近年来最累的一次折腾。有必要好好记录一下。跟了网上N个教程，有好几个都走不通。最后自己彻底整理了一次。把自己的经验和配置分享出来。
首先隆重感谢一下至今素未谋面的老熊，真是好人啊。06年的时候就在plesk服务器换ip的事上，帮了我好大的忙。没想到这次在我卡在第6步，最绝望的时候，这只熊的婀娜身影再一次出现在了我的生活中。还在漆黑的深夜中陪伴在我的身边。以下隆重公布一下他的QQ，造福广大网友！QQ: 499******* （后几位号码，为符合当地莫名其妙的“相关法规”，被系统自动屏蔽。。。）
其次要感谢老熊的同事，百忙中写好的防火墙规则，让我10分钟之内，就被兴奋得昏了头的我，给无情覆盖了。以至于下次人家重写之后，很自觉的在/root下给留了个备份。
1. 先安装openswan
wget http://download.openswan.org/openswan/openswan-2.6.38.tar.gz
tar zxvf openswan-2.6.38.tar.gz
cd openswan-2.6.38
make programs install
gkp说“安装 openswan，记得别用2.6.26，宁可用2.6.24。他和xl2tpd存在严重兼容性bug”
2. 安装xl2tpd
在centos 6官方的yum源中，没有这个软件包。需要安装fedora的epel源。
#CentOS 5用这个地址： rpm -Uvh http://mirrors.ustc.edu.cn/fedora/epel/5/i386/epel-release-5-4.noarch.rpm
yum install xl2tpd -y
如果上面这个安装地址如果不对了。参照这个fedora epel的faq页（https://fedoraproject.org/wiki/EPEL/FAQ/zh-cn#How_can_I_install_the_packages_from_the_EPEL_software_repository.3F）
安装其他一些linux软件源，参照这页：http://www.esojourn.org/blog/post/repo-sources.php
编辑 /etc/ipsec.conf 注意$esojourn.org替换成你机器的ip。这个IP应该是你的公网IP，不是本机内网IP。
# This file: /usr/local/share/doc/openswan/ipsec.conf-sample
#
# Manual: ipsec.conf.5
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.0/24
# It seems that T-Mobile in the US and Rogers/Fido in Canada are
# using 25/8 as “private” address space on their 3G network.
# This range has not been announced via BGP (at least upto 2010-12-21)
virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12,%v4:25.0.0.0/8,%v6:fd00::/8,%v6:fe80::/10
# 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
#这里注释说可以用auto，但实际上不行。还得老老实实写netkey
# 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=172.16.0.0/24
# leftnexthop=10.22.33.44
# # Right security gateway, subnet behind it, nexthop toward left.
# right=10.12.12.1
# rightsubnet=192.168.0.0/24
# 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=3
rekey=no
ikelifetime=8h
keylife=1h
type=transport
left=$esojourn.org #这里写公网IP，没固定IP的就到花生壳弄个动态域名解析。
leftid=$esojourn.org
leftprotoport=17/1701
right=%any
修改ipsec密钥
www.esojourn.org %any: PSK “myps”
#www.esojourn.org改成公网ip，没固定IP的，就到花生壳弄个动态域名解析。http://www.oray.com/peanuthull/
#myps改成密码
3. 修改/etc/sysctl.conf，开启路由功能
星云说：
将下面两项找到：
net.ipv4.conf.default.rp_filter = 1
改为：
net.ipv4.conf.default.rp_filter = 0
之后先让修改后的配置生效，再测试一下，
service ipsec start
ipsec verify
如果行了就这样了，但实际上，我反复测试之后，干脆在sysctl.conf最下面，加了这一段。加好后别忘了再sysctl -p一下。
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.default.log_martians = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.icmp_ignore_bogus_error_responses = 1
如果ipsec verify返回这样，就可以了。
Version check and ipsec on-path [OK]
Linux Openswan U2.6.24/K2.6.32.16-linode28 (netkey)
Checking for IPsec support in kernel [OK]
NETKEY detected, testing for disabled ICMP send_redirects [OK]
NETKEY detected, testing for disabled ICMP accept_redirects [OK]
Checking for RSA private key (/etc/ipsec.secrets) [OK]
Checking that pluto is running [OK]
Pluto listening for IKE on udp 500 [OK]
Pluto listening for NAT-T on udp 4500 [OK]
Two or more interfaces found, checking IP forwarding [OK]
Checking NAT and MASQUERADEing
Checking for ‘ip’ command [OK]
Checking for ‘iptables’ command [OK]
Opportunistic Encryption Support [DISABLED]
我的经验中，这里可能会出这么几个故障：
第一：
这样的话， /etc/xl2tpd/xl2tpd.conf这个文件里
ipsec saref = no
第二：
Checking NAT and MASQUERADEing [OK]
ip转发检查失败。
Paul和Steve说：只要 cat /proc/sys/net/ipv4/ip_forward 返回结果是1就没事。这个我已经证实了。
