Dns And Email

DNS AND E-MAIL By….. SUKANTA BEHERA Reg. No. 07SBSCA048

DNS and E-mail DNS: DNS stands for Domain Name System. It is incredibly Important but completely hidden part of internet. The DNS system forms one of the largest and most active distributed database on the planet. Without DNS, the Internet would shout down very quickly. When we use the web or send an e-mail message, we can see it, we use a domain name to do it. For example:The URL http://www.gmail.com contains the domain name gmail.com. So the e-mail address is like that “[email protected].”

Human-readable names like “gmail.com” is easy for us to remember, but its difficult to remember the IP addresses that’s why it becomes in human-readable names. But when we enter the name of any domain the domain name servers converts it into corresponding IP address. IP address To keep all of the machines on the Internet straight, each machine is assigned a unique address called an IP address. IP stands for Internet protocol, and these address are 32-bit numbers normally expressed as four “octal” in a “dotted decimal number.” A typical IP address

looks like this: 70.123.252.42 The four numbers in an IP address are called octets because they can have values between 0 and 256 (28 possibilities per octet). Every machine on the Internet has its own IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when you dial in. That IP address is unique for your session and may be different the next time you dial in. In this way, an ISP only needs one IP address for each modem it supports, rather than for every customer.

Domain Names If we had to remember the IP addresses of all of the Web sites we visit every day. Human beings just are not that good at remembering strings of numbers. We are good at remembering words, however, and that is where domain names come in. For example: www.google.com - a typical name www.yahoo.com - the world's best-known name www.mit.edu - a popular EDU name

DNS Services

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Hostname to IP address translation Host aliasing  Canonical and alias names Mail server aliasing Load distribution  Replicated Web servers: set of IP addresses for one canonical name

DNS Infrastructure root DNS server

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Host at cis.poly.edu wants IP address for gaia.cs.umass.e du Infrastructure:   

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Client resolver Local DNS server Authoritative DNS Server Root DNS Server

2

3 4

TLD DNS server

5 local DNS server 1

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7

6

authoritative DNS server

requesting host cis.poly.edu gaia.cs.umass.edu

Distributed, Hierarchical Database Root DNS Servers

com DNS servers

ca DNS servers

edu DNS servers

TLD Servers

usask.ca poly.edu umass.edu yahoo.com amazon.com ucalgary.ca DNS servers DNS servers DNS servers DNS servers DNS servers DNS servers

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Root servers and TLD servers typically do not contain hostname to IP mappings; they contain mappings for locating authoritative servers.

DNS: Root name servers 

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contacted by local name server that can not resolve name root name server: contacts authoritative name server if name mapping not known  gets mapping  returns mapping to local name server 

TLD and Authoritative Servers 

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Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.  Network solutions maintains servers for com TLD  Educause for edu TLD Authoritative DNS servers: organization’s 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 

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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.  Reduces lookup latency for commonly searched hostnames

DNS records DNS: distributed db storing resource records RR (RR) format: (name, value, type, ttl) 

Type=A

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Type=CNAME  name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com  value is cannonical name

name is hostname  value is IP  Type=NS address  name is domain (e.g. foo.com)  Type=MX  value is IP address  value is name of of authoritative mailserver associated name server for with name this domain 

E-mail (Electronic Mail) E-mail stands for electronic mail it is a exchange of computer-stored messages by telecommunication. E-mail messages are commonly stored in ASCII text. However, we can send non-text files, such as graphic images and sound files, as attachments sent in binary streams. E-mail was one of the first uses of the Internet and is still the most popular use. A large percentage of the total traffic over the Internet is email. E-mail can also be exchanged between online service provider users and in networks other than the Internet, both public and private.

E-mail is one of the protocols included with the Transport Control Protocol/Internet Protocol (TCP/IP) suit of protocols. A popular protocol for sending e-mail is Simple Mail Transfer Protocol and a popular protocol for receiving it is POP3.

