COMS/CSEE 4140 Networking Laboratory Lecture 09 Salman Abdul Baset Spring 2008 Announcements Prelab 8 and Lab report 7 due next week before your lab slot Weekly.
Download ReportTranscript COMS/CSEE 4140 Networking Laboratory Lecture 09 Salman Abdul Baset Spring 2008 Announcements Prelab 8 and Lab report 7 due next week before your lab slot Weekly.
COMS/CSEE 4140 Networking Laboratory Lecture 09 Salman Abdul Baset Spring 2008 Announcements Prelab 8 and Lab report 7 due next week before your lab slot Weekly project reports due before Friday 5pm 2 Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool 3 DNS History hosts.txt file Paul Mockapetris (<1983) Download a single file (hosts.txt) from a central server with FTP Names in hosts.txt are not structured. The hosts.txt file still works on most operating systems. It can be used to define local names. DNS RFC 882, 883 – updated by RFC 1032, 1033 DNS implementation Written by four Berkeley students (client/server) Renamed BIND in 1985 (Berkeley Internet Name Daemon) 4 Domain Names and IP addresses People prefer to use easy-to-remember names instead of IP addresses Domain names are alphanumeric names for IP addresses e.g., neon.ece.utoronto.ca, www.google.com, ietf.org The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses How important is DNS? Imagine what happens when the local DNS server is down. 5 Design Principles of DNS Name space (domain namespace) Distributed (Delegation) Be maintained in a distributed manner Size of DNS database and frequency of updates Per organization – can additional layers of hierarchy Names for hosts, email servers, and others Name space and protocols Hierarchical and logical tree Notion of address in various protocols Names of hosts can be assigned without regard of location on a link layer network, IP network or autonomous system In practice, allocation of the domain names generally follows the allocation of IP address, e.g., All hosts with network prefix 128.143/16 have domain name suffix virginia.edu 6 DNS Usage Assumptions Size of the total database Proportional to number of hosts in the Internet ~110 million gTLDs (as of April 7, 2008) Slow (days) and fast (min/sec) changing data Access is critical than update guarantees! Each organization is responsible for providing redundant DNS servers for its hosts. 7 Elements of DNS Domain Name Space Name Servers Specification for tree structured name space Resource Records (RR) Hold information about part of domain’s tree structure called Zone Resolvers Programs that extract information from name servers in response to client requests 8 Resolver and Name Server 1. HTTP IP address (128.143.71.21) Resolver Hostname (neon.tcpip-lab.edu) 3. Hostname (neon.tcpip-lab.edu) IP address (128.143.71.21) 2. An application program on a host accesses the domain system through a DNS client, called the resolver Resolver contacts DNS server, called name server DNS server returns IP address to resolver which passes the IP address to application Name server 9 DNS Name Hierarchy DNS hierarchy can be represented by a tree Root and top-level domains are administered by an Internet central name registration authority (ICANN) Below top-level domain, administration of name space is delegated to organizations Each organization can delegate further . (root) org gov edu Top-level Domains com Siblings should have different names uci.edu toronto.edu Managed by UofT math.toronto.edu ece.toronto.edu Managed by ECE Dept. neon.ece.toronto.edu 10 Domain Name System . Each node in the DNS tree represents a DNS name Each branch below a node is a DNS domain. DNS domain can contain hosts or other domains (subdomains) edu virginia.edu www.virginia.edu cs.virginia.edu Example: DNS domains are ., edu, virginia.edu, cs.virginia.edu neon.cs.virginia.edu 11 Domain Names Hosts and DNS domains are named based on their position in the domain tree Every node in the DNS domain tree can be identified by a unique Fully Qualified Domain Name (FQDN). The FQDN gives the position in the DNS tree. Note the dot cs.virginia.edu or cs.virginia.edu. A FQDN consists of labels (“cs”,“virginia”,”edu”) separated by a period (“.”) There can be a period (“.”) at the end. Each label can be up to 63 characters long FQDN contains characters, numerals, and dash character (“-”) 12 FQDNs are not case-sensitive Top-level domains (TLDs) IANA classifies TLD into three types gTLD (generic): 3-character code indicates the function of the organization ccTLD (country code): 2-character country or region code Examples: us, va, jp, de iTLD (infrastructure): A special domain (in-addr.arpa) used for IP address-to-name mapping Pseudo domains Used primarily within the US .com, .net etc.