multi output >> *.xml

[GuangLightman]

There are several lines like this:
cat >> /usr/lib/firewalld/services/pptpd.xml
This will output to xml file many times when you run the script several times.

PS:
It is handy to show how to add multiple vpn users in chap-secrets file.

Also: .
Based on my experience I encountered several issues.

• Please refer to this blog, http://leijingwei.com/post/87.html, CentOS 7.3 安装 xl2tpd 失败，Requires: kmod(l2tp_ppp.ko).
  Due to xl2tpd-1.3.8-1.el7.x86_64 与 CentOS 7.3不兼容
  I solved by:

wget http://dl.fedoraproject.org/pub/epel/testing/7/x86_64/x/xl2tpd-1.3.8-2.el7.x86_64.rpm 
rpm -ivh xl2tpd-1.3.8-2.el7.x86_64.rpm

• I commented crtscts, lock, modem options in /etc/ppp/options.xl2tpd. Details can be found at https://bbs.archlinux.org/viewtopic.php?id=211885.

• I add the below lines(not sure whether it is necessary, but it solved my problem.)

firewall-cmd --permanent --zone=public --add-port=500/udp
firewall-cmd --permanent --zone=public --add-port=4500/tcp

Thanks for your contributions for this nice script.