, but what really annoys
> me is the way the iMacs respond to a missing DHCP server.
> They seem to pick an IP address at random! It may not even be
> in the same subnet mask as the original! This is throwing my local
> network : file sharing (even through appletalk) and ethernet printing
> .
>
> All I want it to do, is to carry on using the last address it
> recieved through DHCP until I can get them to run a generator at the
> base station and the server returns. Any ideas? Anyone?

Not much you can do about the address assignment, that's the way DHCP
works. I don't know why they pick the number they do. I suppose in
part it's because they have to assign SOME address. There are utilities
to force DHCP to renew the DHCP lease. Also you can have the DHCP
server assign longer leases. The machine can't use the last address if
it doesn't still have a lease for it, it would cause problems if the
address has been reassigned.

--
Clark Martin
Redwood City, CA, USA Macintosh / Internet Consulting

"I'm a designated driver on the Information Super Highway"