Translation with Bash

To save ourselves from manual hex conversion every time we update our infrastructure, I wrote a small utility script. It's bi-directional: give it a semicolon-separated list of domains to get the hex code, or paste a hex string to see what's hidden inside.

#!/bin/bash
# RFC 3397 Converter for Freebox DHCP Option 119

encode_rfc() {
    local input=$1
    local hex_result=""
    IFS=';' read -ra ADDR <<< "$input"
    for domain in "${ADDR[@]}"; do
        IFS='.' read -ra LABELS <<< "$domain"
        for label in "${LABELS[@]}"; do
            len=$(printf "%02x" ${#label})
            hex_label=$(echo -n "$label" | xxd -p)
            hex_result+="${len}${hex_label}"
        done
        hex_result+="00"
    done
    echo "$hex_result"
}

decode_rfc() {
    local hex=$1
    local i=0
    local result=""
    while [ $i -lt ${#hex} ]; do
        len_hex=${hex:$i:2}
        len=$((16#$len_hex))
        i=$((i + 2))
        if [ "$len" -eq 0 ]; then
            result+=";"
        else
            label_hex=${hex:$i:$((len * 2))}
            label=$(echo -n "$label_hex" | xxd -r -p)
            result+="$label"
            i=$((i + (len * 2)))
            next_len_hex=${hex:$i:2}
            if [ "$next_len_hex" != "00" ] && [ "$i" -lt ${#hex} ]; then
                result+="."
            fi
        fi
    done
    echo "${result%;}"
}

# Main logic
INPUT=$1
if [[ -z "$INPUT" ]]; then
    echo "Usage: $0 <string_or_hex>"
    exit 1
fi

if [[ "$INPUT" =~ ^[0-9a-fA-F]+$ ]]; then
    decode_rfc "$INPUT"
else
    encode_rfc "$INPUT"
fi

Example

Once set in the Freebox, the option should look like: