FortiGate
FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
akushwaha
Staff
Staff
Article Id 424086
Description This article describes an issue where an IPsec Site-to-Site tunnel is not coming up because of a mismatch in the tunneling protocol.
Scope FortiGate.
Solution

Topology:

 

s2s.png

 

On FGT-1:

FGT-1 # diagnose vpn ike gateway list

vd: root/0
name: IPSEC
version: 2
interface: port1 3
addr: 10.109.19.195:9591 -> 10.109.19.199:4442
tun_id: 10.109.19.199/::10.109.19.199
remote_location: 0.0.0.0
network-id: 0
transport: TCP   <------------------- TCP protocol is used on FGT-1.
created: 12s ago
pending-queue: 0
PPK: no
IKE SA: created 1/1
IPsec SA: created 1/1

id/spi: 11 b956228fd5f53990/0000000000000000
direction: responder
status: connecting, state 3, started 12s ago

 
On FGT-2:

FGT-2 # diagnose vpn ike gateway list

vd: root/0
name: IPSEC
version: 2
interface: port1 3
addr: 10.109.19.199:500 -> 10.109.19.195:500
tun_id: 10.109.19.195/::10.109.19.195
remote_location: 0.0.0.0
network-id: 0
transport: UDP   <-----------------UDP protocol is used on FGT-2.
created: 2s ago
pending-queue: 0
PPK: no
IKE SA: created 1/1
IPsec SA: created 1/1

id/spi: 3 7c5f9b27372b5747/0000000000000000
direction: responder
status: connecting, state 3, started 2s ago

 

Diagnostics:

Enable IKE debug:

diagnose debug reset

diagnose debug reset
diagnose debug application ike -1
diagnose debug console timestamp enable
diagnose debug enable

 

To disable debug:

    diagnose debug disable

Debug output :

 

ike V=root:0:1411c654163ab506/0000000000000000:81: SA proposal chosen, matched gateway IPSEC
ike V=root:0:IPSEC:IPSEC: created connection: 0x55aa85712940 3 10.109.19.195->10.109.19.199:500.
ike V=root:0:IPSEC: duplicate connection detected on insert, dropping this connection
ike V=root:0:IPSEC: reset TCP ports
ike V=root:0:1411c654163ab506/0000000000000000:81: failed to create a connection
ike V=root:could not send IKE msg (RETRANSMIT_SA_INIT): still waiting for tcp-transport(vd=0, vrf=0, intf=3:3, 10.109.19.195:1897->10.109.19.199:4442 sock=42 refcnt=1 ph1=0x55aa85708ba0)


This issue is occurring due to a mismatch in the protocol used for the IPsec tunnel on the firewall. To resolve it, change the transport port on FGT-1 to UDP:

 

config vpn ipsec phase1-interface
    edit "IPSEC" 
        set transport udp

    next

end