ECE5650
Basic Network Services (II)
FTP, Email, and DNS
2: Application Layer
1
Recap: HTTP and Web
 HTTP request msg format and method
types:

GET, POST, HEAD, PUT, DELATE
 HTTP response msg format and status
codes
 Cookies and their usage:

Persistent vs Non-Persistent cookies
2: Application Layer
2
Examples ofInternet Services
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

SMTP, POP3, IMAP
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.9 Building a Web
server
 2.5 DNS
2: Application Layer
3
FTP: the file transfer protocol
user
at host
FTP
FTP
user
client
interface
file transfer
local file
system
FTP
server
remote file
system
 transfer file to/from remote host
 client/server model
client: side that initiates transfer (either to/from
remote)
 server: remote host
 ftp: RFC 959
 ftp server: port 21 for control, port 20 for data

2: Application Layer
4
SFTP: secure file transfer protocol
user
at host
file transfer over SSH
SFTP
SFTP
SFTP
user
client
server
interface
local file
system
remote file
system
 All communication (login, control and data are secured)
 transfer file to/from remote host
 same as FTP client/server model
 network protocol designed by the IETF to provide secure
file transfer and manipulation facilities over the secure
shell (SSH) protocol.
2: Application Layer
5
FTP: separate control, data connections
TCP control connection
port 21
 FTP client contacts FTP server




at port 21, specifying TCP as
transport protocol
Client obtains authorization over
control connection
Client browses remote directory
by sending commands over the
persistent control connection.
When server receives a
command for a file transfer or
directory listing, the server
opens a TCP data connection to
client
After transferring one file,
server closes connection.
FTP
client
TCP data connection
port 20
FTP
server
 Server opens a second TCP data
connection to transfer another
file.
 Control connection: “out of band”

HTTP sends control info in-band
 FTP server maintains “state”:
current directory, earlier
authentication
2: Application Layer
6
FTP commands, responses
Sample commands:
Sample return codes
 sent as ASCII text over
 status code and phrase (as
control channel
 USER username
 PASS password

 LIST return list of file in

 RETR (Get) filename

 STOR (Put) filename

current directory
retrieves (gets) file
stores (puts) file onto
remote host
in HTTP)
331 Username OK,
password required
125 data connection
already open;
transfer starting
425 Can’t open data
connection
452 Error writing
file
2: Application Layer
7
All FTP commands (RFC 959)
 Access control commands:
 USER, PASS, ACT, CWD, CDUP, SMNT, REIN, QUIT.
 Transfer parameter commands:
 PORT, PASV, TYPE STRU, MODE.
 Service commands:
 RETR, STOR, STOU, APPE, ALLO, REST, RNFR, RNTO,
ABOR, DELE, RMD, MRD, PWD, LIST, NLST, SITE,
SYST, STAT, HELP, NOOP.
 www.faqs.org/rfcs/rfc959.html
2: Application Layer
8
FTP Summary
 FTP/SFTP is used to transfer files between hosts
 FTP is an out-of-band protocol: control is sent over
server port 21 while data is sent over server port
20.
 Control connection is persistent and the FTP server
must maintain the state of the user.
 Data connection is non-persistent and initiated by
FTP server.
2: Application Layer
9
Electronic Mail
outgoing
message queue
user mailbox
user
agent
Three major components:
 user agents
 mail servers
mail
server
SMTP
 simple mail transfer
protocol: SMTP
User Agent
 a.k.a. “mail reader”
 composing, editing, reading
mail messages
 e.g., Eudora, Outlook, elm,
Netscape Messenger
 outgoing, incoming messages
stored on server
SMTP
mail
server
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
user
agent
2: Application Layer
10
Electronic Mail: mail servers
user
agent
Mail Servers
 mailbox contains incoming
messages for user
 message queue of outgoing
(to be sent) mail messages
 SMTP protocol between mail
servers to send email
messages
 client: sending mail
server
 “server”: receiving mail
server
mail
server
SMTP
SMTP
mail
server
user
agent
SMTP
user
agent
mail
server
user
agent
user
agent
user
agent
2: Application Layer
11
Electronic Mail: SMTP [RFC 2821]
 uses TCP to reliably transfer email message from client
to server, port 25
 direct transfer: sending server to receiving server
 three phases of transfer
 handshaking (greeting)
 transfer of messages
 closure
 command/response interaction
 commands: ASCII text
 response: status code and phrase
 messages must be in 7-bit ASCII
2: Application Layer
12
Scenario: Alice sends message to Bob
1) Alice uses UA to compose
message and “to”
bob@wayne.edu
2) Alice’s UA sends message
to her mail server; message
placed in message queue
3) Client side of SMTP opens
TCP connection with Bob’s
mail server
1
user
agent
2
mail
server
3
4) SMTP client sends Alice’s
message over the TCP
connection
5) Bob’s mail server places the
message in Bob’s mailbox
6) Bob invokes his user agent
to read message
mail
server
4
5
6
user
agent
2: Application Layer
13
Sample SMTP interaction
C:
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
S:
C:
S:
telnet smtp.wayne.edu 25
220 mirapointmr3.wayne.edu
HELO alice
250 Hello alice, pleased to meet you
MAIL FROM: <alice@crepes.fr>
250 alice@crepes.fr... Sender ok
RCPT TO: <bob@wayne.edu>
250 bob@wayne.edu ... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 wayne.edu closing connection
2: Application Layer
14
Try SMTP interaction for yourself:
 telnet ece.eng.wayne.edu 25
 see 220 reply from server
 enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands
above lets you send email without using email client
(reader)
2: Application Layer
15
SMTP Mail message format
SMTP: protocol for
exchanging email msgs
RFC 822: standard for text
message format:
 header lines, e.g.,
To:
 From:
 Subject:
different from SMTP
commands. SMTP msg goes
into the DATA command

