Weight: 3
Candidates should be aware of the commonly available MTA programs and be able to perform basic forward and alias configuration on a client host. Other configuration files are not covered.
Key Knowledge Areas
- Create e-mail aliases.
- Configure e-mail forwarding.
- Knowledge of commonly available MTA programs (
postfix,sendmail,exim) (no configuration)
Terms and Utilities
~/.forward- sendmail emulation layer commands
newaliasesmailmailqpostfixsendmailexim
MTAs
Email is an integral part of many GNU/Linux and Unix systems. Each user do have a mail box and can send / receive email to other local users. This is done via MTAs (Mai Transfer Agents). In other words, MTAs are programs which handle emails in your operating system. They can receive and dispatch emails locally and over the network. There are different options for MTAs. In this section we will do a quick review on them and you will see how you can send emails to other uses (or over the internet) and how you can check your local mails.
sendmail
Is one of the oldest options available. It is huge and kind of difficult to configure and not that security oriented. Because of these, few systems use it as default their MTA.
exim
It aims to be a general and flexible mailer with extensive facilities for checking incoming e-mail. It is feature rich with ACLs, authentication and many other features.
postfix
This is a new alternative to sendmail and uses easy to understand configuration files. It supports multiple domains, encryption, etc. Postfix is what you see on most distros as the default MTA.
Most desktop distros do not install MTAs by default. If you want, I suggest installing the
postfix(andmailxorbsd-mailx) via your package manager.
sendmail emulation layer
As you already know, sendmail is the oldest MTA alive and therefore, many other MTAs try to comply with it and has a sendmail emulation layer to keep themselves backward compatible with sendmail. Thats why you can type sendmail on whatever distro you are or use the mailq and check your mail regardless of your MTA choice.
aliases
There are some mail aliases on the system. Defined in /etc/aliases.
$ cat /etc/aliases
#
# Aliases in this file will NOT be expanded in the header from
# Mail, but WILL be visible over networks or from /bin/mail.
#
# >>>>>>>>>> The program "newaliases" must be run after
# >> NOTE >> this file is updated for any changes to
# >>>>>>>>>> show through to sendmail.
#
# Basic system aliases -- these MUST be present.
mailer-daemon: postmaster
postmaster: root
# General redirections for pseudo accounts.
bin: root
daemon: root
adm: root
lp: root
sync: root
shutdown: root
halt: root
mail: root
news: root
uucp: root
operator: root
games: root
www: webmaster
webmaster: root
[ .... ]
This tells the system if there is a message for news, it should be delivered to root and if the email is written to www it should be delivered to webmaster.
In case of any change in this file, you need to run the newaliases command.
sending mail
It is possible to send an email from the command line using the mail command:
[jadi@funlife ~]$ mail news
Subject: Email to news user
hahah.. we know where this will go.
this will go to root and then to jadi!
Hi Jadi!
Cc:
[jadi@funlife ~]$ mail
Mail version 8.1.2 01/15/2001. Type ? for help.
"/var/mail/jadi": 12 messages 12 new
>N 1 root@funlife Sat Jan 02 08:50 39/1373 apt-listchanges: news for f
N 2 root@funlife Sat Jan 02 09:01 165/7438 apt-listchanges: news for f
N 3 jadi@funlife Sat Jan 02 19:58 18/640 *** SECURITY information fo
N 4 jadi@funlife Sat Jan 02 20:04 18/631 *** SECURITY information fo
N 5 jadi@funlife Sun Jan 03 10:15 18/664 *** SECURITY information fo
N 6 root@funlife Mon Jan 04 12:42 27/941 Cron <jadi@funlife> /home/j
N 7 root@funlife Mon Jan 04 17:11 26/845 apt-listchanges: news for f
N 8 root@funlife Tue Jan 05 18:42 27/945 Cron <jadi@funlife> /home/j
N 9 root@funlife Wed Jan 06 09:17 46/1788 apt-listchanges: news for f
N 10 root@funlife Thu Jan 07 12:42 27/945 Cron <jadi@funlife> /home/j
N 11 root@funlife Thu Jan 07 18:42 27/943 Cron <jadi@funlife> /home/j
N 12 jadi@funlife Thu Jan 7 19:53 17/478 Email to news user
& 12
Message 12:
From jadi@funlife Thu Jan 7 19:53:08 2016
X-Original-To: news
To: news@funlife
Subject: Email to news user
Date: Thu, 7 Jan 2016 19:53:08 +0330 (IRST)
From: jadi@funlife (jadi)
hahah.. we know where this will go.
this will go to root and then to jadi!
Hi Jadi!
& d
& q
Held 11 messages in /var/mail/jadi
local forwards
We saw that it is possible to forward emails using the /etc/aliases. That file is not writable by normal users so what a normal user like jadi should do?
Each user can create a .forward file in her own directory and all mail targeted to that user will be forwarded to that address.
You can even put a complete email address like
[email protected]in your.forwardfile.
It also can send email from the command line or even within your scripts by issuing something like echo -e "email content" | mail -s "email subject" "[email protected]".
mailq
This command lists the mail queue. Each entry shows the queue file ID, message size, arrival time, sender, and the recipients that still need to be delivered. If mail could not be delivered upon the last attempt, the reason for failure is shown. The sysadmin can use this command to check the status of emails still in the queues.
$ mailq
-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
AA52C228E6B 468 Thu Jan 7 19:59:41 jadi@funlife
(connect to alt2.gmail-smtp-in.l.google.com[2404:6800:4003:c01::1a]:25: Network is unreachable)
[email protected]
-- 0 Kbytes in 1 Request.
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