Порт 53 (DNS)

Port 53 TCP is primarily used for the Domain Name System (DNS) protocol, a critical component of the internet infrastructure. DNS translates human-readable domain names (like google.com) into IP addresses that computers use to locate each other on the network. While DNS primarily uses UDP on port 53 for queries due to its speed and efficiency, TCP on port 53 is used for larger DNS messages, specifically zone transfers (AXFR) and requests exceeding the size limit of UDP packets. Historically, DNS was initially designed with simplicity in mind, but the increasing complexity of the internet and the need for more robust features led to the adoption of TCP for specific operations.

Technically, when a DNS client (resolver) needs to send a request larger than the UDP packet size limit (typically 512 bytes, though EDNS0 extensions allow for larger UDP packets), it will switch to TCP. Zone transfers, where a secondary DNS server replicates the entire DNS database from a primary server, almost always use TCP due to the large amount of data being transferred. The DNS protocol operates on a client-server model. The client sends a query to the server, and the server responds with the requested information. This exchange is defined by a specific message format that includes headers, questions, answers, authority records, and additional records. TCP provides a reliable, connection-oriented transport mechanism, ensuring that large DNS messages are delivered completely and in the correct order, which is essential for zone transfers.

## Firewall Recommendations

For internal networks, allowing outbound TCP port 53 to trusted DNS servers is essential for DNS resolution. Inbound TCP port 53 should generally be blocked unless the server is acting as a DNS server. If acting as a DNS server, carefully configure access control lists (ACLs) to restrict zone transfers to authorized secondary servers only. Implement rate limiting to mitigate DoS attacks. Keep DNS server software up to date with the latest security patches. Consider implementing DNSSEC (Domain Name System Security Extensions) to authenticate DNS responses and prevent cache poisoning. Monitoring DNS traffic for suspicious activity can help detect and prevent attacks. Limit the recursion ability of your DNS servers to only serve your internal networks. Consider using a dedicated DNS firewall to provide an additional layer of security. Ensure proper logging is enabled and analyzed regularly.