Setting Up A Remote SMTP Relay In Postfix On Linux Or Unix

Hey there,

Too much to do today to write anything with gristle ;)

I did, however, run into an issue with Postfix email SMTP relaying that drove me nuts until I found this link on the web about how to configure postfix to use a remote smtp relay host

I've paraphrased below to save my saviour some bandwidth :)

I hadn't dealt with Postfix in forever and, naturally, assumed that the whole setup was far more complicated than it was, and it ended up costing me a bit of time. Once again, life knocks me down a peg. Pretty soon I won't be sitting so high and mighty ;)

Here's the solution (from the link above) Easy as Pie:

This is a public service message, because when I queried google "configure postfix remote smtphost", I couldn't find anything useful to the question I was trying to answer. 

So you're running Linux, and your ISP is filtering traffic on port 25, thus blocking you from sending outgoing mail, and so you want to configure your mailserver to relay all your mail out through your ISP's server, like they want you to. If you're running postfix, this turns out to be very easy -- in /etc/postfix/main.cf, just set the variable: 

 relayhost = smtp.yourisp.com
then restart postfix (/etc/init.d/postfix restart, as root). 

You're welcome. 

, Mike

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Shell Script To Send Mail Using Bash File Descriptors

Hey There,

It's the weekend again, and I thought I'd wrap up this week's postings with quick script that, again, demonstrates a great way to take advantage of networking with bash using file descriptors on Linux or Unix. If you find this sort of thing fascinating (which I seem to ;), be sure to check out our orignal post on networking with bash file descriptors and the follow up regarding more things you can do with bash networking.

Note: Interestingly enough, apart from file descriptors 0, 1 and 2, you should also stay away from file descriptor 5. It seems bash uses this as its default file descriptor when a child process is created. Thankfully, I chose the number 9 (No Beatles pun intended ;)

Today's script sends email from the shell, directly to port 25 out, and is simple to run. It only takes a few arguments: Your From address, your To address, your mail server or relay, your domain and your message text (which you can put in any sort of file, as long as it's readable).

Ex:

host # ./mail.sh me@xyz.com you@xyz.com xyz.com localhost fileName

Hope you enjoy it and find some good use for it. If anything, it might make shooting emails out from the shell when you have a good idea much easier (before you have to fuss with Windows and lose your train of thought - Does that happen to everyone else, or just me ;).

Have a great weekend :)





This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License

#!/bin/bash

#
# mail.sh
#
# 2008 - Mike Golvach - eggi@comcast.net
#
# Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
#

if [ $# -ne 5 ]
then
 echo "Usage: $0 FromAdress ToAdress Domain MailServer MailText"
 exit 1
fi

from=$1
to=$2
domain=$3
mailserver=$4
mailtext=$5

if [ ! -f $mailtext ]
then
 echo "Cannot find your mail text file.  Exiting..."
 exit 1
fi

exec 9<>/dev/tcp/$mailserver/25
echo "HELO $domain" >&9
read -r temp <&9
echo "$temp"
echo "Mail From: $from" >&9
read -r temp <&9
echo "$temp"
echo "Rcpt To: $to" >&9
read -r temp <&9
echo "$temp"
echo "Data" >&9
read -r temp <&9
echo "$temp"
cat $mailtext >&9
echo "." >&9
read -r temp <&9
echo "$temp"
echo "quit" >&9
read -r temp <&9
echo "$temp"
9>&-
9<&-
echo "All Done Sending Email. See above for errors"
exit 0

, Mike

linux unix internet technology

Checking If A Mail Server Is An Open Relay Using Perl and Berkeley Sockets!

SPAM is common enough nowadays that you can probably find some version of this basic check somewhere on the web (probably a lot of places), but our intent here is to take a look at what's behind the pretty looking web pages and/or software, used in determining whether or not the mail server you're runnning (or any mail server, for that matter) is an open relay.

Below is a little script I put together using Perl (and its Socket module - based on Berkeley Sockets) that will do this check for you. In a future post, we'll explore Berkeley Sockets programming at a more granular level (as it's far outside the scope of this simple blog post), but for now check out the script below:





This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License

#!/bin/perl

#
# 2007 - Mike Golvach - eggi@comcast.net
#
# Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License
#

use Socket;

$host = "whateverMailServerYouWantToCheck.xyz.com";
$relaydomain = "xyz.com";
$sender = "eggi@noOneWillEverPickThisDomain.com";
$adressee = "info@hopefullyThisMailServerIsNotInTheDomainWereTesting.zyx.com";
$protocol = "tcp";
$port = 25;

if ( $port =~ /\D/) {
$port = getservbyname($port, $protocol);
}
unless ( $port ) {
print "No port : ${port}/$protocol\n";
}
$inet_address = inet_aton($host) || die "No host: ${host}!";
$port_address = sockaddr_in($port, $inet_address);
$protocol_num = getprotobyname('$protocol');
socket(SOCKET, PF_INET, SOCK_STREAM, $protocol_num) || die "Socket: $!";
connect(SOCKET, $port_address) || die "Socket $!";
send(SOCKET, $line, 'SOCK_STREAM') == length($line) || die "Cannot Send Message!: $!\n";
print SOCKET "HELO $relaydomain\n";
print SOCKET "MAIL From: $sender\n";
print SOCKET "RCPT To: $adressee\n";
print SOCKET "DATA\n";
print SOCKET "Subject: If this didn't get rejected - you may be running an open relay!\n";
print SOCKET ".\n";
print SOCKET "quit\n";
close(SOCKET) || die "Close $!";
exit(0);

At a very basic level, there are just a few things (covered in the script above) that you need to do to quickly determine if a mail server is an open relay. First, make sure you're sending from a different domain than the mail server's domain. Second, connect to the mail server and attempt to send mail to another (different) domain. If this works, the mail server is acting as an open relay.

Of course, like I said, it might be easier to just go to any number of sites on the web and have that free service do the check for you. But, in any event, this way can be more fun and you can customize it to do whatever (hopefully, good and decent) things you want it to :)

, Mike

linux unix internet technology