Technical Tip: FQDN based firewall policies are not working intermittently

[rk1]

Description

This article describes FQDN address objects that are used in firewall policies that are not working intermittently.

Scope

FortiGate.

Solution

When a FQDN-based destination address object in firewall policies is used, whenever incoming traffic coming from LAN to WAN, it should hit the configured firewall policy with the FQDN destination object, if all the other required fields match the firewall policy.

If the traffic is not hitting the expected FQDN-based firewall policy, follow the below steps:

1. Check if the FortiGate can resolve the domain:
   
   

  

It is also possible to verify the DNS cache using the commands below on FortiGate :

diagnose firewall fqdn list

For v7.0 and later:

   diagnose firewall fqdn list-all

Note: The above commands were replaced in newer versions of firmware as follows:

diagnose  firewall  fqdn list ?

list-ip     List IP FQDN.

list-mac    List MAC FQDN.

list-all    List FQDN.

diagnose test application dnsproxy

For v7.0 and later:

diagnose firewall fqdn list-ip
diagnose test application dnsproxy 6

2. If FortiGate can resolve to an IP address, make sure the DNS settings on FortiGate and the client machine are the same.
    
3. If the DNS settings configured on FortiGate and the client machine are different, configure the FortiGate or client machine to use the same DNS server and flush the client DNS cache using "ipconfig /flushdns" and check if that resolves the issue.
4. If the issue persists after configuring the same DNS server settings on both FortiGate and client machines and if the destination FQDN resolves to a different IP very frequently, try with a wildcard FQDN object instead of the full FQDN.
5. Sometimes, the default TTL (time-to-live) value of the FQDN is very small, so we might observe that IP resolution on the endpoint and FortiGate are different at times, even when using the same DNS Servers. It is good to increase the cache-ttl value for that FQDN on the FortiGate.

config firewall address

    edit "example.com”

        set type fqdn

        set fqdn "example.com"

        set cache-ttl 86400    <- {0 - 86400 in seconds}, where 0 means default.

    next

end

If the firewall fqdn address cache-ttl is set to 0 (default), the cache information will be ignored, and the global dns-cache-ttl (default = 1800s) will be used.

About Wildcard FQDNs:

• Support for wildcard FQDN addresses in firewall policy has been included in v6.2.2.
• When the wildcard FQDN has been configured, it will show as an unresolved FQDN in the firewall address list.
• As compared to the standard FQDNs, the wildcard FQDN does not use system DNS settings (Network -> DNS).
• The wildcard FQDN is updated when a DNS query is made from a host connected to FortiGate (DNS traffic passing through a FortiGate).
• If the query matches the wildcard FQDN, the IP address is added to the cache for that object on the FortiGate. Make sure that the DNS traffic is passing through the FortiGate.

• Note that the dns-udp session helper is configured by default. If an administrator removes the dns-udp session helper, wildcard FQDNs will not be resolved when devices behind FortiGate attempt DNS queries.

config system session-helper
...
    edit 14
        set name dns-udp
        set protocol 17
        set port 53
    next
end

Related documents:

Support for wildcard FQDN addresses in firewall policy 6.2.2

Technical Tip: Using wildcard FQDN

Technical Tip: FQDN Wildcard address is not resolved when using it in Firewall Policies