Encodes a list of domain names into the format specified in Section 4.1.4 of "Domain Names - Implementation And Specification" (RFC 1035).

Convert-DomainListToByteArray.ps1

<#
.SYNOPSIS
    Convert domain name list to byte array
.DESCRIPTION
    Encodes a list of domain names into the format specified in Section 4.1.4 of "Domain Names - Implementation And Specification" (RFC 1035).
.EXAMPLE
    PS C:\> Convert-DomainListToByteArray ('foo.example.com', 'example.com')
.PARAMETER DomainList
    Specifies a List of domains
.INPUTS
    [string[]] DomainList
.OUTPUTS
    [byte[]]
.LINK
    https://datatracker.ietf.org/doc/html/rfc1035#section-4.1.4
.AUTHOR
    mkht
.DATE
    05/09/2021
.LICENSE
    MIT License
#>
function Convert-DomainListToByteArray {
    [CmdletBinding()]
    [OutputType([byte[]])]
    param (
        [Parameter(Mandatory, ValueFromPipeline, Position = 0)]
        [ValidateNotNullOrEmpty()]
        [string[]]$DomainList
    )

    Begin {
        $IdnMapping = [System.Globalization.IdnMapping]::new()
        $PointerHash = @{}
        $Writer = [System.IO.BinaryWriter]::new([System.IO.MemoryStream]::new())
    }

    Process {
        foreach ($Domain in $DomainList) {
            while (-not [string]::IsNullOrWhiteSpace($Domain)) {
                # Convert internationalized domain names to Punycode
                $Domain = $IdnMapping.GetAscii($Domain.Trim("`0", '.').Trim())

                # Search duplicates part of a domain name
                if ($PointerHash.ContainsKey($Domain)) {
                    if ($PointerHash[$Domain] -ge 0 -and $PointerHash[$Domain] -le 0x3fff) {
                        # Compression pointer (2 bytes big endian)
                        $Pointer = [System.BitConverter]::GetBytes([ipaddress]::NetworkToHostOrder([Int32]((0xc0 -shl 8) + $PointerHash[$Domain])))[2..3]
                        $Writer.Write([byte[]]$Pointer)
                        break
                    }
                }

                if ($Domain.IndexOf('.') -le 0) {
                    # Top Level domain
                    $PointerHash[$Domain] = $Writer.BaseStream.Position
                    $TLDBytes = [System.Text.Encoding]::UTF8.GetBytes($Domain)
                    $Writer.Write([byte]$TLDBytes.Length)
                    $Writer.Write([byte[]]$TLDBytes)
                    # End flag
                    $Writer.Write([byte]0x00)
                    break
                }
                else {
                    # Sub Level domain
                    $PointerHash[$Domain] = $Writer.BaseStream.Position
                    $SplitDomain = $Domain.Split('.', 2, 1)
                    $Domain = $SplitDomain[1]
                    $LLD = $SplitDomain[0].Trim("`0").Trim()
                    $LLDBytes = [System.Text.Encoding]::UTF8.GetBytes($LLD)
                    $Writer.Write([byte]$LLDBytes.Length)
                    $Writer.Write([byte[]]$LLDBytes)
                }
            }
        }
    }

    End {
        $Writer.BaseStream.ToArray()
        $Writer.Close()
    }
}