DNS and email

• DNS
• electronic mail:
  • addressing
  • format
  • content encoding: MIME
  • Simple Mail Transfer Protocol, SMTP
  • access: POP, IMAP

Domain Name System

• www.ics.hawaii.edu, maru.ics.hawaii.edu, www.ibm.com
• hierarchical name space:
  • top-level domains: .com, .edu, .gov, .mil, and all two-letter country prefixes
  • second-level domains: yahoo.com, hawaii.edu, stanford.edu
  • third-level domains: ...
• each name at each level may or may not correspond to an actual IP address
• each name at each level may or may not be further subdivided into lower-level domains
• the same component (e.g., www) can be used within different DNS names

In-Class Exercise

• whole class jointly
• name as many top-level domains as possible

DNS Protocol

• each DNS name corresponds to a Resource Record
• "A" RRs provide name to IP translations
• DNS servers are configured to serve (part of) a DNS (sub)tree: a zone
• each zone has at least 2 DNS servers
• example: hawaii.edu is a zone, ics.hawaii.edu could be another zone
• servers may also cache information outside their zone, and forward it, but they are not authoritative about that information

DNS Lookup

• host requests translation (over TCP or UDP)
• server can:
  • provide translation from its authoritative data
  • provide translation from its cache
  • forward the request to another server closer to the destination (in one of its neighboring zones)
  • provide the address of another server closer to the destination
• a client can try different suffixes if the name itself doesn't work (e.g. .ics.hawaii.edu or .hawaii.edu)

Comparison of DNS and IP hierarchies

• often, the IP and DNS hierarchies match with the administrative structures, e.g. ICS has ics.hawaii.edu and the 128.171.20.0/24 subnets, under hawaii.edu and 128.171.0.0/16
• but this doesn't have to be the case:
  • an IP that belongs to UH (128.171.X.Y) could have a different DNS name
  • a DNS name ending in .hawaii.edu need not have a UH IP address
  • one IP address can have many different domain names, e.g. www.ics.hawaii.edu and iwi.ics.hawaii.edu are both 128.171.10.250
  • www.rcuh.com at 207.175.158.132

User-level NS translations

• nslookup name gives us the translation, main name, and aliases (available on uhunix)
• forward and reverse translation
• nslookup gives access to different resource records, can be aimed at different servers, etc
• also dig -x dns or dig -x ip (linux, unix?, win?)
• DNS is case-independent

Electronic Mail

• originally, distributing text to local computer users ("memos")
• evolved to transferring to users on other computers
• terminology of paper mail: to, from, cc
• an address uniquely identifies a user (before the @ sign), and a computer (after the @ sign)
• the user name is assigned locally
• the computer name is assigned according to DNS (hierarchically)

Electronic Mail Format

• an electronic mail message has a header followed by a blank line, followed by an arbitrary body
• the header has entries, one per line
• each entry has the field name, followed by a colon (":"), followed by the field value
• what other protocol does this resemble?
• there is no content length: the length is given by the length of the file or of the transfer
• header fields (case independent) include To, From, Cc
• mailers generally ignore header fields they don't understand

Multipurpose Internet Mail Extension

• ASCII body is fine for text, not for graphics or audio
• there is no single standard for video/audio, nor for non-ASCII (non-English) text
• MIME is an extension that adds a few header field (upward and downward compatible) and allows sending of arbitrary types
• the contents should still be ASCII (characters in the range 32 to 127)
• MIME-Version: 1.0 header field is followed by Content-Type: ..., which can be Multipart to have multiple attachments
• plain text often precedes the first attachment

Electronic Mail Transfer

• SMTP: simple mail transfer protocol, port 25
• mail generated by a mail client
• mail client transfers the message to a mail agent on the same machine (mail transfer program)
• mail agent contacts the mail server (or mail agent) on the remote machine to deliver the message
• user uses mail client to access email stored locally
• many mail agents will forward email from any machine, not just the local host, or may expand a mailing list or convert an external to an internal address

POP, IMAP

• Post Office Protocol, POP
• POP server runs on computer with mailbox
• user contacts POP server, authenticates, retrieves mail
• POP-2 and POP-3 are newer, more secure, better versions
• Internet Message Access Protocol, IMAP
• same model as POP, but more flexibility:
  • client can request a message without deleting it from the server
  • client can request only the headers
  • allows email access from many different computers