OpenVPN routing table change

Question

Here is a copy of my routing table when I'm not connected to my VPN:

===========================================================================
Interface List
 12...00 ff cc b0 c2 44 ......TAP-Windows Adapter V9
 11...00 25 22 32 39 42 ......Realtek RTL8168D/8111D Family PCI-E Gigabit Ethernet NIC (NDIS 6.20)
  1...........................Software Loopback Interface 1
===========================================================================

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0         10.0.0.1         10.0.0.3     10
         10.0.0.0    255.255.255.0         On-link          10.0.0.3    266
         10.0.0.3  255.255.255.255         On-link          10.0.0.3    266
       10.0.0.255  255.255.255.255         On-link          10.0.0.3    266
        127.0.0.0        255.0.0.0         On-link         127.0.0.1    306
        127.0.0.1  255.255.255.255         On-link         127.0.0.1    306
  127.255.255.255  255.255.255.255         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link          10.0.0.3    266
  255.255.255.255  255.255.255.255         On-link         127.0.0.1    306
  255.255.255.255  255.255.255.255         On-link          10.0.0.3    266
===========================================================================
Persistent Routes:
  None

IPv6 Route Table
===========================================================================
Active Routes:
 If Metric Network Destination      Gateway
  1    306 ::1/128                  On-link
  1    306 ff00::/8                 On-link
===========================================================================
Persistent Routes:
  None

Here is a copy of when I am connect to my VPN:

===========================================================================
Interface List
 12...00 ff cc b0 c2 44 ......TAP-Windows Adapter V9
 11...00 25 22 32 39 42 ......Realtek RTL8168D/8111D Family PCI-E Gigabit Ethernet NIC (NDIS 6.20)
  1...........................Software Loopback Interface 1
===========================================================================

IPv4 Route Table
===========================================================================
Active Routes:
Network Destination        Netmask          Gateway       Interface  Metric
          0.0.0.0          0.0.0.0         10.0.0.1         10.0.0.3     10
          0.0.0.0        128.0.0.0       10.158.1.5       10.158.1.6     30
         10.0.0.0    255.255.255.0         On-link          10.0.0.3    266
         10.0.0.3  255.255.255.255         On-link          10.0.0.3    266
       10.0.0.255  255.255.255.255         On-link          10.0.0.3    266
       10.158.1.1  255.255.255.255       10.158.1.5       10.158.1.6     30
       10.158.1.4  255.255.255.252         On-link        10.158.1.6    286
       10.158.1.6  255.255.255.255         On-link        10.158.1.6    286
       10.158.1.7  255.255.255.255         On-link        10.158.1.6    286
        127.0.0.0        255.0.0.0         On-link         127.0.0.1    306
        127.0.0.1  255.255.255.255         On-link         127.0.0.1    306
  127.255.255.255  255.255.255.255         On-link         127.0.0.1    306
        128.0.0.0        128.0.0.0       10.158.1.5       10.158.1.6     30
     208.23.71.92  255.255.255.255         10.0.0.1         10.0.0.3     10
        224.0.0.0        240.0.0.0         On-link         127.0.0.1    306
        224.0.0.0        240.0.0.0         On-link          10.0.0.3    266
        224.0.0.0        240.0.0.0         On-link        10.158.1.6    286
  255.255.255.255  255.255.255.255         On-link         127.0.0.1    306
  255.255.255.255  255.255.255.255         On-link          10.0.0.3    266
  255.255.255.255  255.255.255.255         On-link        10.158.1.6    286
===========================================================================
Persistent Routes:
  None

IPv6 Route Table
===========================================================================
Active Routes:
 If Metric Network Destination      Gateway
  1    306 ::1/128                  On-link
  1    306 ff00::/8                 On-link
===========================================================================
Persistent Routes:
  None

10.0.0.1 is my default gateway
10.0.0.3 is my internal IP
208.23.71.92 is my address I am assigned by my VPN provider (not my real one)

My question is, what does the following entry that OpenVPn added do?

     208.23.71.92  255.255.255.255         10.0.0.1         10.0.0.3     10

Since I am 208.23.71.92 why would that need to be routed to my default gateway?

Answer 1

You say that you are "208.23.71.92", but your routing tables don't support that assertion, or at least your computer is not "208.23.71.92". I suspect that the remote VPN endpoint might be doing NAT and is thus providing you an apparent external address of 208.23.71.92 when using the tunnel.

The entry in question points a specific route for that IP address out your gateway. This is neccessary because the VPN tunnel will publish a default route (ie all traffic without a specific route will go out the VPN). When the VPN adds this default route, connectivity to the VPN would be lost - as its trying to reach the endpoint of the VPN through the VPN !!!!

By putting in the more specific route of your physical gateway for the VPN endpoint, traffic for the VPN endpoint is always routed out your Internet connection, thus a path can exist over which to route your VPN traffic.