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Amazon EC2 is a great resource for cheap virtual servers to do simple things, like DNS or (low bandwidth) VPNs. I had the need this morning to set up a DNS server for a company which needed to blacklist a list of domains. The simplest way to do this is by editing all the computers’ hostfiles, but that method leaves a lot to be desired. Namely, blocking entire domains (as opposed to single subdomains), and deploying changes. Centralizing in a single place makes the job instant, immediate, and in the end, faster.

The following are the steps I used to set this up on an EC2 server. All command line instructions are followed by a single command you can run to execute the step. There is a full script below, at the end of the post, containing all steps from when you first login to SSH ("Login to root") to the end.

I am not going to go into the details of setting up an EC2 instance, as that information can be found elsewhere. I will also be skipping over some of the more obvious steps. Just create a default EC2 instance with the “Amazon Linux AMI”, and I will list all the changes that need to be made beyond that.

<?php
//Get old domains
$BlockedFile='/var/named/blacklisted.conf';
$CurrentZones=Array();
foreach(explode("\n", file_get_contents($BlockedFile)) as $Line)
        if(preg_match('/^zone "([\w\._-]+)"/', $Line, $Results))
                $CurrentZones[]=$Results[1];

//List domains
if(isset($_REQUEST['List']))
        return print implode('<br>', $CurrentZones);

//Get new domains
if(!isset($_REQUEST['Domains']))
        return print 'Missing Domains';
$Domains=$_REQUEST['Domains'];
if(!preg_match('/^[\w\._-]+(,[\w\._-]+)*$/uD', $Domains))
        return print 'Invalid domains string';
$Domains=explode(',', $Domains);

//Remove domains
if(isset($_REQUEST['Remove']))
{
        $CurrentZones=array_flip($CurrentZones);
        foreach($Domains as $Domain)
                unset($CurrentZones[$Domain]);
        $FinalDomainList=array_keys($CurrentZones);
}
else //Combine domains
        $FinalDomainList=array_unique(array_merge($Domains, $CurrentZones));

//Output to the file
$FinalDomainData=Array();
foreach($FinalDomainList as $Domain)
        $FinalDomainData[]=
                "zone \"$Domain\" { type master; file \"blacklisted.db\"; };";
file_put_contents($BlockedFile, implode("\n", $FinalDomainData));

//Reload named
print `sudo /var/www/html/AddRules/restart_named`;
?>

To permanently set “localhost” as the resolver DNS, add “DNS1=localhost” to “/etc/sysconfig/network-scripts/ifcfg-eth0”. I have not yet confirmed this edit.

Soon after setting up this DNS server, it started getting hit by a DNS amplification attack. As the server is being used as a client’s DNS server, turning off recursion is not available. The best solution is to limit the people who can query the name server via an access list (usually a specific subnet), but that would very often not be an option either. The solution I currently have in place, which I have not actually verified if it works, is to add a forced-forward rule which only makes external requests to the name server provided by Amazon. To do this, get the name server’s IP from /etc/resolv.conf (it should be commented from an earlier step). Then add the following to your named.conf in the “options” section.

After I added this rule, external DNS requests stopped going through completely. To fix this, I turned “dnssec-validation” to “no” in the named.conf. Don’t forget to restart the service once you have made your changes.

#User defined variables
VARIABLES_SET=0; #Set this to 1 to allow the script to run
YOUR_DOMAIN="localhost";
YOUR_IP="1.1.1.1";
BLOCKED_ERROR_MESSAGE="Domain is blocked";
ADDRULES_USERNAME="YourUserName";
ADDRULES_PASSWORD="YourPassword";

#Confirm script is ready to run
if [ $VARIABLES_SET != 1 ]; then
    echo 'Variables need to be set in the script';
    exit 1;
fi
if [ `whoami` != 'root' ]; then
    echo 'Must be root to run script. When running the script, add "sudo" before it to' \
        'run as root';
    exit 1;
fi

#Allow root login
cat /home/ec2-user/.ssh/authorized_keys > /root/.ssh/authorized_keys;
perl -pi -e 's/^\s*#?\s*PermitRootLogin.*$/PermitRootLogin yes/igm' /etc/ssh/sshd_config;
service sshd reload;

