OpenVPN bypass on some ports
After a little more searching, I've found this thread : https://forum.linode.com/viewtopic.php?p=50114&sid=b440414422596bb7dbc96cf7c9ee511f#p50114
I've now modified my "route-up" OpenVPN script as follow, and it's finally working ! I've removed all the others messy rules (iptable PREROUTING, MASQUERADE, etc).
Here is my final "route-up" script :
ip route flush table 100
ip route flush cache
ip rule add from x.x.x.x table 100
ip route add table 100 to y.y.y.y/y dev ethX
ip route add table 100 default via z.z.z.z
Where x.x.x.x is my server's public IP, y.y.y.y/y is the subnet of my server's public IP address, ethX is my server's public Ethernet interface, and z.z.z.z is the default gateway.
Hope this may help someone else.
After going through the same ordeal myself I found at least one problem with the route-up script.
iptables -t mangle -A PREROUTING ...
iptables -t mangle -A OUTPUT ...
Read about why here: http://www.iptables.info/en/structure-of-iptables.html
I didn't have to turn on IP forwarding.