• The Mysterious Microsoft DHCP Option ID 81

    April 16, 2020

    DHCP

    Recently I worked on a project where I needed to create a PowerShell script to automate the creation of numerous Microsoft DHCP servers and all their scopes, scope options, reservations, all settings. The original DHCP servers were in an old Active Directory domain, and a new AD Forest was created with a different networking scheme. I used the output from the DHCP documentation script to gather all the info needed, but there was one scope option that had me stumped.

    Almost every DHCP Scope had an Option ID of 81, and the Value number was different for a lot of the scopes.

    Scope Options

    Option Name 00081
    Vendor Standard
    Value 7
    Policy Name None

    There is no Option ID 81 in the Predefined Options and Values, as shown in Figure 1.

    Figure 1
    Figure 1

    Here is what I found out.

    When a new DHCP Scope is created (at least on Windows Server 2016 or 2019), there is no scope Option ID 81, as shown in Figure 2.

    Figure 2
    Figure 2

    Why? Apparently, because Name Protection is enabled by default on a new scope, as shown in Figure 3.

    Figure 3
    Figure 3

    If Name Protection is disabled, as shown in Figure 4, Option ID 81 mysteriously appears, as shown in Figure 5.

    Figure 4
    Figure 4
    Figure 5
    Figure 5

    What happens if Name Protection is enabled again, as shown in Figure 6?

    Figure 6
    Figure 6

    Option 81 is still there, as shown in Figure 7, with a different numeric Value.

    Figure 7
    Figure 7

    Why all the fuss? I am trying to recreate all the scopes and related settings with just changing the network data. I see an Option 81 with no way to set it or control it.

    The only information I can find on Option ID 81 from Microsoft states that option is used for the client to tell DHCP what its FQDN is.

    Integrating DHCP with DNS

    When a DHCP server registers and updates DNS pointer (PTR) and address (A) resource records on behalf of its DHCP-enabled clients, it uses the information contained within an additional DHCP option: the Client FQDN option (option 81), which permits a client to provide its FQDN and any instructions to the DHCP server that is used to process DNS dynamic updates on its behalf.

    If you like to be confused, read the information at this link.

    2.2.7 DHCPv4 Option Code 81 (0x51) – Client FQDN Option

    I decided just to ignore Option ID 81, just like I do Option ID 51, in the script.

    BTW, I had to review why I skipped Option ID 51 and came across an interesting (to me) article.

    YOU SHOULD CARE ABOUT DHCP OPTION 51

    What I also found out about Option ID 81, is that by selecting or unselecting the various options, the numeric Value changes with no discernable pattern (at least to me).

    I have no clue why Option ID 81 exists. All the DNS settings for a scope are obtained by running Get-DHCPServerV4DnsSetting -ScopeId, as shown in Figure 8.

    Figure 8
    Figure 8

    For me, for this project, I will just ignore Option ID 81 and get on with life. The DHCP documentation script is updated in Version 1.43 to ignore Option ID 81.

    Thanks

    Webster

     







    About Carl Webster

    Carl Webster is an independent consultant specializing in Citrix, Active Directory, and technical documentation. Carl (aka “Webster”) serves the broader Citrix community by writing articles (see CarlWebster.com) and by being the most active person in the Citrix Zone on Experts Exchange. Webster has a long history in the IT industry beginning with mainframes in 1977, PCs and application development in 1986, and network engineering in 2001. He has worked with Citrix products since 1990 with the premiere of their first product – the MULTIUSER OS/2.

    View all posts by Carl Webster

    No comments yet.

    Leave a Reply