SMTP

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SMTP

• Tapu Ahmed
• Jeremy Nunn

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Basics

• Responsible for electronic mail delivery.
• Simple ASCII protocol that runs on top of TCP/IP.
• Uses reserved port number 25.

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SMTP Model
A
B
You want to send an E-Mail from A to B. Both A
and B are simple workstations.
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A submits source and destination. If both are
valid, server gives the go-ahead signal
We will first attempt the destination server to
see if it will accept mail.
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• The exchange of mails using TCP/IP is performed
  by a message transfer agent (MTA).
• An MTA is responsible for routing mails to their
  proper destinations.
• MTA uses the Mail Exchange (MX) record from a DNS
  server to determine location.
• In essence, the SMTP protocol describes how two
  MTAs communicate with each other using a single
  TCP connection.

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Queue of mail
Queue of mail
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Topics for SMTP

• SMTP Description
• Primarily RFC 821 and 822
• Message formats
• Extensions
• SMTP Applications
• Purpose
• Operations
• Unique problems encountered.

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Topics for SMTP continued

• Security and performance issues
• Hacking
• End-end delivery system performance
• Spamming issues
• Looking Ahead
• Future standardizations

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RFC 821

• A Description of SMTP
• Objective is to deliver mail reliably and
  efficiently.
• Points of interest
• Mailing/sending
• Forwarding
• Relaying
• Opening/closing

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• SMTP Procedure
• MAIL command
• Clear buffer and get ready to receive mail
• Gives sender ID
• RCPT command
• Gives receiver information
• DATA command
• Send the data

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• S MAIL FROM group_at_yahoo.com
• R OK
• S RCPT TO person1_at_yahoo.com
• R OK
• S RCPT TO person2_at_yahoo.com
• R ERROR no such user here
• S RCPT TO person3_at_yahoo.com
• R OK
• S DATA
• send mails
• Ending signature
• R OK

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• RFC 821 also provides
• Verification
• SMTP commands and syntax structure
• State diagrams
• Sequencing of commands and replies
• Extensions
• RFC 1869
• EHLO command
• MAIL, RCPT, DATA can all receive additional
  values.

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RFC (2)822

• Describes message formatting for SMTP.
• RFC 822 is the standard for the format of
  internet text messages.
• RFC 2822 new standard
• Points of interest
• Message specifications
• Date/time specifications
• Address specifications
• Different RFC extensions

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• Limitations
• 998 character/line max. 78 min.
• Only ASCII characters
• Header
• Composed of a field name
• Terminates by a , and ends with CRLF.
• Address Specification
• Individual or an entire mailbox
• Occurs in multiple fields to indicate sender or
  receiver

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• Date and Time Specification
• Must be semantically valid.
• Added on through numerous headers.
• Extensions
• RFC 2045 and 2046
• Describe mechanisms for transmission

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SMTP Applications

• Several SMTP applications exist, too numerous to
  enumerate
• qmail
• Configuration is unique (one file to one config
  value)
• Places many files in the root of your system
• Awkward license prevents distributing modified
  source or binaries
• Developer/owner is a busy guy
• sendmail
• Past and current versions (8) have been
  notoriously insecure
• Remote root exploits, etc.

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SMTP Applications

• Postfix
• Uses sensible defaults
• Good security track record
• Easier to configure
• Can query LDAP to pick up new/modified users
• Microsoft Exchange
• Very easy to use on a Windows network
• Integrates with Active Directory (uses LDAP)
• Not the best security

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Security and Performance

• Who needs security?
• When SMTP was initially developed, little (none
  maybe?) emphasis was placed on security
• Design was built on the idea of cooperation and
  trust
• Didnt anticipate spam
• Mail Relay
• Relay is sending mail from one mail server to
  another
• Most SMTP servers didnt check authenticity of
  users

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Security and Performance

• Bulk mails
• Unsolicited bulk mailers take advantage of this
• Decreases performance of server for rightful
  users
• Relay Restrictions
• Verify that the computer is on the servers local
  network
• Require a local domain return address
• Do not accept mail from other open relay servers

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Security and Performance

• How about a new SMTP?
• The problem is obvious, so lets change the
  protocol
• No guaranteed way to implement without creating
  incompatibilities
• We like the idea of cooperation
• Maybe theres another way

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Security and Performance

• Other Security Measures
• Limit the use of commands
• Check the validity of the envelope
• Limit the size of the email
• Limit the number of emails that may be sent in a
  given amount of time
• Log everything
• POP-before-SMTP Authentication
• Note that SMTP has no mechanism for privacy
  (encryption).
• This has to be done at a higher level if needed
• Currently is application specific

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Future Projections

• Current projects include
• Sender Policy Framework (SPF)
• Only certain servers are allowed to forward mail
  from certain domain names
• Easy to check
• New DNS Blackhole Lists
• Narrowly identify specific invalid senders
• Will be viable once a large enough number of
  servers support this
• Spam Filtering
• Intelligent, self-learning software
• Good job of identifying unwanted messages that
  get through
• Greylisting
• Receiving servers make sending servers wait
• Spammers probably wont come back
• More time means more chances to add the offender
  to the blackhole list