Ubuntu 15.10搭建IPSec L2TP服务器

以下步骤完全使用于Ubuntu 14.04版本

首先安装以下所需包

sudo apt-get install openswan xl2tpd ppp lsof
！注意！Ubuntu 15.10会提示无法找到openswan包，解决办法是用喜欢的编辑器编辑以下文件，实例用vim
sudo vim /etc/apt/sources.list
在文件最后添加一行
deb http://us.archive.ubuntu.com/ubuntu precise main universe
保存后，执行下面命令
sudo apt-get update
sudo apt-get install openswan

防火墙和转发

设置允许转发数据包

iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+

将上面的%SERVERIP%替换成服务器IP，eth+换成服务器的网卡名。！注意！不一定所有的网卡都是eth开头的

执行下列命令允许内核转发，并且禁用ICP重定向

echo "net.ipv4.ip_forward = 1" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.accept_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.all.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.rp_filter = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.accept_source_route = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.conf.default.send_redirects = 0" |  tee -a /etc/sysctl.conf
echo "net.ipv4.icmp_ignore_bogus_error_responses = 1" |  tee -a /etc/sysctl.conf

如果有其他网络接口，执行同样的设置

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done

应用以上设置

sysctl -p

保证每次开机这些设置生效

for vpn in /proc/sys/net/ipv4/conf/*; do echo 0 > $vpn/accept_redirects; echo 0 > $vpn/send_redirects; done
iptables -t nat -A POSTROUTING -j SNAT --to-source %SERVERIP% -o eth+

配置OpenSwan(IPSEC)

用编辑器打开以下文件

/etc/ipsec.conf 

# /etc/ipsec.conf - Openswan IPsec configuration file

# This file:  /usr/share/doc/openswan/ipsec.conf-sample
#
# Manual:     ipsec.conf.

version    2.0    # conforms to second version of ipsec.conf specification

# basic configuration
config setup
    dumpdir=/var/run/pluto/
    #in what directory should things started by setup (notably the Pluto daemon) be allowed to dump core?
    nat_traversal=yes
    #whether to accept/offer to support NAT (NAPT, also known as "IP Masqurade") workaround for IPsec
    virtual_private=%v4:10.0.0.0/,%v4:192.168.0.0/,%v4:172.16.0.0/,%v6:fd00::/,%v6:fe80::/
    #contains the networks that are allowed as subnet= for the remote client. In other words, the address ranges that may live behind a NAT router through which a client connects.
    oe=off
    protostack=netkey
    #decide which protocol stack is going to be used.

conn L2TP-PSK-NAT
    rightsubnet=vhost:%priv
    also=L2TP-PSK-noNAT

conn L2TP-PSK-noNAT
    authby=secret
    #shared secret. Use rsasig for certificates.
    pfs=no
    #Disable pfs
    rekey=no
    dpddelay=
    dpdtimeout=
    dpdaction=clear
    auto=add
    #start at boot
    keyingtries=
    #Only negotiate a conn.  times.
    ikelifetime=8h
    keylife=1h
    type=transport
    #because we use l2tp as tunnel protocol
    left=%SERVERIP%
    leftprotoport=/%any
    right=%any
    rightprotoport=/%any
    forceencaps=yes
    #force all to be nat'ed. because of iOS

将上述%SERVERIP%换成服务器IP

共享密钥

用编辑器打开以下文件

/etc/ipsec.secrets

添加以下格式的内容

%SERVERIP%  %any:   PSK "69EA16F2C529E74A7D1B0FE99E69F6BDCD3E44"

%SERVERIP%为服务器IP地址。PSK为共享密钥，可以用以下命令生成随机密钥

openssl rand -hex 

验证OpenSwan(IPSEC)设置

ipsec verify

我的机器示例输出如下

Linux Openswan U2.6.37/K4.2.0--generic (netkey)
Checking for IPsec support in kernel                            [OK]
 SAref kernel support                                           [N/A]
 NETKEY:  Testing XFRM related proc values                      [OK]
    [OK]
    [OK]
Hardware RNG detected, testing if used properly                 [FAILED]

  Hardware RNG is present but 'rngd' or 'clrngd' is not running.
  No harware random used!

Checking that pluto is running                                  [OK]
 Pluto listening for IKE on udp                              [OK]
 Pluto listening for NAT-T on udp                           [OK]
Two or more interfaces found, checking IP forwarding            [OK]
Checking NAT and MASQUERADEing                                  [OK]
Checking for 'ip' command                                       [OK]
Checking /bin/sh is not /bin/dash                               [WARNING]
Checking for 'iptables' command                                 [OK]
Opportunistic Encryption Support                                [DISABLED]

配置xl2tpd

用编辑器编辑下述文件

/etc/xl2tpd/xl2tpd.conf  

将内容替换为以下内容

[global]
ipsec saref = yes
saref refinfo = 

[lns default]
ip range = 172.16.1.2-172.16.1.100
local ip = 172.16.1.1
refuse pap = yes
require authentication = yes
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes

配置PPP

编辑以下文件

/etc/ppp/options.xl2tpd 

将文件内容替换为以下内容

require-mschap-v2
ms-dns 8.8.8.8
ms-dns 8.8.4.4
auth
mtu
mru
crtscts
hide-password
modem
name l2tpd
proxyarp
lcp-echo-interval
lcp-echo-failure 

