IP: tunnel transformation

modulename: xfrm4_tunnel.ko

and/or

tunnel4.ko

configname: CONFIG_INET_TUNNEL

Linux Kernel Configuration
└─>Networking
└─>Networking options
└─>IP: tunnel transformation
In linux kernel since version 2.6.12  
Support for generic IP tunnel transformation, which is required by
the IP tunneling module as well as tunnel mode IPComp.

If unsure, say Y.

source code:
net/ipv4/xfrm4_tunnel.c
selects
CONFIG_XFRM