4343 X2 – Outline The Domain Name System The Web

4343 X2 – The Domain Name System Using IP addresses as absolute machine addresses on the Internet is not very practical. Computers can frequently change IPs, rendering using an IP address to access the machine useless. A system of using a name to access the machine was devised to overcome this problem.

4343 X2 – DNS The name of the machine is related to the IP of the machine, but the IP can be changed without altering the name. A Domain Name System machine (sometimes known as the Domain Name Server) is responsible for keeping track of the relationships between IP addresses and system names.

4343 X2 – The DNS Name Space The original name space was split into over 200 top-level domains, including com, edu, int (international), net (network providers), org, gov (US only), mil (US only) and at least one domain for each country. Recently (2000), biz, info, name and pro were introduced at the top level.

4343 X2 – Domain Records Each domain record stores the following information: –Domain Name (the name of the domain) –Time to Live (how stable the record is) –Class –Type (A, MX, NS, CNAME, PTR, etc) –Value

4343 X2 – Domain Types SOA = State of Authority: Information about the domain A = IP address of the host MX = Mail exchange: a server willing to accept mail for this domain NS = Name of a server for this domain CNAME = Canonical name PTR = Pointer: an alias for an IP address

4343 X2 – Name Servers We need more than just one name server for the whole Internet. The net is split into non-overlapping zones, each of which has a primary name server. The primary name server gets its information off of disk, and shares that information with secondary name servers. Some name servers for a zone can be located outside of the zone.

4343 X2 – Name Zones

4343 X2 – Resolving Domain Names The request is passed from Name Server to Name Server until it arrives at the zone where the machine should reside. The local name servers can give an answer about the machine in question. It should be noted that an authoritative record is one that comes from the name authority and is always correct, while cached information from a secondary name server may be incorrect.

4343 X2 – I will skip the basics, as you should all know them by now… :-) I will talk about: –SMTP –POP3 –IMAP

4343 X2 – Simple Mail Transfer Protocol The mailer daemon waits for a connection on port 25 of the machine. The daemon understands SMTP, The protocol can easily be used with a standard ACSII text connection. Telnet to port 25 of a Unix machine to check it out. Note that Extended SMTP (ESMTP) is now often used to overcome some problems with SMTP. Most server differentiate which one you are going to used based on your “HELLO” to the daemon.

4343 X2 – POP3 Post Office Protocol version 3 A protocol used to get the messages from the server to the client program. This protocol is also ASCII based, and you can play with it using a telnet session to port 110 of your nearest POP3 server. You can read your is ASCII format if you wish…

4343 X2 – POP3 Advantages We can download a copy of the to our local machines, leaving a copy on the server. Having a copy on the server is helpful if we want to access from multiple machines but still have access to all of our s. This will begin to cause problems when the mailbox on the POP3 server gets too full.

4343 X2 – IMAP Internet Message Access Protocol Does not assume that the s will be downloaded, like with POP3, but assumes that they will stay on the server in multiple different mailboxes. IMAP can also accept outgoing (unlike POP3) IMAP has many more commands, and is more complex.

4343 X2 – The Web We use URLs (Uniform Resource Locators) to specify information about the data we want to access: –the protocol (http, nntp, ftp, etc) –the name of the machine (dragon.acadiau.ca) –the file containing the page (~dbenoit/index.html) How does it all work?

4343 X2 – How it works. 1.Browser determines the URL 2.Browser gets IP address for server 3.Browser makes a connection to server 4.Browser sends a request for the page 5.Server sends the page back 6.The TCP connection is released 7.Browser displays the page

4343 X2 – Web server machine

4343 X2 – Markup Languages The WWW began with straight text, but then moved to HTML. HTML is a subset of SGML. HTML moved through several versions until HTML 4. Now, the standard is XHTML, essentially HTML 4 written in XML. We are not sure where things will go from here…