Troubleshooting Tip: Troubleshooting IPsec Site-to-Site Tunnel Connectivity

msingh_FTNT · ‎02-18-2021

Description

This article describes how to troubleshoot basic IPsec tunnel issues and understand how to collect data required by TAC to investigate the VPN issues.

The process responsible for negotiating phase-1 and phase-2: 'IKE'.

Use the following steps to assist with resolving a VPN tunnel that is not active or passing traffic.

Scope

FortiGate.

Solution

Step 1: What type of tunnel have issues:

FortiOS supports:

Site-to-Site VPN.
Dial-Up VPN.

Step 2: Is Phase-2 Status 'UP':

No (SA=0) - Continue to Step 3.
Yes (SA=1) - If traffic is not passing, - Jump to Step 6.
Flapping - SA is flapping between the 'UP' and 'Down' states - Jump to Step 7.

How to identify if Phase 2 is 'Up' or 'Down':

Phase-2 status can be found from both GUI and Command-Line.

From GUI:

When Phase2 is Down:

When Phase2 is Up:

One more way to check the IPsec monitor status from GUI is by clicking the up or inactive name under status in IPsec tunnel. Image attached below.

From CLI:

Execute the command 'diagnose vpn tunnel list name <phase1-name>' <- To view the phase1 status for a specific tunnel.

('diagnose vpn tunnel list', can also be executed to view the phase2 status of all tunnels).

When Phase2 is Down:

When Phase2 is Up:

Step 3: Is IKE Phase1 up:

No (State – 'Connecting') - Continue to Step 4.
Yes (State – 'Established') - Jump to Step 5.

Execute the command 'diagnose vpn ike gateway list name <phase1-name>' <- To view the phase1 status for a specific tunnel.

('diagnose vpn tunnel list' can also be executed to view the phase2 status of all tunnels).

When Phase1 is Down:

When Phase1 is Up:

Step 4: Analyze the IKE phase 1 messages on the responder for a solution. [Phase 1 not up]:

Troubleshooting the IKE Phase 1 problem is best handled by reviewing VPN status messages on the responder firewall.

The responder is the 'receiver' side of the VPN receiving tunnel setup requests.

The initiator is the side of the VPN that sends the initial tunnel setup requests.

Checklist:

Check if there is any other device upstream of the firewall.
Check if the VPN Gateway is configured to use the correct outgoing interface.
Check that the remote IP is configured correctly.
Run packet capture on the outgoing interface and confirm it is possible to see traffic from the remote peer. If not:

Make sure that IKE traffic on port 500/4500 is allowed in the network device connected upstream.

Packet capture can be run from the CLI or the GUI:

GUI:

In v7.2 and v7.4, diagnostics are an option instead of packet capture under the network section. Select diagnostics and then select new packet capture.

CLI:

diagnose sniffer packet any 'host <remote-peer-ip> and port (500 or 4500)' 6 0 l, control + c to stop

If it is possible to see traffic on port 500/4500, follow the steps below to troubleshoot this issue:
Run the below commands(on the receiver) to capture the IKE logs and initiate tunnel/traffic from the remote end.

diagnose debug console timestamp enable

diagnose vpn ike log-filter dst-addr4 10.10.100.109 <- 10.10.100.109 is the remote gateway.

diagnose debug application ike -1
diagnose debug application fnbamd -1
diagnose debug enable

To disable debugs:

diagnose debug reset

diagnose debug disable

Note:

Starting from v7.4.1, the 'diagnose vpn ike log-filter dst-addr4' command has been changed to 'diagnose vpn ike log filter rem-addr4'.

diagnose vpn ike log filter rem-addr4 10.10.100.109
diagnose debug application ike -1

diagnose debug application fnbamd -1

diagnose debug console timestamp enable

diagnose debug enable

To disable debugs:

diagnose debug reset

diagnose debug disable

Note:

Try to run the packet capture and the logs at the same time.
If VDOMs are enabled, make sure to be in the VDOM context and then execute the above commands.

If there are more active tunnels on VDOM, is possible to reduce a possibly large debug output with the following command (FortiOS 7.4.x) and filter the debugged tunnel by name, for example:

diagnose vpn ike log filter name <name of tunnel>

Step 5: Phase1 has been established but Phase2 is down.

Checklist:

Confirm if the Encryption and hash algorithms match on both the receiver and initiator.
Check if PFS is enabled, if yes, make sure the configuration is matched on both the units.
Make sure, that the quick mode selectors (interesting traffic) match on both units.
If Phase-2 is still not up, run the packet capture on port 500/4500 and run the below commands.

diagnose vpn ike gateway list (Or diagnose vpn ike gateway list name <tunnel-name>)

diagnose vpn ike log-filter dst-addr4 10.10.100.109 <- 10.10.100.109 is the remote gateway.

diagnose debug console timestamp enable
diagnose debug application ike -1

diagnose debug enable

To disable debugs:

diagnose debug reset

diagnose debug disable

Note:

If VDOMs are enabled, make sure to be in the VDOM context and then execute the above commands.

Packet capture can be collected as shown below:

Step 6: Phase2 is up but traffic is not passing.

Once the tunnel is up, traffic will be encapsulated in ESP (Encapsulating Security Payload) protocol and sent to the remote peer.

Checklist:

1. Make sure the quick mode selector defined in Phase2 is configured properly to allow the traffic flow, which is having the issue.

For example:

Phase 2 define below allows traffic between – 192.168.1.0/24 and 192.168.2.0/24.

