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I've purchased a VPN solution, it works correctly when I have "redirect-gateway def1" in the configuration file (routing all traffic through the VPN).

However when I remove that line from the configuration file, I am still able to ping-out of the machine (ping -I tap0), however I cannot ping the IP assigned to the machine (it's a public ip), i get the error: Destination Host Unreachable.

I only want to have certain applications sending traffic through the VPN tunnel (eg: ZNC, irssi), all of which i can select which IP they use. However they can't recieve any data, making the tunnel essentially useless to me when disabling redirect-gateway.

Any ideas on how to allow specific applications use the tunnel, without of forcing everything to go through it?

My configuration file is as follows:

dev tap
remote #.#.#.#
float #.#.#.#
port 5129
comp-lzo
ifconfig #.#.#.# 255.255.255.128
route-gateway #.#.#.#
#redirect-gateway def1
secret key.txt
cipher AES-128-CBC

The output of ifconfig -a when the tunnel is connected:

tap0      Link encap:Ethernet  HWaddr 00:ff:47:d3:6d:f3  
          inet addr:#.#.#.#  Bcast:#.#.#.#  Mask:255.255.255.255
          inet6 addr: <snip> Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:612 errors:0 dropped:0 overruns:0 frame:0
          TX packets:35 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:25704 (25.1 KiB)  TX bytes:6427 (6.2 KiB)

EDIT: the Bcast:#.#.#.# (ifconfig) is different from route-gateway #.#.#.# (openvpn) if that makes any difference.

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2 Answers

I think u can do that using iptables:

#!/bin/bash
# Create new routing table
echo 200 vpn >> /etc/iproute2/rt_tables

# assign iptables mark "1" to this table
ip rule add fwmark 1 table vpn

#next hop for the new routing table will be trough openvpn
ip route add table vpn eql nexthop via <remotevpnip> dev tap0

#with iptables, add mark "1" to the traffic that must be sent trough the vpn
# Sample using the -m owner iptables module
iptables -t mangle -A OUTPUT -p tcp -m owner --uid-owner <user> -j MARK --set-mark 0x01

This works for udp & tcp traffic, not icmp.

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You are probably missing a route on your box for the VPN. Something like route add [NETWORK ON OTHER SIDE OF VPN] gw [VPN address] should do the trick.

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Hi, Thanks for the suggestion, however I don't control the VPN server itself, only the client. Would your suggestion make a difference, as the tunnel works when everything is routed through it? As I don't wish to route everything through the tunnel. Thanks –  jinjin Dec 18 '09 at 22:50
    
This is a change on YOUR computer, not on the VPN. The problem is that your computer doesn't know how to find addresses on the other side of the VPN, so you have to tell it where they are. It will work unless the VPN owners have set up the VPN such that it can never be used unless all traffic goes through it (which is possible for security reasons). –  Catherine MacInnes Dec 19 '09 at 0:16
    
Cool, I didn't relise that - I've just tried adding the remote/float as the first IP, but then I can't ping out using tap0 either, Is there a specific IP from the conf file I should be using in the route add, or any way to check if they are forcing all traffic to go through the VPN? Thanks for your help! –  jinjin Dec 19 '09 at 0:36
    
well the gateway is the route gateway, the network is the network on the other side, which isn't in the config. Also, just noticed that the mask on your tap0 interface is wrong. –  Catherine MacInnes Dec 19 '09 at 1:49
    
Is there any way of changing the Mask? i thought it should take whatever the configuration file gives it, but it obviously isn't doing that. Should I use "ifconfig #.#.#.# MASK" That #.#.#.#? –  jinjin Dec 19 '09 at 12:02
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