Created on
02-18-2021
08:48 AM
Edited on
03-24-2025
01:14 AM
By
Anthony_E
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:
Execute the command 'diagnose vpn tunnel list name <phase1-name>' <- To view the phase1 status for a specific tunnel.
- 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.
Troubleshooting the IKE Phase 1 problem is best handled by reviewing VPN status messages on the responder firewall.
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:
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 fnbamd -1
diagnose debug enable
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
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 application ike -1
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.
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
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.
Note:
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
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.
Step 8: Logs to be collected and attached to TAC case.
Checklist:
SSH Session 1:
diagnose debug application ike -1
diagnose debug enable
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
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.
To flush tunnel:
diagnose vpn tunnel flush <my-phase1-name>
diagnose vpn ike gateway clear name <my-phase1-name>
Related articles:
Technical Tip: How to configure VPN Site to Site between FortiGates (Using VPN Setup Wizard)
Troubleshooting Tip: IPsec VPNs tunnels
Technical Tip: Setting multiple DNS server for IPSec dial-up VPN
Technical Tip: NAT-traversal comparison between site-to-site and dial-up” dynamic” tunnels
Technical Tip: FortiGate Hub with multiple IPSec Dial-up phase1 using IKEv2 and PSK authentication
Technical Tip: How to configure multiple VPN tunnels from the same ISP to the same remote peer ISP.
Technical Tip: IPsec dial-up full tunnel with FortiClient
Technical Tip: Differences between Aggressive and Main mode in IPSec VPN configurations
Technical Tip: Dynamic routing (BGP) over IPsec tunnel
Technical Tip: OSPF with IPSec VPN for network redundancy
Technical Tip: Dynamic dial-up VPN with OSPF
Technical Tip: Fortinet Auto Discovery VPN (ADVPN)
Technical Tip: 'set net-device' new route-based IPsec logic
Technical Tip: Simple OCVPN deployment
Technical Tip: SD-WAN integration with OCVPN
Technical Tip: Configure IPsec VPN with SD-WAN
Technical Tip: SD-WAN with DDNS type IPsec
Technical Tip: SD-WAN primary and backup IPsec tunnel scenario
Troubleshooting Tip: IPsec VPN Phase 1 Process - Aggressive Mode
Technical Tip: How to configure IPsec VPN Tunnel using IKE v2