Computer Networks Application layer Overview q DNS

Computer Networks Application layer

Overview q DNS -- Domain Name System q E-mail q Terminal Access: TELNET Application layer -- May 2004

Domain names q IP-addresses not user friendly readable names telnet 134. 58. 42. 36 telnet nix. cs. kuleuven. ac. be q Overview o Definition of Internet domain names o Translating domain names into IP addresses Application layer -- May 2004

Domain names q Internet domain names o Hierarchical structure nix. cs. kuleuven. ac. be country code (Belgium) university (academic) K. U. Leuven dept. Computer science name of computer system Application layer -- May 2004

Domain names q 3 groups Top Level Domains (TLD) o 2 -letter country codes (ISO 3166) o generic names (similar organisations) • • • commercial organisations org non-commercial organisations (bv. Vzw) international organisations (nato, EU, …) net companies offering network services NEW: name, biz, info, … o names of organisations within USA • edu • gov • mil universities US government US army Application layer -- May 2004

Domain names q View on Internet domain name space Application layer -- May 2004

Domain names q Each TLD: o administrator (assign names within domain) o “be”: • till jan. 2000: dept. Computer science • now: vzw DNS BE (Ispa, Agoria, Beltug) q Each organisation with a domain name: o creates new names within its domain o E. g. . kuleuven. ac. be and kulak. ac. be q Hierarchical name structure + delegation = workable structure Application layer -- May 2004

Translating domain names q Database o not centralised (bottleneck!) o name server • Translate domain name IP-address o hierarchy of name server q For each domain (zone): separate (primary) name server o Zone = sub tree of name space tree o Primary name server • Gets info from a zone file on disk Application layer -- May 2004

Domain names q Internet domain names <> zones Application layer -- May 2004

Translating domain names Name server of cs. kuleuven. ac. be Systems/subdomains of cs. kuleuven. ac. be type IP-adres nix A 134. 58. 42. 36 idefix A 134. 58. 41. 7 droopy A 134. 58. 41. 10 stevin A 134. 58. 41. 16 . . . A = Address Application layer -- May 2004

Translating domain names Name server of kuleuven. ac. be Machines/subdomeinen van kuleuven. ac. be type IP-adres cs NS 134. 58. 39. 1 esat NS … www A … . . . NS = Name. Server Application layer -- May 2004

Translating domain names q Resource records Field Meaning Domain_Name to which the record applies Time_to_live Indicates how stable the record is Class IN for Internet; … Type Kind of record Value appropriate for type Application layer -- May 2004

Translating domain names q Principal DNS Resource record types Type Meaning Value SOA Start of authority Parameters for this zone A IP address of host 32 -bit integer NS Name Server Name of name server for this domain MX Mail exchange Priority, domain willing to accept email CNAME Canonical Name Domain Name PTR Pointer Alias for an IP address HINFO Host description CPU and OS in ascii TXT Text Uninterpreted ascii text Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be ; Authoritative data for cs. kuleuven. ac. be ; $TTL 86400 @ IN ; default TTL = 1 day SOA dns. cs. kuleuven. ac. be. postmaster. cs. kuleuven. ac. be. ( 2002042601 ; serial 10800 ; refresh (3 h) 1800 ; retry (30 m) 3600000 ; expire (>5 w) 86400 ) ; default (1 d) ; ; Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be (cont. ) ; General info for zone CS. KULEUVEN. AC. BE NS snoopy. cs. kuleuven. ac. be. NS dns. cs. kuleuven. ac. be. NS ns 1. kulnet. kuleuven. ac. be. NS ns. be. ubizen. com. NS secdns. eunet. be. ; dns. cs. kuleuven. ac. be is used as secondary for be. and others dns ; A 134. 58. 40. 4 general MX records cs. kuleuven. ac. be. 20 mailrelay. cs. kuleuven. ac. be. MX st. cs. kuleuven. ac. be. MX 100 mail. cc. kuleuven. ac. be. MX 10 st. cs. kuleuven. ac. be. MX 20 mailrelay. cs. kuleuven. ac. be. MX 100 mail. cc. kuleuven. ac. be. Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be (cont. ) calendar CNAME billie. cs. kuleuven. ac. be. lp-kleur CNAME delphi. cs. kuleuven. ac. be. lp-wit CNAME medusa. cs. kuleuven. ac. be. ssh 3600 ; Cnames do not work for MX's ; ftp 3600 CNAME arachne. cs. kuleuven. ac. be. ftp 1800 A 134. 58. 40. 10 MX 18 mail. cs. kuleuven. ac. be. MX 20 mailrelay. cs. kuleuven. ac. be. HINFO Sparc Solaris ; Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be (cont. ) ; our switch/router/firewall/ntp-server ; lswitch-cw A 134. 58. 250. 50 A 134. 58. 39. 254 A 134. 58. 41. 254 A 134. 58. 42. 254 A 134. 58. 43. 254 A 134. 58. 44. 245 A 134. 58. 45. 254 A 134. 58. 46. 254 MX 18 mail. cs. kuleuven. ac. be. MX 20 mailrelay. cs. kuleuven. ac. be. RP hostmaster contact. cs. kuleuven. ac. be. ; Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be (cont. ) snoopy 134. 58. 41. 8 A 134. 58. 42. 3 A 134. 58. 43. 4 A 134. 58. 45. 1 A 134. 58. 46. 63 A 134. 58. 39. 1 MX 10 snoopy. cs. kuleuven. ac. be. MX 18 mail. cs. kuleuven. ac. be. MX ; A 20 mailrelay. cs. kuleuven. ac. be. HINFO Sun. ULTRA 2 Server Solaris Application layer -- May 2004

