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1 DNS dr. C. P. J. Koymans Informatics Institute University of Amsterdam September 16, 2008 dr. C. P. J. Koymans (UvA) DNS September 16, / 46
2 DNS and BIND DNS (Domain Name System) concepts theory BIND (Berkeley Internet Name Domain) implementation Other implementations: djbdns (TinyDNS), PowerDNS, NSD,... practice dr. C. P. J. Koymans (UvA) DNS September 16, / 46
3 Short history of DNS (1) December 1973 HOSTS.TXT (RFC 606) November 1983 DNS invented (RFC 882) October 1984 TLDs defined (RFC 920) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
4 Short history of DNS (2) October 1984 gtlds established.arpa (temporary).gov,.edu,.com,.mil,.org January 1985 SRI runs DNS service.net (forgotten in RFC 920) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
5 Short history of DNS (3) July 1985 cctlds established.us (February 15, 1985).UK,.GB (July 24, 1985).AU (March 5, 1986).NL (April 25, 1986).JP (August 5, 1986) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
6 Short history of DNS (4) November 1987 DNS Specification STD 13, RFC 1034, RFC 1035 November 1988.INT domain established April 1993 InterNIC starts, operated by NSI (Network Solutions Inc.) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
7 Short history of DNS (5) June 1994 Commercial use becomes dominant September 1995 Charging for domain name registration starts dr. C. P. J. Koymans (UvA) DNS September 16, / 46
8 Short history of DNS (6) 1997 Start planning for competition On July 1, 1997, as part of the Administration s Framework for Global Electronic Commerce, the President directed the Secretary of Commerce to privatize the management of the domain name system (DNS) in a manner that increases competition and facilitates international participation in its management. Source: MoU (Memorandum of Understanding; November 1998) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
9 Short history of DNS (7) November 1998 Start of ICANN (Internet Corporation for Assigned Numbers and Names) Responsibilities IP address assignment, via ASO Internet domain names, via GNSO and ccnso Protocol parameters and port numbers, supported by IANA dr. C. P. J. Koymans (UvA) DNS September 16, / 46
10 DNS structure Hierarchical tree root is unnamed ( unlabeled ) Top Level Domains (TLDs) generic TLDs (gtlds) country code TLDs (cctlds) Distributed database dr. C. P. J. Koymans (UvA) DNS September 16, / 46
11 Root servers Status in 2001, according to ICANN official Michael Roberts 13 root servers Most of them located in the US (10) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
12 Root servers map Map provided by ICANN dr. C. P. J. Koymans (UvA) DNS September 16, / 46
13 Root server list (part 1) A NSI (Verisign) Herndon VA (Dulles VA) B USC-ISI Marina del Rey CA C PSInet (Cogent Communications) Herndon VA (*) D University of Maryland College Park MD E NASA (Ames) Mountain View CA F Internet Software Consortium (ISC) Palo Alto CA (*) G DISA (US DOD NOC) Vienna VA H US Army Research Lab (ARL) Aberdeen MD dr. C. P. J. Koymans (UvA) DNS September 16, / 46
14 Root server list (part 2) I NORDUnet (Autonomica) Stockholm (*) J NSI (Verisign) Herndon (Dulles VA) (*) K RIPE (NCC) London (*) L ICANN Marina del Rey (Los Angeles CA) (*) M WIDE Tokyo (*) (*) These nameservers use anycast dr. C. P. J. Koymans (UvA) DNS September 16, / 46
15 Anycast Overloading of IP address Route to nearest instance (BGP metric) Global or local significance dr. C. P. J. Koymans (UvA) DNS September 16, / 46
16 k root server presence Map provided by RIPE dr. C. P. J. Koymans (UvA) DNS September 16, / 46
17 DNS concepts Domain Name Space Resource Records Name Servers Resolvers dr. C. P. J. Koymans (UvA) DNS September 16, / 46
18 Domain names Nodes (internal and leaf) have a label root label is empty (not ) non-root labels must not be empty A domain is a sequence of labels separated by. (dot) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
19 To slash or not to slash Compare domains to pathnames in a filesystem Labels (filenames) separated by / (slash). Absolute versus relative pathnames dr. C. P. J. Koymans (UvA) DNS September 16, / 46
20 To dot or not to dot Absolute domain (FQDN) mail.serv.os3.nl. Relative domain mail mail.serv machine.cs can give problems (why?) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
21 Resource Records (RR s) owner (domain name) type (A, CNAME, MX, NS, PTR, SOA,... ) class (IN, CH) ttl (time to live (in cache)) resource data (depends on type) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
22 A record Address record translates domain name to IP address mail.serv.os3.nl Multihomed hosts have several A records Routers have multiple A records dr. C. P. J. Koymans (UvA) DNS September 16, / 46
23 CNAME record Canonical name record defines an alias info4u.os3.nl. No other RR s are allowed Does not work for subdomains DNAME record proposed for that dr. C. P. J. Koymans (UvA) DNS September 16, / 46
24 MX record Mail exchanger record defines for a domain mail servers for that domain and the order of their preference lower precedence is preferred MX must not point to a CNAME dr. C. P. J. Koymans (UvA) DNS September 16, / 46
