Encapsulate ESP packets within TCP headers

FortiOS includes a proprietary solution to support the encapsulation of Encapsulating Security Payload (ESP) packets within Transmission Control Protocol (TCP) headers. This allows ESP packets to be assigned a port number, which enables them to traverse over carrier networks where direct IPsec traffic is blocked or impeded by carrier-grade NAT.

This feature only works with IKE version 2, and it does not support ADVPN.

To configure the TCP port for IKE/IPsec traffic:

config system settings
    set ike-tcp-port <integer>
end

ike-tcp-port <integer>

Set the TCP port for IKE/IPsec traffic (1 - 65535, default = 4500).

To configure ESP encapsulation on the phase 1 interface:

config vpn ipsec phase1-interface
    edit <name>
        set ike-version 2
        set transport {udp | udp-fallback-tcp | tcp}
        set fortinet-esp {enable | disable}
        set fallback-tcp-threshold <integer>
    next
end

transport {udp | udp-fallback-tcp | tcp}	Set the IKE transport protocol. udp: use UDP transport for IKE. udp-fallback-tcp: use UDP transport for IKE, with fallback to TCP transport. tcp: use TCP transport for IKE.
fortinet-esp {enable | disable}	Enable/disable Fortinet ESP encapsulation.
fallback-tcp-threshold <integer>	Set the timeout before IKE/IPsec traffic falls back to TCP, in seconds (1 - 300, default = 15).

TCP encapsulation of IKE and IPsec packets across multiple vendors

FortiOS supports TCP encapsulation of IKE and IPsec packets across multiple vendors. This cross-vendor interoperability ensures that you can maintain a secure and efficient network, while also having the flexibility to choose the hardware that aligns best with your requirements.

IKE and ESP will be encapsulated into TCP and ESP packets will be encapsulated into a real TCP header. This is because Fortinet Inc. ESP encapsulation is not enabled, and therefore, anti-replay does not need to be disabled.

To configure ESP encapsulation for cross-vendor interoperability:

1. Configure the IKE TCP port setting.

2. Configure the IPsec phase 1 settings and do not set fortinet-esp to enable. This is disabled by default.

3. Configure the IPsec phase 2 settings.

Example

In this example, IPsec VPN crosses over a carrier network and UDP packets are not allowed.

To encapsulate ESP packets within TCP headers:

1. On each FortiGate, configure the IKE TCP port setting:

   config system settings
       set ike-tcp-port 1443
   end

2. Disable anti-replay in the global settings on the FGT_B (NAT) FortiGate (see step 7 for more information):

   config system global
       set anti-replay disable
       set hostname "FGT-B"
   end

3. Configure the FGT_A (spoke) FortiGate.

   1. Configure the IPsec phase 1 settings:

      config vpn ipsec phase1-interface
          edit "spoke"
              set interface "wan1"
              set ike-version 2
              set peertype any
              set net-device disable
              set proposal aes128-sha256 aes256-sha256 aes128gcm-prfsha256 aes256gcm-prfsha384 chacha20poly1305-prfsha256
              set transport tcp
              set fortinet-esp enable
              set remote-gw 173.1.1.1
              set psksecret **********
          next
      end

   2. Configure the IPsec phase 2 settings:

      config vpn ipsec phase2-interface
          edit "spoke"
              set phase1name "spoke"
              set proposal aes128-sha1 aes256-sha1 aes128-sha256 aes256-sha256 aes128gcm aes256gcm chacha20poly1305
              set src-subnet 10.1.100.0 255.255.255.0
          next
      end

      IKE and ESP will be encapsulated into TCP, and ESP packets encapsulated into a fake TCP header.

4. Configure the FGT_C (spoke) FortiGate.

   1. Configure the IPsec phase 1 settings:

      config vpn ipsec phase1-interface
          edit "Spoke"
              set interface "wan1"
              set ike-version 2
              set peertype any
              set net-device disable
              set proposal aes128-sha256 aes256-sha256 aes128gcm-prfsha256 aes256gcm-prfsha384 chacha20poly1305-prfsha256
              set transport udp-fallback-tcp
              set fortinet-esp enable
              set fallback-tcp-threshold 10
              set remote-gw 173.1.1.1
              set psksecret **********
          next
      end

   2. Configure the IPsec phase 2 settings:

      config vpn ipsec phase2-interface
          edit "Spoke"
              set phase1name "Spoke"
              set proposal aes128-sha1 aes256-sha1 aes128-sha256 aes256-sha256 aes128gcm aes256gcm chacha20poly1305
              set src-subnet 192.168.4.0 255.255.255.0
          next
      end

      IKE will use UDP encapsulation first. If it fails to establish in 10 seconds, it will fall back to TCP. ESP packets are encapsulated into a fake TCP header.

