15-441 Computer Networking Lecture 13 – DNS

15-441 Computer Networking Lecture 13 – DNS

Outline DNS Design DNS Today Lecture 13: 02-22-2005 2

Naming How do we efficiently locate resources? DNS: name IP address Challenge How do we scale these to the wide area? Lecture 13: 02-22-2005 3

Obvious Solutions (1) Why not centralize DNS? Single point of failure Traffic volume Distant centralized database Single point of update Doesn’t scale! Lecture 13: 02-22-2005 4

Obvious Solutions (2) Why not use /etc/hosts? Original Name to Address Mapping Flat namespace /etc/hosts SRI kept main copy Downloaded regularly Count of hosts was increasing: machine per domain machine per user Many more downloads Many more updates Lecture 13: 02-22-2005 5

Domain Name System Goals Basically a wide-area distributed database Scalability Decentralized maintenance Robustness Global scope Names mean the same thing everywhere Don’t need Atomicity Strong consistency Lecture 13: 02-22-2005 6

Programmer’s View of DNS Conceptually, programmers can view the DNS database as a collection of millions of host entry structures: /* DNS host entry structure */ struct hostent { char *h name; /* official domain name of host */ char **h aliases; /* null-terminated array of domain names */ int h addrtype; /* host address type (AF INET) */ int h length; /* length of an address, in bytes */ char **h addr list; /* null-terminated array of in addr structs */ }; in addr is a struct consisting of 4-byte IP address Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS host name. gethostbyaddr: query key is an IP address. Lecture 13: 02-22-2005 7

DNS Message Format 12 bytes Identification Flags No. of Questions No. of Answer RRs No. of Authority RRs No. of Additional RRs Name, type fields for a query Questions (variable number of answers) RRs in response to query Answers (variable number of resource records) Records for authoritative servers Additional “helpful info that may be used Authority (variable number of resource records) Additional Info (variable number of resource records) Lecture 13: 02-22-2005 8

DNS Header Fields Identification Used to match up request/response Flags 1-bit to mark query or response 1-bit to mark authoritative or not 1-bit to request recursive resolution 1-bit to indicate support for recursive resolution Lecture 13: 02-22-2005 9

DNS Records RR format: (class, name, value, type, ttl) DB contains tuples called resource records (RRs) Classes Internet (IN), Chaosnet (CH), etc. Each class defines value associated with type FOR IN class: Type A Type CNAME name is hostname value is IP address Type NS name is domain (e.g. foo.com) value is name of authoritative name server for this domain name is an alias name for some “canonical” (the real) name value is canonical name Type MX Lecture 13: 02-22-2005 value is hostname of mailserver associated with name 10

Properties of DNS Host Entries Different kinds of mappings are possible: Simple case: 1-1 mapping between domain name and IP addr: Multiple domain names maps to the same IP address: eecs.mit.edu and cs.mit.edu both map to 18.62.1.6 Single domain name maps to multiple IP addresses: kittyhawk.cmcl.cs.cmu.edu maps to 128.2.194.242 aol.com and www.aol.com map to multiple IP addrs. Some valid domain names don’t map to any IP address: for example: cmcl.cs.cmu.edu Lecture 13: 02-22-2005 11

DNS Design: Hierarchy Definitions root org edu com uk net gwu ucb cmu cs cmcl ece bu mit Each node in hierarchy stores a list of names that end with same suffix Suffix path up tree E.g., given this tree, where would following be stored: Fred.com Fred.edu Fred.cmu.edu Fred.cmcl.cs.cmu.edu Fred.cs.mit.edu Lecture 13: 02-22-2005 12

DNS Design: Zone Definitions Zone contiguous section of name space E.g., Complete tree, single node or subtree root org ca A zone has an associated edu com uk net gwu ucb set of name servers cmu cs bu mit Must store list of names and tree links ece Subtree Single node cmcl Complete Tree Lecture 13: 02-22-2005 13

DNS Design: Cont. Zones are created by convincing owner node to create/delegate a subzone Records within zone stored multiple redundant name servers Primary/master name server updated manually Secondary/redundant servers updated by zone transfer of name space Zone transfer is a bulk transfer of the “configuration” of a DNS server – uses TCP to ensure reliability Example: CS.CMU.EDU created by CMU.EDU administrators Who creates CMU.EDU or .EDU? Lecture 13: 02-22-2005 14

