Internet Protocol Security (IPsec) 网际网路安全协定

前言

越来越多公司用 IPSEC 来建立 Site to Site VPN,普遍应用於透过 Internet 建立连线,因为价钱便宜,设定上相对简单得多。IPSEC VPN 用 IKE (Internet Key Exchange) 进行参数交换和建立连线,因此在设定 IPSEC VPN 前应先了解 IKE 的运作。

IKE (Internet Key Exchange)

IKE 又分为 Phase 1 和 Phase 2,现在来看看 Phase 1 和 Phase 2 分别做了些什麽。

Phase 1

IKE Phase 1 的工作主要是认证 (authenticate) 对方,什麽是认证呢?很简单,我是 Site A 的 Router,你说你是 Site B 的 Router 想和我建立 VPN,我怎知道你真的是 Site B 而不是其他公司呢?其中一个常用的方法是用 Share Secret 来认证,即是说 Site A 和 Site B 的 Router 都知道同一组密码,证明是自己人,就通过认证了。另外,Phase 1 还会用透过 Diffie-Hellman 来建立一组 Key,这组 Key 是用来为 Phase 2 的资讯进行加密,即是说 Phase 1 的工作就是为 Phase 2 准备一条加密管道,让 Phase 2 用这条管道去交换资讯。

Phase 2

至於 Phase 2 则是真正建立 IPSEC VPN 管道让资讯在这条管道上传送。记着 Phase 2 是建立在 Phase 1 之上的,没有 Phase 1 就没有 Phase 2。

设定

现在我们试为以下网络的 Site A 和 Site B 设定 Site to Site IPSEC VPN。

ipsec

Router 的起始设定如下:

hostname R1
!
interface Ethernet0/0
 ip address 192.168.13.1 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.10.1 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 192.168.13.3
hostname R2
!
interface Ethernet0/0
 ip address 192.168.23.2 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.20.1 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 192.168.23.3
hostname R3
!
interface Ethernet0/0
 ip address 192.168.13.3 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.23.3 255.255.255.0

所以,R1 能够 Ping 通 192.168.23.2,但就去不到 192.168.20.1,因为我们假设 192.168.10.0 / 24 和 192.168.20.0 / 24 都是 Internal IP。

R1#ping 192.168.23.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.23.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/49/92 ms
R1#
R1#ping 192.168.20.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.20.1, timeout is 2 seconds:
UUUUU
Success rate is 0 percent (0/5)

Step 1: 设定 Interesting Traffic

所谓 Interesting Traffic 就是需要建立 VPN 的 Traffic,在本例子中,R1 的 Interesting Traffic 就是由 192.168.10.0 / 24 到 192.168.20.0 / 24 的 Traffic,而 R2 则刚好相反,是由 192.168.20.0 / 24 到 192.168.10.0 / 24,我们用 Access List 把这些 Traffic Define 好。

R1(config)#ip access-list extended VPN-Traffic
R1(config-ext-nacl)#permit ip 192.168.10.0 0.0.0.255 192.168.20.0 0.0.0.255
R1(config-ext-nacl)#exit
R2(config)#ip access-list extended VPN-Traffic
R2(config-ext-nacl)#permit ip 192.168.20.0 0.0.0.255 192.168.10.0 0.0.0.255
R2(config-ext-nacl)#exit

Step 2: 设定 IKE Phase 1 参数

R1(config)#crypto isakmp policy 1
!encryption 可使用 DES, 3DES 或 AES,其中以 AES Security 最高
R1(config-isakmp)#encryption aes
!hash 可选 SHA 或 MD5,SHA Security 较高
R1(config-isakmp)#hash md5
!用 Pre Share Key 的方法来认证对方
R1(config-isakmp)#authentication pre-share
!DH Group 用作产生 Symmetric Key,一般使用 Group 2
R1(config-isakmp)#group 2
!管道的 Lifetime,预设为 86400
R1(config-isakmp)#lifetime 30000
R2(config)#crypto isakmp policy 1
R2(config-isakmp)#encryption aes
R2(config-isakmp)#hash md5
R2(config-isakmp)#authentication pre-share
R2(config-isakmp)#group 2
R2(config-isakmp)#lifetime 30000

Step 3: 设定 IKE Phase 2 参数

可以在 AH Transform 选择一款,在 ESP Cipher 选择一款,在 ESP Auth 选择一款,基本上选用越多就越安全但比较耗资源。最後选择是否要做 Compression。所以最多可选 4 个 Option。无论选多少个,两边 Peer 的选择必需相同。

