DOMAIN NAME SYSTEM (DNS) (see RFC 1034 and RFC 1035) DNS protocol is utilized to identify servers by their IP addresses and aliases given their registered name. The request is usually simple, including just the name of the server. The response however is usually very complex because it contains all the addresses and aliases that the server might have. Because of this a compression algorithm is utilized in all cases to reduce the number of redundant data and the size of the datagrams. UDP is utilized to send and receive DNS requests.
DNS MESSAGE FORMAT Header Question Answer Authority Additional DNS HEADER FORMAT OCTET 1,2 ID OCTET 3,4 QR(1 bit) + OPCODE(4 bit)+ AA(1 bit) + TC(1 bit) + RD(1 bit)+ RA(1 bit) + Z(3 bit) + RCODE(4 bit) OCTET 5,6 QDCOUNT OCTET 7,8 ANCOUNT OCTET 9,10 NSCOUNT OCTET 11,12 ARCOUNT QUESTION FORMAT OCTET 1,2,…n QNAME OCTET n+1,n+2 QTYPE OCTET n+3,n+4 QCLASS ANSWER, AUTHORITY, ADDITIONAL FORMAT OCTET 1,2,..n NAME OCTET n+1,n+2 TYPE OCTET n+3,n+4 CLASS OCTET n+5,n+6,n+7,n+8 TTL OCTET n+9,n+10 RDLENGTH OCTET n+11,n+12,….. RDATA
DNS SESSION (example) SEND 7E FF 03 00 21 45 00 00 40 00 02 00 00 3C 11 E0 30 CE D9 8F 1F C7 B6 78 CB 04 6D 00 35 00 2C 0D 54 00 02 01 00 00 01 00 00 00 00 00 00 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 C7 00 7E Start 7E Address FF 03 SEP 00 21 IP Header 45 00 00 40 00 02 00 00 3C 11 E0 30 CE D9 8F 1F C7 B6 78 CB 04 6D UDP Header 00 35 00 2C 0D 54 DNS Header 00 02 01 00 00 01 00 00 00 00 00 00 DNS Message 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 FCS C7 00 Stop 7E IP Header VER=4 IHL=5 TOS=0 TOL=64 ID=2 FLG=00 FRO=00 TTL=60 PRO=17 IP_SUM=E030 SRC=206.217.143.31. DEST=199.182.120.203. OPT=00000000 UDP Header SRC_PORT=046D DEST_PORT=0035 UDP_LEN=002C UDP_SUM=0D54 DNS Header ID=2 QR=0 OPCODE=0 AA = 0 TC=0 RD = 1 RA=0 Z =0 RCCODE=0 QDCOUNT=1 ANCOUNT=0 NSCOUNT=0 ARCOUNT=0 DNS Message QNAME=04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 QTYPE=0001 QCLASS=0001 Client sends a UDP/IP packet with a DNS question (QR=0) as a standard query (OPCODE=0) with one entry (QDCOUNT=1). It does not include any resource in neither one of the answer, authority or additional records (ANCOUNT=0 NSCOUNT=0 ARCOUNT=0). The QNAME specifies the domain name of the resource the client is searching for (QNAME = popd.ix.netcom.com.). Note that the periods in the domain name are replaced by the length of the name that follows. The type and class of resource the client is searching for are QTYPE=1 (Host Address), QCLASS=1 (Internet). RECV 7E 21 45 00 01 5F F6 79 40 00 F7 11 ED 98 C7 B6 78 CB CE D9 8F 1F 00 35 04 6D 01 4B 49 AA 00 02 85 80 00 01 00 03 00 06 00 06 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 C0 0C 00 05 00 01 00 00 00 3C 00 19 04 70 6F 70 64 04 62 65 73 74 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 C0 30 00 05 00 01 00 00 00 00 00 06 03 69 78 36 C0 3A C0 55 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 06 C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 31 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 32 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 33 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 34 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 0C 09 64 66 77 2D 69 78 6E 73 31 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 0C 09 64 66 77 2D 69 78 6E 73 32 C0 3A C0 77 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 CB C0 89 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 CA C0 9B 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 01 C0 AD 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 02 C0 BF 00 