Advertise and route your VPN routes (ppp0) through a Tailscale node

tailscale-advertise-vpn-routes.sh

#!/bin/bash

mask2cdr ()
{
   # Based on https://stackoverflow.com/questions/20762575/explanation-of-convertor-of-cidr-to-netmask-in-linux-shell-netmask2cdir-and-cdir
   local mask=$1

   # In RFC 4632 netmasks there's no "255." after a non-255 byte in the mask
   local left_stripped_mask=${mask##*255.}
   local len_mask=${#mask}
   local len_left_stripped_mask=${#left_stripped_mask}

   local conversion_table=0^^^128^192^224^240^248^252^254^
   local number_of_bits_stripped=$(( ($len_mask - $len_left_stripped_mask)*2 ))
   local signifacant_octet=${left_stripped_mask%%.*}

   local right_stripped_conversion_table=${conversion_table%%$signifacant_octet*}
   local len_right_stripped_conversion_table=${#right_stripped_conversion_table}
   local number_of_bits_from_conversion_table=$((len_right_stripped_conversion_table/4))
   echo $(( $number_of_bits_stripped + $number_of_bits_from_conversion_table ))
}

sysctl net.ipv4.ip_forward=1
sysctl net.ipv6.conf.all.forwarding=1

routes="$(netstat -rn | grep ppp0 | awk '{print $1 " " $3}' | while read ip ipRange; do printf "%s," $(echo $ip/$(mask2cdr $ipRange)); done)"

echo tailscale up --advertise-routes=${routes::-1}
tailscale up --advertise-routes=${routes::-1}