After I dist-upgraded my Debian Squeeze to Jessie, I couldn’t connect to any machine on the remote LAN anymore after connecting via OpenVPN. The VPN connection to the OpenVPN server on my Debian server was successful, but I couldn’t ping or access machines on the remote network.
I found the solution to my problem by chance while searching for another error I found in the OpenVPN server logfile:
Fri Jan 22 16:20:40 2016 us=402991 pc/126.96.36.199:60860 MULTI: bad source address from client [fe80::487c:a5ff:f332:3df7], packet dropped Fri Jan 22 16:20:44 2016 us=603301 pc/188.8.131.52:60860 UDPv4 WRITE  to [AF_INET]184.108.40.206:60860: P_DATA_V1 kid=0 DATA len=52
While routinely checking the OpenVPN server configuration I noticed, that IP forwarding had been turned off. I don’t know how or why this happened during apt-get dist-upgrade, but after re-enabling it, I was able to connect to the remote machines instantly:
cat /proc/sys/net/ipv4/ip_forward # 0 echo 1 > /proc/sys/net/ipv4/ip_forward cat /proc/sys/net/ipv4/ip_forward # 1