Lecture 7 Application layer protocols_v1_Part1.pdf

Lecture 7 Application-Layer Protocols Dr Vicky Liu Lecturer/Unit Coordinator [email protected] 7. Application HTTP DNS Telnet SSH DHCP SMTP FTP … 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical 2 Outline • Domain Name System (DNS) • Hypertext Transfer Protoco...

Lecture 7 Application-Layer Protocols Dr Vicky Liu Lecturer/Unit Coordinator [email protected] 7. Application HTTP DNS Telnet SSH DHCP SMTP FTP … 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical 2 Outline • Domain Name System (DNS) • Hypertext Transfer Protocol (HTTP) • Simple Mail Transfer Protocol (SMTP) • File Transfer Protocol (FTP) • Telnet and Secure Socket Shell (SSH) • Dynamic Host Configuration Protocol (DHCP) 3 DNS • The service converts a human readable name into an IP address on a network. • It can be thought of as a White Pages or Yellow Pages directory service. DNS is like the phone book of the Internet. • DNS is a name-to-address resolution protocol that keeps a list of computer names and their IP addresses. – Using DNS, a user can use a computer’s name instead of using its IP address. • Applications that use DNS include World Wide Web (WWW), email, and instant messages. 4 Visiting a Web Site • When you enter library.qut.edu.au in your web browser, the DNS client service contacts the DNS server specified in the IP configuration of your operating system and requests that the name be resolved to an IP address • Once the IP address for the website is returned, your computer can contact the web server to request a web page. 5 DNS Structure - Root Servers • The DNS is a hierarchical naming system. • The top of that hierarchy is the root domain. • The root domain contains all top-level domains of the Internet. • Root servers are a network of hundreds of servers for redundancy in many countries around the world. 6 DNS Structure - Root Servers (cont.) • They are configured in the DNS root zone as 13 named authorities. – This does not mean that there are only 13 physical servers; each operator uses redundant computer equipment to provide reliable service – As of 2020-07-21, the root server system consists of 1086 instances operated by the 12 independent root server operators 7 DNS Structure - TLDs • • • • When you put all the names of a branch together, separated by periods, you have the fully qualified domain name (FQDN). The top of that hierarchy is the root domain. The root domain contains all top-level domains (TLDs) of the Internet. The two main types of TLDs – Country-code TLDs (ccTLDs) – Generic TLDs (gTLDs) 8 DNS Structure - ccTLDs • Specify with two-letter country code for countries – – – – Australia (.au), New Zealand (.nz), United Kingdom (.uk) … 9 DNS Structure - gTLDs • gTLDs represent with a set of three or more letter code the categories of the organisation – – – – – – – government (.gov) education (.edu) commercial (.com) military (.mil) organisation (.org) network (.net) .… 10 How do you choose between ccTLD and gTLD? 11 DNS Structure – 2nd level domains • Second-level domains are usually the name of a company or institution. • The subdomain level is optional and can consist of names separated by a period. • The host level represents individual computers hosting network services. • For example, in www.qut.edu.au, 'au' is the toplevel domain name, 'edu' is the second-level domain, 'qut' is the subdomain, and 'www' is the hostname. 12 DNS Server • DNS servers are composed of the following: – DNS zones – a database of primarily hostname and IP address pairs – Resource records – the unit of information entry in DNS zone files – Cache – results of queries are cached so that if the same query occurs again, the local DNS server can respond without having to contact another server – Root hints – file containing a list of all IP addresses of Internet root servers – DNS Server service – runs in the background and listens for DNS queries on UDP port 53 13 DNS Client • The DNS client is responsible for communicating with a DNS server to resolve computer and domain names to IP addresses • The DNS client is referred to as a “resolver” • An operating system (OS) must be configured at least one address of a DNS server that it can query 14 DNS Client In Windows, the first DNS server configured is called the preferred DNS server and the second one is the alternate DNS server 15 Authoritative and non-authoritative answers • An authoritative answer is when the DNS server hosting the DNS record responses to the DNS query. • An authoritative DNS server is used by domain name owners to store DNS records. It provides authoritative answers to DNS resolvers. • A DNS server provides answers for which they are not authoritative, the queried DNS record is not in its zone file. This is known as a nonauthoritative answer. 16 Iterative and recursive query in DNS • Recursive query – A query that demands a resolution or the answer – The initial request the resolver makes to the local server is a recursive query. The local DNS server must provide the information requested by the resolver • Iterative query – A query that does not demand resolution – When the local server issues queries to other servers, the other servers only provide information if they have it 17 Name Resolution Process DNS Server Root Local DNS Server 4. Query .com TLD server 1. Query for www.microsoft.com TLD Server 5 Try one of microsoft.com servers 8. The destination address is 203.0.19.190 .com DNS Server Visit www.microsoft.com Web Server microsoft.com 18 www.microsoft.com 1. 2. 3. 4. 5. 6. 7. 8. Query the IP address for www.microsoft.com Query to root server: If local DNS server does not recognize the domain name, then it queries the root server. Try one of these .com TLD servers Query .com TLD server: Contact one of the microsoft.com DNS servers Query to the microsoft.com DNS server The query is resolved Then the resolver returns the resolved IP address to the requester 19 Summary of DNS • Functionality of DNS • Hierarchical structure of DNS – Root, TLDs, subdomain-level domains • Roles for DNS clients and servers • Basic process of a DNS name resolution 20