How mailx -r works? Post: 302832565

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Top Forums Shell Programming and Scripting How mailx -r works? Post 302832565 by bakunin on Monday 15th of July 2013 07:42:12 AM

07-15-2013

Registered User

First, you have to distinguish between mail as a (network) protocol and the means (clients, servers, MTAs, ...) to use this protocol.

The mail protocol - SMTP - allows to send messages with any name as sender. As "mailx" is a very basic client it does nothing to authenticate a user at all, so: yes, in the way you presented it it would be possible to send mail using any arbitrary mail address as sender. Yes, this would work for other email addresses too, provided that your mail server (more precisely: mail transfer agent, MTA) is set up this way. In fact this is the way professional spammers work: by setting up a MTA of their own and configuring it in a sufficiently crooked way.

This is the reason why you should not trust incoming mail (solely) based on the mail address of the sender: because this could be forged.

Now, in a common setup the MTA would not allow to send mail with an arbitrary sender address and/or has rules to rewrite address information when handling mail messages. Your MTA (probably "sendmail") is perhaps set up only rudimentary to allow sending diagnostic information via mail to the admins. This is not uncommon: if a filesystem becomes nearly full or a similar condition takes place the server might generate a mail message to the admin(s). For this the MTA is installed and set up with minimal effort. This does not automatically mean it could send mail anywhere outside the companies network. If so: this is a security deficit and should be taken care of.

I hope this helps.

bakunin

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femail

FEMAIL(8)						    BSD System Manager's Manual 						 FEMAIL(8)

NAME

     femail -- accept mail on behalf of a real MTA

SYNOPSIS

     femail [-46tv] [-f from] [-F name] [address ...]

DESCRIPTION

     femail is intended to be used on servers that do not run a real MTA or inside chroot(8) environments.  It features a sendmail(8)-compatible
     command line interface and forwards mail via SMTP to the host given in the config file, an environment variable or localhost.

     The options are as follows:

     -4        Only use IPv4.

     -6        Only use IPv6.

     -F name   Set the sender's name to name.

     -f from   Set the sender address to from.	Normally, femail tries to parse the sender from the message, and uses login@hostname if that is
	       not present.

     -t        Read recipients from the message given on stdin, in addition to the recipients given on the command line.

     -v        Enable verbose operation.

CONFIG FILE

     The config file, /etc/femail.conf, consists of simple name=value pairs.  The supported settings are as follows:

     smtphost  Specify the server femail should send the messages to.

     smtpport  Specify the port femail uses on smtphost to send the messages.  Default is 25.

     myname    The hostname femail uses.  Defaults to the machine's hostname.

ENVIRONMENT

     SMTPHOST	      Forward mail to the given host instead of the local host.  Only consulted when not set in the config file.

     SMTPPORT	      Connect to the given port instead of 25.	Only consulted when not set in the config file.

SEE ALSO

     mail(1), sendmail(8)

AUTHORS

     femail was written by Henning Brauer.

BSD
								  August 9, 2005							       BSD

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