How E-mail Works Every day, the citizens of the Internet send each other billions of e-mail messages. If you're online a lot, you yourself may send a dozen or more e-mails each day without even thinking about it. Obviously, e-mail has become an extremely popular communication tool. Have you ever wondered how e-mail gets from your computer to a friend halfway around the world? What is a POP3 server, and how does it hold your mail? The answers may surprise you, because it turns out that e-mail is an incredibly simple system at its core. In this article, we'll take an in-depth look at e-

Steps in sending & Receiving E-mail

Step A: Sender creates and sends an email The originating sender creates an email in their Mail User Agent (MUA) and clicks 'Send'. The MUA is the application the originating sender uses to compose and read email, such as Eudora, Outlook, etc. Step B: Sender's MDA/MTA routes the email The sender's MUA transfers the email to a Mail Delivery Agent (MDA). Frequently, the sender's MTA also handles the responsibilities of an MDA. Several of the most common MTAs do this, including send mail. The MDA/MTA accepts the email, then routes it to local mailboxes or forwards it if it isn't locally addressed. In our diagram, an MDA forwards the email to an MTA and it enters the first of a series of "network clouds," labeled as a "Company Network" cloud.

Step C: Network Cloud An email can encounter a network cloud within a large company or ISP, other largest network cloud in existence: the Internet. The network cloud may encompass a multitude of mail servers, DNS servers, routers, lions, tigers, bears (wolves!) and other devices and services too numerous to mention. These are prone to be slow when processing an unusually heavy load, temporarily unable to receive an email when taken down for maintenance, and sometimes may not have identified themselves properly to the Internet through the Domain Name System (DNS) so that other MTAs in the network cloud are unable to deliver mail as addressed. These devices may be protected by firewalls, spam filters and mail ware detection software that may bounce or even delete an email. When an email is deleted by this kind of software, it tends to fail silently, so the sender is given no information about where or when the delivery failure occurred. Email service providers and other companies that process a large volume of email often have their own, private network clouds. These organizations commonly have multiple mail servers, and route all email through a central gateway server

Step D: Email Queue The email in the diagram is addressed to someone at another company, so it enters an email queue with other outgoing email messages. If there is a high volume of mail in the queue—either because there are many messages or the messages are unusually large, or both—the message will be delayed in the queue until the MTA processes the messages ahead of it.

Step E: MTA to MTA Transfer When transferring an email, the sending MTA handles all aspects of mail delivery until the message has been either accepted or rejected by the receiving MTA. As the email clears the queue, it enters the Internet network cloud, where it is routed along a host-to-host chain of servers. Each MTA in the Internet network cloud needs to "stop and ask directions" from the Domain Name System (DNS) in order to identify the next MTA in the delivery chain. The exact route depends partly on server availability and mostly on which MTA can be found to accept email for the domain specified in the address. Most email takes a path that is dependent on server availability, so a pair of messages originating from the same host and addressed to the same receiving host could take different paths. These days, it's mostly spammers that specify any part of the path, deliberately routing their message through a series of relay servers in an attempt to obscure the true origin of the message. To find the recipient's IP address and mailbox, the MTA must drill down through the Domain Name System (DNS), which consists of a set of servers distributed across the Internet. Beginning with the root name servers at the top-level domain (.tld), then domain name servers that handle requests for domains within that .tld, and eventually to name servers that know about the local domain

Step F: Firewalls, Spam and Virus Filters The transfer process described in the last step is somewhat simplified. An email may be transferred to more than one MTA within a network cloud and is likely to be passed to at least one firewall before it reaches it's destination. An email encountering a firewall may be tested by spam and virus filters before it is allowed to pass inside the firewall. These filters test to see if the message qualifies as spam or malware. If the message contains malware, the file is usually quarantined and the sender is notified. If the message is identified as spam, it will probably be deleted without notifying the sender. Spam is difficult to detect because it can assume so many different forms, so spam filters test on a broad set of criteria and tend to misclassify a significant number of messages as spam, particularly messages from mailing lists. When an email from a list or other automated source seems to have vanished somewhere in the network cloud, the culprit is usually a spam filter at the receiver's ISP or company. This explained in greater detail in Virus Scanning and Spam Blocking.

Delivery In the diagram, the email makes it past the hazards of the spam trap...er...filter, and is accepted for delivery by the receiver's MTA. The MTA calls a local MDA to deliver the mail to the correct mailbox, where it will sit until it is retrieved by the recipient's MUA. RFCs Documents that define email standards are called "Request For Comments (RFCs)", and are available on the Internet through the Internet Engineering Task Force (IETF) website. There are many RFCs and they form a somewhat complex, interlocking set of standards, but they are a font of information for anyone interested in gaining a deeper understanding of email.