Translating domain names q Part of DNS database for cs. kuleuven. ac. be (cont. ) panoramix A 134. 58. 45. 66 MX 10 panoramix. cs. kuleuven. ac. be. MX 18 mail. cs. kuleuven. ac. be. MX 20 mailrelay. cs. kuleuven. ac. be. RP pv contact. cs. kuleuven. ac. be. ; HINFO i 86 Linux ubiz A 134. 58. 39. 91 MX 18 mail. cs. kuleuven. ac. be. MX 20 mailrelay. cs. kuleuven. ac. be. RP pv contact. cs. kuleuven. ac. be. HINFO IBM_Think. Pad ; Application layer -- May 2004 Windows 2000

Translating domain names q Part of DNS database for. be $ORIGIN. $TTL 86400 be ; 1 day IN SOA master. ns. dns. be. tech. dns. be. ( 2002052301 ; serial 3600 ; refresh (1 hour) 1800 ; retry (30 minutes) 3600000 ; expire (5 weeks 6 days 16 hours) 600 ; minimum (10 minutes) ) NS dns. cs. kuleuven. ac. be. NS master. ns. dns. be. NS woluwe. ns. dns. be. NS hasselt. ns. dns. be. NS vilvoorde. ns. dns. be. Application layer -- May 2004

Translating domain names q Part of DNS database for. be (cont. ) $ORIGIN be. 007 ns 3. asp NS ns 4. asp NS ns. belnet NS ns 1. surfnet. nl. NS ns 3. belnet A 134. 58. 40. 4 NS ns. be. ubizen. com. NS ac NS dns. cs. kuleuven. ac NS ns. yournamehosting. com. NS ns. yournamewebhosting. com. $ORIGIN ac. be. dns. cs. kuleuven $ORIGIN be. huens verbaeten Application layer -- May 2004

Translating domain names q Name server of TLD names o = root name server o Known IP-address o Contains only references to TLD name servers Application layer -- May 2004

Resolving domain names www. cs. vu. nl 130. 37. 24. 11 Example : www. cs. vu. nl Root-NS Lokale NS NS (nl) (cs. kuleuven. ac. be) NS (vu. nl) 130. 37. 24. 11 NS (cs. vu. nl) Application layer -- May 2004

Resolving domain names q Iterative (see previous slide) o Name server returns reference 2 schemes q Recursive o Scheme of forwarding requests to “better” name servers o Correct figure? q Comparison? Application layer -- May 2004

Resolving domain names q Cumbersome? o Name servers: “cache” o Translations are cached during the specified time (TTL) q Vulnerable? o Name server can crash o For each domain: many name servers • One primary: returns authoritative records • At least one secondary: return cached records, possibly out of date Application layer -- May 2004

DNS Security q Can we trust the DNS Replies? Normal situation. Application layer -- May 2004

DNS Security q Can we trust the DNS Replies? Normal situation. An attack based on breaking into DNS and modifying Bob's record. Application layer -- May 2004

