DNS performs a crucial role in enabling user access to network resources so that users need not remember IP addresses and individual computers need not store a huge volume of domain names mapped to IP addresses. DNS employs a client/server model; a DNS server resolves a query for a DNS client by looking up the domain in its cache and if necessary sending queries to other servers until it can respond to the client with the corresponding IP address.
The DNS structure of domain names is hierarchical: the top-level domain (TLD) in a domain name can be a generic TLD (gTLD): com, edu, gov, int, mil, net, or org (gov and mil are for the United States only) or a country code (ccTLD), such as au (Australia) or us (United States). ccTLDs are generally reserved for countries and dependent territories; they are sometimes used in an unrelated context.
A fully qualified domain name (FQDN) includes at a minimum a host name, a second-level domain, and a TLD to completely specify the location of the host in the DNS structure. For example, www.paloaltonetworks.com is an FQDN.
Wherever a Palo Alto Networks firewall uses an FQDN in the user interface or CLI, the firewall must resolve that FQDN using DNS. Depending on where the FQDN query originates, the firewall determines which DNS settings to use to resolve the query. The firewall uses DNS in a number of ways:
You must configure your firewall with at least one DNS server so it can resolve hostnames. Configure primary and secondary DNS servers or a DNS proxy object that specifies such servers, as shown in Use Case 1: Firewall Requires DNS Resolution for Management Purposes. Security policies, reporting, and services often require DNS, as shown in Use Case 2: ISP Tenant Uses DNS Proxy to Handle DNS Resolution for Security Policies, Reporting, and Services within its Virtual System. The firewall can act as a DNS server for a client, as shown in Use Case 3: Firewall Acts as DNS Proxy Between Client and Server. You can configure an Anti-Spyware profile to Use DNS Queries to Identify Infected Hosts on the Network. You can Enable Passive DNS Collection for Improved Threat Intelligence. You can Enable DNS Proxy and then enable evasion signatures for threat prevention. When you Configure an Interface as a DHCP Server, the firewall acts as a DHCP Server and sends DNS information to its DHCP clients so the provisioned DHCP clients can reach their respective DNS servers.

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