Random Password Generator

Posted on June 30th, 2008 in General,Linux System Administration,Security by Brandon

There are times when I’ve been focusing on programming all day, and it is easier to write a program to do something trivial, then it is to just do it the simple way. Today was such a day. Instead of typing some random character to make up new user’s password, I wrote a script to do it for me:

#!/usr/bin/perl
#################################################################
## Quick Random Password Generator
## Author: Brandon Checketts
## http://www.brandonchecketts.com/
#################################################################

my $length = $ARGV[0] || 10;

my $charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";

my $pw = "";
for (my $i=0; $i < $length; $i++) {
    my $pos = rand(length($charset));
    $pw .= substr($charset, $pos, 1);
}
print "\\nRandom Password: $pw\\n\\n";

Of course you can modify the default length and/or characters to make something more suitable for your use.

Sample Usage:

[root@dev ~]# ~/bin/pwgen

Random Password: mYTZrSpE8B

[root@dev ~]# ~/bin/pwgen 20

Random Password: EoSQpypmeK3SZCVPodaM

Postfix Vacation Message

Posted on June 26th, 2008 in General,Linux System Administration,Mail by Brandon

The idea of a vacation message is kindof odd to me, but I had one client request it today, so took a look at configuring it. On RHEL/CentOS distros, the ‘vacation’ binary is distributed with sendmail, and is not available with postfix, so you have to build it yourself. Fortunately, it is about the easiest thing I have ever compiled.

[root@host ~]# yum install gdbm-devel
[root@host ~]# cd /usr/local/src/
[root@host ~]# wget http://internap.dl.sourceforge.net/sourceforge/vacation/vacation-1.2.7.0-rc2.tar.gz
[root@host ~]# tar -xvzf vacation*
[root@host ~]# cd vacation-1.2.7.0-rc2
[root@host ~]# make
[root@host ~]# make install

That’s it. Not even a configure script. That should install the vacation binary in /usr/bin/vacation.

Now just create a vacation message by putting a ‘.vacation.msg’ in the user’s home directory with the auto-reply content:

Subject: On vacation message.

I'm on vacation and will not be reading my mail for a while.
Your mail will be dealt with when I return.

And finally, create a .forward file that tells your mail program to deliver to the vacation program:

\myuser, "|/usr/bin/vacation  myuser"

That should be it. I tested and verified that it works. Note that you have to provide a to: header with the recipient’s address.

Next, I might try some experiments to see if I can get it to work with virtual users.

Checking MySQL Replication

Posted on June 23rd, 2008 in General,Linux System Administration,PHP,Programming by Brandon

MySQL replication is pretty easy to set up, but needs a few extra things to make it more reliable. I wrote this quick PHP script to alert me when replication has failed and is more than 5 minutes behind the master.

<?php

$user = 'username';
$pass = 'password';
$host = 'localhost';
// Grant this user permission to check the status with this mysql statement
// GRANT REPLICATION CLIENT on *.* TO 'user'@'host' IDENTIFIED BY 'password';

$threshold = 300;

$db = mysql_connect($host, $user, $pass);

$result = mysql_query('SHOW SLAVE STATUS');
if (!$result) {
    // Make sure that your user has the 'REPLICATION CLIENT' privlege
    echo "Error 'SHOW SLAVE STATUS' command failed\n";
    echo mysql_error()."\n";
    exit(1);
}

$status = mysql_fetch_array($result);

if (!isset($status['Seconds_Behind_Master'])) {
    echo "Error: Seconds_Behind_Master column not found in result\n";
    print_r($status);
    exit(2);
}

if ($status['Seconds_Behind_Master'] > $threshold) {
    $minutes = floor($status['Seconds_Behind_Master'] / 60);
    echo "Error: Slave is $minutes minutes behind the master server\n";
    exit(3);
}

exit(0);
?>

This script is intended to be run periodically from cron. It doesn’t generate any output unless something is wrong. The behavior of cron is that when a script generates output, it will email the output to the user, so make sure that you have mail on your system configured to send you the cron output correctly. The script also exits with a non-zero status on each error, so you might include this in a more complicated script that attempts to do something else based on the status.

I use something like this in a non-privileged user’s crontab:

*/15 * * * /usr/bin/php /path/to/check_replication.php

Getting Dkimproxy Installed and Configured

Posted on June 21st, 2008 in Encryption,General,Linux System Administration,Mail,Spam by Brandon

Dkimproxy is a great program for getting Postfix to both sign and validate DomainKeys and DKIM messages. Prior to dkimproxy, one would have used dk-filter and dkim-filter which did DomainKeys and DKIM signing separately. dkimproxy is a newer version that combines the functionality into one program. Installing it can be a bit complicated because it isn’t available in most distro repositories, and requires several Perl modules that need to be installed. Configuring it can be difficult as well, because it involves making changes DNS and postfix, in addition to its own configuration.

I wrote these steps below as I went through a recent installation for a customer

You can install the required Perl modules through the RPM Forge Repository if you have it enabled with the command (Thanks JohnB for mentioning that):

yum install perl-Net-Server perl-Error perl-Mail-DKIM

Otherwise, you can install them manually with CPAN. First install the openssl-devel package (You’ll need it for CPAN to install Mail::DKIM)

yum install openssl-devel

Now install the required Perl modules

# perl -MCPAN -e shell
> install Net::Server
> install Error
> install Mail::DKIM

Download and install the actual dkimproxy code:

cd /usr/local/src
wget http://internap.dl.sourceforge.net/sourceforge/dkimproxy/dkimproxy-1.0.1.tar.gz
tar -xvzf dkimproxy-1.0.1.tar.gz
cd dkimproxy-1.0.1
./configure --prefix=/usr/local/dkimproxy
make
make install

You should now have the program installed in /usr/local/dkimproxy. A sample init file is included, so we can copy it into place also:

cp /usr/local/src/dkimproxy-1.0.1/sample-dkim-init-script.sh /etc/init.d/dkimproxy

Create a ‘dkim’ user and group, but lock the password:

useradd -d /usr/local/dkimproxy dkim
passwd -l dkim

That should be enough to get dkimproxy running, but it isn’t configured yet.

