Khung 14, bạn sẽ thấy máy chủ BRUCE gửi một sự thừa nhận. Bạn biết giá trị của lĩnh vực xác nhận rằng BRUCE đã thừa nhận tất cả các byte được gửi bởi máy chủ Rollins. Nếu bạn có số thứ tự ban đầu được gửi bởi Rollins Khung 12 và thêm byte để nó, bạn sẽ có được một số xác nhận của | Upper-Layer Protocols 245 The following is a description of the fields in the DNS message format. Identification. The 16-bit identification field allows a host to match DNS questions with responses. Flags. The Flags field is broken down into several smaller field entries QR Bit 16 . A 0 in the question response QR field indicates that the DNS message is a question a 1 indicates it is a response. Opcode Bits 17-20 . A 0 indicates a standard query a 1 indicates an inverse query and a 2 indicates a server status request. AA Bit 21 . The authoritative answer AA field is the DNS authority field. This indicates that the answer is from an authoritative server for the particular domain. TC Bit 22 . The truncated TC bit indicates that the reply is truncated to 512 bytes. RD Bit 23 . The Recursion Desired RD bit allows two types of DNS questions recursive and nonrecursive. A recursive question indicates to a name server that it should handle the resolution of the information asked for in the question section of the message. A nonrecursive question indicates to the name server that it should only return information to the host about where best to locate an answer for information about the domain in question. RA Bit 24 . The Recursive Available RA bit is set to 1 if a server supports recursion. This bit will be set on all recursive answers. Zero field Bits 25-27 . These three bits are set to 0. RC Bits 28-31 . The Return Code RC indicates the status of the returned answer from a name server. A 0 indicates no error and a 3 indicates an error. Name errors are sent only by servers that are authoritative for a domain. They indicate that the name does not exist. Number of questions. The number of questions is typically only 1. Number of answers resource records. This indicates the number of resource records present in the answer. Number of authoritative resource records. This indicates the number of authoritative resource records present in the answer. Number of additional resource
