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    Next-Generation Firewall

    DNS

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    DNS

    Learn about how Domain Name System (DNS) translates a domain name to an IP address, thus enabling user access to network resources without users needing to remember IP addresses.
    Where Can I Use This?What Do I Need?
    • NGFW (Managed by PAN-OS or Panorama)
    Domain Name System (DNS) is a protocol that translates (resolves) a user-friendly domain name, such as www.paloaltonetworks.com, to an IP address so that users can access computers, websites, services, or other resources on the internet or private networks.
    DNS performs a crucial role in enabling user access to network resources so that users don't need to remember IP addresses, and individual computers don't need to 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.
    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.
    A DNS record of an FQDN includes a time-to-live (TTL) value, and by default the firewall refreshes each FQDN in its cache based on that individual TTL provided the DNS server, as long as the TTL is greater than or equal to the Minimum FQDN Refresh Time you configure on the firewall, or the default setting of 30 seconds if you don’t configure a minimum. Refreshing an FQDN based on its TTL value is especially helpful for securing access to cloud platform services, which often require frequent FQDN refreshes to ensure highly available services. For example, cloud environments that support autoscaling depend on FQDN resolutions for dynamically scaling services up and down, and fast resolutions of FQDNs are critical in such time-sensitive environments.
    By configuring a minimum FQDN refresh time, you limit how small a TTL value the firewall honors. If your IP addresses don’t change very often you may want to set a higher Minimum FQDN Refresh Time so that the firewall doesn’t refresh entries unnecessarily. The firewall uses the higher of the DNS TTL time and the configured Minimum FQDN Refresh Time.
    For example, two FQDNs have the following TTL values. The Minimum FQDN Refresh Time overrides smaller (faster) TTL values.
    TTL
    If Minimum FQDN Refresh = 26
    Actual Refresh Time
    FQDN A
    20
    26
    FQDN B
    30
    30
    The FQDN refresh timer starts when the firewall receives a DNS response from the DNS server or DNS proxy object that is resolving the FQDN.
    Additionally, you can set a stale timeout to configure how long the firewall continues to use stale (expired) FQDN resolutions in the event of an unreachable DNS Server. At the end of the stale timeout period, if the DNS server is still unreachable, the stale FQDN entries become unresolved (the firewall removes stale FQDN entries).
    Beginning with PAN-OS 11.2.1 and later releases, you can use encrypted DNS for DNS proxy. You can also use encrypted DNS for the management interface, whether the management interface connects to DNS servers or uses a DNS proxy. Encrypted DNS increases user privacy and security for DNS traffic between a client and server by preventing man-in-the-middle attacks. Encrypted DNS occurs between these devices:
    • For the management interface, encrypted DNS occurs between the firewall and the DNS server.
    • For DNS proxy, you can configure encrypted DNS in both directions; you specify one or more types of encrypted DNS the firewall will accept from the DNS client, and you specify just one type of encrypted DNS the firewall will use with the DNS server.
    The firewall supports two DNS encryption types: DNS over HTTPS (DoH) and DNS over TLS (DoT). You have the option for the firewall to fall back on traditional DNS (cleartext) if the DNS server rejects encrypted DNS or times out (receives no response from the primary or secondary DNS server within the configured TCP timeout period). When encrypted DNS is configured for an interface, that interface must not host other traffic on TCP ports 443 or 853.
    The following firewall tasks are related to DNS:

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