ECE5650

2: Application Layer 1

ECE5650

Basic Network Services (II)FTP, Email, and DNS


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


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

2.7 Socket programming with TCP

2.8 Socket programming with UDP

2.9 Building a Web server


FTP: the file transfer protocol

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

file transfer FTPserver

FTPuser

interfaceFTP

client

local filesystem

remote filesystem

user at host


SFTP: secure file transfer protocol

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.

file transfer over SSH SFTPserver

SFTPuser

interfaceSFTPclient

local filesystem

remote filesystem

user at host


FTP: separate control, data connections 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.

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

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


FTP commands, responsesSample commands: sent as ASCII text over

control channel USER username PASS password LIST return list of file in

current directory RETR (Get) filename

retrieves (gets) file STOR (Put) filename

stores (puts) file onto remote host

Sample return codes status code and phrase

(as in HTTP) 331 Username OK,

password required 125 data connection

already open; transfer starting

425 Can’t open data connection

452 Error writing file


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


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.


Electronic MailThree major components: user agents mail servers 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

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragentmail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


Electronic Mail: mail serversMail 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

mailserver

useragent

useragent

useragentmail

server

useragent

useragent

mailserver

useragent

SMTP

SMTP

SMTP


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


Scenario: Alice sends message to Bob1) Alice uses UA to compose

message and “to” [email protected]

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

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

useragent

mailserver

mailserver user

agent

1

2 3 4 56


Sample SMTP interaction C: telnet smtp.wayne.edu 25 S: 220 mirapointmr3.wayne.edu C: HELO alice S: 250 Hello alice, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 wayne.edu closing connection


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)


SMTP Mail message formatSMTP: protocol for

exchanging email msgsRFC 822: standard for text

message format: header lines, e.g.,

To: From: Subject:different from SMTPcommands. SMTP msg goesinto the DATA command

body the “message”, 7-bit

ASCII characters only

header

body

blankline


Message format: multimedia extensions MIME: multimedia mail extension, RFC 2045, 2056 additional lines in msg header declare MIME content

type

From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg

base64 encoded data ..... ......................... ......base64 encoded data

multimedia datatype, subtype,

parameter declaration

method usedto encode data, quoted-

printable is another method

MIME version

encoded data where each line is 78 7-bit ASCII characters including CRLF


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.

Value Char Value Char Value Char Value Char0 A 16 Q 32 g 48 w1 B 17 R 33 h 49 x2 C 18 S 34 i 50 y3 D 19 T 35 j 51 z4 E 20 U 36 k 52 05 F 21 V 37 l 53 16 G 22 W 38 m 54 27 H 23 X 39 n 55 38 I 24 Y 40 o 56 49 J 25 Z 41 p 57 5

10 K 26 a 42 q 58 611 L 27 b 43 r 59 712 M 28 c 44 s 60 813 N 29 d 45 t 61 914 O 30 e 46 u 62 +15 P 31 f 47 v 63 /

Base64 Encoding Table

Example: input is ABA ASCII hex=0x41B ASCII hex=0x4216-bits + 8 padded 0s:0100 0001 0100 0010 0000 00004 values that are 6-bits each:010000-010100-001000-00000016-20-8-6 padded 0sBase64 output is QUI=


Mail access protocols

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.

useragent

sender’s mail server

useragent

SMTP SMTP accessprotocol

receiver’s mail server


POP3 protocolauthorization 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 download-and-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: list S: 1 498 S: 2 912 S: . C: retr 1 S: Return-Path: <fromuser@..> S: Received: from b1 by d1 ... S: more data.. S: . C: dele 1 C: retr 2 S: <message 1 contents> S: . C: dele 2 C: quit S: +OK POP3 server signing off

C: telnet ece.eng.wayne.edu 110S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on


POP3 (more) and IMAPMore 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


Email Summary SMTP and POP3 uses

persistent connections SMTP requires message

(header & body) to be in 7-bit 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

Comparison with HTTP: 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


DNS: Domain Name SystemPeople: 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.


DNS Why not centralize

DNS? single point of

failure traffic volume distant centralized

database maintenance

doesn’t scale!

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. [email protected], wayne.edu

Load distribution 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.


Root DNS Servers (13 servers labeled A-M)

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS serversyahoo.com

DNS serversamazon.comDNS servers

pbs.orgDNS servers

Distributed, Hierarchical Database

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).

Top-Level Domain Servers

(TLDs)Authoritative DNS servers


DNS: Root name servers

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA Mt View, CAf Internet Software C. Palo Alto, CA (and 17 other locations)

i Autonomica, Stockholm (plus 3 other locations)

k RIPE London (also Amsterdam, Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MDj Verisign, ( 11 locations)

There are 13 root DNS server world wide that are labeled A-M: map of root DNS, as of Oct 2006.


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


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.


requesting hostX

Y

root DNS server

local DNS server

1

23

4

5

6

authoritative DNS server

78

TLD DNS server

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.

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 Y8- local DNS return IP of Y to X

Query 1 is recursiveQueries 2, 4 and 6 are iterative

Example of recursive+iterative DNS query - typically used


requesting host

requested host

root DNS server

local DNS server

1

2

45

6

authoritative DNS server

7

8

TLD DNS server

3

Recursive and Iterative DNS queries

recursive query: puts burden of

name resolution on contacted name server

heavy load?

iterative query: reply is directly

returned to requesting server

“I don’t know this name, but ask this server” Example of pure recursive DNS

query - not typically used


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


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


DNS recordsDNS: distributed db storing resource records (RR)

Type=NS name is domain (e.g. foo.com) value is hostname of authoritative

name server for this domain always in non-authoritative DNSs to

point to authoritative DNSs

RR format: (name, value, type, ttl)

Type=A name is hostname value is IP address always in authoritative DNS may be cached in non-

authoritative 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=MX value is name of mailserver

associated with name that is usually an alias name

company can have a web server and a mail server with the same alias name. e.g. [wayne.edu mail.wayne.edu, MX]TTL is time to live of the RR and determines

when an RR should be removed from cache.


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.


DNS protocol, messagesDNS 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


DNS protocol, messages

Name, type fields for a query

RRs in responseto query

records forauthoritative servers

additional “helpful”info that may be used


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?


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 141.217.1.15 NS2.WAYNE.EDU 141.217.1.13 DNS.MERIT.NET NS2.CS.WAYNE.EDU 141.217.16.10


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


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


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

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