Details

Configuring hosts that run a BIND 9.X implementation to only accept DNS traffic on a DNS interface allows a system firewall to be configured to limit the allowed incoming ports/protocols to 53/tcp and 53/udp. Sending outgoing DNS messages from a random port minimizes the risk of an attacker guessing the outgoing message port and sending forged replies.

The TCP/IP stack in DNS hosts (stub resolver, caching/resolving/recursive name server, authoritative name server, etc.) could be subjected to packet flooding attacks (such as SYNC and smurf), resulting in disruption of communication. By implementing a specific set of firewall rules that limit accepted traffic to the interface, these risk of packet flooding and other TCP/IP based attacks is reduced.

Solution

Configure the OS firewall to only allow incoming DNS traffic on ports 53/tcp and 53/udp.
Add the following rules to the host firewall rule set:

# iptables -A INPUT -i [DNS Interface] -p tcp –dport 53 -j ACCEPT
# iptables -A INPUT -i [DNS Interface] -p udp –dport 53 -j ACCEPT
# iptables -A INPUT -i [DNS Interface] -j DROP

Note: If the system is not using an IPTables firewall, the appropriate firewall rules that limit traffic to ports 53/tcp and 53/udp should be configured on the active firewall.

Supportive Information

The following resource is also helpful.

• https://dl.dod.cyber.mil/wp-content/uploads/stigs/zip/U_BIND_9-x_V2R2_STIG.zip

This security hardening control applies to the following category of controls within NIST 800-53: Configuration Management.This control applies to the following type of system Unix.

References

• 800-53|CM-6b.
• CAT|II
• CCI|CCI-000366
• Rule-ID|SV-207536r612253_rule
• STIG-ID|BIND-9X-001004
• STIG-Legacy|SV-86995
• STIG-Legacy|V-72371
• Vuln-ID|V-207536

Source

• Tenable.com/audits