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To address this issue, the IETF RFC 2671 "Extension Mechanisms for DNS (EDNS0)" defines a method to extend the UDP buffer size to 4096 bytes to allow for DNSSEC and larger query responses. DNS servers get numerous connections per second and using TCP can add too much overhead. However, using UDP messages are preferable to using TCP for large DNS messages is due to the fact that TCP connections can consume computing resources for each connection. TCP port 53 can be used in the cases where the DNS responses greater than 512 bytes. DNSSEC (Defined in RFC 4033, RFC 4034, and RFC 4035) requires the ability to transmit larger DNS messages because of the extra key information contained in the query responses. One of the key issues mentioned is that DNSSEC can cause DNS replies to be larger than 512 bytes. This article covered many of the caveats that organizations run into as they move to deploy DNSSEC. In the recent edition of the IP Journal there was an article by a friend of mine, Stephan Lagerholm, of Secure64 and the Texas IPv6 Task Force, titled " Operational Challenges When Implementing DNSSEC".
![hack router port 53 tcp hack router port 53 tcp](https://miloserdov.org/wp-content/uploads/2018/01/03-1.png)
I love reading The IP Journal and have read it since the first issue in 1998. There are two good reasons that we would want to allow both TCP and UDP port 53 connections to our DNS servers. However, the practice of denying TCP port 53 to and from DNS servers is starting to cause some problems. This is double-protection in case the DNS server accidentally allowed transfers.Ĭonfiguring your DNS servers to permit zone transfers to only legitimate DNS servers has always been and continues to be a best practice. This can be configured in the BIND zone file using any one of these forms of the allow-transfer command as shown below.Īllow-transfer įurthermore, most organizations have also used firewalls to block TCP port 53 to and from their DNS servers and the Internet. However, most organizations have configured their DNS servers to prevent zone transfers from unintended DNS servers. If the organization's firewall protecting the authoritative DNS server allowed the TCP port 53 packets and the DNS server was configured to allow zone transfers to anyone, then this dig command would be successful. Zone transfers take place over TCP port 53 and in order to prevent our DNS servers from divulging critical information to attackers, TCP port 53 is typically blocked. You can use the dig command to gather information from a server for a specific zone file. However, hackers often try to perform a zone transfer from your authoritative DNS servers to gain access to even more information. Attackers can use a variety of techniques to retrieve DNS information through queries. Public information contained a target's servers is valuable to an attacker and helps them focus their attacks. ĭNS can be used by attackers as one of their reconnaissance techniques.
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Now with the impending deployment of DNSSEC and the eventual addition of IPv6 we will need to allow our firewalls for forward both TCP and UDP port 53 packets. The reality is that DNS queries can also use TCP port 53 if UDP port 53 is not accepted. Security practitioners for decades have advised people to limit DNS queries against their DNS servers to only use UDP port 53.