What is IKE (Internet Key Exchange)? How to configure IPSec site-to-site?

IKE (Internet Key Exchange) Phase 1

The main reason for IKE phase 1 is to establish a secure for IKE phase 2 or IPsec. let understand Phase 1 in steps 

• The first step is Negotiation between future neighbors or peers have traffic to be 

Step 1: Negotiation

the neighbor (peer) which having traffic and want to protect will initiate the initiate INK phase 1 negotiation. Peers will negotiate about the following:

• Hashing: peers will use a hashing algorithm to verifying the integrity and use MD5 or SHA.
• Authentication: peers have to show their identity to prove who he is. Peers use shared keys or digital certificates.
• DH (Diffie Hellman) group : DH group determines how strong the key is and it's used in the exchange process of the key. The higher number means more secure but it takes longer to compute.
• Lifetime: lifetime means how long does the IKE phase 1 tunnel stand up? The shorter the lifetime means the more secure it is because rebuilding the tunnel means we will also use new keying material. Each the vendor uses a different lifetime; a default value is 86400 seconds (1 day).
• Encryption: algorithm we use for encryption DES, 3DES, or AES.

Step 2: DH Key Exchange

After completing the negotiation process, now both the peers will know what kind of policy they have to use. Now they will use the DH the group for negotiating and to exchange keying material and in the end result both peers will have a shared key.

Step 3: Authentication

in the previous step, both peers authenticate each other using authenticate method in which they agreed on in the negotiation process. Once the authentication is successful it’s mean we that both peers can send and receive on this tunnel (the tunnel is completed).

The three steps above can be completed using two different modes:

• Main mode

• Aggressive mode 

IKE Phase 2

IPSec tunnel or IKE phase 2 is used to protect user data. In the IKE phase 2 tunnel, there is only one mode for building the IPSec tunnel is called Quick mode.

As peers negotiate in IKE phase 1, our IKE phase 2 (IPSec) peers will be negotiated.

• IPsec Protocol: AH or ESP
• Encapsulation Mode: transport or tunnel mode.
• Encryption: what encryption algorithm  DES, 3DES, or AES.
• Authentication: what authentication algorithm is MD5 or SHA.
• Lifetime: how long is the IKE phase 2 tunnel valid? When the tunnel is about to expire, we will refresh the keying material.

(Optional) DH exchange: used for PFS (Perfect Forward Secrecy).

IPsec Protocols

AH, and ESP is the two protocols that we use to protect user data. Both of them can be used in transport or tunnel mode, let’s see all the possible options.

Authentication Header Protocol     

AH offers authentication and integrity which is good but it doesn’t provide any encryption. AH protects IP packet by calculating a hash value over almost all fields in the IP header. The fields it excludes are the ones that can be changed in transit (TTL and header checksum).

Transport Mode

Transport mode is very simple, it just adds an AH header just after the IP header. Here’s an example of an IP packet that carries some TCP traffic:

Tunnel Mode

In tunnel mode, we add a new IP header on top of the original IP packet. So this could be useful when you are using private IP addresses and you need to tunnel your traffic over the Internet. Yes we can use AH but it doesn’t provide encryption

In simple words, IPsec uses three main protocols to create a security framework

• ESP Encapsulation security payload is used for providing encryption, authentication, and sharing data.
• AH, Authentication Header use provides a framework for authentication and sharing data.
• IKE Internet Key Exchange offers a framework for negotiating security parameters and establishing authenticating keys. 

Let's take a look at our topology: 

Goal:

• configure a site-to-site VPN 
• configure IPSec VPN on router 1 and router 2
• enable IKE policy on both the routers
• verify site-to-site IPSec VPN
• Test IPSec VPN.
• configure an authentication type pre-shared keys
• use AES 256 encryption, SHA Hash algorithm, and DH group 3 key exchange 
• configure lifetime of one hour

R1(config)#interface serial 4/0

R1(config-if)#ip address 1.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#interface fastethernet 0/0

R1(config-if)#ip address 10.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#no keepalive

R1(config-if)#exit

R1(config)#interface loopback 0

R1(config-if)#ip address 192.168.10.1 255.255.255.0

R1(config-if)#no shutdown

R1(config-if)#exit

R2(config-if)#interface serial 4/1

R2(config-if)#ip address 3.3.3.1 255.0.0.0

R2(config-if)#no shutdown

R2(config-if)#interface serial 4/2

R2(config-if)#ip address 4.4.4.1 255.0.0.0

R2(config-if)#no shutdown

R3(config)#interface serial 4/1

R3(config-if)#ip address 3.3.3.2 255.0.0.0

R3(config-if)#no shutdown

R3(config-if)#interface fastethernet 0/0

R3(config-if)#ip address 30.1.1.1 255.0.0.0

R3(config-if)#no shutdown

R3(config-if)#no keepalive

R3(config-if)#exit

R1(config)#ip route 0.0.0.0 0.0.0.0 serial 4/0

R1(config)#exit

R2(config)#ip route 1.0.0.0 255.0.0.0 serial 4/0

R2(config)#ip route 10.0.0.0 255.0.0.0 serial 4/0

R2(config)#ip route 30.0.0.0 255.0.0.0 serial 4/1

R2(config)#ip route 3.0.0.0 255.0.0.0 serial 4/1

R2(config)#ip route 4.0.0.0 255.0.0.0 serial 4/2

R2(config)#ip route 40.0.0.0 255.0.0.0 serial 4/2

R2(config)#exit

R3(config)#ip route 0.0.0.0 0.0.0.0 serial 4/1

R3(config)#exit

R1#ping 30.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 72/92/156 ms

R3#ping 10.0.0.0

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.0.0.0, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 52/56/60 ms

R1(config)#access-list 100 permit ip 192.168.10.0 0.0.0.255 192.168.20.0 0.0.0.255

R1(config)#access-list 100 permit ip 192.168.20.0 0.0.0.255 192.168.10.0 0.0.0.255

R1(config)#crypto isakmp enable

R2(config)#crypto isakmp enable

R1(config)#crypto isakmp policy 10

R1(config-isakmp)#authentication pre-share

R1(config-isakmp)#encryption aes 256

R1(config-isakmp)#hash sha

R1(config-isakmp)# group 3

R1(config-isakmp)#lifetime 3600

R1(config-isakmp)#end

R3(config)#crypto isakmp enable

R3(config)#crypto isakmp policy 10

R3(config-isakmp)#authentication pre-share

R3(config-isakmp)#encryption aes ?

  128  128 bit keys.

  192  192 bit keys.

  256  256 bit keys.

  <cr>

 

R3(config-isakmp)#encryption aes 256

R3(config-isakmp)#hash sha?

sha  sha256  sha384  sha512

 

R3(config-isakmp)#hash sha

R3(config-isakmp)#group 3

R3(config-isakmp)#lifetime 3600

R3(config-isakmp)#do show crypto isakmp policy

 

Global IKE policy

Protection suite of priority 10

        encryption algorithm:   AES - Advanced Encryption Standard (256 bit keys).

        hash algorithm:         Secure Hash Standard

        authentication method:  Pre-Shared Key

        Diffie-Hellman group:   #3 (1536 bit)

        lifetime:               3600 seconds, no volume limit

R1(config)#crypto isakmp key 0 cisco123 address 2.2.2.2

R3(config-isakmp)#crypto isakmp key 0 cisco123 address 1.1.1.1

R1(config)#crypto ipsec transform-set ip_set esp-aes 256 esp-sha-hmac

R3(config)#crypto ipsec transform-set ip_set esp-aes 256 esp-sha-hmac

R3(cfg-crypto-trans)#exit

R1(config)#crypto map CRMAP 10 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)#match address 100

R1(config-crypto-map)#set peer 2.2.2.2

R1(config-crypto-map)#set transform-set ip_set

R1(config-crypto-map)#exit

R1(config)#interface serial 4/0

R1(config-if)#crypto map CRMAP

*Jul 30 14:16:13.923: %CRYPTO-6-ISAKMP_ON_OFF: ISAKMP is ON

R1(config-if)#END

R3(config)#crypto map CRMAP 10 ipsec-isakmp

% NOTE: This new crypto map will remain disabled until a peer

        and a valid access list have been configured.

R3(config-crypto-map)#match address 100

R3(config-crypto-map)#set peer 1.1.1.1

R3(config-crypto-map)#set transform-set ip_set

R3(config-crypto-map)#end

R3(config)#interface serial 4/1

R3(config-if)#crypto map CRMAP

R1#show crypto ipsec transform-set

Transform set default: { esp-aes esp-sha-hmac  }

   will negotiate = { Transport,  },

 

Transform set ip_set: { esp-256-aes esp-sha-hmac  }

   will negotiate = { Tunnel,  },

R3(config)#do show crypto ipsec transform-set

Transform set default: { esp-aes esp-sha-hmac  }

   will negotiate = { Transport,  },

 

Transform set ip_set: { esp-256-aes esp-sha-hmac  }

   will negotiate = { Tunnel,  },

 

R1#show crypto map

Crypto Map IPv4 "CRMAP" 10 ipsec-isakmp

        Peer = 2.2.2.2

        Extended IP access list 100

            access-list 100 permit ip 192.168.10.0 0.0.0.255 192.168.30.0 0.0.0.255

        Current peer: 2.2.2.2

        Security association lifetime: 4608000 kilobytes/3600 seconds

        Responder-Only (Y/N): N

        PFS (Y/N): N

        Transform sets={

                ip_set:  { esp-256-aes esp-sha-hmac  } ,

        }

        Interfaces using crypto map CRMAP:

                Serial4/0

 R1#ping 2.2.2.2 source 10.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:

Packet sent with a source address of 10.1.1.1

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 56/109/208 ms

R1#ping 192.168.30.1 so

R1#ping 192.168.30.1 source 1.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.30.1, timeout is 2 seconds:

Packet sent with a source address of 1.1.1.1

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 52/68/88 ms

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