PPP VPN split-network/split-DNS script for OSX

ip-up

#!/bin/sh
####################################################
##                                                ##
## PPP VPN split-network/split-DNS script for OSX ##
## by Aaron Meriwether                            ##
##                                                ##
## installation:                                  ##
## sudo cp thisfile /etc/ppp/ip-up                ##
## sudo chmod 755 /etc/ppp/ip-up                  ##
##                                                ##
####################################################

##################
## begin config...

# Define per-VPN-server settings here...
SERVERS[0]="vpn.example.org" # must match the server name set in Network Preferences.
NETWORKS[0]="192.168.2.0/24 192.168.3.0/24" # only networks besides the VPN LAN itself!
DOMAINS[0]="example.org" # domains which should use the VPN's DNS resolver when connected.

SERVERS[1]="vpn.mit.edu"
NETWORKS[1]="" # an empty network list is acceptable if you are only interested in the DNS aspects
DOMAINS[1]="staff.mit.edu athena.mit.edu"

SERVERS[2]="vpn.rogers.com"
NETWORKS[2]="172.16.7.128/25 205.150.67.0/24"
DOMAINS[2]="" # likewise, an empty DNS list is acceptable if you are only interested in the network aspects

# set to /dev/null if you object to debugging output.
DEBUGFILE=/tmp/ip-up-debug.txt

## end config...
#################


# When the ppp link comes up, this script is called with the following
# parameters
#       $1      the interface name used by pppd (e.g. ppp3)
#       $2      the tty device name
#       $3      the tty device speed
#       $4      the local IP address for the interface
#       $5      the remote IP address
#       $6      the parameter specified by the 'ipparam' option to pppd
#               (when used with a VPN configured via Network Preferences,
#                $6 seems to contain the IP address of the gateway over
#                which the VPN traffic passes.  This might be useful in
#                ip-up scripts which need to avoid breaking the route
#                used by pppd itself.)

PATH=/usr/bin:/bin:/usr/sbin:/sbin

# Generic array operation.
# e.g.: indexOf "something" "${myArray[@]}"
# note the double quotes around the array and the use of [@] not [*]
indexOf() {
  local s="$1"; shift; local a=( "$@" ) i
  for (( i = 0; i < ${#@}; i++ )); do
    if [ "${a[$i]}" == "$s" ]; then echo $i; return; fi
  done
}

# Given an interface name, find the corresponding scutil service key.
if2service() {
  local i
  for i in $(echo "list State:/Network/Service/[^/]+/PPP" | scutil | cut -d/ -f4); do
    if [ "$(echo show State:/Network/Service/$i/PPP | scutil | grep InterfaceName | cut -d' ' -f5)" == "$1" ]; then echo $i; return; fi
  done
}

# Given a network service key (specifically a PPP service), get the server name (as set in Network Preferences).
getPppServerName() {
  echo show Setup:/Network/Service/$1/PPP | scutil | grep CommRemoteAddress | cut -d' ' -f5
}

echo "$(date +'%Y-%m-%d %T %Z') $0 1=$1 2=$2 3=$3 4=$4 5=$5 6=$6" >> $DEBUGFILE
SERVICE="$(if2service $1)"
echo "Service: $SERVICE" >> $DEBUGFILE
SERVER="$(getPppServerName $SERVICE)"
echo "Server Name: $SERVER" >> $DEBUGFILE
i=$(indexOf "$SERVER" "${SERVERS[@]}")
if [ -z "$i" ]; then
  echo "No special configuration found for this VPN server." >> $DEBUGFILE
  exit
fi

if [ -n "$(echo show State:/Network/Service/$SERVICE/IPv4 | scutil | grep OverridePrimary)" ]; then
  echo "Clearing the 'Send all traffic over VPN connection' flag" >> $DEBUGFILE
  scutil <<-END
	get State:/Network/Service/$SERVICE/IPv4
	d.remove OverridePrimary
	set State:/Network/Service/$SERVICE/IPv4
	END
fi

if [ -n "${NETWORKS[$i]}" ]; then
  for n in ${NETWORKS[$i]}; do
    echo "Adding route for network $n" >> $DEBUGFILE
    route -n add -net $n $5
  done
fi

if [ -n "${DOMAINS[$i]}" ]; then
  echo "Limiting DNS over the VPN to these domains: ${DOMAINS[$i]}"
  scutil <<-END
	get State:/Network/Service/$SERVICE/DNS
	d.add SupplementalMatchDomains * ${DOMAINS[$i]}
	set State:/Network/Service/$SERVICE/DNS
	END
fi

Nice script, only I have problem running it under El Capitan getting the split-dns to work. I can see the SupplementalMatchDomains being set initially after the ipsec tunnel is up. But after a few seconds the PPP somehow seems to reset this change. From /var/log/ppp.log:

Tue May 17 13:39:56 2016 : Received acsp/dhcp dictionaries
Tue May 17 13:39:56 2016 : Committed PPP store on install command

Have you found any way to work around this?

I have the same problem like @aequitas: its working directly after the ppp connection comes up, after 2 seconds the chage is reverted by the system. Any solution?

just as a workaround I added a "sleep 20" between line 103 and 104. With this its working. I think the problem is Mac OS related:

Fri Oct 13 12:18:00 2017 : l2tp_wait_input: Address added. previous interface setting (name: en4, address: 10.18.89.2), current interface setting (name: ppp0, family: PPP, address: 192.168.1.1, subnet: 255.255.255.0, destination: 10.255.255.0).
Fri Oct 13 12:18:00 2017 : Committed PPP store on install command
Fri Oct 13 12:18:00 2017 : L2TP port-mapping for en4, interfaceIndex: 0, Protocol: None, Private Port: 0, Public Address: 0, Public Port: 0, TTL: 0.
Fri Oct 13 12:18:00 2017 : L2TP port-mapping for en4 inconsistent. is Connected: 1, Previous interface: 6, Current interface 0
Fri Oct 13 12:18:03 2017 : sent [IP data <src addr 192.168.1.1> <dst addr 255.255.255.255> <BOOTP Request> <type INFORM> <client id 0x08000000010000> <parameters = 0x6 0x2c 0x2b 0x1 0xf9 0xf>]
Fri Oct 13 12:18:09 2017 : sent [IP data <src addr 192.168.1.1> <dst addr 255.255.255.255> <BOOTP Request> <type INFORM> <client id 0x08000000010000> <parameters = 0x6 0x2c 0x2b 0x1 0xf9 0xf>]
Fri Oct 13 12:18:12 2017 : sent [IP data <src addr 192.168.1.1> <dst addr 255.255.255.255> <BOOTP Request> <type INFORM> <client id 0x08000000010000> <parameters = 0x6 0x2c 0x2b 0x1 0xf9 0xf>]
Fri Oct 13 12:18:15 2017 : No DHCP server replied

It seems that the Mac OS links the ppp interface to the en4 interface with some inconsistencies, and that is generating a new DHCP BOOTP request.

In the end it resets the changes of the script here. So my workaround is just a delayed run of the setting by the script.