DNS Security q How Trudy spoofs Alice's ISP. o Reply faster than DNS server! Application layer -- May 2004

DNS Security q Solution: DNSsec o New records • KEY: public key of zone • SIG: signed (with secret key) hash of set of resource records o Signed Replies from servers • Set of resource records (RRSets) + SIG Application layer -- May 2004

DNS security q Resource Record set for bob. com: o A-record(s) for bob. com o Public key for bob. com o Signature of hash of A record & Key record, signed with signature of com Application layer -- May 2004

Overview q DNS -- Domain Name System q E-mail q Terminal Access: TELNET Application layer -- May 2004

E-mail outgoing message queue user mailbox q Major components: o User agent o Mail servers user agent mail server • = Transfer agents SMTP o Protocols • Transfer: SMTP • Access: POP, IMAP POP SMTP mail server Application layer -- May 2004 user agent IMAP user agent POP user agent 32

E-mail: user agent q Internet E-mail addresses pv@cs. kuleuven. be Pierre. Verbaeten@cs. kuleuven. be nachtradio@vrt. be o after @: domain name o before @: • login_name • First. Name. Last. Name • Name_of_Group Application layer -- May 2004

E-mail: user agent q E-mail programs o Often (mostly) graphical user interface o functionality: • • • receive & read messages compose & send a message reply to a message forward a message Store messages in folders maintain file with addresses o E. g. : Netscape, Exchange, Eudora, pine, elm, . . . Application layer -- May 2004

E-mail: user agent q E-mail program: Netscape Application layer -- May 2004

E-mail: message q RFC 822: standard for text message format: q Structure of a message o Header From: sender To: destination Cc: copy to … Subject: header o Blank line body o Message body q Must be ASCII!!! Application layer -- May 2004

E-mail: message Subject: mail voor luce Date: Mon, 20 May 2002 10: 21: 34 +0200 From: Jean Huens To: Pierre Verbaeten CC: Bart Swennen Pierre, vermits jullie in Leuven wonen kan je voor Luce ook een postbox. be adres krijgen. info op www. postbox. be Jean Application layer -- May 2004

E-mail: message q RFC 822 Header fields, related to transport Header Meaning To: E-mail address(es) of primary recipient(s) Cc: E-mail address(es) of secondary recipient(s) Bcc: E-mail address(es) for blind carbon copies From: Person who created the message Sender: E-mail address of actual sender Received: Line added by each transfer agent along the route Return-Path Can be used to identify a path back to the sender Application layer -- May 2004

E-mail: message q Other RFC 822 header fields Header Meaning Date: The date and time the message was sent Reply-To: E-mail address to which the reply should be sent Message-Id: Unique number for referencing the message later In-Reply-To: Message-Id of the message to which this is a reply References: Other relevant message-Ids Keywords: User chosen keywords Subject: Short summary of the message for the one-line display X-* User defined header Application layer -- May 2004

E-Mail: mail servers q mail server = system responsible for the E-mail handling within an organisation: o Talks smtp with other mail servers o Stores received messages for local users in its mailbox o Sends messages for remote users to the appropriate mail server o Accepts mail from local user agents user agent mail server SMTP user agent Application layer -- May 2004 user agent 40

E-Mail: smtp protocol q Defined in RFC 821 q Interaction in command/response mode Sending server or User agent commands server client response q three phases of transfer reliable transfer o handshaking (greeting) o transfer of messages o closure q messages must be 7 -bit ASCII Application layer -- May 2004 41

E-Mail: smtp protocol q try smtp interaction for yourself: o telnet servername 25 o wait for reply from server: 220 … o enter commands: HELO, MAIL FROM, RCPT TO, DATA, QUIT send an email without using email client (reader) Application layer -- May 2004

E-Mail: smtp protocol: example q Connect to mail server …. billie$ telnet mail. cs. kuleuven. ac. be 25 Trying 134. 58. 41. 11. . . Connected to mail. cs. kuleuven. ac. be. Escape character is '^]'. 220 iris. cs. kuleuven. ac. be ESMTP Mail Transfer Agent ; Wed, 22 May 2002 16: 31: 50 +0200 (MEST) q Dialogue with mail server Application layer -- May 2004