header
blank
line
body
 body

the “message”, 7-bit
ASCII characters only
2: Application Layer
16
Message format: multimedia extensions
 MIME: multimedia mail extension, RFC 2045, 2056
 additional lines in msg header declare MIME content
type
MIME version
method used
to encode data, quotedprintable is another method
multimedia data
type, subtype,
parameter declaration
From: alice@crepes.fr
To: bob@wayne.edu
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
.........................
......base64 encoded data
encoded data where each line is 78
7-bit ASCII characters including CRLF
2: Application Layer
17
Base64 encoding scheme (RFC 2045)
 Encoding (not encrypting) method
 input: 8 bit ASCII data
 output: one of the 64 values listed in the
Base64 encoding table and the “=“
character
 Process:





group the input data in chunks of 3-bytes
or 24-bits each
break each 24-bit chunk into 4 values
that are 6-bits each
use the Base64 encoding table to find the
character of each value
if any chunk is less than 3 bytes then pad
it with 0s and use the “=“ for any of these
6 0s. A 6-bit value with all 0s and at least
1 non-padded 0 will be an “A”
Smallest base64 encoded output is 4
characters.
Base64 Encoding Table
Value Char
0A
Value Char
16 Q
Value Char
32 g
Value Char
48 w
1B
17 R
33 h
49 x
2C
18 S
34 i
50 y
3D
19 T
35 j
51 z
4E
20 U
36 k
52 0
5F
21 V
37 l
53 1
6G
22 W
38 m
54 2
7H
23 X
39 n
55 3
8I
24 Y
40 o
56 4
9J
25 Z
41 p
57 5
10 K
26 a
42 q
58 6
11 L
27 b
43 r
59 7
12 M
28 c
44 s
60 8
13 N
29 d
45 t
61 9
14 O
30 e
46 u
62 +
15 P
31 f
47 v
63 /
Example: input is AB
A ASCII hex=0x41
B ASCII hex=0x42
16-bits + 8 padded 0s:
0100 0001 0100 0010 0000 0000
4 values that are 6-bits each:
010000-010100-001000-000000
16-20-8-6 padded 0s
Base64 output is QUI=
2: Application Layer
18
Mail access protocols
user
agent
SMTP
SMTP
sender’s mail
server
access
protocol
user
agent
receiver’s mail
server
 SMTP: delivery/storage to receiver’s server
 Mail access protocol: retrieval from server



