Week 3: . Johan Bollen Old Dominion University Department of Computer Science

Transcription

1 Week 3: Johan Bollen Old Dominion University Department of Computer Science jbollen September 11, 2003 Page 1

2 Demo SSH Not always available on windows machines: install and download September 11, 2003 Page 2

3 Demo traceroute, ping and host We did last week? September 11, 2003 Page 3

4 1. The internet s killer-app (a) Like regular mail (snailmail) but better! i. cheaper ii. Quicker iii. requires little to no overhead iv. Archiving (b) Probably most popular i. Enormous volume ii. Lays foundation for myriad other functions iii. Oldest iv. Defines internet culture (c) However! i. Fraud ii. The attachment scourge iii. Security iv. SPAM 2. Key concepts: (a) address and account (b) SMTP (c) POP (d) IMAP (e) Netiquette September 11, 2003 Page 4

5 User IDs, Passwords and addresses 1. User IDs and accounts (a) Context is UNIX system (b) Each user has unique ID and account on system (c) typically: i. initial of first name ii. last name iii. 8 characters 2. Passwords (a) Account access is restricted to user (b) Use of passwords (c) Security considerations September 11, 2003 Page 5

6 addresses 1. Natural consequence of UNIX account and user ID management (a) user ID + host identify user (b) use to combine both (c) host is identified by hostname (not IP?) 2. jbollen@cs.odu.edu (a) user ID = jbollen (b) hostname = cs.odu.edu (c) combined= address 3. addres is thus not uniquely tied to individual (a) Many possible hosts (b) Many possible user names (c) None need to reflect identity of user 4. Determine someone s adddress? September 11, 2003 Page 6

7 message components September 11, 2003 Page 7

8 Writing an 1. To: field 2. Cc: field 3. Bcc: field 4. Attachments 5. Body: (a) Netiquette i. Salutation ii. Spelling? iii. Appropriate language iv. NO SHOUTING (b) Keep in mind who will receive (c) Treat as business mail (d) Emotionally charged s i. Lack of body language is dangerous ii. 10 minute rule iii. Never attribute to malice that which Signature (a) Most clients have specific setting (b) Keep it concise (c) Be a minimalist 7. Forwarding (a) Message (b) To other accounts:.forward September 11, 2003 Page 8

9 Attachments 1. File is attached to (a) Receiver can save to local system (b) Convenient way to share files 2. Not part of original intent (a) Attachment is sent as part of message (b) Convert original file to text representation 3. I recommend against attachments (a) Burden on slow connections (mail bomb) (b) Security risk (c) Often unnecessary! (d) Requires applications to decode and display (e) Alternative: send URL September 11, 2003 Page 9

10 inner workings 1. Post system (a) Origin (b) Destination (c) Delivery 2. Internet: (a) Computers are intermediaries (b) user computer computer user (c) Actual system: i. Mail clients ii. Mail servers iii. Mail accounts iv. SMTP/POP protocols September 11, 2003 Page 10

11 The system in a nutshell jbollen@cs.odu.edu September 11, 2003 Page 11

12 Simple Mail Transfer Protocol 1. Set on its path 2. Client and mail server handshake and communicate 3. Simple command set communicated on port 25 of mail server: (a) HELO domain.name (b) MAIL FROM: (c) RCPT TO: yourfriend (d) DATA (e) Terminate with. and enter (f) QUIT September 11, 2003 Page 12

13 Post Office Procol 1. Used to retrieve from mail server to local host 2. Client and mail server handshake and communicate on port Set of commands to retrieve s (a) USER yourlogin (b) PASS pasword (c) STAT (d) LIST (e) RETR number (f) DELE number (g) QUIT September 11, 2003 Page 13

14 Some notes on SMTP and POP 1. No security (a) Passwords and logins transfered unprotected (b) SMTP does not require authentication (c) SPAM heaven! 2. SMTP + POP (a) Standard configuration: i. Load to local host ii. Remove from mail server iii. no longer accessible from anywhere iv. No backups (b) Other possibilities i. Telnet to mailserver ii. Read using PINE iii. Always accessible given telnet + internet (c) IMAP 3. Mailserver problems (a) Does not exist or defunct: bounced mail (b) Need to be reliable systems (c) Sufficient space? (d) Other than that: anyone can run mailserver September 11, 2003 Page 14

15 IMAP September 11, 2003 Page 15

16 IMAP explained 1. Internet Message Access Protocol 2. Intended to solve POP s problems (a) POP is simple (b) download and delete (c) or leave in place (d) Synchronization problems between several systems 3. IMAP (a) RFC: in-notes/rfc3501.txt (b) Leave messages on server (c) Only download those you want to read (d) Folder management on server and client 4. Comparison (a) POP: periodic contact, IMAP continous (b) IMAP Mail remains on server (c) Folders, address books, etc are centralized on server 5. Compatibility (a) PINE (b) Outlook express (c) Netscape (d) Eudora (e) Ximian September 11, 2003 Page 16

17 Where do s go? Shortly: to your accounts mail spool 1. disk space allocated for s 2. accessible through POP 3. /var/mail/jbollen 4. Flat ASCII file structure (a) parsing is done by POP or local client (b) Attachments? go there too: encoded BASE64 + Mine headers i. Content-type ii. Content-Transfer-Encoding iii. boundary delimiters (c) see: us/~neal/attachments.html September 11, 2003 Page 17

18 filtering One big inbox? 1. hundreds of messages can accumulate 2. need to organize (a) not part of protocols (b) mail spool is simple flat text file (c) Responsibility of client 3. Most clients can filter (a) Define set of filter rules on To:, From, etc fields (b) Depending on match, move message to separate folder 4. procmail (a) UNIX tool (b) independent from client (c) use script file: RE and actions September 11, 2003 Page 18

19 Mailing lists 1. is essentially a point-to-point system (a) Communication between two individuals (b) Set up to simulate regular mail 2. Need to broadcast messages (a) Group discussion (b) Announcements 3. Mailing lists (a) Managed by list owner (b) Subscription based (c) Messages are broadcast via mailing list server (d) Always check cc:!!! September 11, 2003 Page 19