E-Mail: smtp protocol : example q Dialogue with mail server HELO ubiz. cs. kuleuven. ac. be 250 iris. cs. kuleuven. ac. be Hello billie. cs. kuleuven. ac. be [134. 58. 41. 39], pleased to meet you MAIL FROM Pierre. Verbaeten@cs. kuleuven. ac. be 501 5. 5. 2 Syntax error in parameters scanning "FROM" MAIL FROM: Pierre. Verbaeten@cs. kuleuven. ac. be 250 2. 1. 0 Pierre. Verbaeten@cs. kuleuven. ac. be. . . Sender ok RCPT TO: Jean. Huens@cs. kuleuven. ac. be 250 2. 1. 5 Jean. Huens@cs. kuleuven. ac. be. . . Recipient ok DATA 354 Enter mail, end with ". " on a line by itself q Enter Message (header + body) Application layer -- May 2004

E-Mail: smtp protocol : example q Enter Message (header + body) Subject: test Date Wed, 22 May 2002 16: 35: 00 From: Pierre. Verbaeten@cs. kuleuven. ac. be To: Jean. Huens@cs. kuleuven. ac. be ingetikte mail Pierre. 250 2. 0. 0 g 4 MEXXG 06443 Message accepted for delivery QUIT 221 2. 0. 0 iris. cs. kuleuven. ac. be closing connection Connection closed by foreign host. billie$ Application layer -- May 2004

E-Mail: smtp protocol q summary o Simple text based protocol: 7 -bit ascii • Line with “. ” only to indicate end of message • This string excluded inside message o smtp requires reliable connections • TCP: reliable byte stream o ESMTP extensions • Negotiate special services o other contents: voice, video, … MIME extensions Application layer -- May 2004 46

E-Mail: esmtp extensions q Approach o EHLO command: extensions available at server? o Symbolic name for each extension o Optional parameters for MAIL FROM: & RCPT TO q Some extensions o 8 BITMIME: 8 bit data, still lines o BINARYMIME: arbitrary data o SIZE: server informs client of max size of message o CHECKPOINT: support for resuming broken sessions o DSN: delivery status notification Application layer -- May 2004

E-Mail: MIME extensions q MIME: multimedia mail extension, RFC 1341, 1521 q additional lines in message header declare MIME content type q Content encoded into ascii text MIME version method used to encode data multimedia data type, subtype, encoded data From: Pierre. Verbaeten@cs. kuleuven. ac. be To: hilde@yahoo. com Subject: Picture MIME-Version: 1. 0 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data Application layer -- May 2004 48

E-Mail: MIME extensions q RFC headers related tp MIME Header Meaning MIME-version Identifies MIME version Content-Description Human-readable string telling what is in the message Content-Id Unique identifier Content-Transfer-Encoding How the body is encoded for translation Content-Type Nature of the message Application layer -- May 2004

E-Mail: MIME extensions q Mime types/subtypes RFC 1521 Type text Subtype Description Plain Unformatted text Richtext Text including simple formatting commands Gif Still picture in GIF format Jpeg Still picture in JPEG forma Audio Basic Audible sound Video Mpeg Movie in … format Application Octet-stream An uninterpreted byte sequence Postscript A printable document in Postscript RFC 822 A MIME RFC 822 message Partial Message split for transmission External-body Message itsef must be fetched over the net Mixed Independent parts in the specified order Alernative Same message in different formats Parallel Parts must be viewed simultaneously Digest Each part is a complete RFC 822 message Image Message Multipart Application layer -- May 2004

E-Mail: MIME extensions From: Pierre. Verbaeten@cs. kuleuven. ac. be To: hilde@yahoo. com Subject: Picture of Sarah MIME-Version: 1. 0 Content-Type: multipart/mixed; boundary=98766789 --98766789 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain Dear Hilde, Please find a picture of Sarah --98766789 Content-Transfer-Encoding: base 64 Content-Type: image/jpeg base 64 encoded data. . . . . base 64 encoded data --98766789 -- Application layer -- May 2004 51

E-Mail: MIME extensions q Transfer Encoding o SMTP requires ascii only in message o So conversions (encode – decode) necessary q Encoding schemes o Just ascii o 8 -bit ascii: violation; does not always work! o ascii + simple markup language (html) o Base 64 encoding (ascii armor) • 24 bits broken up in 4 * 6 bits • Each group of 6 bits represented as one ascii character o Quoted-printable • 7 -bit ascii • Char > 127 “=“ + char value as 2 hex digits Application layer -- May 2004

