Introduction to Sendmail

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Introduction to Sendmail
The Internets Mail Transport Agent
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OReillys Bat Book3rd Edition
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This is what happens if you learn to much about
Sendmail
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Sendmail
It's been said that you aren't a real Unix system
administrator until you've edited a sendmail.cf
file. It's also been said that you're crazy if
you've attempted to do so twice.
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What is Sendmail?

• Definition Sendmail is the most widely used Mail
  Transport Agent (MTA) on the internet
• MTAs send mail from one machine to another.
• Sendmail is not a client program, which you use
  to read your email.
• Sendmail is a behind-the-scenes program which
  actually moves email over networks or the
  Internet.
• Normally it runs as a background daemon
• Can be run out of the super daemon (xinetd)

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Other MTAs

• Postfix
• Qmail
• MS Exchange
• CCMail
• Lotus Notes
• .etc.

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What is SMTP

• SMTP Simple Mail Transfer Protocol
• SMTP is the protocol for transporting mail over
  the Internet.
• SMTP is spoken via port 25 of the MTAs. Using the
  basic model described above, a session is
  performed as follows

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Basic SMTP Session

• The local MTA establishes a connection with the
  remote MTA. (They exchange 'hellos.')
• The local MTA informs the remote MTA that it has
  mail from a local user. The remote machine
  acknowledges.
• The local MTA informs the remote MTA to whom the
  mail is addressed on the remote side. The remote
  machine acknowledges.

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Basic SMTP Session

• The local MTA sends the message and the remote
  machine acknowledges its arrival.
• The machines break connection.

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Sendmail Features

• Sendmail uses DNS (Domain Naming System)
• But not 100 dependent Joe_at_192.168.1.1
• DNS does provide Mail Exchange (MS) Info
• Sendmail can do a DNS double-tap
• Lookup up who the client says they are
• Default is mail relay off
• From A to C through B
• Realtime Blackhole Lists (RBL)

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Sendmail Anti-Spam Enhancements

• Mailscanner
• Minimal anti-spam
• Anti-virus integration (scan in/outbound)
• http//www.sng.ecs.soton.ac.uk/mailscanner/
• Or http//www.mailscanner.info
• Spam Assassin
• Rule based heuristic
• Header and text analysis
• Blacklist (RBL)
• Vipul's Razor (http//razor.sf.net)
• http//www.spamassassin.org

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Implementations

• SMTP Gateway
• An SMTP gateway allows users on your network to
  communicate with others on the Internet without
  concern as to which local mail software package
  exists on your network.
• All incoming mail for your network will pass
  through this gateway which converts the message
  into the appropriate format specific to your
  local mail software.
• Similarly, all mail destined for the Internet
  from your network will pass through this gateway
  to be sent across the Internet via SMTP

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Implemetations

• SMTP Relay Warning Will Rogers
• An SMTP relay is a machine that actually sends
  the mail across the Internet.
• A common misconception is that SMTP gateways are
  the same as SMTP relays. This is not always the
  case.
• There are SMTP gateways that act as relays
  themselves, but there are also many that do not.
  If the latter is the case on your network, you'll
  need to bounce your mail off one of the relays.

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Installation Methods

• RPM installation
• Obtained from installation CDs
• Binaries (.tgz)
• Obtained from http//www.sendmail.org
• Source Code
• Obtained from http//www.sendmail.org

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The Pieces

• The binary
• /sbin/sendmail
• The configuration file
• /etc/mail/sendmail.cf
• Supporting files
• /etc/mail/access
• /etc/mail/aliases
• and many more

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More Pieces

• Email messages are stored in the directory
• /var/spool/mail
• There is a separate file for each user
• Email waiting to be sent
• ./var/spool/mqueue
• A log of Email sent and received
• /var/log/mail

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Virtual Email Hosting

• To accept or have all Email for a given domain be
  sent to a server that is always connected to the
  Internet.
• All Email for the given domain is automatically
  forwarded to someuser_at_mydomain.com
• Typically, another server or individual user will
  access the account of someuser_at_mydomain.com
  (usually via a dial-up connection) and retrieve
  all mail.