报错的是Server端的xl2tpd服务，错误日志如下
pppd[1408]: Sent 259920 bytes, received 75036 bytes.
charon: 14[KNL] interface ppp0 deactivated
systemd-networkd: ppp0 : lost carrier
charon: 15[KNL] 192.168.xxx.xxx disappeared from ppp0
xl2tpd: xl2tpd[918]: result_code_avp: result code endianness fix for buggy Apple client. network=xxx, le=3
xl2tpd: xl2tpd[918]: control_finish: Connection closed to xxx.xxx.xxx.xxx, serial 1 ()
pppd[1408]: Terminating on signal 15
pppd[1408]: Modem hangup
pppd[1408]: Connection terminated.
v6 charon: 05[KNL] interface ppp0 deleted解法办法，在xl2tp.conf的global部分中加入即可解决：
ipsec saref = yes