DNS Domain Name Service

1. DNSDomain Name Service References: • Wikipedia • http://www.sans.org/rr/whitepapers/dns/1069.php

2. Domain Name SystemOverview • Maps domain names to IP addresses • Databases that hold Internet host information • Protocol for exchanging host information • Library routines that implement the defined protocol • BIND - Berkeley Internet Name Domain • named - DNS server daemon

3. http://en.wikipedia.org/wiki/File:Domain_name_space.svg DNS Domain Structure

4. . (root) .edu .com .net .redhat .hawaii .fedora .rhn .hcc ssh DNS Domain Structure • Hierarchical • root domain (.) • top-level domains • subdomains • host • FQDN • Distributed • DNS servers • Replicated FQDN:ssh.hcc.hawaii.edu.

5. Terminology • FQDN • ssh.hcc.hawaii.edu. • hierarchy is read from right to left • last "." is frequently (usually) omitted • domain - each node in the DNS database is called a domain • "." is the root node or root domain - top of hierarchy • The separator character between domains is "." • Subdomain • Any domain that has a parent • (i.e., any domain other than ".") • Subdomains can resolve to hosts, even if they have children

6. More Terminology • Resolver • Routines that convert an unqualified domain name into an FQDN • FQDN is passed to DNS to be mapped to an IP address • Always appends the root domain "." to an unqualified domain name first • List additional domain names to try into /etc/resolv.conf file • Case Insensitive • DNS considers upper and lower case letters the same (i.e., DNS is case insensitive)

7. Zones • Administrative groups of domains • A single DNS server is responsible for all domains within a zone. • A zone server also holds pointers to DNS servers that are responsible for the subordinate DNS zones. • Root domain • highest zone, contains information for the root domain • does not contain any hosts • contains pointers (delegates) to the DNS servers for the top-level domains • 13 authoritative name servers - responsible for the root and top-level domains.

8. Delegation of Authority • ICANN - Internet Corporation for Assigned Names and Numbers • Delegates (designates) the (13) authoritative DNS servers for the root and top-level domains • Each top-level authoritative DNS server, then delegates (designates) for its own next higher level subdomains. • Allows local control over segments of DNS database, while making all segments available to the public.

9. Queries Iterative and Recursive • Iterative (non-recursive) queries • Sends domain name to DNS server and requests server to provide: • the IP address of the requested domain, OR • The name of the authoritative DNS server for the requested domain (or a parent). • The server does not query other servers to obtain an answer. • Recursive queries • Sends a domain name to the DNS server and requests the IP address of the domain • The server may need to query other servers to obtain the answer (recursively) • Both types of query can fail to locate the domain

10. Clients and Resolvers • Client example - web browser • Queries resolver for domain name resolution • Resolver recursively initiates iterative queries to the DNS server • DNS servers must answer iterative queries • If not configured to answer recursive queries, DNS servers respond as though they were iterative queries.

11. Recursive Queries

12. DNS Servers • Primary (or master) server • Authoritative server that holds the master copy of zone data • Copies information from the zone or master file - a local file maintained by system administrator • For security and efficiency provide iterative answers only. • Slave (or secondary) servers • Authoritative • Copies information from the master server or another slave server • DNS caches or caching servers • Not authoritative • Store answers to previous queries in cache. • Responds from cache, if available or forwards to an authoritative server.

13. DNS Database • Resource records store node (domain) information • Over 30 types of resource records • Resource record fields: • Name Domain name or IP address • TTL Time to Live (not used in all resource records) • Class Only class supported is 1 for IN Internet • Type Record type • Data Varies based on record type

14. Resource Record Types • AAddress. IPv4 address mapping. Ex: ns IN A 192.168.0.1 • AAAAAddress. IPv6 address mapping. Ex: ns IN AAAA (ipv6 address) • CNAMECanonical Name. Maps alias or nickname. Ex: ftp IN CNAME www.sam.net. • MXMail Exchange. Domain mail destination.Ex: speedy MX 10 mail MX 20 mail.sam.net. • NSNameServer. Name of DNS server.Ex: peach NS ns.max.net.

15. Resource Record Types • PTRPointer. Used for reverse name resolution. (See reverse name resolution later.) Ex: 3 IN PTR grape • TXTText. Quoted character string associated with the domain name. Ex: peach "Refers to the peach subdomain"

16. DNS Query Format • Name Domain name, FQDN, or IP address • Class Class 1 for IN or Internet class • Type Type of Record requested Example request for an address record: fedora.redhat.com IN A

17. DNS Cache • Many DNS servers cache query responses from other DNS servers. • A DNS server will first try to respond to a query from cache. • If there is no cached response available, the DNS server may query other DNS servers, if configured to do so. • Because of DNS caching, it may take several days for DNS changes to propagate throughout the DNS.

18. DNS Response Format • Header Record Information about the query • Query Record Restates the original query • Answer Records Records to answer the query • Authority Records Records for authoritative servers • Additional Records Additional records, such as NS

19. DNS Query/Response Example $ dig fedora.redhat.com ... ;; QUESTION SECTION: ;fedora.redhat.com. IN A ;; ANSWER SECTION fedora.redhat.com. 600 IN CNAME www.redhat.com www.redhat.com. 330 IN A 209.132.177.50 ;; AUTHORITY SECTION redhat.com. 409 IN NS ns1.redhat.com redhat.com. 409 IN NS ns2.redhat.com redhat.com. 409 IN NS ns3.redhat.com ;; ADDITIONAL SECTION ns1.redhat.com. 300 IN A 66.187.233.210 ns2.redhat.com. 600 IN A 66.187.224.210 ns3.redhat.com. 600 IN A 66.187.229.10

20. Reverse Name Resolution • Also referred to as "inverse mapping" or "reverse mapping." • Looks up domain names, given an IP address. • Uses special domain namedin-addr.arpa. • Resource record formats for this domain • Name field holds IP addresses. • Records are indexed hierarchically by IP address. • Data fields holds the corresponding FQDN. • Used for reverse name resolution • Allows server to retrieve and record domain names of clients. • Verification of the domain of email senders and IP address of sending machine.

21. Examples ofReverse Name Resolution $ dig 110.232.187.66.in-add.arpa PTR ... ;; QUESTION SECTION ; 110.232.187.66.in-addr.arpa. IN PTR ;; ANSWER SECTION 110.232.187.66.in-addr.arpa. 540 IN PTR rhn.redhat.com $ dig -x 66.187.232.110 ... ;; QUESTION SECTION ; 110.232.187.66.in-addr.arpa. IN PTR ;; ANSWER SECTION 110.232.187.66.in-addr.arpa. 552 IN PTR rhn.redhat.com $ host 66.187.232.110 110.232.187.66.in-addr.arpa domain name pointer rhn.redhat.com

22. DNS Related Utilities • whois • Look up domains • Different sites have different capabilities • nslookup • Name service lookup • Deprecated, may be removed from future distributions of Linux. • dig • Domain Internet Groper • more current than nslookup.