DNS: Root Name Servers Responsible for “root” zone Approx. 13 root name servers worldwide Currently {a-m}.rootservers.net Local name servers contact root servers when they cannot resolve a name Configured with wellknown root servers Lecture 13: 02-22-2005 15

Servers/Resolvers Each host has a resolver Typically a library that applications can link to Local name servers hand-configured (e.g. /etc/resolv.conf) Name servers Either responsible for some zone or Local servers Do lookup of distant host names for local hosts Typically answer queries about local zone Lecture 13: 02-22-2005 16

Typical Resolution www.cs.cmu.edu Client Local DNS server edu . u m cs. c . w ww .edu u m .c ns1 S N NS ns1.cs.cm Aw u.edu root & edu DNS server ns1.cmu.edu DNS server ad dr ns1.cs.cmu.edu DNS server Lecture 13: 02-22-2005 17 ww IP

Typical Resolution Steps for resolving www.cmu.edu Application calls gethostbyname() (RESOLVER) Resolver contacts local name server (S1) S1 queries root server (S2) for (www.cmu.edu) S2 returns NS record for cmu.edu (S3) What about A record for S3? This is what the additional information section is for (PREFETCHING) S1 queries S3 for www.cmu.edu S3 returns A record for www.cmu.edu Can return multiple A records what does this mean? Lecture 13: 02-22-2005 18

Lookup Methods Recursive query: root name server Server goes out and searches for more info (recursive) Only returns final answer or “not found” 2 iterated query 3 Iterative query: Server responds with as much as it knows (iterative) “I don’t know this name, but ask this server” 4 7 local name server dns.eurecom.fr 1 8 Workload impact on choice? Local server typically does recursive Root/distant server does requesting host iterative surf.eurecom.fr Lecture 13: 02-22-2005 intermediate name server dns.umass.edu 5 6 authoritative name server dns.cs.umass.edu gaia.cs.umass.ed u 19

Workload and Caching Are all servers/names likely to be equally popular? Why might this be a problem? How can we solve this problem? DNS responses are cached Quick response for repeated translations Other queries may reuse some parts of lookup NS records for domains DNS negative queries are cached Don’t have to repeat past mistakes E.g. misspellings, search strings in resolv.conf Cached data periodically times out Lifetime (TTL) of data controlled by owner of data TTL passed with every record Lecture 13: 02-22-2005 20

Typical Resolution www.cs.cmu.edu Client Local DNS server edu . u m cs. c . w ww .edu u m .c ns1 S N NS ns1.cs.cm Aw u.edu root & edu DNS server ns1.cmu.edu DNS server ad dr ns1.cs.cmu.edu DNS server Lecture 13: 02-22-2005 21 ww IP

Subsequent Lookup Example root & edu DNS server ftp.cs.cmu.edu Client Local DNS server ftp .c ftp s.c m u.e d IPa dd r Lecture 13: 02-22-2005 cmu.edu DNS server u cs.cmu.edu DNS server 22

Reliability DNS servers are replicated Name service available if one replica is up Queries can be load balanced between replicas UDP used for queries Need reliability must implement this on top of UDP! Why not just use TCP? Try alternate servers on timeout Exponential backoff when retrying same server Same identifier for all queries Don’t care which server responds Lecture 13: 02-22-2005 23

Reverse DNS unnamed root Task edu arpa Method in-addr 128 cmu Maintain separate hierarchy based on IP names Write 128.2.194.242 as 242.194.128.2.in-addr.arpa cs Why is the address reversed? Managing 2 cmcl 194 242 Given IP address, find its name Authority manages IP addresses assigned to it E.g., CMU manages name space 128.2.in-addr.arpa kittyhawk 128.2.194.242 Lecture 13: 02-22-2005 24

.arpa Name Server Hierarchy in-addr.arpa 128 2 194 kittyhawk 128.2.194.242 a.root-servers.net m.root-servers.net chia.arin.net (dill, henna, indigo, epazote, figwort, ginseng) cucumber.srv.cs.cmu.edu, t-ns1.net.cmu.edu t-ns2.net.cmu.edu mango.srv.cs.cmu.edu (peach, banana, blueberry) At each level of hierarchy, have group of servers that are authorized to handle that region of hierarchy Lecture 13: 02-22-2005 25