Create a key file for your domain

cd /usr/local/dkimproxy/etc/
openssl genrsa -out domain.tld.key 1024
openssl rsa -in domain.tld.key -pubout -out domain.tld.pub

Now create a DNS TXT record for mail._domainkey.domain.tld with the contents of domain.tld.pub. Your public key will span at least two lines, so combine all of the lines of the key together when putting it in your DNS record. The whole DNS record will look something like this:

k=rsa; t=s; p=MFwwDQYJ......0JMCAwEAAQ==

(Note that the key is pretty long and I’ve shortened it here)
You could now confirm the key is correct in your DNS:

[root@host etc]# host -ttxt mail._domainkey.domain.tls
mail._domainkey.domain.tld descriptive text "k=rsa\; t=s\; p=MFwwDQYJ......0JMCAwEAAQ=="

(Note that the key is pretty long and I’ve shortened it here)

Now tell dkimproxy about the key files, and configuration parameters. Create /usr/local/dkimproxy/etc/dkimproxy_out.conf with this content

# specify what address/port DKIMproxy should listen on
listen    127.0.0.1:10027

# specify what address/port DKIMproxy forwards mail to
relay     127.0.0.1:10028

# specify what domains DKIMproxy can sign for (comma-separated, no spaces)
domain    domain.tld

# specify what signatures to add
signature dkim(c=relaxed)
signature domainkeys(c=nofws)

# specify location of the private key
keyfile   /usr/local/dkimproxy/etc/domain.tld.key

# specify the selector (i.e. the name of the key record put in DNS)
selector  mail

And copy the sample inbound config to the real inbound config

cd /usr/local/dkimproxy/etc
cp dkimproxy_in.conf.example dkimproxy_in.conf

Now you should be able to start up dkimproxy, and configure it to start at boot:

/etc/init.d/dkimproxy start
chkconfig dkimproxy on

And the last step is just to modify the postfix configuration to tell it to forward messages sent to port 587 through dkimproxy for signing. I added these three sections to /etc/postfix/master.cf

### dkimproxy filter - see http://dkimproxy.sourceforge.net/postfix-outbound-howto.html
#
# modify the default submission service to specify a content filter
# and restrict it to local clients and SASL authenticated clients only
#
submission  inet  n     -       n       -       -       smtpd
    -o smtpd_etrn_restrictions=reject
    -o smtpd_sasl_auth_enable=yes
    -o content_filter=dksign:[127.0.0.1]:10027
    -o receive_override_options=no_address_mappings
    -o smtpd_recipient_restrictions=permit_mynetworks,permit_sasl_authenticated,reject

# specify the location of the DKIM signing proxy
# Note: the smtp_discard_ehlo_keywords option requires a recent version of
# Postfix. Leave it off if your version does not support it.
dksign    unix  -       -       n       -       10      smtp
    -o smtp_send_xforward_command=yes
    -o smtp_discard_ehlo_keywords=8bitmime,starttls

# service for accepting messages FROM the DKIM signing proxy
127.0.0.1:10028 inet  n  -      n       -       10      smtpd
    -o content_filter=
    -o receive_override_options=no_unknown_recipient_checks,no_header_body_checks
    -o smtpd_helo_restrictions=
    -o smtpd_client_restrictions=
    -o smtpd_sender_restrictions=
    -o smtpd_recipient_restrictions=permit_mynetworks,reject
    -o mynetworks=127.0.0.0/8
    -o smtpd_authorized_xforward_hosts=127.0.0.0/8

If you want it to sign messages sent from the command line sendmail program, modify the pickup service to use the content_filter like this:

pickup    fifo  n       -       n       60      1       pickup
    -o content_filter=dksign:[127.0.0.1]:10027

Finally restart postfix with ‘postfix reload’, and you *should* have a working installation. You can now use my Domainkeys/Dkim validator to test and ensure that it is working.

CentOS Mail Toaster Guide Updated

Posted on June 18th, 2008 in General by Brandon

I’m getting a chance to go through my own guide with several installations that I’m doing.  I noticed that PostfixAdmin has a new version 2.2.0 release available which changes the installation process significantly.   It is much simpler now and my howto guide has been updated to reflect the new changes.   Kudos to the developers who updated the installation process making it much simpler.

Getting Un-locked out of Webmin

Posted on June 5th, 2008 in Linux System Administration,Security by Brandon

Webmin has come a long way in the last year or two. I still (and always will) prefer the command line, but many customers are certainly much more comfortable using a web-interface to configure their server. I have to reset a password every once in a while and have to look up the steps every time. It seems I can never find them quickly, so I’ll put them here so I can find them easily next time.

Blocked IP addresses are stored a text file named /var/webmin/blocked. IP Address should be cleared out after some period of time, but you can hasten the process by clearing it manually:

cp /dev/null /var/webmin/blocked

Webmin passwords are stored in /etc/webmin/miniserv.users. A script for changing a user password is provided with webmin. Just run it like this:

/usr/libexec/webmin/changepass.pl /etc/webmin root