1. Application Services COM211 Communications and Networks CDA College Theodoros Christophides Email: theo_christopher@hotmail.comtheo_christopher@hotmail.com www.cdacollege.ac.cy/site/info-com-technology-ll/

2. Application Services Domain Name Service Proxy Service Mail Service Web Service SSH

3. DNS – Domain Name System A DNS server maintains the name to IP address mapping of the domain for which it is the name server. The DNS server for a domain is registered with the domain registrar and the entry is maintained by the Internet Root- Servers (13) or Country Level Root-Servers. Whenever a server is queried, if doesn’t have the answer, the root servers are contacted. The root servers refer to the DNS server for that domain (in case the domain is a top level domain) or the Country Root Server (in case the domain is country level domain).

4. Internet Naming Hierarchy The silent dot at the end of all addresses.com.net.org.in.tcd www.ac.co.iitk www

5. Proxy A proxy is a host which relays web access requests from clients Used when clients do not access the web directly Used for security, logging, accounting and performance browserproxy web

6. Connections Customers connect to an ISP ISPs connect to backbone Backbone networks ISP networ k Customer Networks Bandwidth-limited links

7. Internet Connections Cost of connections is based on bandwidth Cost of connection is a major part of network cost Organisations only obtain as much bandwidth as they can afford Many organisations in Asia-Pacific only have 64kb/s – 2Mb/s connections (as compared to their counterpart in US and Europe who have bandwidths of 2.4 Gbps – 10 Gbps)

8. What is Web Caching? Storing copies of recently accessed web pages Pages are delivered from the cache when requested again Browser caches Proxy caches

9. Why Cache? Shorter response time Reduced bandwidth requirement Reduced load on servers Access control and logging

10. Popular Proxy Caches Apache proxy MS proxy server WinProxy Squid  Squid is popular because it is powerful, configurable and free Many others

11. Web Server HTTP (Hyper Text Transfer Protocol) is used to transfer web pages from a Web Server to Web Client (Browser) Web Pages are arranged in a directory structure in the Web Server HTTP supports CGI (Common Gateway interface) HTTP supports Virtual Hosting (Hosting multiple sites on the same server) Popular Web Servers  Apache  Windows IIS  IBM Websphere

12. Web Server

13. Email Internet Mail Server Mail Client

14. Mail Architecture

15. Simple Mail Transfer Protocol (SMTP) is used to transfer mail between Mail Servers over Internet Post Office Protocol (PoP) and Interactive Mail Access Protocol (IMAP) is used between Client and Mail Server to retrieve mails The mail server of a domain is identified by the MX record of that domain Popular Mail Servers  Sendmail/Postfix  Microsoft Exchange Server  IBM Lotus

16. Email

17. SSH – Secure Shell SSH is a secure remote virtual terminal application  Provides encrypted communication between untrusted hosts over an insecure network Assumes eavesdroppers can hear all communications between hosts Provides different methods of authentication Encrypts data exchanged between hosts  Intended to replace insecure programs such as rlogin, rsh, etc.  Includes capability to securely transfer file SCP  Includes ability to forward X11 connections and TCP ports securely Very popular and widely used  Not invulnerable!

18. Authentication in SSH1 Three means of authenticating supported by SSH  Simple rhosts User/system names in ~/.rhosts, ~/.shosts Vulnerable to IP/DNS spoofing Requires specific compilation for this mode of operation  Host based Use RSA to verify host keys Use ~/.rhosts file for user authentication  Host and user based RSA host key verification RSA user key verification If authentication fails, client is prompted for password  All communication is encrypted

19. SSH1 Key Exchange Protocol Server has public/private key pair  Client knows server’s public key in advance Must be sent securely in advance Server sends public key and random server key to client  Client verifies public key Client sends random session key encrypted with host and server key  Rest of session is encrypted with session key

20. SSH2 Key Exchange Protocol Diffie-Hellman public key exchange algorithm is used  Public key exchange algorithm – very cool  Two users can exchange a secret key over and insecure link without sharing any prior secrets (!) Digital signature verifies identity of server to client At the end of the key exchange, a secret key is shared  Used for encrypting the rest of the session Supports MD5 checksums for data integrity Supports a variety of encryption mechanisms  IDEA (default), Blowfish, DES, Triple DES, …

21. SSH in Practice Host public/private key is generated when SSH is installed  Public key must be in ~/.ssh/known_hosts on remote systems ssh-keygen command is used to generate users public/private keys  Requires user enter a pass phrase  Public key copied to ~/.ssh/authorized_keys on remote systems Ssh-agent and ssh-add eliminate the need for repeated typing of pass phrase Password authentication is vulnerable to guessing attacks X11 and port forwarding enable encrypted pipe through the Internet  Can be used to securely access insecure application eg. SMTP  Can be used to circumvent firewalls Available as open source software  See OpenSSH Tricky to get working properly Standard with many software distributions

22. Questions?