– .mil and .gov reserved for use by US .local for the Zeroconf protocol Reserved TLD .example, .invalid, .localhost, .test 13 Top-level domains (TLDs) http://www.iana.org/domains/root/db/ iTLD gTLD .aero, .asia, .biz, .cat, .com, .coop, .edu, .gov, .info, .int, .jobs, .mil, .mobi, .museum, .name, .net, .org, .pro, .tel, .travel ccTLD .arpa, .root >200 countries Non-conventional usage del.icio.us, inter.net 14 TLD Distribution 80,000,000 70,000,000 60,000,000 50,000,000 40,000,000 30,000,000 20,000,000 10,000,000 0 .com .net .org .info .biz .us Total 110 million records Source: http://icannwiki.org/Domain_Statistics 15 Hierarchy of Name Servers The resolution of the hierarchical name space is done by a hierarchy of name servers Each server is responsible (authoritative) for a contiguous portion of the DNS namespace, called a zone. Zone is a part of the subtree root server org server uci.edu server edu server gov server com server .virginia.edu server cs.virginia.edu server 16 Authority and Delegation Authority for the root domain is with the Internet Corporation for Assigned Numbers and Names (ICANN) ICANN delegates to accredited registrars (for gTLDs) and countries for country code top level domains (ccTLDs) Authority can be delegated further Chain of delegation can be obtained by reading domain name from right to left. Unit of delegation is a “zone”. 17 DNS Domain and Zones Domain .edu conceptually contains all data for columbia.edu, www.columbia.edu, cs.columbia.edu, yale.edu, etc. Zone for .edu contains nameserver reference for yale.edu and columbia.edu and all data for harvard.edu . edu yale.edu .edu zone columbia.edu www.columbia.edu harvard.edu cs.columbia.edu .edu domain 18 DNS Domain and Zones Each zone is anchored at a specific domain node, but zones are not domains. . (root) Zone .edu A DNS domain is a branch of the namespace A zone is a portion of the DNS namespace generally stored in a file (It could consists of multiple nodes) A server can divide part of its zone and delegate it to other servers .uci.edu math.virginia.edu Zone and domain .virginia.edu cs.virginia.edu Domain 19 Primary & Secondary Name Servers For each zone, there must be a primary name server and a secondary name server The primary server (master server) maintains a zone file which has information about the zone. Updates are made to the primary server The secondary server copies data stored at the primary server. Adding a host: When a new host is added (“gold.cs.virginia.edu”) to a zone, the administrator adds the IP information on the host (IP address and name) to a configuration file on the primary server 20 Root Name Servers The root name servers know how to find the authoritative name servers for all toplevel zones. There are only 13 root name servers Root servers are critical for the proper functioning of name resolution A.ROOT-SERVERS.NET. B.ROOT-SERVERS.NET. C.ROOT-SERVERS.NET. D.ROOT-SERVERS.NET. E.ROOT-SERVERS.NET. F.ROOT-SERVERS.NET. G.ROOT-SERVERS.NET. H.ROOT-SERVERS.NET. I.ROOT-SERVERS.NET. J.ROOT-SERVERS.NET. K.ROOT-SERVERS.NET. L.ROOT-SERVERS.NET. M.ROOT-SERVERS.NET. 198.41.0.4 192.228.79.201 192.33.4.12 128.8.10.90 192.203.230.10 192.5.5.241 192.112.36.4 128.63.2.53 192.36.148.17 192.58.128.30 193.0.14.129 197.7.83.42 202.12.27.33 21 Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool 22 Domain Name Resolution 1. 2. b) 4. If yes, it responds. Otherwise, it will query other name servers, starting at the root tree When the name server has the answer it sends it to the resolver. HTTP IP address (128.143.71.21) Resolver Hostname (neon.tcpip-lab.edu) a) Hostname (neon.tcpip-lab.edu) IP address (128.143.71.21) 3. User program issues a request for the IP address of a hostname Local resolver formulates a DNS query to the name server of the host Name server checks if it is authorized to answer the query. Name server 23 Recursive and Iterative Queries There are two types of queries: Recursive queries Iterative (non-recursive) queries The type of query is determined by a bit in the DNS query Recursive query: When the name server of a host cannot resolve a query, the server issues a query to resolve the query Iterative queries: When the name server of a host cannot resolve a query, it sends a referral to another server to the resolver. 24 Recursive Queries root server 1st query: neon.cs.virginia.edu In a recursive query, the resolver expects the response from the name server Referral to edu name server 2nd query: neon.cs.virginia.edu The root sever sends a referral to the “edu” server. Querying this server yields a referral to the server of “virginia.edu” … and so on Referral to virginia.edu name server Name server query If the server cannot supply the answer, it will send the query to the “closest known” authoritative name server (here: In the worst case, the closest known server is the root server) response edu server 3rd query: neon.cs.virginia.edu Referral to cs.virginia.edu name server virginia.edu server 4th query: neon.cs.virginia.edu Resolver IP address of neon.cs.virginia.edu cs.virginia.edu 25 server Iterative Queries root server This involves more work for the resolver Name server st 1 query In an iterative query, the name server sends a closest known authoritative name server a referral to the root server. referral to root server Resolver qu y: er ne . cs n o l rra to rg .vi u ed in n ed ia . am e u s .ed a i in ve er r u edu server g r i e .v m .cs na n u o ne ed ia . ry: n i e u g u nd q .ed vir rver a i n o i 2 t se g .vir ral s r c e . f n Re neo virginia.edu : y r du e . rd q u e ia 3 rgin server server