Quiz 1
• ANGEL Lessons  Quizzes
• System will stop accepting submissions after
15 minutes
Reading for next class
• Ch. 5, Ch. 6.1-6.9
Research assignment 1
• Due: Monday, 2/1 at 11:59pm
Internet Applications
DNS: domain name system
• IPs: unique identifiers for hosts…
• but nobody can remember them!
• Try both in your browser:
– 146.186.157.6
– www.psu.edu
• The goal of DNS is to translate (“resolve”)
between the two
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  www.psu.edu
– .com  google.com  maps.google.com
• DNS: domain name system
– A hierarchical system to resolve named addresses into
IP addresses
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
com DNS servers
org DNS servers
yahoo.com
amazon.com
DNS servers DNS servers
Authoritative DNS servers
pbs.org
DNS servers
edu DNS servers
mit.edu
psu.edu
DNS servers DNS servers
ist.psu.edu
DNS servers
cse.psu.edu
DNS servers
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 )
e. NASA Mt View, CA
f. Internet Software C.
Palo Alto, CA (and 48 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)
k. RIPE London (17 other sites)
i. Netnod, Stockholm (37 other
sites)
m. WIDE Tokyo
(5 other sites)
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
DNS name resolution example
root DNS server
• host at
cydar.ist.psu.edu wants
IP address for
www.media.mit.edu
2
3
TLD DNS server
4
5
• 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may be out of date!)
local DNS server
1
8
7
6
authoritative DNS server
dns.media.mit.edu
requesting host
cydar.ist.psu.edu
www.media.mit.edu
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?
In-Class Exercise: nslookup
• Task 2
a. Find out IP address of server ist.psu.edu
b. In the command prompt window, type the ip address of
ist.psu.edu. What information did nslookup respond with?
In-Class Exercise: nslookup
• Task 3 (Ch. 4.23)
a. What is the IP address of www.psu.edu?
b. What is the real name of www.psu.edu?
In-Class Exercise: nslookup
• Task 4
– Find out IP address(es) of google.com
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 email 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.
HTTP (hypertext transfer protocol)
• HTTP :
– 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: http://
– An application layer protocol which
is above the TCP/IP protocols
• HTTP commands are encapsulated
into TCP packets.
HTTP Commands
• GET, HEAD, POST, PUT, etc.
• These commands are often hidden from users.
HTTP Status code
• Indicate whether the server handled the
request
SMTP Protocol: Email
• SMTP (Simple Mail Transfer Protocol) commands
– HELO, MAIL, RCPT, DATA, QUIT, VRFY…
Demo: Sending Email via Gmail using
Command Line
1.
2.
3.
4.
5.
6.
7.
8.
9.
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: “235 2.7.0 Accepted”
Type mail from: <person_a@gmail.com>
Type rcpt to: <person_b@gmail.com>
Type data, then enter your message
To finish entering the message, press ENTER, then press ., then
press ENTER again
Type quit
Note: If you want to send email via PSU mail server, in step 1 type the
following instead:
openssl s_client -connect authsmtp.psu.edu:587 -starttls smtp
Other Protocols
• FTP (File Transfer Protocol)
– Was a dominant tool for file transfer.
• Suffers the same security issues as Telnet.
– Replaced with SFTP (Secured FTP).
• Email Access Protocol
– POP3
• Download email messages to a local client.
– IMAP
• Keep email in email server.
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 emails via Openssl
– Find out how to use openssl to send emails through your favorite
email service provider (hotmail, yahoo, gmail, aol, etc.)