Filtering Specific Domains (FQDN/URLs)

Set up a distributed firewall rule to filter specific domains identified with a fully qualified domain name or URl, for example, *.office365.com.

You must set up a DNS rule first, and then the FQDN allowlist or denylist rule below it. NSX uses time to live (TTL) in the DNS response (coming from DNS server to the virtual machine), for keeping the DNS to IP mapping cache entry for the virtual machine (VM). To override the DNS TTL using a DNS security profile, see Configure DNS Security. For FQDN filtering to be effective, virtual machines need to use a DNS server for domain resolution (no static DNS entries), and also need to honor the TTL received in the DNS response. NSX-T Data Center uses DNS Snooping to obtain a mapping between the IP address and the FQDN. SpoofGuard should be enabled across the switch on all logical ports to protect against the risk of DNS spoofing attacks. A DNS spoofing attack is when a malicious VM can inject spoofed DNS responses to redirect traffic to malicious endpoints or bypass the firewall. For more information about SpoofGuard, see Understanding SpoofGuard Segment Profile.

Note: FQDN filtering does not support CNAME records in DNS as the FQDN attribute type entry in context profiles.

This feature works at layer 7 and does not cover ICMP. If a user creates a denylist rule for all services on example.com the feature is working as intended if ping example.com responds, but curl example.com does not.

Selecting a wild card FQDN is a best practice because it includes sub domains. For example, selecting *.example.com, would include sub domains such as americas.example.com and emea.example.com. Using example.com would not include any sub domains.

FQDN-based rules are retained during vMotion for ESXi hosts.

Note: ESXi and KVM hosts are supported. KVM hosts support the FQDN allowlist only. FQDN filtering is available only with TCP and UDP traffic.

Prerequisites

To use a user-defined FQDN, see Add a Custom FQDN.

Procedure

1. From your browser, log in with admin privileges to an NSX Manager at https://<nsx-manager-ip-address>.
2. Navigate to Security > Distributed Firewall.
3. Add a firewall policy section by following the steps in Add a Distributed Firewall. An existing firewall policy section can also be used.
4. Select the new or existing firewall policy section and click Add Rule to create the DNS firewall rule first.
5. Provide a name for the firewall rule, such as DNS rule, and provide the following details:

   Option	Description
   Services	Click the edit icon and select the DNS or DNS-UDP service as applicable to your environment.
   Context Profiles	Click the edit icon and select the DNS context profile. This is system generated context profile, and is available in your deployment by default.
   Applied To	Select a group as required.
   Action	Select Allow.

6. Click Add Rule again to set up the FQDN allowlisting or denylisting rule.
7. Name the rule appropriately, such as, FQDN/URL Allowlist. Drag the rule under the DNS rule under this policy section.
8. Provide the following details:

   Option	Description
   Services	Click the edit icon and select the service you want to associate with this rule, for example, HTTP.
   Context Profiles	Click the edit icon, and Add Context Profile and name the profile. In the Attributes column, select Set > Add Attribute > Domain (FQDN) Name . Select the list of Attribute Name/Values from the predefined list, or create a custom FQDN. See Context Profiles for details. Click Add, and Apply.
   Applied To	Select DFW or a group as required.
   Action	Select Allow, Drop, or Reject.

9. Click Publish.