5. Configure the FGT_D (hub) FortiGate.

   1. Configure the IPsec phase 1 settings:

      config vpn ipsec phase1-interface
          edit "Hub"
              set type dynamic
              set interface "port25"
              set ike-version 2
              set peertype any
              set net-device disable
              set proposal aes128-sha256 aes256-sha256 aes128gcm-prfsha256 aes256gcm-prfsha384 chacha20poly1305-prfsha256
              set dpd on-idle
              set transport tcp
              set fortinet-esp enable
              set psksecret **********
              set dpd-retryinterval 60
          next
      end

   2. Configure the IPsec phase 2 settings:

      config vpn ipsec phase2-interface
          edit "Hub"
              set phase1name "Hub"
              set proposal aes128-sha1 aes256-sha1 aes128-sha256 aes256-sha256 aes128gcm aes256gcm chacha20poly1305
          next
      end

6. Verify the IPsec VPN tunnel state on FGT_D (hub):

   # diagnose vpn ike gateway list
   
   vd: root/0
   name: Hub_0
   version: 2
   interface: port25 33
   addr: 173.1.1.1:1443 -> 173.1.1.2:23496
   tun_id: 173.1.1.2/::10.0.0.4
   remote_location: 0.0.0.0
   network-id: 0
   transport: TCP
   created: 733s ago
   peer-id: 11.101.1.1
   peer-id-auth: no
   nat: peer
   PPK: no
   IKE SA: created 1/1  established 1/1  time 0/0/0 ms
   IPsec SA: created 1/1  established 1/1  time 0/0/0 ms
   
     id/spi: 3 f050ac7a151a3b31/3b46b71108eea2e2
     direction: responder
     status: established 733-733s ago = 0ms
     proposal: aes128-sha256
     child: no
     SK_ei: 619dfbeb679345f7-531692a72da85727
     SK_er: 5b6a1625b2ce71cf-13b339289ca99b9d
     SK_ai: a61818128c0d5390-b6d15cf9eb58e0f6-4e8c552e6265387b-4f79dc3acdd5d092
     SK_ar: 64fb56b13ee65bd2-6ea1fb268b3ffad9-818c8e4d302a1176-c8978a8ce91d9856
     PPK: no
     message-id sent/recv: 11/2
     QKD: no
     lifetime/rekey: 86400/85396
     DPD sent/recv: 0000000c/0000000c
     peer-id: 11.101.1.1
   
   vd: root/0
   name: Hub_2
   version: 2
   interface: port25 33
   addr: 173.1.1.1:1443 -> 173.1.1.2:12186
   tun_id: 10.0.0.4/::10.0.0.6
   remote_location: 0.0.0.0
   network-id: 0
   transport: TCP
   created: 645s ago
   peer-id: 172.16.200.3
   peer-id-auth: no
   nat: peer
   PPK: no
   IKE SA: created 1/1  established 1/1  time 0/0/0 ms
   IPsec SA: created 1/1  established 1/1  time 0/0/0 ms
   
     id/spi: 17 7eb5a40cd324d2fc/f04fec6d8d77d996
     direction: responder
     status: established 645-645s ago = 0ms
     proposal: aes128-sha256
     child: no
     SK_ei: c1fe2027086b046b-0f15c6e2d25a255d
     SK_er: 3eac9a73b4dd2961-900c0af7f0e18abf
     SK_ai: e21ca3934cca7a85-af425d12baf40693-0c30e3f6d98a6a7d-273b33cc49155092
     SK_ar: 1bef95d13784e8e1-9894c1b3628e158a-3cbfe4f7a730d9de-c9150844e3ff2002
     PPK: no
     message-id sent/recv: 10/2
     QKD: no
     lifetime/rekey: 86400/85484
     DPD sent/recv: 0000000b/0000000b
     peer-id: 172.16.200.3

7. Verify the ESP packets sniffed on the NAT device.

   

   In the packet capture, ESP packets are encapsulated into TCP ACK packets with the same sequence number. This is why anti-replay must be disabled on the NAT FortiGate.