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Module 7

Module 7. Advanced Zone Files. Objectives. Understand failover strategies using DNS Understand domain delegation Understand Glue Records Understand the SRV RR Understand the NAPTR RR. Load Balancing/Failover. Basic Strategy Multiple RRs Mail Servers – 2 strategies

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Module 7

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  1. Module 7 Advanced Zone Files

  2. Objectives • Understand failover strategies using DNS • Understand domain delegation • Understand Glue Records • Understand the SRV RR • Understand the NAPTR RR

  3. Load Balancing/Failover • Basic Strategy • Multiple RRs • Mail Servers – 2 strategies • MX natural failover/load balancing • Multiple RRs • RRs delivered in round-robin order • BIND provides alternate controls

  4. Mail Servers – MX RRs ; mail server Resource Records for the zone (domain) 3w IN MX 10 mail.example.com. ; the second mail server has lower priority and is ; external to the zone (domain) - backup IN MX 20 mail.example.net. 3w = TTL Priority 10 simply means you can add a more important mail server with only one change

  5. Mail Servers – Multiple RRs ; zone file fragment IN MX 10 mail.example.com. IN MX 10 mail1.example.com. IN MX 10 mail2.example.com. .... mail IN A 192.168.0.4 mail1 IN A 192.168.0.5 mail2 IN A 192.168.0.6 OR ; zone file fragment IN MX 10 mail.example.com. .... mail IN A 192.168.0.4 IN A 192.168.0.5 IN A 192.168.0.6

  6. Mail Servers • No difference in two approaches • All local mail servers must have access to common mail filestore • Backup servers (priority 20) are configured to forward mail to real (priority 10) mail servers • MX RRs can be set to very high TTLs • A or AAAA RRs may change

  7. Mail Server Reverse Map ; reverse-map file fragment ; for 0.168.192.IN-ADDR.ARPA .... 4 PTR mail.example.com. 5 PTR mail.example.com. 6 PTR mail.example.com.

  8. Other Services • Typically uses multiple A (AAAA) RRs • Multiple RRs called RRsets • In case of web - browser will do IP failover in 2 – 3 minutes • Dynamic updating of IP with short TTL will failover in 30 minutes (MSIE) • DNS load balances on IP address • Works for flat loads • If transaction loads vary must use load-balancer

  9. Load Balance – Multiple RRs ; example.com zone file fragment .... ftp IN A 192.168.0.4 ftp IN A 192.168.0.5 ftp IN A 192.168.0.6 www IN A 192.168.0.7 www IN A 192.168.0.8 ; example.com zone file fragment .... ftp IN A 192.168.0.4 IN A 192.168.0.5 IN A 192.168.0.6 www IN A 192.168.0.7 IN A 192.168.0.8

  10. Parent and Child domains • Parent of any domain is the next level in the hierarchy • Parent contains NS RRs which are not Authoritative • Child contains NS RRs which are Authoritative

  11. Parent and Child domains

  12. DNS Subdomain Delegation • www.ramq.example.com • Two methods • Full delegation • Needs separate name servers • Complete control to delegated authority • Method used by domain system • Multiple zone files • Virtual subdomain • Does not need name servers • Single zone file to maintain

  13. Parent Zone File ; IPv4 zone file for example.com $TTL 2d ; default TTL for zone $ORIGIN example.com. ; base domain-name ; Start of Authority record defining the key characteristics of the zone (domain) @ IN SOA ns1.example.com. hostmaster.example.com. ( 2003080800 ; se = serial number 12h ; ref = refresh 15m ; ret = update retry 3w ; ex = expiry 2h ; min = minimum ) ; name servers Resource Records for the domain IN NS ns1.example.com. ; the second name server is ; external to this zone (domain). IN NS ns2.example.net. ; mail server Resource Records for the zone (domain) 3w IN MX 10 mail.example.com. ; the second mail server has lower priority and is ; external to the zone (domain) IN MX 20 mail.example.net. ; domain hosts includes NS and MX records defined previously ; plus any others required ns1 IN A 192.168.254.2 mail IN A 192.168.254.4 joe IN A 192.168.254.6 www IN A 192.168.254.7 ; aliases ftp (ftp server) to an external location ftp IN CNAME ftp.example.net.

  14. DNS Subdomain Delegation ;; subdomain definitions in the same zone file; $ORIGIN directive simplifies and clarifies definitions$ORIGIN ramq.example.com. ; all subsequent RRs use this ORIGIN; two name servers for the subdomain@ IN NS ns3.ramq.example.com.; the preceding record could have been written without the $ORIGIN as; ramq.example.com. IN NS ns3.ramq.example.com.; or @ IN NS ns3; the second name server points back to preceding ns1 IN NS ns1.example.com.; A records for name server ns3 required - the glue recordns3 IN A 10.10.0.24 ; glue record; the preceding record could have been written as; ns3.ramq.example.com. A 10.10.0.24 if it's less confusing

  15. Child Zone File ; zone file for subdomain us.example.com $TTL 2d ; zone default of 2 days $ORIGIN ramq.example.com. IN SOA ns3.ramq.example.com. hostmaster.ramq.example.com. ( 2003080800 ; serial number 2h ; refresh = 2 hours 15m ; update retry = 15 minutes 3w12h ; expiry = 3 weeks + 12 hours 2h20m ; minimum = 2 hours + 20 minutes ) ; subdomain name servers IN NS ns3.ramq.example.com. IN NS ns1.example.com. ; see following notes ; subdomain mail server IN MX 10 mail.ramq.example.com. ; A records for preceding name servers ns3 IN A 10.10.0.24 ns1.example.com. IN A 192.168.0.3 ; 'glue' record ; A record for preceding mail server mail IN A 10.10.0.25 ; next record defines our ftp server ftp IN A 10.10.0.28

