1 Application Layer Mahalingam Mississippi State University, MS September 15, 2014
2 Outline 1 DNS Records DNS Components 2 Message Transfer Fetching s 3
3 Applications We will focus on 3 applications DNS
4 Domain Name Space DNS Records DNS Components Generic Countries int com edu gov mil org net jp us nl... sun yale acm ieee ac co oce vu eng cs eng jack jill keio nec cs ai linda cs csl flits fluit robot pc24
5 Domain Name Space DNS Records DNS Components DNS Root at the top of the inverted tree Top level domains generic (gtld) and country-code (cctld) New TLDs added rarely Names below TLDs can be purchased (if available) DNS is a distributed database of DNS records A database for every zone
6 Domain Name Service DNS Records DNS Components DNS (domain name service) provides domain-name to IP look-up Naming according to organizational boundaries - not physical IP addresses depend on physical location (issued by a ISP) DNS is an application uses UDP as the transport layer DNS servers queried by DNS clients.
7 DNS Zone DNS Records DNS Components name under the control of an entity msstate.edu is a zone Owner of zone msstate.edu authorized to create any name ending with msstate.edu. Can also delegate some names to other entities For example, cse.msstate.edu has been delegated A delegated name becomes a zone Root delegated gtld edu. gtld edu delegated msstate.edu. msstate.edu delegated cse.msstate.edu need not delegate all names (for example, eng.msstate.edu is not delegated, and so is still under the control of msstate.edu.
8 DNS Database DNS Records DNS Components Zone owner responsible for creating the zone database Zone database (or zone file, or, zone master file) is a list of DNS records Each record indicates name, type, and value There can be multiple records for the same name and type. DNS servers host the database typically every zone has one or more zone DNS servers
9 DNS Queries DNS Records DNS Components DNS servers can be queried by specifying a name and type For example, name disney.cse.msstate.edu, type A (A is for address) The response is a value corresponding to the name and type Possibly multiple values if multiple records exist for the same name and type. Query: name=cse.msstate.edu, type=a Response: name=cse.msstate.edu, type=a, value=
10 DNS Lookup DNS Records DNS Components Every zone provides it s parent information regarding how to reach the zone server. cse.msstate.edu provides information regarding name servers for cse.msstate.edu to parent zone msstate.edu Consequently root zone knows how to reach it s child edu edu knows how to reach it s child zone msstate.edu msstate.edu knows it s child zone cse.msstate.edu, etc.
11 Recursive and Iterative Queries DNS Records DNS Components Queries could be recursive or non-recursive Query by Q for name disney.cse.msstate.edu, type A Recursive: Q root edu msstate.edu cse.msstate.edu msstate.edu edu root Q Iterative: Q root ; Root Q response is direction to reach edu) Q edu direction to reach msstate.edu) Q msstate.edu direction to reach cse.msstate.edu Q cse.msstate.edu final (authoritative) answer to query Pros and Cons?
12 DNS Resource Record DNS Records DNS Components Each resource record is a five-tuple Domain Name TTL - how stable the record is Class - For Internet, it is IN Type - Many types (next slide) Value - Value corresponding to the type
13 Type Field in Resource Records DNS Records DNS Components SOA (start of authority) - details regarding primary source of info, contact addresses, serial number, flags and time-outs A (Address) - 32 bit IP address - there could be more that one A record (multiple IPs) MX (Mail Exchange) - Mail server for this domain NS (Name Server) CNAME - aliases (eg disney.cse.msstate.edu IN CNAME disney.cs.msstate.edu) PTR - allows for reverse-lookup HINFO - additional info about the machine TXT - Whatever else you want to add!
14 Sample DNS Database Entry DNS Records DNS Components
15 DNS Components DNS Records DNS Components Domain Registrars: sell domain names Zone authority/owner: Creates DNS records pertaining to a zone all records entered in a zone master file Authoritative DNS servers (zone servers): Servers run by the authority. Serves DNS records from the zone master file(s) Root name servers are authoritative for root domain. A gtld name server is authoritative for the gtld. Local DNS servers: typically operated by ISPs (performs iterative queries, caches responses) Stub resolvers: in every host (queries the local DNS server) dig (DNS query tool. Download, install and play with it).
16 Authoritative Name Servers DNS Records DNS Components root servers: A.ROOT-SERVERS.NET, B.ROOT-SERVERS.NET, M.ROOT-SERVERS.NET edu name servers: a.gtld-servers.net., c.gtld-servers.net,... msstate.edu: ns1.msstate.edu, trantor.umd.edu, ns2.msstate.edu. cnn.com: twdns-01.ns.aol.com, twdns-02.ns.aol.com,...
