+-----------+
                   |172.20.15.1|
                   +-----------+
        route:192.168.10.0/24 via 172.20.15.150
wanted: SNAT 172.20.15.160 to 172.20.15.1 destination 192.168.10.0/24
                          |
                          |
       +------------------+
       |                  |
+-------------+    +-------------+    +---------------+
|172.20.15.160|    |172.20.15.150|----|192.168.10.0/24|
+-------------+    +-------------+    +---------------+
GW:172.20.15.1     rule: allow from
                   172.20.15.1 to
                   192.168.10.0/24