L2TP是常用的一种point-site的VPN。而目前在Azure上的VPN Gateway只支持IPsec和SSTP两种。如果客户需要L2TP服务器，需要自己在VM中搭建。本文将介绍如何在Azure上搭建基于CentOS65的L2TP服务器。
一、在Azure上创建VM
1. 创建CentOS6.5的VM
在Azure上创建虚拟机时选择CentOS6.5：
具体创建的过程就不再介绍了。
2. 给VM创建Endpoint
L2TP Over IPSec将使用UDP500和UDP4500两个端口，是标准的IPSEC的NAT-T端口。另外22端口也是需要的。具体的配置如下：
二、在VM上安装软件
通过SSH登陆VM。
1. 检查版本，配置YUM源
cat /etc/redhat-release CentOS release 6.5 (Final)
确认软件版本是CentOS6.5
vim /etc/yum.repos.d/epel.repo [epel]
name=epel
baseurl=http://mirrors.sohu.com/fedora-epel/6/$basearch
enabled=
gpgcheck=
2. 安装软件
a. 首先通过Yum安装：
yum install -y ppp iptables make gcc gmp-devel xmlto bison flex xmlto libpcap-devel lsof
b. 然后安装IPSEC：
下载最新的IPSEC：
wget https://download.openswan.org/openswan/openswan-2.6.49.tar.gz --no-check-certificate
解压：
tar vxf openswan-2.6..tar.gz
安装：
make programs install
c. 通过Yum安装epel里的xl2tpd:
yum install -y xl2tpd
三、配置各种服务
1. IPSEC的配置
vim /etc/ipsec.conf # /etc/ipsec.conf - Openswan IPsec configuration file
# This file: /usr/local/share/doc/openswan/ipsec.conf-sample
#
# Manual: ipsec.conf.
version 2.0 # conforms to second version of ipsec.conf specification
# basic configuration
config setup
# Do not set debug options to debug configuration issues!
# plutodebug / klipsdebug = "all", "none" or a combation from below:
# "raw crypt parsing emitting control klips pfkey natt x509 dpd private"
# eg:
# plutodebug="control parsing"
# Again: only enable plutodebug or klipsdebug when asked by a developer
#
# enable to get logs per-peer
# plutoopts="--perpeerlog"
#
# Enable core dumps (might require system changes, like ulimit -C)
# This is required for abrtd to work properly
# Note: incorrect SElinux policies might prevent pluto writing the core
dumpdir=/var/run/pluto/
#
# NAT-TRAVERSAL support, see README.NAT-Traversal
nat_traversal=yes
# exclude networks used on server side by adding %v4:!a.b.c./
# It seems that T-Mobile in the US and Rogers/Fido in Canada are
# using / as "private" address space on their 3G network.
# This range has not been announced via BGP (at least upto --)
virtual_private=%v4:10.0.0.0/,%v4:192.168.0.0/,%v4:172.16.0.0/,%v4:25.0.0.0/,%v6:fd00::/,%v6:fe80::/
# OE is now off by default. Uncomment and change to on, to enable.
oe=off
# which IPsec stack to use. auto will try netkey, then klips then mast
protostack=netkey
#protostack=auto
# Use this to log to a file, or disable logging on embedded systems (like openwrt)
#plutostderrlog=/dev/null
# Add connections here
# sample VPN connection
# for more examples, see /etc/ipsec.d/examples/
#conn sample
# # Left security gateway, subnet behind it, nexthop toward right.
# left=10.0.0.1
# leftsubnet=172.16.0.0/
# leftnexthop=10.22.33.44
# # Right security gateway, subnet behind it, nexthop toward left.
# right=10.12.12.1
# rightsubnet=192.168.0.0/
# rightnexthop=10.101.102.103
# # To authorize this connection, but not actually start it,
# # at startup, uncomment this.
# #auto=add
conn L2TP-PSK-NAT
rightsubnet=vhost:%priv
also=L2TP-PSK-noNAT
conn L2TP-PSK-noNAT
authby=secret
pfs=no
auto=add
keyingtries=
rekey=no
ikelifetime=8h
keylife=1h
type=transport
left=10.215.92.31 #AWS EC2 Internal IP
leftprotoport=/
right=%any
rightprotoport=/%any
dpddelay=
dpdtimeout=
dpdaction=clear
2. 配置IPSEC的Security
vim /etc/ipsec.secrets # This file holds shared secrets or RSA private keys for inter-Pluto
# authentication. See ipsec_pluto() manpage, and HTML documentation.
# RSA private key for this host, authenticating it to any other host
# which knows the public part. Suitable public keys, for ipsec.conf, DNS,
# or configuration of other implementations, can be extracted conveniently
# with "ipsec showhostkey".
# this file is managed with debconf and will contain the automatically created RSA keys
#include /var/lib/openswan/ipsec.secrets.inc
%any %any: PSK "azure"
3. 配置sysctl.conf文件
vim /etc/sysctl.conf net.ipv4.ip_forward =
net.ipv4.conf.default.rp_filter =
net.ipv4.conf.all.send_redirects =
net.ipv4.conf.default.send_redirects =
net.ipv4.conf.all.log_martians =
net.ipv4.conf.default.log_martians =
net.ipv4.conf.default.accept_source_route =
net.ipv4.conf.all.accept_redirects =
net.ipv4.conf.default.accept_redirects =
net.ipv4.icmp_ignore_bogus_error_responses =
通过下面的命令，是配置生效：
sysctl -p
service ipsec start
ipsec verify
4. 配置xl2tpd
vim /etc/xl2tpd/xl2tpd.conf ;
; This is a minimal sample xl2tpd configuration file for use
; with L2TP over IPsec.
;
; The idea is to provide an L2TP daemon to which remote Windows L2TP/IPsec
; clients connect. In this example, the internal (protected) network
; is 192.168.1.0/. A special IP range within this network is reserved
; for the remote clients: 192.168.1.128/
; (i.e. 192.168.1.128 ... 192.168.1.254)
;
; The listen-addr parameter can be used if you want to bind the L2TP daemon
; to a specific IP address instead of to all interfaces. For instance,
; you could bind it to the interface of the internal LAN (e.g. 192.168.1.98
; in the example below). Yet another IP address (local ip, e.g. 192.168.1.99)
; will be used by xl2tpd as its address on pppX interfaces.
[global]
; listen-addr = 192.168.1.98
;
; requires openswan-2.5. or higher - Also does not yet work in combination
; with kernel mode l2tp as present in linux 2.6.+
; ipsec saref = yes
; Use refinfo of if using an SAref kernel patch based on openswan 2.6. or
; when using any of the SAref kernel patches for kernels up to 2.6..
; saref refinfo =
;
; force userspace = yes
;
; debug tunnel = yes
ipsec saref = no
[lns default]
ip range = 192.168.1.128-192.168.1.254
local ip = 192.168.1.99
require chap = yes
refuse pap = yes
require authentication = yes
name = LinuxVPNserver
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
5. 配置ppp的用户名密码
vim /etc/ppp/chap-secrets # Secrets for authentication using CHAP
# client server secret IP addresses
user * azure *
6. 配置iptables
iptables -t nat -A POSTROUTING -s 192.168.1.0/ -o eth0 -j MASQUERADE
iptables -I FORWARD -s 192.168.1.0/ -j ACCEPT
iptables -I FORWARD -d 192.168.1.0/ -j ACCEPT
service iptables save
四、设置服务
service ipsec restart
service xl2tpd restart
service iptables restart
chkconfig xl2tpd on
chkconfig iptables on
chkconfig ipsec on
五、设置IPHONE