i v . cs e am l to n a r r e Ref 4th query: neon.cs.virginia.edu fe Re IP address of neon.cs.virginia.edu cs.virginia.edu 26 server Recursive vs. Iterative Queries Recursive [Con] resource intensive – DNS server maintains state [Pro] Cache namespace – helpful for other queries Iterative [Pro] DNS server never maintains state [Con] Caching is local to a machine – others cannot benefit from it 27 Caching To reduce DNS traffic, name servers caches information on domain name/IP address mappings When an entry for a query is in the cache, the server does not contact other servers Note: If an entry is sent from a cache, the reply from the server is marked as “unauthoritative” 28 Resource Records The database records of the distributed data base are called resource records (RR) Resource records are stored in configuration files (zone files) at name servers. db.mylab.com $TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial 28800 ; refresh 7200 ; retry 604800 ; expire 86400 ; ttl ) ; mylab.com. IN ; localhost PC4.mylab.com. PC3.mylab.com. PC2.mylab.com. PC1.mylab.com. NS PC4.mylab.com. A A A A A 127.0.0.1 10.0.1.41 10.0.1.31 10.0.1.21 10.0.1.11 29 Resource Record Types (Name, Value, Type, TTL) Type=A, name=hostname, value=IPv4 address Type=AAAA, name=hostname, value=IPv6 address Type=NS, name=domain value=hostname of authoritative DNS server Type=CNAME, name=hostname, value=canonical host name nslookup –query=ns google.com (columbia.edu, ns1.columbia.edu, CNAME) Type=MX, name=mail server hostname, value=canonical name of mail server Type=PTR, name=address (IP), value=hostname Type=SRV, name=service name, value=canonical hostname 30 Resource Records db.mylab.com $TTL 86400 mylab.com. IN SOA PC4.mylab.com. hostmaster.mylab.com. ( 1 ; serial Slave refresh time 28800 ; refresh Slave retry time 7200 ; retry Slave expiration time 604800 ; expire 86400 ; ttl Cache time for RR ) ; mylab.com. IN ; localhost PC4.mylab.com. PC3.mylab.com. PC2.mylab.com. PC1.mylab.com. NS PC4.mylab.com. A A A A A 127.0.0.1 10.0.1.41 10.0.1.31 10.0.1.21 10.0.1.11 Max. age of cached data in seconds * Start of authority (SOA) record. Means: “This name server is authoritative for the zone Mylab.com” * PC4.mylab.com is the name server * [email protected] is the email address of the person in charge Name server (NS) record. One entry for each authoritative name server Address (A) records. 31 One entry for each hostaddress DNS Summary Domain name space Domain and zone Zone and resource record 32 DNS Example nslookup google.com 33 DNS Example 34 DNS Example 35 DNS Example Query 1 >nslookup google.com ns.google.com Server: ns1.google.com Address: 216.239.32.10 Name: google.com Addresses: 64.233.167.99, 72.14.207.99, 64.233.187.99 Query 2 >nslookup google.com Server: disco.cs.columbia.edu Address: 128.59.16.7 Non-authoritative answer: Name: google.com Addresses: 64.233.187.99, 64.233.167.99, 72.14.207.99 36 Agenda DNS history DNS concepts Recursive and iterative queries Caching Resource records mDNS King tool 37 mDNS Multicast DNS (mDNS) name-to-address translation on a local network Multicast address: 224.0.0.251 Self-assigned local name It only has significance on the local network. .local. Subdomain Name conflict resolution Default name: <host name>.local, SALMAN_PDA.local SALMAN_PDA.local MDNS: Standard query ANY response SALMAN_PDA.local A 169.254.18.87 PTR SALMAN_PDA.local 38 King Tool How to estimate latency between two arbitrary hosts accurately? C A B C can measure latency between A and B 39 Source: King tool slides by Krishna Gummadi King: A Latency Measurement Tool Estimate latency between arbitrary end hosts Requires no additional infrastructure leverages existing DNS infrastructure enabling a large fraction of Internet hosts to be measured Provides highly accurate latency estimates Fast and light-weight requires only a few DNS queries per estimate We hope that King will be used in many unanticipated ways like in the case of Ping and Traceroute 40 How King Works: The Basic Idea Host A Actual Latency Between End Hosts Host B Latency Estimated By King Name Server near Host A Name Server near Host B Challenge 1: How to find name servers that are close to end hosts Challenge 2: How to estimate latency between two name servers 41 Challenge 2: How do we estimate the latency between name servers? Name Server A Name Server B foo.bar 3. Reply Q: IP addr of xyz.foo.bar 2. Request Q (Forwarded) Our Client C (King) 42 Success of Recursive DNS For King to work, name servers must support recursive queries in a large random sample, > 75% of name servers supported recursion translates to > 90% success rate given a pair as we can measure from A->B, or B->A 43 Example Estimating latency between MIT and google.com salman@irtcluster05:~$ time nslookup mit.edu bitsy.mit.edu Server: bitsy.mit.edu Address: 18.72.0.3#53 Name: mit.edu Address: 18.7.22.69 real 0m0.013s user 0m0.000s sys 0m0.008s salman@irtcluster05:~$ salman@irtcluster05:~$ time nslookup x.google.com bitsy.mit.edu Server: bitsy.mit.edu Address: 18.72.0.3#53 ** server can't find x.google.com.columbia.edu: NXDOMAIN real user sys 0m0.061s 0m0.000s 0m0.008s 44