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.

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 'UP' and 'Down' state - 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:

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:

Is there any other device upstream to the firewall.
Is the VPN Gateway configured to use the correct outgoing interface.
Is the remote IP 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 if IKE traffic on port 500/4500 is allowed in the network device connected upstream

Packet capture can be run from CLI or 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, then 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

Note: Starting from FortiOS 7.4.1, 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 is enabled, make sure to be in the VDOM context and then execute the above commands.

In case there are more active tunnels on VDOM, is possible to reduce possibly large debug output with the below command (FortiOS 7.4.x) and filter the debugged tunnel by e.g. name:

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

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 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 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.

3. 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> )

Note:

If VDOMs is enabled, make sure it is not in the VDOM context and then execute the above command.

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.

Step 7: Troubleshoot IPsec VPN that is flapping.

Checklist:

Does the issue affect one VPN or all configured VPNs:

If all VPN tunnels are affected:
Check the Internet connection.
Run the below command to find out errors/logs associated with the firewall/interface.

diagnose debug crashlog read

diagnose sys top 2 50 <----- Control + 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 is enabled, make sure it is not in the Global context and then execute the above commands.

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

Was the VPN stable for some time and now it is going up and down?
Yes - 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.
No, Collect logs and packet capture as mentioned on 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, control + 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

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

Second Packet capture - ESP Traffic Protocol 50.

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 Note: Dynamic routing (BGP) over IPsec tunnel