iptables NETMAP target...
But indeed better re-IP server side.
Do not use following common subnets:
10.0.0
10.0.1
10.1.1
10.1.10
10.2.0
10.8.0
10.10.1
10.90.90
10.100.1
10.255.255
169.254 # APIPA #
172.16.0
172.16.16
172.16.42
172.16.68
172.19.3
172.20.10 # IPhone built-in hotspot #
192.168.0
192.168.1
192.168.2
192.168.3
192.168.4
192.168.5
192.168.6
192.168.7
192.168.8
192.168.9
192.168.10
192.168.11
192.168.13
192.168.15
192.168.16
192.168.18
192.168.20
192.168.29
192.168.30
192.168.31
192.168.33
192.168.39
192.168.40
192.168.42 # Android USB tethering #
192.168.43 # Android built-in hotspot #
192.168.50
192.168.55
192.168.61
192.168.62
192.168.65
192.168.77
192.168.80
192.168.85
192.168.88
192.168.98
192.168.99
192.168.100
192.168.101
192.168.102
192.168.111
192.168.123
192.168.126
192.168.129
192.168.137 # Windows Phone built-in hotspot #
192.168.168
192.168.178
192.168.190
192.168.199
192.168.200
192.168.220
192.168.223
192.168.229
192.168.240
192.168.245
192.168.251
192.168.252
192.168.254
200.200.200