1. Internet Applications

2. DNS: domain name system
IPs: unique identifiers for hosts…
but nobody can remember them!
Try both in your browser:
The goal of DNS is to translate (“resolve”) between the two

3. Resolve IP Address of URL
URLs are hierarchical
Most significant part on the right
The leftmost segment is the name of individual computer
Examples
.edu  .psu.edu 
.com  google.com  maps.google.com
DNS: domain name system
A hierarchical system to resolve named addresses into IP addresses
DNS uses a large number of servers, organized in a hierarchical fashion and distributed all over the world. The mappings are distributed across the DMS servers.

4. Resolve IP Address of URL
DNS servers: distributed databases to keep information about URL and IP address pairs
Root DNS Servers
Top-level domain servers
org DNS servers
edu DNS servers
DNS uses a large number of servers, organized in a hierarchical fashion and distributed all over the world. The mappings are distributed across the DMS servers.
com DNS servers
psu.edu
DNS servers
mit.edu
DNS servers
pbs.org
DNS servers
yahoo.com
DNS servers
amazon.com
DNS servers
ist.psu.edu
DNS servers
cse.psu.edu
DNS servers
Authoritative DNS servers

5. DNS root servers 13 root name “servers” worldwide
c. Cogent, Herndon, VA (5 other sites)
d. U Maryland College Park, MD
h. ARL Aberdeen, MD
j. Verisign, Dulles VA (69 other sites )
k. RIPE London (17 other sites)
i. Netnod, Stockholm (37 other sites)
e. NASA Mt View, CA
f. Internet Software C.
Palo Alto, CA (and 48 other sites)
m. WIDE Tokyo
(5 other sites)
g. US DoD Columbus, OH (5 other sites)
a. Verisign, Los Angeles CA
(5 other sites)
b. USC-ISI Marina del Rey, CA
l. ICANN Los Angeles, CA
(41 other sites)

6. TLD, authoritative servers
top-level domain (TLD) servers:
responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp
authoritative DNS servers:
organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts

7. DNS name resolution example
root DNS server
host at cydar.ist.psu.edu wants IP address for
Local DNS server
does not strictly belong to hierarchy
has local cache of recent name-to-address translation pairs (but may be out of date!) 2 3
TLD DNS server 4 5
local DNS server 7 6 1 8
authoritative DNS server
dns.media.mit.edu
requesting host
cydar.ist.psu.edu

8. In-Class Exercise: nslookup
nslookup: an application to query DNS
Start "nslookup"
Start  Run  cmd
Type "nslookup"
Query DNS server
Type URL or IP address
Task 1
What is the IP address of your DNS server?

9. In-Class Exercise: nslookup
Task 2
Find out IP address of server ist.psu.edu
non-authoritative: this answer came from the cache of some server rather than from an authoritative DNS server.

10. In-Class Exercise: nslookup
Task 3 (Ch. 4.23)
What is the IP address of
What is the real name of
In the command prompt window, type the ip address of What information did nslookup respond with?

11. In-Class Exercise: nslookup
Task 4
Find out IP address(es) of google.com

12. In-Class Exercise: nslookup
Task 5 (Ch. 4.22)
Each entry in a DNS server has a type
Type A: IPv4 (default)
Type AAAA: IPv6
Type MX: used for lookup name in address
Type CNAME: an alias to another DNS entry
Type set q=MX, press ENTER. Then find out IP address(es) of google.com again.

13. HTTP (hypertext transfer protocol)
The primary transfer protocol that a browser use to interact with a web server
Establish connection, negotiate methods/parameters of data transmission, send/obtain data, close connection
Most URLs contain an explicit protocol reference:
An application layer protocol which is above the TCP/IP protocols
HTTP commands are encapsulated into TCP packets.

14. HTTP Commands GET, HEAD, POST, PUT, etc.
These commands are often hidden from users.

15. HTTP Status code
Indicate whether the server handled the request

16. SMTP Protocol: Email SMTP (Simple Mail Transfer Protocol) commands
HELO, MAIL, RCPT, DATA, QUIT, VRFY…

17. Demo: Sending Email via Gmail using Command Line
Type openssl s_client -connect smtp.gmail.com:465 -crlf -ign_eof
Type helo smtp.gmail.com
Type auth login and you will receive “334 VXNlcm5hbWU6”, which means “Username:”
Using a Base 64 encoder such as this one, encode your user name and enter it. Do the same for your password, which is requested next. If authenticated, you should see: “ Accepted”
Type mail from:
Type rcpt to:
Type data, then enter your message
To finish entering the message, press ENTER, then press ., then press ENTER again
Type quit
Press Ctrl+V+Enter
Press .
Note: If you want to send via PSU mail server, in step 1 type the following instead:
openssl s_client -connect authsmtp.psu.edu:587 -starttls smtp

18. Other Protocols FTP (File Transfer Protocol) Email Access Protocol
Was a dominant tool for file transfer.
Suffers the same security issues as Telnet.
Replaced with SFTP (Secured FTP).
Access Protocol
POP3
Download messages to a local client.
IMAP
Keep in server.

19. After Class Exercise
Just for exploration. No need to turn in anything.
DNS
Use nslookup to resolve several different URLs.
Check the DNS server names in different places.
IST, your dorm, coffee shop, etc.
Figure out which organizations these servers.
Think about what may happen if a DNS is hijacked.
Sending s via Openssl
Find out how to use openssl to send s through your favorite service provider (hotmail, yahoo, gmail, aol, etc.)