DHCP no ping

July 7, 2009 at 01:29:51
Specs: Windows XP
This is a problem that has been annoying me for some time now...

I'm running a network of about 150 winXP clients, about 30 of which are laptops that may connect via two NICs at times (wifi and LAN). I have DHCP and DNS sitting on windows server 2003v2 AD Domain Controller with a scope of 512 addresses.

What seems to happen (still no idea why after many hours poking around) is that suddenly an IP address (often on a wifi NIC, but sometimes on a wired NIC) will become unable to reach the domain controller. By this I mean network shares become inaccessible, users cannot log on and I cannot ping the server from the client or the client from the server.

However I can still ping other clients; from the server and the client with the problem. Furthermore it is often the case that the other NIC on the problematic client will be able to maintain the connection no problem. As it has a different IP address it is able to reach the DC with no problem.

So what is basically happening is that the communication between a specific client and the DC only is being blocked somehow! I'm tearing my hair out! It doesnt make sense.

My only solution so far (which works everytime) is to set up an exclusion on the IP address this is occuring on and then renew the connection on the client. Now, this does not even work first time and I often have to exclude a string of IP addresses temporarily until I find one that allows full communication! Interestingly the DHCP server will happily provide IP addresses (next available in the scope etc) and then be completely unable to ping that IP address or recieve pings from that IP address.

If I try to ipconfig renew the IP on the client without excluding the current IP address from DHCP scope first it will invariably come back with "cannot connect to DHCP server"!

I've considered the possibility the problem lies with the networking hardware, switches etc, but then, i have another server connected to the same switch as the DC which I never fail to ping!

Anyway thats about it.

I would very much appreciate some thoughts or tips on what I can do! I've played with various sniffer programs, netstat, tracert, netsh etc but they dont seemed to have shed any light on whats going (or perhaps I missed something).

Feel free to ask for more details if you're inclined to help! I'd be more than happy to elaborate. This thing is doing my head in!!

See More: DHCP no ping

Report •

July 7, 2009 at 08:11:34
all the signs of a double gateway entry.

solution, assuming you are not routing between your DC and pcs is to remove the gateway entry from the lan card if they are going to be connecting wirelessly to the internet.

I am assuming your wired and wireless are not in the same subnet.

Report •

July 7, 2009 at 13:59:56
Hi wanderer,

You sound like you know what you're on about! Goody!

Double gateway entry? You're suggesting there is a different gateway for each NIC in the laptop. E.g. one on wifi card one on lan card? I didnt notice this as being the case. Otherwise where do you mean?

All clients whether devices or PCs operate on the same subnet. This I cannot change. The wifi APs currently also have DHCP IPs. My current plan is to change their IPs to static because I read somewhere that its possible for DHCP to be not forwarded by Wifi APs if they are DHCP IPd themselves. The wifi network is run by a management unit, on which I tweaked DHCP forwarding settings to hopefully help!

But returning to the gateway thing! There is no routing between DC and PCs, only backbone stuff like switches and fibre. Or should I say thats what exists between the wireless APs and the DC. Then its wifi to the laptops :). I'm currently thinking it is something to do with the APs.

However, again back to the gateway thing, I'd like to try your solution. So you are saying I need to remove the default gateway entry from the LAN card configuration, not the wifi card configuration? Is that right?

There is only one gateway on the network, so they both have the same value, however I imagine you're saying its down to there being too many gateway entries (as opposed to too many gateways).

Sorry if I've rambled. Please continue helping me! I'd love it if you were right and this was all fixed. I'd have to go straight to the pub to celebrate.

Report •

July 7, 2009 at 16:18:56
The problem appears to be that you are allowing users to use the lan which has a gateway to the internet along with wireless which uses the same gateway and subnet as the lan.

The makes these dual homed pcs in the same subnet. This is like having two front doors.. which do you go out?

Gateway is the ip address of your router to the internet.

Usually wifi does not go to your internal network since it is considered insecure when compared to a wired network. This would mean you would have one ip subnet for the lan with gateway and a different ip subnet for the wireless with gateway. The two gateways would not be the same ip.

Easiest solution is to decide which to use and disable the other. There is no good reason for any user to attached to the same network via two different hardware accesses. There is nothing to be gained and everything to lose [confuse windows].

The easiest solution is by policy users don't do this. Either use one or the other but not both. I would disable all wired interfaces on the wireless laptops.

Concerning your questions:
"So you are saying I need to remove the default gateway entry from the LAN card configuration, not the wifi card configuration? Is that right?"

assumption here is you are using the lan for server access and wlan for internet access. Both interfaces are live. Then yes no gateway entry on the lan still gives you access to the servers but internet is via the wlan.

"My current plan is to change their IPs to static because I read somewhere that its possible for DHCP to be not forwarded by Wifi APs if they are DHCP IPd themselves."

You are talking a lot of work and perhaps for nothing since dhcp being routed is not your issue. Most APs allow you to either bridge [passes thru like a switch] or nat which is when they supply dhcp to the requestor. Since they all are on the same subnet this does not matter.

Report •

Related Solutions

July 8, 2009 at 01:45:20

What you say seems logical and I'm gonna have a go at making those changes as soon as possible. I wonder if I can control connection related stuff in group policy...? Otherwise it'll be a fair bit of work to manually change each laptop. Still I'll test it on one and see if it works.

One thing I still do not quite understand, and I'd very much appreciate an explaination, is why exactly I cannot reach the DC because of this issue. Only asking because I thought gateways were for accessing extra-network devices. Server is obviously on same local subnet as clients so I would have thought I should be able to reach it with any connection that has an IP address. Also why would a regular "repair" on my wifi connection not yield a solid connection till I excluded the existing IP it was assigned?

Oh and I think I may have forgotten to mention that when I am connected via wifi but unable to reach the server I have internet access (as well as being able to reach other clients and the other server).

Thanks for the tip regarding the APs, I think I will abandon my plans there.

Report •

July 10, 2009 at 03:49:56
Just a further update on this problem.

It just occured on a desktop machine. On which there is only one network adapter. This unfortunatly seems to suggest the problem isnt just related to conflict between two NIC configs.

I've made some headway on laptops however. On several laptops if I use toshibas ConfigFree tool to switch devices, or repair the wifi device using ConfigFree it has actually solved the issue! What I'm not sure of is, what extra things does it do above what a regular repair does initiated via windows own wireless settings?

Hope someone can help!

Report •

July 10, 2009 at 08:20:50
It does sound like you have deeper tcp/ip configuration problems.

Post a ipconfig /all from one of the dual homed laptops. Don't run a repair. I want to see both interfaces live. Thanks

What is doing your dhcp serving and how many dhcp servers do you have?
You say you have a scope of 512 ips. Are you superscoping two class c subnets or subnetting a class b?

Report •

Ask Question