第三：返回结果里，就没有Two or more interfaces found, checking IP forwarding和Checking NAT and MASQUERADEing这两项。碰上这个我也没办法了。重装了系统，又瞎折腾一通之后就好了。
[ 2012-7-6 ]
第四：更新加入一个新的情况
SAref kernel support [N/A]
NETKEY: Testing XFRM related proc values [OK]
[OK]
[FAILED]
Please enable /proc/sys/net/core/xfrm_larval_drop
or NETKEY will cause non-POSIX compliant long time-outs
Checking that pluto is running [OK]
出这毛病，照着说明，
vi /etc/bashrc
然后把“echo 1 > /proc/sys/net/core/xfrm_larval_drop”这句加进去。以后每次开机自动执行一下。
4. 编辑 /etc/xl2tpd/xl2tpd.conf
vi /etc/xl2tpd/xl2tpd.conf
; 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 192.168.1.0/24. A special IP range within this network is reserved
; for the remote clients: 192.168.1.128/25
; (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 = esojourn.org
;这里写内网IP。或者加;注释掉也问题不大。
;
; requires openswan-2.5.18 or higher – Also does not yet work in combination
; with kernel mode l2tp as present in linux 2.6.23+
; ipsec saref = yes
ipsec saref = no
;这里一般都让写yes，但如果刚才你的ipsec verify中，有报SAref kernel support[N/A]的话，这里就写no。
; Use refinfo of 22 if using an SAref kernel patch based on openswan 2.6.35 or
; when using any of the SAref kernel patches for kernels up to 2.6.35.
; ipsec refinfo = 30
;
; forceuserspace = yes
;
; debug tunnel = yes
[lns default]
ip range = 192.168.7.128-192.168.7.254
;这里写vpn client的ip段
local ip = 192.168.7.1
require chap = yes
refuse pap = yes
require authentication = yes
name = LinuxVPNserver
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
5. 修改 /etc/ppp/options.xl2tpd
vi /etc/ppp/options.xl2tpd
ipcp-accept-local
ipcp-accept-remote
ms-dns 8.8.4.4
ms-dns 8.8.8.8
noccp
auth
crtscts
idle 1800
mtu 1410
mru 1410
nodefaultroute
debug
lock
proxyarp
connect-delay 5000
vi /etc/ppp/chap-secrets
# user server password ip
username * userpass * #改成你的用户名 密码
6. 最关键的iptables配置
做完上面这堆步骤之后，客户端建个连接就可以验证进入vpn主机了。但是无法访问内外网。我就是在这里卡了一个多星期。网上的资料，有的没说这一步。有的说了也没说全。我发个权威的吧。
iptables -I FORWARD -s 192.168.7.0/24 -j ACCEPT
iptables -I FORWARD -d 192.168.7.0/24 -j ACCEPT
iptables-save
service iptables restart
192.168.7.0/24根据实际情况替换。
vi /etc/sysconfig/iptables 看到的应该是类似这样。
最上面先是nat规则，下面是filter规则。
下面filter表里，先把VPN要用到的udp端口1701，500，4500都打开。要用openvp的话，还要开1194。
另外filter表里，一定要有FORWARD规则。这点在网上好几个教程里都没说！坑死人。
*nat
:PREROUTING ACCEPT [39:3503]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A POSTROUTING -s 192.168.7.0/24 -o eth0 -j MASQUERADE
COMMIT
# Completed on Thu Jun 28 15:50:40 2012
# Generated by iptables-save v1.4.7 on Thu Jun 28 15:50:40 2012
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [121:13264]
-A INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 22 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 80 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 53 -j ACCEPT
-A INPUT -p udp -m state –state NEW -m udp –dport 53 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 1194 -j ACCEPT
-A INPUT -p udp -m state –state NEW -m udp –dport 1701 -j ACCEPT
-A INPUT -p udp -m state –state NEW -m udp –dport 500 -j ACCEPT
-A INPUT -p udp -m state –state NEW -m udp –dport 4500 -j ACCEPT
-A INPUT -j REJECT –reject-with icmp-host-prohibited
-A FORWARD -d 192.168.7.0/24 -j ACCEPT
-A FORWARD -s 192.168.7.0/24 -j ACCEPT
-A FORWARD -j REJECT –reject-with icmp-host-prohibited
COMMIT
# Completed on Thu Jun 28 15:50:40 2012
最后
service iptables restart
chkconfig xl2tpd on
chkconfig iptables on
chkconfig ipsec on