R1(config)#crypto ipsec transform-set TS ?
  ah-md5-hmac   AH-HMAC-MD5 transform
  ah-sha-hmac   AH-HMAC-SHA transform
  comp-lzs      IP Compression using the LZS compression algorithm
  esp-3des      ESP transform using 3DES(EDE) cipher (168 bits)
  esp-aes       ESP transform using AES cipher
  esp-des       ESP transform using DES cipher (56 bits)
  esp-md5-hmac  ESP transform using HMAC-MD5 auth
  esp-null      ESP transform w/o cipher
  esp-seal      ESP transform using SEAL cipher (160 bits)
  esp-sha-hmac  ESP transform using HMAC-SHA auth
R1(config)#crypto ipsec transform-set TS esp-3des ah-sha-hmac
R2(config)#crypto ipsec transform-set TS esp-3des ah-sha-hmac

Step 4: 设定 Pre-share Key

假设以 ccie 为 Pre-share Key。

R1(config)#crypto isakmp key 6 ccie address 192.168.23.2
R2(config)#crypto isakmp key 6 ccie address 192.168.13.1

Step 5: 定义 Crypto Map

把我们刚才设定的东西,一次过放进 Crypto Map 之中。最後把 Crypto Map 放上 Interface。

R1(config)#crypto map CMAP 1 ipsec-isakmp
% NOTE: This new crypto map will remain disabled until a peer
        and a valid access list have been configured.
R1(config-crypto-map)#set peer 192.168.23.2
R1(config-crypto-map)#set transform-set TS
R1(config-crypto-map)#match address VPN-Traffic
R1(config-crypto-map)#exit
R1(config)#interface ethernet 0/0
R1(config-if)#crypto map CMAP
R2(config)#crypto map CMAP 1 ipsec-isakmp
% NOTE: This new crypto map will remain disabled until a peer
        and a valid access list have been configured.
R2(config-crypto-map)#set peer 192.168.13.1
R2(config-crypto-map)#set transform-set TS
R2(config-crypto-map)#match address VPN-Traffic
R2(config-crypto-map)#exit
R2(config)#interface ethernet 0/0
R2(config-if)#crypto map CMAP

测试

在 R1 尝试让 192.168.10.1 去 Ping 192.168.20.1,已经可以 Ping 通了。

R1#ping 192.168.20.1 source 192.168.10.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.20.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.10.1
.!!!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 36/72/96 ms

R1#show crypto isakmp sa
dst             src             state          conn-id slot status
192.168.23.2    192.168.13.1    QM_IDLE              1    0 ACTIVE

R1#show crypto ipsec sa

interface: Ethernet0/0
    Crypto map tag: CMAP, local addr 192.168.13.1

   protected vrf: (none)
   local  ident (addr/mask/prot/port): (192.168.10.0/255.255.255.0/0/0)
   remote ident (addr/mask/prot/port): (192.168.20.0/255.255.255.0/0/0)
   current_peer 192.168.23.2 port 500
     PERMIT, flags={origin_is_acl,}
    #pkts encaps: 4, #pkts encrypt: 4, #pkts digest: 4
    #pkts decaps: 4, #pkts decrypt: 4, #pkts verify: 4
    #pkts compressed: 0, #pkts decompressed: 0
    #pkts not compressed: 0, #pkts compr. failed: 0
    #pkts not decompressed: 0, #pkts decompress failed: 0
    #send errors 1, #recv errors 0

     local crypto endpt.: 192.168.13.1, remote crypto endpt.: 192.168.23.2
     path mtu 1500, ip mtu 1500, ip mtu idb Ethernet0/0
     current outbound spi: 0xD2E73BB1(3538369457)

     inbound esp sas:
      spi: 0x80CE4FB7(2161004471)
        transform: esp-3des ,
        in use settings ={Tunnel, }
        conn id: 2001, flow_id: SW:1, crypto map: CMAP
        sa timing: remaining key lifetime (k/sec): (4531862/3488)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     inbound ah sas:
      spi: 0x490FFB61(1225784161)
        transform: ah-sha-hmac ,
        in use settings ={Tunnel, }
        conn id: 2001, flow_id: SW:1, crypto map: CMAP
        sa timing: remaining key lifetime (k/sec): (4531862/3481)
        replay detection support: Y
        Status: ACTIVE

     inbound pcp sas:

     outbound esp sas:
      spi: 0xD2E73BB1(3538369457)
        transform: esp-3des ,
        in use settings ={Tunnel, }
        conn id: 2002, flow_id: SW:2, crypto map: CMAP
        sa timing: remaining key lifetime (k/sec): (4531862/3481)
        IV size: 8 bytes
        replay detection support: Y
        Status: ACTIVE

     outbound ah sas:
      spi: 0x4F72667D(1332897405)
        transform: ah-sha-hmac ,
        in use settings ={Tunnel, }
        conn id: 2002, flow_id: SW:2, crypto map: CMAP
        sa timing: remaining key lifetime (k/sec): (4531862/3479)
        replay detection support: Y
        Status: ACTIVE

     outbound pcp sas:

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2021-07-22

Posted In: 虚拟私人网路 VPN

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