Let's assume that the IP address of the PC having an issue is 10.10.100.100/24.
If this PC is trying to reach any host in the 192.168.2.0/24 network, FortiGate will drop this traffic because the phase2 quick mode selector does not have this source network included in it.

2. Check the IPv4 policies and routes are in place to confirm:

If there is a policy defined for this traffic flow.
If there are any source and destination addresses defined, make sure it is configured to allow this traffic flow.
If the routes are added correctly pointing to the tunnel.
If the debug flow shows that the traffic is allowed but provides an error stating 'No matching IPsec selector, drop', check if NAT is enabled in the firewall policy for outbound IPsec VPN traffic. If it is, try to disable NAT in this firewall policy and test again.

In cases where ping is used as the diagnostic tool to test connectivity between local and remote sites, it will fail despite having the required firewall policy, phase 2 selectors (if defined), and static routes/blackhole routes configured on the FortiGate. However, when running the debug flow, it will show that the traffic is leaving the correct tunnel interface and gateway.
Ping to the FortiGate interface and the remote wan interface works. To resolve this, check the Windows firewall settings on the destination devices whether it is enabled. This is a default behavior on the Windows firewall. To allow ICMP requests for testing connectivity, refer to these articles:
- Technical Tip: Unable to ping Windows Device from FortiGate
- Troubleshooting Tip: Window 10 computer does not reply to ping
If the issue persists:

Enable packet capture for remote peer’s IP address and set protocol to 50(ESP).

Open two SSH sessions and run the below commands:

SSH session 1:

diagnose debug console timestamp enable
diagnose debug flow filter addr <destination-IP>
diagnose debug flow filter proto <1 or 17 or 6> (optional) where 1=ICMP, 6 = TCP, 17 = UDP…
diagnose debug flow show iprope enable
diagnose debug flow trace start 1000

To disable debugs:

diagnose debug reset

diagnose debug disable

Note other protocol numbers can used as well for example OSPF(89).

SSH Session 2:

diagnose vpn tunnel list (Or diagnose vpn tunnel list name <phase2_tunnel_name> ).

SSH Session 3:

To clear the session for the source and destination use the following command:

diagnose sys session filter clear

diagnose sys session filter src <source-IP>

diagnose sys session clear

diagnose sys session filter clear

diagnose sys session filter dst <destination-IP>

diagnose sys session clear

Note:

If VDOMs are enabled, make sure not to be in the VDOM context, and then execute the command above.

Make sure to collect packet capture and the logs mentioned above around the same and attach it to the Fortinet case updates.

Along with this information, attach network topology (if any).

With this information, TAC will try to decrypt the ESP traffic in Wireshark.

If the remote peer is FortiGate as well, take packet capture on this unit as well which will make sure that this unit received the encrypted traffic or if it was lost in the middle.

Note:

Under Dashboard ->IPsec Monitor, the number of encrypts and decrypts for the affected tunnel can be. In case Encrypts are increasing and decrypts are not, this means that maybe the FortiGate is not receiving the return traffic. In case there are decrypts and no encrypts, this means that the FortiGate is receiving the traffic from the peer end, however, due to some reason the FortiGate is not sending the traffic out to its LAN or the traffic is not received from LAN.

There are chances that both encrypts and decrypts are 0. In this case, first, it needs to be checked which side of the IPsec tunnel is initiating the traffic if the traffic is reaching the FortiGate, and if the firewall policies are in place.

Step 7: Troubleshoot the IPsec VPN that is flapping.

Checklist:

Check whether the issue affects one VPN or all configured VPNs. If all VPN tunnels are affected:

Check the Internet connection.
Run the following command to find errors/logs associated with the firewall/interface.

diagnose debug crashlog read

diagnose sys top 2 50 <- Press Ctrl + C to stop (run for 5 iterations).

get system performance status

diagnose hardware sysinfo conserve

diagnose hardware deviceinfo nic <interface-name>

execute tac report

Note:

If VDOMs are enabled, make sure not to be in the Global context and then execute the commands above.

If only one tunnel is flapping, collect the 'VPN Events' log as shown below:

Consider whether the VPN was stable for some time and is only now going up and down.
If so - Investigate for network or unit changes or if any new network equipment has been added to the environment. If so, confirm changes/additions are correct.
If not, collect logs and packet capture as mentioned in Step 4.

Make sure to collect packet capture and all the logs mentioned above around the same and attach it to the Fortinet case updates.

Along with this information, attach network topology (if any). With this information, TAC will investigate this issue.

Step 8: Logs to be collected and attached to TAC case.

Checklist:

SSH Session 1:

diagnose debug console timestamp enable
diagnose debug application ike -1

diagnose debug application fnbamd -1
diagnose debug enable

SSH Session 2:

diagnose debug crashlog read
diagnose sys top 2 50 <- Press Ctrl + C to stop (run for 5 iterations).
get system performance status
diagnose hardware sysinfo conserve
diagnose hardware deviceinfo nic <interface-name>
diagnose vpn ike gateway list
diagnose vpn tunnel list
execute tac report

To disable debugs:

diagnose debug reset

diagnose debug disable

First Packet capture - IKE Traffic on ports 500/4500.

Second Packet capture - ESP Traffic Protocol 50.

Note: Some ISP blocks traffic on port 500, if no ESP packets are being received force NAT-T under phase1>>network.

An ISP can block ESP packets even if the tunnel is up as they can filter traffic based on protocol number even after the tunnel is established.

Screenshot 2025-03-06 162340.jpg

Related articles:

Troubleshooting Tip: Troubleshooting IPsec Site-to-Site Tunnel Connectivity

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