  16. Full Subdomain Delegation • One of the Parent name servers provides NS services (slave) – common but not essential • Mail is also delegated – not essential could use mail.example.com • Needs one DNS server in this case (ns3.ramq.example.com) • Zone file controlled by delegated authority • Can delegate further

  17. Virtual Subdomains $ORIGIN ramq.example.com. IN MX 10 mail ; preceding record could have been written as ; ramq.example.com. IN MX 10 mail.ramq.example.com. ; A record for subdomain mail server mail IN A 10.10.0.28 ; the preceding record could have been written as ; mail.ramq.example.com. A 10.10.0.28 if it's less confusing ftp IN A 10.10.0.29 ; the preceding record could have been written as ; ftp.ramq.example.com. A 10.10.0.29 if it's less confusing .... ; other subdomain definitions as required $ORIGIN mderr.example.com.

  18. Virtual Subdomain • Requires no new name servers • Functionally identical to full delegation • Zone file controlled by main zone administrators • Shows delegation of mail – not essential – could use mail.example.com • Further delegation possible under control of main zone administrators

  19. GLUE Records • Widely and erroneously used term • Glue is a A (AAAA) RR pointing to a authoritative name server for the child zone • Glue is essential • At the parent • For in-zone name servers • All others are technically normal A (AAAA) RRs

  20. Glue – Why Essential? • Client needs A RR to get to host • Client needs a name server to get to host record • Client needs A RR of name server to get name server • Not required if out-of-zone – normal lookup for out-of-zone will get an in-zone name server glue record!

  21. DNS – SRV RR • SRV provides a means to find a host that offers a service within domain • srvce = symbolic name of service (standardized by IANA) _ftp • prot = protocol name _tcp • name = domain (zone name) can be omitted (substitution) srvce.prot.name ttl class rr pri weight port target

  22. DNS – SRV RR • pri = relative priority – lower is more important like the MX RR (0 – 65535) • weight = when pri is the same defines the frequency of delivery of the SRV (0 – 65535) • port = the port number of the service (only time that DNS deals with ports and allows for non-standard ports) srvce.prot.name ttl class rr pri weight port target

  23. DNS – SRV RR • target = name of the host that provides the service srvce.prot.name ttl class rr pri weight port target

  24. SRV RR $ORIGIN example.com. ….. ; left hand name is _http._tcp.example.com = query target _http._tcp SRV 10 1 80 slow.example.com. SRV 10 3 80 fast.example.com. ; if neither slow or fast available, switch to ; an external backup web server but use port 8100 not port 80 SRV 20 0 8100 backup.example.net. slow A 192.168.254.3 fast A 192.168.254.4

  25. SRV RR • Extensive use made by modern services such as: • SIP (voip) • LDAP • Windows AD (kerberos and others) • Web browsers rarely use SRV

  26. NAPTR RRs • NAPTR (Naming Authority Pointer Record) is a general purpose RR for Dynamic Delegation Discover System (DDDS). Sister or SRV. Application unique format. • order = low is highest like MX • pref = if order is same pref (lower is highest) is used to find best RR NAPTR order pref flag params regexp replace

  27. NAPTR RRs • flag = Optional. Enclosed in quotes. Unique to application • params = Optional. Enclosed in quotes. Unique to application. • regexp = regular expression applied to Application User String (AUS) NAPTR order pref flag params regexp replace

  28. NAPTR RRs • replace = Replaces the Application User String (AUS). Dot if not used. NAPTR order pref flag params regexp replace

  29. ENUM Use of NAPTR • ENUM is a service which allows a telephone number to be converted into one or more methods to reach a human • Domain name is .e164.arpa • Assume we want to contact +44-111-555-1212 (AUS = +441115551212) • First Well Known Rule (ENUM specific) creates 2.1.2.1.5.5.5.1.1.1.4.4.E164.ARPA – DNS lookup

  30. ENUM Use of NAPTR

  31. ENUM Use of NAPTR ; zone file fragment for 5.5.5.1.1.1.4.4.E164.ARPA $TTL 2d ; zone TTL default = 2 days or 172800 seconds $ORIGIN 5.5.5.1.1.1.4.4.E164.ARPA. .... 2.1.2.1 NAPTR ( 10 ;order 100 ; preference "U" ; flag "E2U+sip" ;svc "!^\\+44111555(.+)$!sip:7\\1@sip.example.com!" ;ere .) ; replace NAPTR 10 101 "u" "E2U+pres" "!^.*$!mailto:sheila@example.com!" .

  32. ENUM Result • First NAPTR RR gives sip:71212@sip.example.com • "!^\\+44111555(.+)$!sip:7\\1@sip.example.com!" • If this fails second NAPTR gives mailto:sheila@example.com • "!^.*$!mailto:sheila@example.com!" Regular Expression Quick Key ! = delimiter, () group, $ = EoL, \1 = backref (group), ^ = SoL + = 1 or more times, * = 0 or more times \ escaped for DNS only (\\1), on-wire escape char removed (\1)

  33. Quick Quiz • Normal strategy for load balancing? • MX failover/load balancing strategies? • Does virtual subdomain delegation require name servers? • What is the parent of ramq.example.com • Are NS RRs in the parent authoritative • Are GLUE records necessary for out-of-zone name servers? • What do SRV records do?

  34. Zone File Exercise • Zone files for domain delegation • Full delegation • Parent is gov.lc • Child is nic.gov.lc • Mail will go to the parent domain • Two NS servers ns1.nic.gov.lc, ns2.nic.gov.lc

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