DNS servers ignored by XP DHCP clients

[Anonymous]

Customer has a NetVanta 3430 (ver 18.03.01 EFP) serving DHCP.  The symptom is that some computers cannot resolve names following the DHCP transaction, nor show DNS servers (ipconfig /all).  Packet capture reveals DNS servers in the "offer" packet sent to the client from the 3430.  On the surface, it appears that nothing is wrong with the 3430.  However there are some other things to think about:

• Only Windows XP computers seem to exhibit the symptom (for example, Windows 7 clients work fine--even after flushing DNS and releasing/renewing DHCP)
• The same XP computers do not exhibit the symptom when a different DHCP server is used (for example, a Windows sever)
• XP clients not joined to a domain--and therefore not subject to Group Policy--were used to test, and still exhibit the symptom

We tested this in our office lab using an XP machine and a 1638 switch, and the symptom was not present.  However there are some differences, including:

• Lab unit is 1638, not 3430
• Customer DHCP scope includes SIP options

Packet captures and DHCP config sections attached.  Where do we go next with this one?

Note that the same DNS server is defined twice in the Customer's DHCP scope.  This isn't something we would normally do; someone "just tried it" before our capture, and the symptom was present when only one DNS server was offered in DHCP.

-Chris

[Anonymous]

Chris:

Thank you for asking this question.  I'm not sure I follow the exact problem.  In your testing (either in your lab or at the customer location), the DHCP clients on Windows XP did not receive any of the DNS servers configured in the DHCP pool on the DHCP server, or is it that they do receive the proper DNS information, but still cannot resolve domain names?

Levi

[Anonymous]

Thanks for your reply, Levi!

In both the customer and lab networks, the DHCP OFFER packet includes option 6, DNS servers. However, at the customer site (only) no XP clients show DNS servers when you issue "ipconfig /all" (and you can't resolve names). We brought one of the XP clients back to our office lab and DNS servers are listed after DHCP info is received (and you can resolve names).

However, the same XP client DOES show DNS servers after receiving DHCP from a Windows server at the customer site (we turned off DHCP in the 3430 to test).

You'll see that we're including option 43 and 120 in the customer's AOS config snippit. This seems to be causing the problem for XP hosts. Windows 7 seems to ignore them. The options were not in our lab config. Today, I added these options to see if it would also cause the symptom here in the lab, and it did.

Normally this is a non-issue, since we tend to use Voice VLANs and any XP workstations out there will be on a different VLAN where option 43 and 120 are not included in DHCP. But the customer in this case is a small branch office and is not equipped for VLAN. Also, there will be some additional remote offices where we're going to run into the same scenario soon.

Chris

[Anonymous]

Chris:

Thank you for replying so quickly with the requested information.  This appears to be a problem with the Microsoft Windows XP Operating System.  I was able to find this article from their website:  Some DHCP Options are not recognized on a Windows XP SP3-based client computer when the DHCP server....

However, you can configure a workaround for the Microsoft problem by configuring the DHCP server differently.  Here is an example, that should work:

ip dhcp-server pool "NAME"

  network <ip address> <network mask>

  netbios-node-type h-node

  default-router <ip address>

  option 6 hex <value>

  option 43 hex <value>

Levi

[Anonymous · Accepted solution]

While Microsoft has documented a hotfix ( Some DHCP Options are not recognized on a Windows XP SP3-based client computer when the DHCP server...), there are times when applying the hotfix to individual clients is not reasonable. For example, a wireless environment where the client devices are not controlled.

You will need to follow the steps below in order to use numbered options and avoid conflicts with Windows XP devices.

1. Remove the DNS servers from the DHCP pool.

2. Remove Option 43 from the DHCP pool.

3. Configure numbered options, in order, as needed.

Lets say your DNS servers are 10.10.0.30 and 10.10.0.5 such that the DHCP configuration presently resembles the following.

!

ip dhcp-server pool "192.168.168.0/24"

  network 192.168.168.0 255.255.255.0

  dns-server 10.10.0.30 10.10.0.5

  netbios-node-type h-node

  default-router 192.168.168.10

  option 43 ascii 10.10.0.69

!

In order to convert the DNS servers' IP addresses to a hex string, you will want to first convert to binary. This is optional, but definitely helps keep things in order. The IP addresses are 10.10.0.30 and 10.10.0.5 which in binary is 0000.1010.0000.1010.0000.0000.0001.1110 and 0000.1010.0000.1010.0000.0000.0000.0101. Notice the periods used to segment the IP addresses into groups of 4 binary digits (binary 1111 is equivalent to 15 or hex E). Simply convert these groups into hex. For example, the IP address 10.10.0.30 when converted to hex is 0A0A001E. Now convert 10.10.0.5 to hex and apprend this to the first DNS server; the entire string would look like 0A0A001E0A0A0005. The DHCP pool is shown below.

!

ip dhcp-server pool "192.168.168.0/24"

  network 192.168.168.0 255.255.255.0

  netbios-node-type h-node

  default-router 192.168.168.10

  option 6 hex 0A0A001E0A0A0005

  option 43 ascii 10.10.0.69

!

[Anonymous]

:

I went ahead and flagged this post as “Assumed Answered.”  If any of the responses on this thread assisted you, please mark them as either Correct or Helpful answers with the applicable buttons.  This will make them visible and help other members of the community find solutions more easily.  If you still need assistance, I would be more than happy to continue working with you on this - just let me know in a reply.

Levi

[Anonymous]

I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post and unmark it and select another in its place with the applicable buttons.  If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.

Thanks,

Noor