X.25 protocol support

modulename: hdlc_x25.ko

configname: CONFIG_HDLC_X25

Linux Kernel Configuration
└─>Device Drivers
└─>Network device support
└─>Wan interfaces support
└─>X.25 protocol support
In linux kernel since version 2.6.12  
Generic HDLC driver supporting X.25 over WAN connections.

If unsure, say N.

source code:
drivers/net/wan/hdlc_x25.c