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DarkVex
Asked: 2017-03-04 07:17:48 +0800 CST

OpenVPN with same subnet on two remote sites

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i have some issue with an OpenVPN concentrator while using TUN device. The scenario is that:

OpenVPN Server
VPN Subnet: 10.10.10.1/24

PC1 OpenVPN Client
VPN IP: 10.10.10.50
PC Local subnet/IP: 192.168.20.100

PC2 OpenVPN Client
VPN IP: 10.10.10.60
PC Local subnet/IP: 192.168.30.100

RemoteSite1
VPN IP: 10.10.10.70
Local Subnet: 192.168.80.0/24

RemoteSite2
VPN IP: 10.10.10.80
Local Subnet: 192.168.80.0/24

The PC1 connect to RemoteSite1 and the PC2 connect to RemoteSite2 but while using iroute one of the PCs reach the wrong remote site. This setup works perfectly while using TAP device. In order to avoid the network conflicts on the OpenVPN server there are not the routing rule for the remote sites, there is a specific lookup routing table for each PC like that:

root@openvpn~ # ip rule sh
0:  from all lookup local 
1:  from 10.10.10.50 lookup 1024
1:  from 10.10.10.60 lookup 1034
10: from all to 10.10.10.0/24 lookup main

And under each table: root@openvpn~ # ip route show table 1024 192.168.80.0/24 via 10.10.10.70 dev tap0

root@openvpn~ # ip route show table 1034
192.168.80.0/24 via 10.10.10.80 dev tap0

How can archieve the same setup but with TUN and iroute?

routing openvpn iproute2 iproute

1 Answers

  1. Best Answer

    I don't think you can do that.

    TUN is routed (OSI layer 3), and it isn't possible to have two identical routes for a single subnet in this way (you would need at least some kind of difference in metric). TAP operates at OSI layer 2, where MAC lookups are possible, which is how your setup is working (when it works).

    For TUN to work in this situation, your best bet might be to split your /24 into two /25 networks, and assign one side to each location.

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