Internet Address and Domain Name Service (DNS) CS587x Lecture 5 Department of Computer Science Iowa State University

What to cover today Internet Address Domain Name Service IPv4 CIDR Name Resolution Load Balancing

Internet Addressing IPv4 Classified IP address Each address is represented by 4 bytes Four numbers, 0-255, separated by dots Classified IP address Class+Network ID+Host ID

Special IP Addresses Some special addresses Reserved addresses 0.0.0.0 - “this host” 255.255.255.255 - “all hosts” 127.0.0.1 - “localhost” Reserved addresses Can be used locally (behind Network Address Translator, for example) 192.168.0.0-192.168.255.255 172.16.0.0-172.31.255.255 10.0.0.0-10.255.255.255 Not routed through the Internet

IP Address Space Exhaustion Restricting IP addresses to 32 bits imposes a major limitation in the number of hosts on the Internet IPv6 calls for 128 bits address, but requires significant changes throughout much of the Internet Some solutions Assign IP to machines dynamically For an ISP, at any given time, only a small percentage of its customers are connected to the network Computers not visible to the Internet can share the same block of IP address

Classless Interdomain Routing (CIDR) CIDR uses flexible block sizes for address allocation CIDR allows the division between the network and host portions of the IP addresses to occur at any point in the 32-bit number The size of a block of IP addresses could be any power of 2 A CIDR network is identified by a network address and a mask length that indicates how many bits are devoted to the network part of the address 204.70.2.0/23: the 23-bit network address leaves 9 of the 32 bits for representing 512 hosts on that network 129.186.0.0/16 (ISU), 192.188.162.0/24 (ISU Research Park), 63.224.0.0/13 (USWest)

Compatibility with Existing Addresses The addresses allocated with class can be extended with mask Class A address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/8 Class B address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/16 Class C address, a#.b#.c#.d# can be recognized as a#.b#.c#.d#/24

Advantages of CIDR Flexible allocation of IP address blocks allows more efficient use of 32-bit address space An organization needing 512 addresses could be assigned with a 23-bit mask, rather than an entire class B network (65536 addresses) ISPs can now aggregate their networks into larger blocks for the purpose of routing Suppose an ISP is assigned the 12.0.0.0/8 network. This block of addresses could be divided into smaller blocks and allocated to specific customers of this ISP E.g., one customer can have 12.45.0.0/16 and another might have 12.194.34.0/23 network The allocation of address blocks may depend on the size of the customer

Routing with CIDR Routers normally do not remember each individual IP Given an IP packet, routers determine the block it belongs to and send the packet to the ISP who are responsible for this block The ISP needs to know how to reach each of its own separate networks Advantage: Internet routers need only to know how to reach ISPs Hierarchical addressing Similar to postal office

Domain Name Service Routers need an address to route while people need a host name to remember Host Names yield information to people IP addresses yield information to routers Solution: give each IP address a name popeye.cs.iastate.edu  129.186.3.1 www.myown.com  111.222.333.444 Questions: Given an IP, how to find out its hostname? Given a hostname, how to find out its IP?

DNS: History 1970’s ARPANET All host-address mappings were in hosts.txt (in /etc/hosts) Changes were submitted to SRI-NIC by email New versions of hosts.txt were updated periodically from SRI Administrators could pick names at their discretion As the internet grew this system broke down because of Traffic and load: SRI couldn’t handled the load Reliability: The system was unreliable since there was a single point of contact Consistency and confliction: Names were not unique and many hosts had inaccurate copies of hosts.txt Internet growth was threatened DNS was created in 1983 (RFCs 1034 and 1035), modified, updated, and enhanced by subsequent RFCs

DNS Concepts Provide a lookup mechanism for object translation (IP address  hostname) DNS is implemented as a globally distributed, loosely coherent, scalable, reliable, dynamic database DNS consists of three components Namespace Domain Name Servers DNS queries (issued by clients) gethostbyname() gethostbyaddr()

