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Table of Contents

Cookbook

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DNS filter

You can apply DNS category filtering to control user access to web resources. You can customize the default profile, or create your own to manage network user access and apply it to a firewall policy, or you can add it to a DNS server on a FortiGate interface.

DNS filtering has the following features:

  • FortiGuard Filtering: filters the DNS request based on the FortiGuard domain rating.
  • Botnet C&C domain blocking: blocks the DNS request for the known botnet C&C domains.
  • External dynamic category domain filtering: allows you to define your own domain category.
  • DNS safe search: enforces Google, Bing, and YouTube safe addresses for parental controls.
  • Local domain filter: allows you to define your own domain list to block or allow.
  • External IP block list: allows you to define an IP block list to block resolved IPs that match this list.
  • DNS translation: maps the resolved result to another IP that you define.

The following sample topology is used in the topics of this section. It includes an internal network and a FortiGate that is used as a gateway device that all DNS traffic traverses.

Note

Some features of this functionality require a subscription to FortiGuard Web Filtering.

DNS filter behavior in proxy mode

In cases where the DNS proxy daemon handles the DNS filter and if DNS caching is enabled (this is the default setting), then the FortiGate will respond to subsequent DNS queries using the result in the DNS cache and will not forward these queries to a real DNS server.

There are two options to disable this behavior:

  • Disable DNS caching globally.
  • Remove the DNS filter profile from the proxy mode firewall policy or from the DNS server configured on a FortiGate interface.
To disable DNS caching globally:
config system dns
    set dns-cache-limit 0
end
Note

There will be a performance impact to DNS queries since each query will not be cached, and will be forwarded to a real DNS server.

The following topics provide information about DNS filters:

DNS filter

You can apply DNS category filtering to control user access to web resources. You can customize the default profile, or create your own to manage network user access and apply it to a firewall policy, or you can add it to a DNS server on a FortiGate interface.

DNS filtering has the following features:

  • FortiGuard Filtering: filters the DNS request based on the FortiGuard domain rating.
  • Botnet C&C domain blocking: blocks the DNS request for the known botnet C&C domains.
  • External dynamic category domain filtering: allows you to define your own domain category.
  • DNS safe search: enforces Google, Bing, and YouTube safe addresses for parental controls.
  • Local domain filter: allows you to define your own domain list to block or allow.
  • External IP block list: allows you to define an IP block list to block resolved IPs that match this list.
  • DNS translation: maps the resolved result to another IP that you define.

The following sample topology is used in the topics of this section. It includes an internal network and a FortiGate that is used as a gateway device that all DNS traffic traverses.

Note

Some features of this functionality require a subscription to FortiGuard Web Filtering.

DNS filter behavior in proxy mode

In cases where the DNS proxy daemon handles the DNS filter and if DNS caching is enabled (this is the default setting), then the FortiGate will respond to subsequent DNS queries using the result in the DNS cache and will not forward these queries to a real DNS server.

There are two options to disable this behavior:

  • Disable DNS caching globally.
  • Remove the DNS filter profile from the proxy mode firewall policy or from the DNS server configured on a FortiGate interface.
To disable DNS caching globally:
config system dns
    set dns-cache-limit 0
end
Note

There will be a performance impact to DNS queries since each query will not be cached, and will be forwarded to a real DNS server.

The following topics provide information about DNS filters: