Question

How to forward traffic from Pub IP of VPN Gateway A to VPN Gateway B internal Host

Posted July 31, 2020 192 views
VPN

I setup a IPSec connection from a VPN GW-A to VPN GW-B, hosts in VPN A subnet can send packets to hosts in VPN B subnet and vice-verse.

VPN GW-A is Ubuntu Server.

I need to connect to GW-A pub IP on port 9876 and route/forward to host in the GW-B subnet to the same port 9876.

What i did

net.ipv4.ip_forward=1
net.ipv6.conf.all.forwarding=1

-A PREROUTING -d GW-A/32 -p tcp -m tcp –dport 9876 -j DNAT –to-destination HOST-B:9876
-A POSTROUTING -s HOST-B/32 -d GW-A/32 -p tcp -m tcp –sport 9876 –dport 9876

My tcpdump log when try to connect to GW-A on port 9876 is

12:07:51.830469 IP CLIENT.60364 > GW-A.9876: Flags [SEW], seq 2029591603, win 8192, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
12:07:51.830520 IP GW-A.60364 > HOST-B.9876: Flags [SEW], seq 2029591603, win 8192, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
12:07:57.898877 IP CLIENT.60364 > GW-A.9876: Flags [S], seq 2029591603, win 8192, options [mss 1460,nop,nop,sackOK], length 0
12:07:57.898928 IP GW-A.60364 > HOST-B.9876: Flags [S], seq 2029591603, win 8192, options [mss 1460,nop,nop,sackOK], length 0

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