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Re: Why is exim installed by default?

On 16/10/11 05:47 PM, Andrew Wood wrote:
> Why is an MTA (exim) installed by deafult on Squeeze even if the 'Mail
> Server' option is not selected during installation? Does it actually
> serve any purpose on an out of the box basic installation?
> Andrew
Exim4 is installed by default in Debian distributions because it is the
default MTA (Mail Transport Agent) for Debian distributions. There are
two versions of exim4: exim4-daemon-light and exim4-daemon-heavy. The
exim4-daemon-light version is installed by default.

Like Unix systems, Linux systems defaults to sending system
notifications via email. Since emails can be sent, copied, cc'ed, bcc'ed
and redirected to any email address both locally and remotely it makes
automatic system email notification easy and flexible.

"Lightweight Exim MTA (v4) daemon

Exim (v4) is a mail transport agent. This package contains the exim4
daemon with only basic features enabled. It works well with the
standard setups that are provided by Debian and includes support for
TLS encryption and the dlopen patch to allow dynamic loading of a
local_scan function."

"Exim MTA (v4) deamon with extended features, including exiscan-acl
Exim (v4) is a mail transport agent. This package contains the exim4
daemon with extended features. In addition to the features already
supported by exim4-daemon-light, exim4-daemon-heavy includes LDAP,
sqlite, PostgreSQL and MySQL data lookups, SASL and SPA SMTP authentication,
embedded Perl interpreter, and the content scanning extension
(formerly known as "exiscan-acl") for integration of virus scanners
and spamassassin."

Yes exim4 does server a purpose and from a system administrator’s and
security point view it is very important.
Exim4 is used by several processes to send notifications via email to
the local root user. These notifications include but are not limited to:
failed login attempts, failed login of sudo users/non-users, cron jobs
such as log rotation, kernel errors, date backup status, etc. Other
processes can also be configured to send their notifications to the root
user by email via exim4.

By default the root user is not permitted to receive emails for security
reasons so emails are redirected to a non-root user. This is configured
in the /etc/aliases file in the format "root: mike". Where all emails
sent to the root user will be re-directed to the local system user
"mike". By default "mike" would be the first non-root user added to the
system during installation. This can be changed by editing the
/etc/aliases file and changing "mike" to any other user on the system or
any valid email address. If a valid email address is used then exim4 has
to be configured to send emails to MTAs on the Internet or a smarthost.

Local emails can be read after logging in as the redirected user "mike"
using email clients such as the default mail client by typing "mail"
[enter] in a consul or at the desktop using "kmail" or "Evolution".

Exim4 can also be configured to use "maildir" which stores each email
message in a separate file as opposed to the default which stores all
email messages in one file. It can also be configured to send the emails
to any other MTA on the local network or Internet if these networks are
contactable. Exim4 can be replaced by any other MTA that is compatible
with the distribution.

I recommend at the very least to leave the default exim4-daemon-light
package installed. System emails can be checked periodically at stated
above. On an average computer the resources it uses is negligible and
the system email notifications are invaluable. Unless system maintenance
or security is not an issue.

One very important notification is low or zero disk space on the root
"/" partition. Low or zero space on the root "'/" partition can cause
the OS to behave in an unpredictable manor and some processes will
behave strangely or not execute at all. This condition can easily occur
on a "desktop" or "laptop" system where "/tmp" is normally located on
the root "/" partition.


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