POP3: Post Office Protocol version 3 [RFC 1939] uses
port 110
• authorization (agent <-->server) and download
IMAP: Internet Mail Access Protocol [RFC 1730]
• more features (more complex)
• manipulation of stored msgs on server
HTTP: Hotmail , Yahoo! Mail, etc.
2: Application Layer
19
POP3 protocol
authorization phase
 client commands:
 user: declare username
 pass: password
 server responses


+OK (Server accepted prior command)
-ERR (server rejected prior command)
transaction phase, client:
 list: list message numbers
 retr: retrieve message by number
 dele: delete
 quit
UIDL: “unique-ID listing” list unique
message ID for all of the messages present
in the users mailbox. Useful for downloadand-keep by keeping a file that lists the
messages retrieved in earlier sessions, the
client can use the UIDL command to
determine which messages on the server
have already been seen.
 “Received:” in the msg indicates the SMTP
servers that forwarded the msg

C:
S:
C:
S:
C:
S:
telnet ece.eng.wayne.edu 110
+OK POP3 server ready
user bob
+OK
pass hungry
+OK user successfully logged on
C:
S:
S:
S:
C:
S:
S:
S:
S:
C:
C:
S:
S:
C:
C:
S:
list
1 498
2 912
.
retr 1
Return-Path: <fromuser@..>
Received: from b1 by d1 ...
more data..
.
dele 1
retr 2
<message 1 contents>
.
dele 2
quit
+OK POP3 server signing off
2: Application Layer
20
POP3 (more) and IMAP
More about POP3
 Previous example uses
“download-and-delete”
mode.
 User cannot re-read
the deleted e-mail.
 “Download-and-keep”:
copies of messages on
different clients
 POP3 is stateless
across sessions
IMAP
 Keep all messages in
one place: the server
 Allows user to
organize messages in
folders
 IMAP keeps user state
across sessions:

names of folders and
mappings between
message IDs and folder
name
2: Application Layer
21
Email Summary
Comparison with HTTP:
 SMTP and POP3 uses




persistent connections
SMTP requires message
(header & body) to be in 7bit ASCII
SMTP server uses
CRLF.CRLF to determine
end of message
download-and-delete vs
download-and-keep in POP3
All data communications
are insecure by default
 HTTP: pull data from web
server
 SMTP: push data to mail
server
 both have command/response
interaction, status codes
 HTTP: each object
encapsulated in its own
response msg
 SMTP: multiple objects sent in
one multipart msg
 SMPT msg must be in 7-bit
ASCII while HTTP has no
restriction
2: Application Layer
22
DNS: Domain Name System
People: many identifiers:

SSN, name, passport #
Internet hosts, routers:


IP address (32 or 128
bit) - used for
addressing datagrams
“canonical name”, e.g.,
ww.yahoo.com - used by
humans
Q: map between IP
addresses and name ?
Domain Name System
(DNS) is:
1- distributed database
implemented in hierarchy of
many name servers
2- application-layer protocol:
host, routers and name
servers communicate to
resolve names (address/name
translation). DNS protocol
uses UDP transport protocol
and port 53.
3- employed by other application
layer protocols (HTTP, SMTP,
FTP) to resolve host names.
2: Application Layer
23
DNS
DNS services
 Hostname to IP address translation
 Host aliasing

Canonical (actual) and alias names
(user-friendly): cwis-1.wayne.edu for
alias www.wayne.edu
 Mail server aliasing:


mail server and web server can share
the same alias name.
E.g. czxu@wayne.edu, wayne.edu
 Load distribution

