1 DNS

2 BIND DNS –Resolve names to IP address –Resolve IP address to names (reverse DNS) BIND –Berkeley Internet Name Domain system Version 4 is still in use, but should be considered obsolete Version 8 improves efficiency, security, and robustness Version 9 is a total rewrite and supports threads, multiprocessor and more

3 Who needs DNS? DNS defines –A hierarchical namespace for hosts and IP addresses –A distributed database of hostname and address information –A “resolver” to query this database –Improved routing for –A mechanism for finding services on a network –A protocol for exchanging naming information

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6 DNS Lookup Application –Becomes DNS client –Sends request to local DNS server Local server –If answer known, returns response –If answer unknown Starts at top-level server Follows links Returns response Called name resolution

Resource Records Name: The domain name or IP address TTL: Time to Live –Indicate the maximum amount of time a server may keep a record in cache before checking whether a newer one is available Class: Always IN for the Internet Type: Record type Data: Varies with record type 7

8 DNS Types Each entry in server consists of –Domain name –DNS type for name –Value to which name corresponds During lookup, client must supply –Name –Type Server –Matches both name and type

9 Example DNS Types Type A (Address) –Name-to-address mapping Type MX (Mail eXchanger) –Value is IP address of computer with mail server for name Type CNAME (Computer NAME) –Canonical name (for aliases) –Used to establish alias (www) SOA (Start of Authority) –Indicate authority for this zone data NS (Name Server) –A name server for this zone PTR –IP-Address to domain name mapping

Reverse Name Resolution To look up domain names given an IP address Implemented by means of special domains –in-adde.arpa 10

Reverse name resolution in-addr.arpa domain 11 IP address: Domain: in-addr.arpa

12 DNS Record Types

13 Components of BIND Daemon named Library routines –Resolve hosts queries by contacting the servers of the DNS distributed database Command-line interface: –nslookup, dig, and host

14 Masters, Slaves, and the Authorities Authoritative Servers –Master Server (keeps official copy of zone info on disk) –Slave server (gets copy of zone info from master via zone transfer) Cache Servers –Never authoritative –Load “root” domain servers but all others are accumulated in memory only Resource Records –stored in zone data –retrieved by resolvers sending queries to nameservers –Different types of resource records: A, CNAME, MX… –Each resource record has a TTL specified in the zone data

15 Name Server Taxonomy

IN SOA beast.TCNJ.EDU. admin.beast.TCNJ.EDU. ( 5923 ; serial number ; Refresh 3 hours 3600 ; Retry 1 hour ; Expire 168 hours/1 week ) ; Minimum 12 hour IN NS beast.TCNJ.EDU. IN NS seuss.TCNJ.EDU. IN NS snuffy.TCNJ.EDU. localhost IN A beast IN A beast IN MX 20 beast.TCNJ.EDU. TCNJ.EDU. IN MX 20 beast.TCNJ.EDU. lion IN A lion IN MX 20 beast.TCNJ.EDU. tsclion IN CNAME lion.TCNJ.EDU. sa.tcnj.edu. IN NS cartman.sa.tcnj.edu. cartman.sa.tcnj.edu. IN A

in-addr.arpa. IN SOA beast.TCNJ.EDU. ssivy.beast.TCNJ.EDU. ( 5774 ; serial number ; Refresh 3 hours 3600 ; Retry 1 hour ; Expire 168 hours/1 week ) ; Minimum 12 hour IN NS beast.Trenton.EDU. IN NS snuffy.Trenton.EDU. IN NS seuss.Trenton.EDU. localhost IN A IN-ADDR.ARPA. PTR beast.TCNJ.EDU IN-ADDR.ARPA. PTR lion.TCNJ.EDU IN-ADDR.ARPA. cartman.sa.tcnj.edu.

IN SOA beast.trenton.edu. admin.beast.tcnj.edu. ( 3 ; Serial number ; Refresh rate in seconds for secondary servers 3600 ; Retry in seconds after failure ; Expire in seconds 86400) ; Default time-to-live in seconds IN NS beast.tcnj.edu. 1 IN PTR localhost.

19 DNS Query

20 DNS Server Architecture

21 Example - /etc/named.conf

22 Example - /etc/named.conf cont’d

23 /etc/named.conf cont’d

24 Security Features in named.conf