http://blog.lincoln.hk/blog/2013/05/17/softether-on-vps-using-local-bridge
I set softether referring above link. And I set an L2TP vpn.

My "ifconfig"

eth0 (I replaced my server ip with 1.2.3.4）

Link encap:Ethernet HWaddr fa:16:3e:3c:b0:a0
inet addr:1.2.3.4 Bcast:1.2.3.255 Mask:255.255.254.0
inet6 addr: fe80::f816:3eff:fe3c:b0a0/64 Scope:Link

lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host

tap_soft
Link encap:Ethernet HWaddr 00:ac:ab:80:3c:c7
inet addr:10.10.10.10 Bcast:10.255.255.255 Mask:255.0.0.0
inet6 addr: fe80::2ac:abff:fe80:3cc7/64 Scope:Link

My dnsmasq.conf

interface=tapsoft
dhcp-range=tapsoft,10.10.10.10,10.10.10.20,12h
dhcp-option=tap_soft,3,10.10.10.10

My ufw setup

ufw default reject incoming
ufw default allow outgoing
ufw allow 500/udp
ufw allow 4500/udp
ufw allow 1701

I can't connect to L2TP when I enable ufw. But it's ok when ufw disabled.

Thanks!