CONFIG_XFRM (not configurable)

modulename: xfrm4_policy.ko

and/or

flow.ko

and/or

xfrm_policy.ko

configname: CONFIG_XFRM

Linux Kernel Configuration
└─>Networking
└─>Networking options
└─>XFRM
In linux kernel since version 2.6.12  
source code:
net/ipv4/

depends
CONFIG_NET

is selected by
CONFIG_XFRM_USER