DNS Namespace The namespace must be able to scale Solution: make namespace hierarchical by naming objects based on location (within country, set of organizations, set of companies, etc) unit within that location (company within set of company, etc) object within unit (name of person in company)

Hierarchical Organization of Host names root edu com gov mil org net fr cn The first level names are called “Top Level Domains” Depth of tree is arbitrary (limit 128) No restriction on the amount of branch Domains are subtrees e.g. iastate.edu and cs.iastate.edu Name collision avoided e.g. iastate.edu and iastate.com iastate mit cs eece popeye

Hierarchical Administration of Host Names root root edu edu com com gov gov mil mil org org net net fr cn iastate mit Each zone corresponds to an administrative authority that is responsible for that portion of the hierarchy Zones are “administrative spaces” Zone administrators are responsible for portion of a domain’s name space Authority is delegated from a parent and to a child cs eece popeye

Domain Name Servers Name servers, who answer “DNS” questions, are organized in hierarchies Each server has authority over a portion of the hierarchy A server maintains only a subset of all names Each server contains all the records for the hosts in its zone Each server may know other servers who are responsible for the other portions of the hierarchy Every server knows the root Root server knows about all top-level domains

DNS Protocol Govern the communication between a DNS client and a DNS server A DNS client sends a query to a DNS server, which returns a response with the requested information DNS primarily uses UDP for sending queries and responses, although TCP may also be used DNS queries can be Recursive : such queries request the receiving DNS server resolve the entire request itself Iterative : such queries request the receiving DNS server respond directly to the DNS client with the IP address of the next DNS server in the hierarchy Root server handles only iterative queries

Example of Recursive Query root name server popeye.cs.iastate.edu wants IP address of www.berkeley.edu 1. Contacts its local DNS server, 129.186.3.1 2. 129.186.3.1 contacts root name server, if necessary 3. Root name server contacts authoritative name server, ns1.berkeley.edu, if necessary 2 4 3 5 local name server 129.186.3.1 authorititive name server ns1.berkeley.edu 1 6 requesting host popeye.cs.iastate.edu www.berkeley.edu

Example of Iterated Queries Contacted server replies with name of server to contact “I don’t know this name, but you can ask this server” root name server 2 iterated queries 3 4 5 intermediate name server (com server) local name server 129.186.3.1 7 6 1 8 authoritative name server www.myown.com requesting host popeye.cs.iastate.edu www.sales.myown.com

DNS query is expensive Resolving an IP hostname may invoke several messages Solution: caching previous query results The cached mapping can be used The cached results can be associated with TTL to reduce the risk of using expired information

DNS-based Web Server Load Balancing A popular web site can be replicated in different geographical locations in order to provide better service to a diverse set of clients One web site can be associated with more than one IP addresses For example, all hosts in the acme.com may have the same IP address The return of an IP address may depend on where a query is sent from

Load Balancing (1) DNS can return an IP address based on where queries come from Direct HTTP requests to a collection of web servers that provide access to the same content

Example: www.akamai.com From Ames C:\>ping www.akamai.com Pinging a1440.g.akamai.net [209.152.119.232] with 32 bytes of data: Request timed out. Ping statistics for 209.152.119.232: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss), From the NY area 63.240.15.146 From the UK 194.82.174.224

Load Balancing (2) A query to a host name may return several IP address e.g., www.big.com may correspond to four machines with IP addresses 1.2.3.1, 1.2.3.2, 1.2.3.3, 1.2.3.4 By default, the requesting client uses the first IP address Heavy load on the server 1.2.3.1 DNS can vary the order of the IP addresses for each query The response to the second query could be 1.2.3.2, 1.2.3.3, 1.2.3,4, 1.2.3.1.

DNS Summary Internet address and CIDR DNS is a crucial part of the internet Namespace is hierarchical Globally distributed and locally managed DNS performance is enhanced by caching DNS can help balance web server workload