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