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[This isn't strictly a Solaris admin question, sorry.]

We're using Sun's DHCP server to provide config to a bunch of Windows
clients. This isn't any kind of HA environment - they're desktop boxes
which can be rebooted, though it would be a pain to reboot them all.
Currently we have a single DHCP server which is due to be retired.
I'm planning on having a pair of boxes which have DHCP servers
configured (identically) on them. If the live box goes down we'd then
start the server on the other one.

The question is: do I need to worry about keeping the DHCP tables
coherent between the boxes (bearing in mind that this is *not* an HA
environment)? My initial guess was that I don't: clients will just
ask to renew their current addresses, and the new box will pick up the
allocations that way. I think it won't hand out duplicate addresses
because it will ping before handing out an address and will find a
machine responding. The bad case is if some machine is temporarily
unresponsive when it won't respond to the ping & will then get unhappy
later. This will probably be few machines though and I can live with
that, especially as we're expecting server failure to be a rare event
(they're reasonably redundant machines).

But perhaps I am confused about things...

--tim

Hi Tim,

you may want to test the free available ISC DHCP-Server. It uses a state
exchange protocol to keep primary and backup dhcp server up to date.

Whenever the primary fails the backup will continue to serve your
clients until the primary comes back.

Tim Bradshaw wrote:
> [This isn't strictly a Solaris admin question, sorry.]
>
> We're using Sun's DHCP server to provide config to a bunch of Windows

I'm not shure whether Sun's DHCP server may also run in primary/backup
mode. We are using the ISC implementation on our Suns. You can compile
the source with gcc or Sun's CC.

http://www.isc.org

> The question is: do I need to worry about keeping the DHCP tables
> coherent between the boxes (bearing in mind that this is *not* an HA
> environment)?

Configure a primary and a backup server. On your routers configure both
server ip addresses as "ip helper" (cisco IOS).

>
> But perhaps I am confused about things...
>
> --tim
>

Regards
CB

Tim Bradshaw wrote:
> [This isn't strictly a Solaris admin question, sorry.]
>
> We're using Sun's DHCP server to provide config to a bunch of Windows
> clients. This isn't any kind of HA environment - they're desktop boxes
> which can be rebooted, though it would be a pain to reboot them all.
> Currently we have a single DHCP server which is due to be retired.
> I'm planning on having a pair of boxes which have DHCP servers
> configured (identically) on them. If the live box goes down we'd then
> start the server on the other one.
>
> The question is: do I need to worry about keeping the DHCP tables
> coherent between the boxes (bearing in mind that this is *not* an HA
> environment)? My initial guess was that I don't: clients will just
> ask to renew their current addresses, and the new box will pick up the
> allocations that way. I think it won't hand out duplicate addresses
> because it will ping before handing out an address and will find a
> machine responding. The bad case is if some machine is temporarily
> unresponsive when it won't respond to the ping & will then get unhappy
> later. This will probably be few machines though and I can live with
> that, especially as we're expecting server failure to be a rare event
> (they're reasonably redundant machines).
>

You do need to provide some level of coherency, which could be as simple
as exporting out the configuration every so often (dhcpconfig -X <some
file> -a ALL -m ALL -o ALL) and copying it to the backup server so that
you can import it (dhcpconfig -I <some file>) when needed. Or you place
the data stores on some HA NFS service and share them (which we do
support) or in a SAN that's dual-ported and can be failed over.

If your backup server doesn't have any idea about existing allocations,
you'll get into this scenario:

- primary goes down
- start server on secondary
- running clients continue to use existing leases
- as leases expire (they can't be extended by the secondary since it
doesn't know about them at all), or clients which weren't running are
started, they request an address; the server picks an address out of the
pool, and attempts to ping to be sure it's free. If nobody responds,
fine, it can be used; if someone does respond, that address gets marked
as unusable, and is taken out of service until the administrator marks
it as available again. Pool of addresses shrinks, perhaps to the point
where you no longer have enough addresses for all your clients. You can
write scripts to go out and reap the addresses which have been marked
unusable to work around this, but at that point you probably should have
just been doing the dhcpconfig export/import I describe above.

Not that you probably want it, but we do sell a thing called the Netra
HA suite which provides a real high availability option,
http://www.sun.com/software/netrahasuite/index.xml.

Dave

On 2007-02-27 17:38:40 +0000, Dave Miner <dave.miner@sun.com> said:

> You do need to provide some level of coherency, which could be as
> simple as exporting out the configuration every so often (dhcpconfig -X
> <some file> -a ALL -m ALL -o ALL) and copying it to the backup server
> so that you can import it (dhcpconfig -I <some file>) when needed. Or
> you place the data stores on some HA NFS service and share them (which
> we do support) or in a SAN that's dual-ported and can be failed over.

Thanks! So if I put the store on a netapp (if we lose that then it's
basically game over since it serves filespace to all the PCs anyway),
then I should be OK? I'd assumed that the usual NFS locking issues
would mean I'd just be immediately doomed if I did that, but things are
probably better than they once were in that respect. So that's a good,
cheap answer.

>
> If your backup server doesn't have any idea about existing allocations,
> you'll get into this scenario:
> [lots of unusable addresses]

That sounds familiar.

> Not that you probably want it, but we do sell a thing called the Netra
> HA suite which provides a real high availability option,
> http://www.sun.com/software/netrahasuite/index.xml.

Well I might want it (I'm interested in HA) but the chances of the
client paying for it are, well, zero (even if it were 83 old pence).

--tim

Tim Bradshaw wrote:
> On 2007-02-27 17:38:40 +0000, Dave Miner <dave.miner@sun.com> said:
>
>> You do need to provide some level of coherency, which could be as
>> simple as exporting out the configuration every so often (dhcpconfig
>> -X <some file> -a ALL -m ALL -o ALL) and copying it to the backup
>> server so that you can import it (dhcpconfig -I <some file>) when
>> needed. Or you place the data stores on some HA NFS service and share
>> them (which we do support) or in a SAN that's dual-ported and can be
>> failed over.
>
> Thanks! So if I put the store on a netapp (if we lose that then it's
> basically game over since it serves filespace to all the PCs anyway),
> then I should be OK? I'd assumed that the usual NFS locking issues
> would mean I'd just be immediately doomed if I did that, but things are
> probably better than they once were in that respect. So that's a good,
> cheap answer.
>

Yes, that's what the shared data store concept is intended to allow, and
part of the reason why we've never pursued implementing the failover
protocol draft that the ISC server has.

Dave

'Internet Systems Consortium, Inc.
(http://www.isc.org/index.pl?/sw/dhcp/)
Please find their mailing list, I configured dhcp failover years ag
following their guide

--
yinlee
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