dco: fix source IP selection

[LGA1150]

If the local option is present, DCO should use it as the source IP, or Linux may pick a different IP as the source IP, breaking the connection.

[schwabe]

Could you add the scenario when this happens with a small config that allows us to reproduce this issue?

[LGA1150]

update: manually rebased to master branch

OS/Kernel: OpenWrt 22.03/Linux 5.10.104 (Minimal kernel config to reduce size), musl libc 1.2.2
OpenVPN: 2.6.12
DCO module: 20240712

There are 2 network interfaces wan and lan. Both have static IPs:

wan: 192.168.190.105/24
lan: 192.168.8.1/24

server.conf:

local 192.168.190.105
tls-server
allow-compression no
ca /etc/openvpn/pki/ca.crt
cert /etc/openvpn/pki/issued/server.crt
dev tun
dh /etc/openvpn/pki/dh.pem
keepalive 10 60
key /etc/openvpn/pki/private/server.key
server 10.200.200.0 255.255.255.0
topology subnet
tun-mtu 1448
verb 5

client.ovpn:

remote 192.168.190.105
tun-mtu 1448
allow-compression no
client
dev tun
tls-client
# ca, cert, key omitted

Client is at lan side with IP 192.168.8.2.
The initial handshake went through, but once the data path is offloaded, DCO transmit path picked the lan interface IP 192.168.8.1 as the source IP, breaking the connection. Adding disable-dco to server.conf fixes it.

With some debugging I found that local option was not passed to DCO via the netlink attr OVPN_NEW_PEER_ATTR_LOCAL_IP. , so Linux picked lan interface IP as the source IP.

[LGA1150]

Update: found a similar bug when multihome option is enabled. I will push a new patch later.

[LGA1150]

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• https://sourceforge.net/p/openvpn/mailman/message/59119029/