25 NS record Name server record defines a cut (zone) Must list at least two name servers Makes DNS distributed Delegates responsibility NS record must not point to a CNAME dr. C. P. J. Koymans (UvA) DNS September 16, / 46
26 PTR record A pointer record literally points to an arbitrary point Mostly used for reverse lookup mail.serv.os3.nl. But lookup works via in-addr.arpa in-addr.arpa. dr. C. P. J. Koymans (UvA) DNS September 16, / 46
27 SOA record Start Of Authority record administrates important zone parameters hostname of master server ns1.os3.nl. address (in dot form) of responsible person hostmaster.os3.nl. numerical parameters dr. C. P. J. Koymans (UvA) DNS September 16, / 46
28 Numerical SOA params (recommended values) The SOA record itself can have a low TTL (for instance 3600 = 1 hour) Serial (YYYYMMDDnn) Refresh (86400 = 1 day) Retry (7200 = 2 hours) Expire ( = 1000 hours 40 days) Minimum ( = 2 days, but... ) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
29 Numerical SOA params (OS3 example) These values are quite low (during IP migration) Serial ( ) Refresh (3600 = 1 hour) Retry (1800 = 30 minutes) Expire (21600 = 6 hours) Minimum (3600 = 1 hour, but... ) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
30 Minimum Different interpretations Minimal TTL allowed (never used this way) Default TTL, if TTL not specified (BIND 8) TTL for caching negative replies (BIND 9) BIND 9 uses global $TTL for default TTL dr. C. P. J. Koymans (UvA) DNS September 16, / 46
31 Resource Record sets (RRsets) Grouping of a set of RRs with the same owner, class and type All RRs in an RRset must have the same TTL DNSSEC signs complete RRsets with RRSIG RRs which might make the RRSIG RR an exception to the item before :) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
32 Name servers and zones Zones are created by cuts (delegations) Cuts are defined by NS records inside parent zone non-authoritative by definition Glue A records sometimes needed dr. C. P. J. Koymans (UvA) DNS September 16, / 46
33 Bootstrap issues Hint file for root RR s Glue for child zones Glue NS records (stub server) Glue A records (for servers inside the child zone) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
34 Name server types Master (primary) Slave (secondary) Stub (limited secondary) Stealth (secondary that is not listed) Caching-only (never authoritative) Forward-only (using forwarders ) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
35 Recursion and iteration Recursive Server follows referrals itself and Often doesn t have authoritative data Iterative Server answers with authoritative data or Server passes referrals back to clients dr. C. P. J. Koymans (UvA) DNS September 16, / 46
36 Resolver Library doing domain name lookup Uses /etc/resolv.conf Contacts a recursive nameserver Does not follow referrals itself dr. C. P. J. Koymans (UvA) DNS September 16, / 46
37 Caching Necessary for performance Negative caching adds more functionality See RFC 2308 Lots of subtleties dr. C. P. J. Koymans (UvA) DNS September 16, / 46
38 Common mistakes See RFC 1912 and also RFCs 2181 and 4697 Using CNAMEs in MX and NS records Forgetting the final. Lame delegation Lack of human coordination dr. C. P. J. Koymans (UvA) DNS September 16, / 46
39 DNS Message format Header Question section Answer section Authority section Additional section dr. C. P. J. Koymans (UvA) DNS September 16, / 46
40 Header ID Flags QDcount ANcount NScount ARcount dr. C. P. J. Koymans (UvA) DNS September 16, / 46
41 Flags QR: Query or Response (bit 0) AA: Authoritative Answer (bit 5) RD: Recursion Desired (bit 7) RA: Recursion Available (bit 8) Result Code (bits 12-15)... (some more) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
42 Queries In most cases QDcount is 1 Query consists of QNAME (sequence of labels, coded with length/value) QTYPE (2 bytes) QCLASS (2 bytes, almost always IN) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
43 Answers, Authorities and Additionals Each of these are a list of resource records NAME, TYPE, CLASS (as in queries) TTL (4 bytes) RDLENGTH (2 bytes) RDATA (RDLENGTH bytes) dr. C. P. J. Koymans (UvA) DNS September 16, / 46
44 DNS limitations DNS is usually based on UDP RFC 1035 maximum size is 512 bytes of DNS content Option to use TCP was present from the start but was not recommended for ordinary use DNS has weak security DNS packets can easily be spoofed Initially no support for message authentication except for a clear text transaction ID dr. C. P. J. Koymans (UvA) DNS September 16, / 46
45 Message Authentication TSIG mechanism added in RFC 2845 calculates HMAC-MD5 over the complete DNS packet adds this as a pseudo -TSIG-RR uses secret keys may use a pseudo -TKEY-RR for key exchange (RFC 2930) SIG(0) mechanism added in RFC 2931 uses public keys uses DNSSEC mechanisms extends DNSSEC to cover complete DNS packets dr. C. P. J. Koymans (UvA) DNS September 16, / 46
46 Extension Mechanisms for DNS EDNS0 Specified in 2671 Necessary for DNSSEC Extends maximum size of UDP-based requests and responses Extends possible flags, result codes and label types Uses a pseudo -OPT-RR dr. C. P. J. Koymans (UvA) DNS September 16, / 46
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