Why not centralize
DNS?
 single point of failure
 traffic volume
 distant centralized
database
 maintenance
doesn’t scale!
Replicated Web servers: a set of IP
addresses for one canonical name.
DNS returns the list of IPs for a
name but rotated by 1 each time so
the user can use the first listed IP.
2: Application Layer
24
Distributed, Hierarchical Database
Root DNS Servers (13 servers labeled A-M)
Top-Level Domain
Servers (TLDs)
com DNS servers
Authoritative
DNS servers
yahoo.com amazon.com
DNS servers DNS servers
org DNS servers
pbs.org
DNS servers
edu DNS servers
poly.edu
umass.edu
DNS serversDNS servers
Each Client uses a local DNS server that does not belong to the
hierarchy:
 The local DNS is usually assigned by the DHCP server as part of
the temporary IP assignment (run command: “ipconfig /all” to find
your local DNS server).
2: Application Layer
25
DNS: Root name servers
There are 13 root DNS server world wide that are
labeled A-M: map of root DNS, as of Oct 2006.
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 11 locations)
k RIPE London (also Amsterdam, Frankfurt)
i Autonomica, Stockholm (plus 3 other
locations)
m WIDE Tokyo
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA (and
17 other locations)
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
2: Application Layer
26
TLD and Authoritative Servers
 Top-level domain (TLD) servers: responsible for com,
org, net, edu, etc, and all country code top-level
domains (ccTLD) us, ca, in, cn, jp.


Network solutions maintains servers for com TLD
Educause for edu TLD
 Authoritative DNS servers: organization’s with public
names has DNS servers, providing authoritative
hostname to IP mappings for organization’s servers
(e.g., Web and mail).

Can be maintained by organization or service provider
2: Application Layer
27
Local Name Server
 Does not strictly belong to hierarchy
 Each ISP (residential ISP, company,
university) has one.

Also called “default name server”
 When a host makes a DNS query, query is
sent to its local DNS server

Acts as a proxy, forwards query into hierarchy.
2: Application Layer
28
Example of Typical DNS request
Client X wants IP address for Y
 Steps performed:
1- Client sends DNS request to the local DNS
server to search on its behalf (recursive
query)
2- local DNS contacts one of the root DNSs to
resolve hostname Y.
root DNS server

3- root DNS returns the TLD DNS IP to
local DNS
4- local DNS contacts one of the TLDs
to get an Authoritative DNS nam
5- TLD returns IP of authoritative DNS
to local DNS
6- local DNS contacts authoritative DNS
to resolve X
7- authoritative DNS returns IP of Y
8- local DNS return IP of Y to X
Query 1 is recursive
Queries 2, 4 and 6 are iterative
2
3
TLD DNS server
4
5
local DNS server
1
8
7
6
authoritative DNS server
requesting host
X
Y
Example of recursive+iterative DNS
query - typically used
2: Application Layer
29
Recursive and Iterative DNS
queries
root DNS server
recursive query:
2
 puts burden of name
resolution on
contacted name
server
 heavy load?
iterative query:
3
7
TLD DNS
server
local DNS server
5
1
4
8
 reply is directly
returned to
requesting server
 “I don’t know this
name, but ask this
server”
6
authoritative DNS server
requesting host
requested host
Example of pure recursive DNS
query - not typically used
2: Application Layer
30
DNS: caching and updating records
 once (any) name server learns mapping, it caches
mapping
 cache entries timeout (disappear) after some
time
 TLD servers typically cached in local name
servers
• Thus root name servers not often visited
Client may also cache DNS names
 update/notify mechanisms under design by IETF


RFC 2136

http://www.ietf.org/html.charters/dnsind-charter.html
2: Application Layer
31
hosts file
 local file that is checked by the client DNS of the OS before
sending a DNS request. It can speed the web access.
 If the requested name is found in the hosts file then its
corresponding IP is used.
 Can be used to create custom (name-IP) entries.
 File Location:


windows XP: C:\WINDOWS\system32\drivers\etc
most UNIX and Linux: /etc
 File Structure:


<IP address><space><name><space><# comment>
Example of an entry: 127.0.0.1
localhost #default entry
2: Application Layer
32
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)

Type=A
 name is hostname
 value is IP address
 always in authoritative DNS
 may be cached in nonauthoritative DNSs

