topic Re: DHCP Server netmask. How to define? in General Topics

DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 15:46:47 GMT

Working with the built-in DHCP server on a PA200. Running PanOS 5.0.4

Eth1/3 has the address of 172.168.1.1/16

I have devices with static IPS in the 172.168.1.0/24 range.

I'd like to have the clients in the 172.168.2.0/24 range so I'll need the netmask to be 255.255.0.0, however, there's no way to set that (that I can see) on the PAN.

Setting the DHCP range to 172.168.2.1-172.168.2.254 results in a netmask of 255.255.255.0 on the client. This breaks all connectivity as the gateway is still 172.168.1.1

I've tried setting the range to 172.168.1.250-172.168.2.254 (forcing the netmask to be different for it to work) and still the client gets a 255.255.255.0 netmask.

I looked at the CLI manual and can't find any useful information there.

Does DCHP only hand out a netmask of 255.255.255.0 no matter what?

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 16:46:16 GMT

Could you set the IP space to the /16 you want, and then define specific reservations for the IP space you already have allocated?

Re: DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 16:54:00 GMT

Thanks for the link. It is helpful.

I need an accessible network range of 172.168.1.1-172.168.2.254 (/22 allows 172.168.0.0-172.168.3.254, correct?)

I need DHCP leases in the range of 172.168.2.1.-172.168.2.254 (easy enough to assign)

How do I tell the the DHCP server to give out a netmask besides 255.255.255.0?

Re: DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 16:55:14 GMT

Assigning a /16 DHCP pool still results in a 255.255.255.0 netmask

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 16:59:46 GMT

If that's the case then this sounds like a bug to me... in theory the CIDR notation on the pool assignment should be parsed and the PA should 'figure out' the netmask to dole out to client.

Just my two cents though.

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 17:01:02 GMT

In-house here we have a PA doing DHCP that crosses a subnet boundary just like you're trying to set up... let me take a look at what netmask the clients are getting from our PA as well

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 17:06:51 GMT

OK I just confirmed that we are doing DHCP with a /23 subnet, and our clients ARE getting the correct subnet mask via DHCP (255.255.254.0). Maybe PA doesn't parse the /16 right?

There's a bug with PA DHCP by the way, where the built in PA DHCP server doesn't NACK a client if the client requests an old IP... make SURE your DHCP client is doing the DORA process (confirm it via Wireshark if you have to even).

The bug's supposed to be fixed in 4.1.13

Re: DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 17:16:09 GMT

Stuck in 172.168.1.0/23 with a reserved address of 172.168.1.1 and client is still getting a 255.255.255.0 netmask.

I thought it odd that it didn't do what I was expecting, but presumed that I was missing something.

Thanks very much for your assistance!

Are you running PANOS 4.1? I've got 5.0.4 on mine.

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 17:34:23 GMT

I'm running 4.1 right now... we're taking a "wait and see" approach on 5.0 right now. You might have found a bug in 5.0's DHCP implementation possibly....

Re: DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 18:05:01 GMT

Ours isn't in service yet, so we're going with the latest and 'greatest'

I'll ping support and see what they say.

Re: DHCP Server netmask. How to define?

Nathan.S — Wed, 01 May 2013 18:30:17 GMT

Changing the IP structure all around seemed to have resolved it.

Eth1/3 172.22.1.1/22

DHCP: 172.22.1.0/22

Reserved address of 172.22.1.1

Client got 172.22.1.0 with a netmask of 255.255.252.0

It would appear that the DHCP settings were 'stuck' and changing everything around completely fixed it.

Re: DHCP Server netmask. How to define?

ericgearhart — Wed, 01 May 2013 18:34:08 GMT

Go look at your config audits and compare the versions! I wonder if the way that PA tries to "overlay" diffs was flaking out in this case.

A "full commit" might have fixed it too. Hm.