SonicOS 7.1 DNS Guide

Configuring DNS Settings > Configuring DNS for IPv4 > DNS Rebinding and Cache Lookup
Table of Contents

DNS Rebinding and Cache Lookup

This section provides settings related to the prevention of DNS rebinding attacks using FQDN address objects.

DNS Binding For FQDN

To enable DNS binding for FQDN

  1. Navigate to NETWORK | DNS > Settings.
  2. Scroll to the DNS Rebinding and Cache Lookup section.
  3. Under the DNS Binding for FQDN heading, select FQDN Object Only Cache DNS Reply from Sanctioned Server. This option is not selected by default.
  4. Click Accept.

Enabling DNS Host Name Lookup over TCP for FQDN

By default, DNS queries are sent over UDP. The DNS response can include a Truncated flag if the response length exceeds the maximum allowed by UDP.

When the Enable DNS host name lookup over TCP for FQDN option is:

  • Enabled and the Truncated flag is set in the DNS response, SonicOS sends an additional DNS query over TCP to determine the full DNS response for multiple IP addresses.
  • Disabled, DNS queries are sent over UDP, and SonicOS only processes the IP addresses in the DNS response packet, although the Truncated flag is set in the response.

The DNS query times out after one second if no DNS response over TCP is received from the DNS server.

This option is used to gain more IP addresses when sending DNS queries from FQDN over TCP while the Security Appliance receives DNS responses over UDP.

To enable DNS host name lookup over TCP for FQDN

  1. Navigate to Network | DNS > Settings.
  2. Under the DNS host name lookup over TCP for FQDN heading, select Enable DNS host name lookup over TCP for FQDN. This option is not selected by default.
  3. Click Accept.

DNS Cache Lookup

With the DNS Cache Lookup feature, you can view the cached names and IP addresses from DNS resolution. To show the contents of the general DNS cache, click Lookup DNS Cache. A pop-up displays the cache contents.

What

DNS Server name:

  • Forward DNS cache, the host name.
  • Reverse DNS cache, a string representation of the IP address.
DNS Name Domain name, such as www.SonicWall.com, or IP address.
IP Address Resolved IP address.
TTL (secs) Time to Live; the TTL value from the DNS response.
flush Clicking this flushes the server’s DNS cache entry
flush all Clicking this flushes all DNS cache entry of all listed servers