Prefetching Name servers can add additional data to response Typically used for prefetching CNAME/MX/NS typically point to another host name Responses include address of host referred to in “additional section” Lecture 13: 02-22-2005 26

Mail Addresses MX records point to mail exchanger for a name E.g. mail.acm.org is MX for acm.org Addition of MX record type proved to be a challenge How to get mail programs to lookup MX record for mail delivery? Needed critical mass of such mailers Lecture 13: 02-22-2005 27

Outline DNS Design DNS Today Lecture 13: 02-22-2005 28

Root Zone Generic Top Level Domains (gTLD) .com, .net, .org, etc Country Code Top Level Domain (ccTLD) .us, .ca, .fi, .uk, etc Root server ({a-m}.root-servers.net) also used to cover gTLD domains Load on root servers was growing quickly! Moving .com, .net, .org off root servers was clearly necessary to reduce load done Aug 2000 Lecture 13: 02-22-2005 29

New gTLDs .info general info .biz businesses .aero air-transport industry .coop business cooperatives .name individuals .pro accountants, lawyers, and physicians .museum museums Only new one actives so far .info, .biz, .name Lecture 13: 02-22-2005 30

New Registrars Network Solutions (NSI) used to handle all registrations, root servers, etc Clearly not the democratic (Internet) way Large number of registrars that can create new domains However NSI still handles A root server Lecture 13: 02-22-2005 31

Recent Measurements No centralized caching per site Each machine runs own caching local server Why is this a problem? How many hosts do we need to share cache? recent studies suggest 10-20 hosts “Hit rate for DNS 80% 1 - (#DNS/#connections) Is this good or bad? Most Internet traffic is Web What does a typical page look like? average of 4-5 imbedded objects needs 4-5 transfers This alone accounts for 80% hit rate! Lower TTLs for A records does not affect performance DNS performance really relies more on NS-record caching Lecture 13: 02-22-2005 32

Tracing Hierarchy (1) Dig Program Allows querying of DNS system Use flags to find name server (NS) Disable recursion so that operates one step at a time unix dig norecurse @a.root-servers.net NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: edu. 172800 IN NS L3.NSTLD.COM. edu. 172800 IN NS D3.NSTLD.COM. edu. 172800 IN NS A3.NSTLD.COM. edu. 172800 IN NS E3.NSTLD.COM. edu. 172800 IN NS C3.NSTLD.COM. edu. 172800 IN NS F3.NSTLD.COM. edu. 172800 IN NS G3.NSTLD.COM. edu. 172800 IN NS B3.NSTLD.COM. All .edu names handled by setNS of servers edu. 172800 IN M3.NSTLD.COM. Lecture 13: 02-22-2005 33

Tracing Hierarchy (2) 3 servers handle CMU names unix dig norecurse @e3.nstld.com NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cmu.edu. 172800 IN cmu.edu. 172800 IN cmu.edu. 172800 IN NS NS NS CUCUMBER.SRV.cs.cmu.edu. T-NS1.NET.cmu.edu. T-NS2.NET.cmu.edu. Lecture 13: 02-22-2005 34

Tracing Hierarchy (3 & 4) 4 servers handle CMU CS names unix dig norecurse @t-ns1.net.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cs.cmu.edu. 86400 cs.cmu.edu. 86400 cs.cmu.edu. 86400 cs.cmu.edu. 86400 IN IN IN IN NS NS NS NS MANGO.SRV.cs.cmu.edu. PEACH.SRV.cs.cmu.edu. BANANA.SRV.cs.cmu.edu. BLUEBERRY.SRV.cs.cmu.edu. Quasar is master NS for this zone unix dig norecurse @blueberry.srv.cs.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cs.cmu.edu. 300 IN SOA QUASAR.FAC.cs.cmu.edu. Lecture 13: 02-22-2005 35

DNS (Summary) Motivations large distributed database Scalability Independent update Robustness Hierarchical database structure Zones How is a lookup done Caching/prefetching and TTLs Reverse name lookup What are the steps to creating your own domain? Lecture 13: 02-22-2005 36

Announcements Summer research positions Srini wireless networking Dave overlay networking Talk to us for details Email us resume Lecture 13: 02-22-2005 37