Troubleshooting Tip: DNS resolution over IPsec/SSL VPN

Description

This article describes how to troubleshoot when the hostname is not accessible over an IPsec VPN tunnel or an SSL VPN connection.

Scope

FortiGate.

Solution

If resources are not accessible across a VPN tunnel by hostname, try the following steps:

1. Make sure to set up the DNS server properly when configuring SSL or IPSec VPN. In this example, server.abcd.local, which resolves to 10.1.2.3, will be used.
2. Make sure it is possible to ping the IP address 10.1.2.3.
3. Confirm to ping using FQDN: ping server.abcd.local.
4. Check if it is possible to ping using the hostname of the ping server.
5. If it is not possible to ping via hostname, configure the DNS suffix in the SSL and IPsec VPN configuration as below.
6. Disconnect from the VPN and reconnect to retrieve the new VPN client configuration.

Note: Making changes to VPN configuration can interrupt VPN connectivity.
Take a configuration backup and have administrative access to FortiGate that does not depend on VPN.

For SSL VPN:

config vpn ssl settings

    set dns-suffix abcd.local
    set dns-server1 10.1.2.3

end

For IPsec IKEv1 VPN:

See this related document: IPsec DNS suffix.

config vpn ipsec phase1-interface

    edit <IKEV1 TUNNEL NAME>

        set type dynamic
        set mode-cfg enable
        set unity-support enable  <----- This needs to be enabled to use the 'set domain' command.
        set dns-mode manual      
        set ipv4-dns-server1 10.1.2.3

        set domain abcd.local     <----- This sets the default DNS domain for VPN clients.

end

Note:

IKEv1 is the default IKE version for tunnels created using the IPsec Tunnel Wizard in the GUI. The 'set domain' configuration will be available only for IKEv1. It requires the configuration 'set type dynamic', 'set mode-cfg enable', and 'set unity-support enable'.

For IPsec IKEv2 VPN:

The DNS-suffix in IKEv2 is supported starting from FortiOS v7.6.4. The DNS-suffix in IKEv2 is not supported on v7.4.11 and lower versions.

Troubleshooting:

If the access is still not working, verify if the DNS query is coming from the client end by performing a sniffer or Packet Capture:

diagnose sniffer packet any "host <client private IP over SSLVPN/IPsec> and port 53" 4 200 l

If there is no DNS query from the client, verify that the domain is configured correctly. If the DNS query is not being replied to, verify further if it is allowed by FortiGate by running Debugging the packet flow.

If a DNS query is being replied to, SYN is sent to the resolved domain, but it is not being replied to; verify if NAT is required on the policy being used.

If a DNS query is being replied with an error code like 'No such name', disable IPv6 on a virtual interface so the system stops attempting IPv6 lookups.

Related articles:

• Technical Tip: How to set DNS suffix for VPN SSL and IPsec in the FortiGate configuration
• Technical Tip: Configuring split-dns on FortiGate for split-tunnel IPsec Dialup VPN