Type=CNAME
 name is alias name for some
“canonical” (the real) name
www.ibm.com is really
servereast.backup2.ibm.com
 value is canonical name
 used by all hosts
Type=NS
 Type=MX
 name is domain (e.g. foo.com)
 value is name of mailserver
 value is hostname of
associated with name that is
authoritative name server for this
usually an alias name
domain
 company can have a web server and a
 always in non-authoritative DNSs
mail server with the same alias
to point to authoritative DNSs
name. e.g.
TTL is time to live of the RR and determines
[wayne.edu mail.wayne.edu, MX]
when an RR should be removed from cache.
2: Application Layer
33

DNS records with DNS servers
 Authoritative DNSs for an institution:
 must contain Type A RRs for the institution’s public names
and IPs.
 may contain Type MX RRs for the institution’s public mail
server names and IPs.
 may contain Type CNAME RRs if the institution has
Canonical names for its alias names.
 TLD DNSs
 contain Type NS RRs with each organization’s public name is
mapped to its authoritative DNS server names. There is
usually a primary and secondary authoritative DNS servers.
 contain Type A RRs with the Authoritative DNS server
name and IP address.
2: Application Layer
34
DNS protocol, messages
DNS protocol : query and reply messages, both with
same message format
msg header
 identification: 16 bit #,
query and reply msgs use
the same #
 flags:
 query or reply 1 bit
flag
 recursion desired or
available 1 bit
 reply is authoritative
2: Application Layer
35
DNS protocol, messages
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used
2: Application Layer
36
Inserting records into DNS
 Example: just created startup “Network Utopia”
 Register name networkuptopia.com at a registrar
(e.g., Network Solutions)


Need to provide registrar with names and IP addresses of
your authoritative name server (primary and secondary)
Registrar inserts two RRs into the com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
 Put in authoritative server Type A record for
www.networkuptopia.com and Type MX record for
networkutopia.com
 How do people get the IP address of your Web site?
2: Application Layer
37
nslookup command and whois DB
 used to displays information that you can use to diagnose Domain







Name System (DNS) infrastructure. Contacts the specified DNS
server to retrieve requested records.
nslookup <domain or IP to find> <DNS server name>
Example: nslookup wayne.com
whois database can be used to locate the corresponding registrar,
DNS server and IPs for a particular domain.
Only registrars accredited by the Internet Corporation for Assigned
Names and Numbers (ICANN - non-profit org) are authorized to
register .aero, .biz, .com, .coop, .info, .museum, .name, .net, .org, or .pro
names.
.com whois database: http://www.internic.net/whois.html
.edu whois database http://whois.educause.net/index.asp
wayne.edu DNS name servers:
NS.WAYNE.EDU
NS2.WAYNE.EDU
DNS.MERIT.NET
NS2.CS.WAYNE.EDU
141.217.1.15
141.217.1.13
141.217.16.10
2: Application Layer
38
DNS Vulnerabilities
 DDoS bw-flooding attack against DNS server.
 A large scale attack on 13 DNS root servers on Oct 21,
2002 by using ICMP ping messages
 Block ICMP ping packets in packet filtering
 DNS queries attack
 Hard to be filtered
 Mitigated by caching in local DNS servers
 Man-in-the-middle attack
 Trick a server into bogus records into its cache
 Hard to implement, because it needs to intercept packets
 Reflection attack on other hosts
 Send queries with spoofed source addr of a target
server
2: Application Layer
39
DNS Summary
 DNS services:
 Hostname to IP address translation
 Host aliasing, Mail server aliasing, Load distribution
 DNS is hierarchical and distributed
 root DNS vs TLD vs Authoritative DNS vs local DNS
 recursive vs iterative DNS query
 DNS cache: local server caches TLDs so that root




servers are rarely visited
DNS record types: A, NS, CNAME, MX
DNS Query and Reply msg format is the same
nslookup command and the whois database
DNS vulnerabilities
2: Application Layer
40
Examples ofInternet Services
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

SMTP, POP3, IMAP
 2.5 DNS
 2.6 P2P and File Sharing
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.9 Building a Web
server
2: Application Layer
41