forwarding packets between two openvpn tunnel networks on the same host

Question

I have two p2p tunnels to two different servers on a host, tun1 and tun2: (host is Ubuntu server 18.04)

(Server 1: IP1) <--> (Host: IP0) <--> (Server 2: IP2)

For tun1 to server 1 I have:

proto       udp
mode        p2p
remote      IP1
rport       4856
local       IP0
lport       4856
dev-type    tun
tun-ipv6
resolv-retry infinite
dev  tun1
comp-lzo
persist-key
persist-tun
cipher aes-256-cbc
ifconfig 192.168.76.2  192.168.76.3
secret /etc/openvpn/key.key

for tun2 to server 2 I have:

proto       udp
mode        p2p
remote      IP2
rport       4857
local       IP0
lport       4857
dev-type    tun
tun-ipv6
resolv-retry infinite
dev  tun2
comp-lzo
persist-key
persist-tun
cipher aes-256-cbc
ifconfig 192.168.77.2  192.168.77.3
secret /etc/openvpn/key.key

I want to forward packets received on tun1 with dst=IP4 to tun2 so I added a static route:

ip route add IP4/32 via 192.168.77.3

Ip forwarding is also enabled.

sysctl -w net.ipv4.ip_forward=1

No Iptables rules existed. All chains have ACCEPT behavior.

All tunnel interfaces are connected to their corresponding servers and up. When I send packets from Server 1 with dst=IP4 to the tunnel, packets are received in host but they are not forwarded to tun2 and I cannot see them in Server 2 using tcpdump. Any idea?

Answer 1

I have to explicitly enable forwarding on tun1 (incoming traffic):

sysctl -w net.ipv4.conf.tun1.forwarding=1

sysctl -w net.ipv4.ip_forward=1 is not enough (actually it is not needed).