E-mail: access protocols q E-mail servers o send / receive messages o Communicate with user agents user agent SMTP mail server IMAP SMTP Simple Mail Transfer Protocol POP Post Office Protocol IMAP Internet Mail Access Protocol Application layer -- May 2004 user agent

E-mail: access protocols q 2 configurations • • UA on same system as mail server UA on PC, using POP to access mail server Application layer -- May 2004

E-mail: access protocols: POP 3 q POP: Post Office Protocol o authorisation of agent (at server) o Download: retrieve & delete at server <> retrieve only o OK for users using fixed UA q RFC 1939 q Example: use POP 3 to fetch 3 messages Application layer -- May 2004 55

E-mail: access protocols q POP: Post Office Protocol [RFC 1939] o authorisation of agent (at server) o Download: retrieve & delete at server <> retrieve only o OK for users using fixed UA q IMAP: Internet Mail Access Protocol [RFC 1730] o more features & more complex o Supports users reading mail from different UAs o manipulation of stored messages on server q Web based user agents o HTTP: Hotmail , Yahoo, … Application layer -- May 2004 56

E-mail: access protocols q Comparison: POP 3 <> IMAP Application layer -- May 2004

E-mail gateways q Interconnect different E-mail systems Internet OSI X 400 RFC 822 Application layer -- May 2004

E-mail security q Requirements: o Privacy o Authentication o Integrity o Non repudiation q Systems o PGP – Pretty Good Privacy o PEM – Privacy Enhanced Mail o S/Mime Application layer -- May 2004

E-mail security: PGP q Designed by Zimmerman (1995) o Promoter of privacy o <> US government q Open source software q Uses existing cryptographic algorithms o IDEA – International Data Encryption Algorithm • Timeline: DES IDEA AES o RSA o MD 5 Application layer -- May 2004

E-mail security: PGP q Operation of PGP: mail message P from Alice to Bob q Decentralised storage of public keys: trust? Application layer -- May 2004

E-mail security: PEM – S/MIME q PEM – Privacy enhanced mail o Internet standard: RFC 1421 – 1424 o Similar to PGP o Keys: X. 509 certificates from CAs in strict hierarchy o Single root CA manager? Trust? q S/MIME o Internet standard: RFC 2632 – 2643 o Integration with MIME o No restrictions on CAs Application layer -- May 2004

Overview q DNS -- Domain Name System q E-mail q Terminal Access: TELNET Application layer -- May 2004

Terminal Access: TELNET q History o Oldest Internet application o 1983 final form issued as RFC 854 and RFC 855 o Still useful application o Basis for other protocols q From old to current environment …. Application layer -- May 2004

Terminal Access: TELNET q Original environment Application layer -- May 2004

Terminal Access: TELNET q Current environment server PC or workstation Application layer -- May 2004

Terminal Access: TELNET q Local login o Characters typed • • From terminal To terminal driver To Operating system To application Application programs -------------- o For some characters • Interpretation by OS Terminal driver Application layer -- May 2004 Operating system

Terminal Access: TELNET q Remote login Telnet Server Telnet Client -------------- Application programs -------------- Operating system TCP IP Terminal driver ------- IP Data link Physical Pseudoterminal driver Different character sets!! Application layer -- May 2004

Terminal Access: TELNET q Network Virtual Terminal Telnet Client Telnet Server ------- Local computer ------- NVT character set Character set Pseudoterminal driver Remote computer Character set Application layer -- May 2004

Terminal Access: TELNET q NVT character set o Data characters • NVT ascii = ‘ 0’ + 7 bit US Ascii o Remote control characters • ‘ 1’ + 7 bits • Examples – Option negotiation characters – Characters to control remote application » Erase character » Erase line » Interrupt process Application layer -- May 2004

Terminal Access: TELNET q Phases of operation o Connection management • Connection request and termination • TCP is used o Negotiation • Determine a mutually agreeable set of characteristics: – Line length, terminal type, terminal speed o Control • Exchange of control information and commands: end of line, interrupt process o Data • Transfer of data Application layer -- May 2004

Terminal Access: TELNET q RLOGIN o Remote login program designed for BSD Unix q SSH o Secure remote login program o Offers: • • Authentication: uses public key encryption Privacy: exchanged data are encrypted Integrity Tunneling Application layer -- May 2004

Computer Networks Application layer -- May 2004