17 DNS Query and Response DNS Records DNS Components Application (for example, browser) requires IP address of groklaw.net Application makes a call to a stub resolver: gethostbyname(groklaw.net) Stub resolver checks in its cache. If A-type record for groklaw.net is found, gethostbyname() returns with the record Else sends query to a local DNS server (LDNS) If LDNS has the answer in it s cache it responds to stub resolver s query.
18 DNS Query and Response DNS Records DNS Components If not found in cache, LDNS maskes iterative queries LDNS sends a query groklaw.net A to a root server Root server sends a response with info regarding ANSs for net LDNS sends the query groklaw.net A to a net name server, and receives info regarding ANSs for groklaw.net LDNS sends a query groklaw.net A to the ANS and receives the desired response. LDNS (caches answer and) sends the response to the stub resolver, gethostbyname(groklaw.net) returns (recall hostent structure returned by gethostbyname())
19 DNS Query/Response Format DNS Records DNS Components Both query and response have the same format Five sections: HEADER QUESTION ANSWER AUTHORITY ADDITION
20 dig DNS Records DNS Components Windows: dig NS (query for root name servers) dig edu NS dig msstate.edu A dig msstate.edu NS
21 dig groklaw.net DNS Records DNS Components QUESTION SECTION: groklaw.net IN A ANSWER SECTION: groklaw.net IN A AUTHORITY: groklaw.net IN NS ns.unc.edu. groklaw.net IN NS ns2.unc.edu. ADDITION: ns.unc.edu IN A ns2.unc.edu IN A
22 Message Transfer Fetching s
23 Basic Components Message Transfer Fetching s Mail clients and servers Two types of clients and servers (sending and receiving) SMTP (simple mail transfer protocol) servers for sending mail IMAP/POP3 servers for making received mails accessible to clients SMTP servers accept s from sending clients Receiving clients fetch from IMAP/POP3 servers Web based services are also popular. RFC 822 specifies formats (for sending to SMTP servers)
24 RFC 822 Header Message Transfer Fetching s
25 RFC 822 Header - Other Fields Message Transfer Fetching s
26 MIME Extensions Message Transfer Fetching s Only printable characters are permitted in RFC 822 format. MIME - for Content description and encoding Content transfer encoding usually base-64 encoding.
27 Message Transfer Fetching s RFC 822 Header MIME Extensions
28 MIME Types Message Transfer Fetching s
29 Message Transfer Fetching s SMTP - Simple Message Transfer Protocol TCP, port 25 Supported commands HELP HELO MAIL FROM RCPT TO DATA
30 SMTP Restrictions Message Transfer Fetching s Max Message length - 64 KB Recursive mailing lists ESMTP (Extended SMTP) New command EHLO - if not accepted, defaults to SMTP.
31 Sending an Message Transfer Fetching s To send an to What is the mail server for somewhere.com? Make a DNS query for (somewhere.com, MX ) e.g., somewhere.com, 82345, IN, MX, mail.something.net Make DNS query for (mail.something.net, A) to determine IP address of mail server e.g. response, mail.something.net,3245, IN, A, Connect to SMTP server (port 25) at IP address use RCPT TO: xyz to check if xyz is a valid address in the domain somewhere.com send in RFC 822 format.
32 Fetching s Message Transfer Fetching s POP3 IMAP Webmail
33 Mail Transfer Modes Message Transfer Fetching s SMTP Internet Message transfer agent User agent (a) Sending host Permanent connection Mailbox Receiving host SMTP Internet Message transfer agent POP3 server POP3 User agent (b) Sending host Mailbox ISP's machine Dial-up connection User's PC
34 POP3 / IMAP Message Transfer Fetching s POP - Post Office Protocol - RFC 1939 POP3 commands - USER, PASS, LIST, RETR, DELE, QUIT IMAP - Internet Message Access Protocol - RFC 2060 Complex, partial message downloads MS Exchange server, GroupWise, Simbra
36 A Good Tutorial HTTP Made Really Easy
37 GET To fetch a page from cse.msstate.edu perform DNS query to determine IP address. Connect to port 80 and send a HTTP query. GET / HTTP/1.1\r\n Host: cse.msstate.edu\r\n User-Agent: Mozilla/5.0 Accept: Accept-Language: Accept-Charset: Keep-Alive: 300\r\n Connection: keep-alive\r\n \r\n First two lines are mandatory in every request. Why is second line (Host:) mandatory? Multiple websites may be hosted in a single machine (same IP and port).
38 Header in HTTP Response HTTP/ OK Date: Server:Apache X-Powered-By:PHP / OpenSSL Keep-Alive:timeout=15 Connection: Keep-Alive Transfer-Encoding: Content-Type: \r\n
39 HTTP Status codes
40 HTTP Request Methods
41 HTTP Headers
42 URLs - Uniform Resource Locators
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