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When the length of the answer exceeds 512 bytes and both client and server support Extension Mechanisms for DNS (EDNS), larger UDP packets may be used. [41] Use of DNS over UDP is limited by, among other things, its lack of transport-layer encryption, authentication, reliable delivery, and message length.
One concern about the move from TCP to UDP is that TCP is widely adopted and many of the "middleboxes" in the Internet infrastructure are tuned for TCP and rate-limit or even block UDP. Google carried out a number of exploratory experiments to characterize this and found that only a small number of connections were blocked in this manner. [3]
Moreover, DNS messages carried by UDP were restricted to 512 bytes, not considering the Internet Protocol (IP) and transport layer headers. [3] Resorting to a virtual circuit transport, using the Transmission Control Protocol (TCP), would greatly increase overhead. This presented a major obstacle to adding new features to DNS.
The actual data transfer process begins by the client sending a query (opcode 0) with the special query type AXFR (value 252) over the TCP connection to the server. Although DNS technically supports AXFR over User Datagram Protocol (UDP), it is considered not acceptable due to the risk of lost, or spoofed packets.
The Link-Local Multicast Name Resolution (LLMNR) is a protocol based on the Domain Name System (DNS) packet format that allows both IPv4 and IPv6 hosts to perform name resolution for hosts on the same local link.
Microsoft's DNS client also has optional support for local caching, in the form of a DNS Client service (also known as DNSCACHE). Before they attempt to directly communicate with DNS servers, the library routines first attempt to make a local IPC connection to the DNS Client service on the machine. If there is one, and if such a connection can ...
When an endpoint receives a TCP segment with the ECE bit it reduces its congestion window as for a packet drop. It then acknowledges the congestion indication by sending a segment with the CWR bit set. A node keeps transmitting TCP segments with the ECE bit set until it receives a segment with the CWR bit set.
The server acknowledges this request by sending SYN-ACK back to the client. The client responds with an ACK, and the connection is established. This is called the TCP three-way handshake, and is the foundation for every connection established using the TCP protocol. A SYN flood attack works by not responding to the server with the expected ACK code