Monthly Archives: January 2019

Setting a DHCP Option Value to Hex Bytes

So sometimes, some annoying times, you have a Vendor scoped DHCP option that you need to set and it is hex bytes. This can be quite frustrating as the GUI doesn’t provide an option to set the value (at least I haven’t found a way) and even when it does, you have to set it using the hex value. The second problem being, I don’t speak Hex.

The way I’d always done it before, and that comes up when I search for it right now uses netsh.

netsh dhcp server scope set optionvalue 125 ENCAPSULATED 000003045669643A697070686F6E652E6D6974656C2E636F6D3B73775F746674703D3139322E3136382E312E313B63616C6C5F7372763D3139322E3136382E312E312C3139322E3136382E312E323B

This is fine if you already have the hex value, or the way to get it. It just isn’t idea if you want to do it for a ton of scopes, or if you don’t have the hex value.

Format-Hex will take a string and make it into an array of hex bytes nicely, so you can either use Powershell to produce that hex string

$hex  = [convert]::ToChar(0) + [convert]::ToChar(0)+ [convert]::ToChar(3) +  [convert]::ToChar(4) + ";sw_tftp=;call_srv=,;"  | Format-Hex
$string = ($hex.Bytes|ForEach-Object ToString X2) -join ''

and Set-DhcpServerv4OptionValue will take that array and save it into the DHCP server for us, so you can then bulk setup scopes:

$hex  = [convert]::ToChar(0) + [convert]::ToChar(0)+ [convert]::ToChar(3) +  [convert]::ToChar(4) + ";sw_tftp=;call_srv=,;"  | Format-Hex
Set-DhcpServerv4OptionValue -ComputerName dhcpserver -ScopeId -OptionID 125 -Value $hex.Bytes

Replace the dchpserver with your actual dhcp server name, the scope ID would be the IP of the scope, etc… The nice thing in powershell is we can then script this and loop over scopes. Building the string per scope and setting it, which would be more complicated with the netsh version. Also sense it is in plain text instead of HEX it is far easier to read and update in the future.

We can reverse the process though it is slightly messier

$value = Get-DhcpServerv4OptionValue -computername dhcpserver -scopeid -OptionId 125 
($value.value|ForEach-Object {[char][byte]"$_"}) -join ''

This will grab the value as an array of strings, in the form of “0x00”, convert that to bytes, then convert that to chars, then join it all back together into a string for viewing.