添加用户（用户认证）

编辑以下文件添加用户

/etc/ppp/chap-secrets

格式内容如下

# Secrets for authentication using CHAP
# client       server  secret                  IP addresses
alice          l2tpd   0F92E5FC2414101EA            *
bob            l2tpd   DF98F09F74C06A2F             *

可以使用以下脚本自动添加用户名和密码。复制内容到本地文件。重命名为addvpnuser.sh

#this script add vpn user to /etc/ppp/chap-secret
#you must be root to run this script

if [ $EUID -ne  ]
then
    echo "You must be root to run this script."
    exit -
fi

if ! getopts ":a:d:q:" opt
then
    echo "Usage: ./addvpnuser.sh -a username | -d username | -q username"
    echo "-a for add, -d for delete and -q for query user's password ."
    exit -
fi

OPTIND=

while getopts ":a:d:q:" opt
do
    case $opt in
        a)
            #add user switch is triggered.

            #check the username parameter is correct.
            if [[ $OPTARG = -* ]]
            then
               echo "Option -a needs an username as a parameter."
               ((OPTIND--))
               continue
            fi

            #check whether the user to be added is existed.
            if grep -qP "^$OPTARG\t" /etc/ppp/chap-secrets
            then
                echo "cannot add user '$OPTARG' because the user has already existed."
                continue
            fi
            #get a random number less than .
            n=$RANDOM
            n=$(( n %  ))

            #VPN Server Type.
            TYPE="l2tpd"

            #get the server external IP.
            IP=`wget http://ipinfo.io/ip -qO -`

            #get the PRE PSK key of the VPN.
            PREKEY=`cat /etc/ipsec.secrets | tail - | awk '{print $4}' | cut -d\" -f2`

            #calculate user's password using the following algorithm.
            PW=`echo -n $OPTARG$RANDOM | md5sum | cut -c$n-$(( n+ ))`
            echo -e "$OPTARG\t$TYPE\t$PW\t*" >> /etc/ppp/chap-secrets

            #successfully added user to configure file.
            #print out user information. copy and send to him.
            if [ $? -eq  ]
            then
                echo "Type: $TYPE"
                #IP=`ifconfig eth0 | grep "inet addr" | awk '{print $2}' | cut -d: -f2`
                echo "VPN Server IP: $IP"
                echo "User: $OPTARG Password: $PW"
                echo "PRE PSK KEY: $PREKEY"
                echo "Done."
            else
                #failed to write the file.
                echo "cannot write to configure file."
            fi
            ;;
        d)
            #echo "-d was triggered. Parameter: $OPTARG"
            #check the username parameter is correct.
            if [[ $OPTARG = -* ]]
            then
               echo "Option -d requires an username as a parameter."
               ((OPTIND--))
               continue
            fi

            #check whether the user to be deleted is exist.
            if ! grep -qP "^$OPTARG\t" /etc/ppp/chap-secrets
            then
                echo "user '$OPTARG' does not exist."
                continue
            fi

            #maybe can use $$(pid) instead
            RANDOM1=$RANDOM
            RANDOM2=$RANDOM

            #delete and overwrite file.
            grep -vP "^$OPTARG\t" /etc/ppp/chap-secrets > /etc/ppp/chap-secrets$RANDOM1$RANDOM2
            mv /etc/ppp/chap-secrets$RANDOM1$RANDOM2 /etc/ppp/chap-secrets

            #check result.
            if [ $? -eq  ]
            then
                echo "user '$OPTARG' has been successfully deleted."
            else
                echo "cannot write to file. Please check it."
            fi
            ;;
        q)
            #check the username parameter is correct.
            if [[ $OPTARG = -* ]]
            then
               echo "Option -q requires an username as a parameter."
               ((OPTIND--))
               continue
            fi
            grep -P "^$OPTARG\t" /etc/ppp/chap-secrets | awk '{print $1, $3}' >/dev/null
            ;;

        \?)
            echo "Invalid option -$OPTARG"
            ;;
        :)
            echo "Option -$OPTARG requires an username as a parameter."
            ;;
    esac
done

保存后为文件添加可执行权限

chmod  addvpnuser.sh

执行示例

#添加VPN用户
./addvpnuser.sh -a [username]
#删除VPN用户
./addvpnuser.sh -d [username]
#查询VPN用户密码
./addvpnuser.sh -q [username]

测试

执行下列命令重启IPsec和xl2tpd服务

/etc/init.d/ipsec restart
/etc/init.d/xl2tpd restart

防火墙

如果开启了ufw防火墙的话，则需要开放udp 500, udp 4500, udp 1701三个端口

ufw allow /udp
ufw allow /udp
ufw allow /udp

注意

配置过程中发现按照上述步骤操作，通过Android或者iOS设备可以连接到VPN服务器，但是不能连接到互联网。经过调试发现是由于Ubuntu开启了ufw(防火墙)，数据包被防火墙屏蔽掉了。解决办法如下

打开以下文件

/etc/default/ufw

将下列行中的DROP替换成ACCEPT

DEFAULT_FORWARD_POLICY="DROP"
替换成
DEFAULT_FORWARD_POLICY="ACCEPT"

调试

出现无法连接或者无法联网的情况，下列命令可以帮助调试

sudo tcpdump -i ppp0
sudo tail -f /var/log/auth.log
sudo tail -f /var/log/syslog