01 00 01 00 00 1C 20 00 04 CE D6 62 21 C0 D7 00 01 00 01 00 00 1C 20 00 04 CE D6 62 22 C8 4C 7E IP Header VER=4 IHL=5 TOS=0 TOL=351 ID=63097 FLG=02 FRO=00 TTL=247 PRO=17 IP_SUM=ED98 SRC=199.182.120.203. DEST=206.217.143.31. OPT=00000000 UDP Header SRC_PORT=0035 DEST_PORT=046D UDP_LEN=014B UDP_SUM=49AA DNS Header ID=2 QR=1 OPCODE=0 AA=1 TC=0 RD=1 RA=1 RCODE=0 QDCOUNT=1 ANCOUNT=3 NSCOUNT=6 ARCOUNT=6 Server sends a response (QR=1) to the client standard query (OPCODE=0). Server is an authority for the domain name (AA=1) and can support recursive queries (RA=1). No errors occurred in the client's query (RCODE=0). The response has 1 entry in the question section (QDCOUNT=1), 3 resource records in the answer section (ANCOUNT=3), 6 resource records in the authority section (NSCOUNT=6) and 6 resource records in the additional records section (ARCOUNT=6). Note that offsets are used to replace domain names and reduce the size of the DNS message. Start 7E SEP 21 IP Header 45 00 01 5F F6 79 40 00 F7 11 ED 98 C7 B6 78 CB CE D9 8F 1F UDP Header 00 35 04 6D 01 4B 49 AA DNS Header 00 02 85 80 00 01 00 03 00 06 00 06 QUESTION 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 ANSWER Name [C0 0C] (offset to position 12 of the DNS message) Type [00 05] Class [00 01] TTL [00 00 00 3C] RDLENGTH [00 19] RDDATA [04 70 6F 70 64 04 62 65 73 74 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00] C0 30 00 05 00 01 00 00 00 00 00 06 03 69 78 36 C0 3A C0 55 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 06 AUTHORITY C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 31 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 32 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 33 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 06 03 6E 73 34 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 0C 09 64 66 77 2D 69 78 6E 73 31 C0 3A C0 3A 00 02 00 01 00 00 1C 20 00 0C 09 64 66 77 2D 69 78 6E 73 32 C0 3A ADDITIONAL C0 77 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 CB C0 89 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 CA C0 9B 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 01 C0 AD 00 01 00 01 00 00 1C 20 00 04 C7 B6 78 02 C0 BF 00 01 00 01 00 00 1C 20 00 04 CE D6 62 21 C0 D7 00 01 00 01 00 00 1C 20 00 04 CE D6 62 22 FCS C8 4C Stop 7E QUESTION popd.ix.netcom.com QTYPE=1 QCLASS=1 ANSWERS NAME: .popd.ix.netcom.com RDDATA: .popd.best.ix.netcom.com TYPE=5 CLASS=1 TTL=60 NAME: .popd.best.ix.netcom.com RDDATA: .ix6.ix.netcom.com TYPE=5 CLASS=1 TTL=0 NAME: .ix6.ix.netcom.com RDDATA: 199.182.120.6. TYPE=1 CLASS=1 TTL=7200 AUTORITIES NAME: .ix.netcom.com RDDATA: .ns1.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 NAME: .ix.netcom.com RDDATA: .ns2.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 NAME: .ix.netcom.com RDDATA: .ns3.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 NAME: .ix.netcom.com RDDATA: .ns4.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 NAME: .ix.netcom.com RDDATA: .dfw-ixns1.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 NAME: .ix.netcom.com RDDATA: .dfw-ixns2.ix.netcom.com TYPE=2 CLASS=1 TTL=7200 ADDITIONAL RECORDS NAME: .ns1.ix.netcom.com RDDATA: 199.182.120.203. TYPE=1 CLASS=1 TTL=7200 NAME: .ns2.ix.netcom.com RDDATA: 199.182.120.202. TYPE=1 CLASS=1 TTL=7200 NAME: .ns3.ix.netcom.com RDDATA: 199.182.120.1. TYPE=1 CLASS=1 TTL=7200 NAME: .ns4.ix.netcom.com RDDATA: 199.182.120.2. TYPE=1 CLASS=1 TTL=7200 NAME: .dfw-ixns1.ix.netcom.com RDDATA: 206.214.98.33. TYPE=1 CLASS=1 TTL=7200 NAME: .dfw-ixns2.ix.netcom.com RDDATA: 206.214.98.34. TYPE=1 CLASS=1 TTL=7200 DNS Address = 199.182.120.6.