Comment 3 for bug 771148

I agree with Andreas; this is a bug in openvpn. I have local office gateway servers with an internal PCI ADSL modem that runs embedded Linux and is configured in RFC 1483 LLC Bridge mode. The Host PC uses PPPoE to connect to the ISP via the embedded ADSL modem over the single ATM LLC available from BT in the U.K.

This results in the host PC creating the ppp0 interface which is the default route:

$ ip route show
default dev ppp0 scope link
62.3.82.17 dev ppp0 proto kernel scope link src 82.71.24.87
...

The OpenVPN server's configuration includes:

push "redirect-gateway def1"

When the local office connects it reports:

ovpn-01linode[24920]: TUN/TAP device tun0 opened
ovpn-01linode[24920]: TUN/TAP TX queue length set to 100
ovpn-01linode[24920]: do_ifconfig, tt->ipv6=0, tt->did_ifconfig_ipv6_setup=0
ovpn-01linode[24920]: /sbin/ifconfig tun0 10.254.252.18 pointopoint 10.254.252.17 mtu 1500
ovpn-01linode[24920]: NOTE: unable to redirect default gateway -- Cannot read current default gateway from system

There is (an invasive) patch in Debian bug #592088 by Lionel Elie Mamane in comment 10. A better solution that doesn't involve trying to get the OpenVPN project to substantially change the semantics of "redirect-gateway" to "redirect-default-route" is in a short PPP script mentioned by Vladislav Naumov in comment 20, and originally from the OpenVPN mailing list in 2007:

/etc/ppp/ip-up.d/default-gateway:
---------------8<--------------
#!/bin/bash

if [ $(ip route list exact default |\
  awk '/^default/ {print $2}') = dev ];
then
         IF=$(ip route | awk '/^default/ {print $3}')
         GW=$(ip address show $IF |\
  awk '/peer/ {print $4}' | cut -d"/" -f1)
         ip route replace default via $GW dev $IF
fi
-----------8<------------------

Now, when the PPP interface comes up

 default dev ppp0 scope link

is replaced by the PPP script with a route that includes the ISP gateway IP address:

 default via 62.3.82.19 dev ppp0

This allows OpenVPN to recognise and replace the default route when it starts:

default via 10.254.252.17 dev tun0
10.254.252.17 dev tun0 proto kernel scope link src 10.254.252.18