#Install services
yum -y install bind httpd php;
chkconfig httpd on;
chkconfig named on;
service httpd start;
service named start;

#Set the DNS server to be usable by other computers
perl -pi -e 's/^(\s*(?:listen-on port 53|allow-query)\s*{).*$/$1 any; };/igm' \
    /etc/named.conf;
service named reload;

#Create/link the blacklist files
echo -ne '\ninclude "/var/named/blacklisted.conf";' >> /etc/named.conf;
touch /var/named/blacklisted.conf;

#Create the blacklist zone file
echo -ne "\$TTL 14400
@       IN SOA $YOUR_DOMAIN. $YOUR_DOMAIN ( 2003052800  86400  300  604800  3600 )
@       IN      NS   $YOUR_DOMAIN.
@       IN      A    $YOUR_IP
*       IN      A    $YOUR_IP" > /var/named/blacklisted.db;

#Fix the permissions on the blacklist files
chgrp named /var/named/blacklisted.*;
chmod 660 /var/named/blacklisted.*;

#Set the server’s domain resolution name servers
perl -pi -e 's/^(?!;)/;/gm' /etc/resolv.conf;
echo -ne '\nnameserver localhost' >> /etc/resolv.conf;

#Run a test
echo 'zone "example.com" { type master; file "blacklisted.db"; };' >> \
    /var/named/blacklisted.conf;
service named reload;
FOUND_IP=`dig -t A example.com | grep -ioP "^example\.com\..*?"'in\s+a\s+[\d\.:]+' | \
     grep -oP '[\d\.:]+$'`;
if [ "$YOUR_IP" == "$FOUND_IP" ]
then
  echo 'Success: Example domain matches your given IP' > /dev/stderr;
else
  echo 'Warning: Example domain does not match your given IP' > /dev/stderr;
fi

#Have the server return a message when a blacklisted domain is accessed
echo "$BLOCKED_ERROR_MESSAGE" > /var/www/html/index.html;
perl -0777 -pi -e 's~(<Directory "/var/www/html">.*?\n\s*AllowOverride).*?\n~$1 All~s' \
     /etc/httpd/conf/httpd.conf;
echo -n 'RewriteEngine on
RewriteCond %{REQUEST_URI} !index.html
RewriteCond %{REQUEST_URI} !AddRules/
RewriteRule ^(.*)$ /index.html [L]' > /var/www/html/.htaccess;
service httpd graceful;

#Create a script that allows apache to refresh the name server’s settings
mkdir /var/www/html/AddRules;
echo '/sbin/service named reload' > /var/www/html/AddRules/restart_named;
chmod 755 /var/www/html/AddRules/restart_named;

echo 'apache ALL=(root) NOPASSWD:/var/www/html/AddRules/restart_named
Defaults!/var/www/html/AddRules/restart_named !requiretty' >> /etc/sudoers;

#Create a script that allows the user to add, remove, and list the blacklisted domains
echo -n $'<?php
//Get old domains
$BlockedFile=\'/var/named/blacklisted.conf\';
$CurrentZones=Array();
foreach(explode("\\n", file_get_contents($BlockedFile)) as $Line)
        if(preg_match(\'/^zone "([\\w\\._-]+)"/\', $Line, $Results))
                $CurrentZones[]=$Results[1];

//List domains
if(isset($_REQUEST[\'List\']))
        return print implode(\'<br>\', $CurrentZones);

//Get new domains
if(!isset($_REQUEST[\'Domains\']))
        return print \'Missing Domains\';
$Domains=$_REQUEST[\'Domains\'];
if(!preg_match(\'/^[\\w\\._-]+(,[\\w\\._-]+)*$/uD\', $Domains))
        return print \'Invalid domains string\';
$Domains=explode(\',\', $Domains);

//Remove domains
if(isset($_REQUEST[\'Remove\']))
{
        $CurrentZones=array_flip($CurrentZones);
        foreach($Domains as $Domain)
                unset($CurrentZones[$Domain]);
        $FinalDomainList=array_keys($CurrentZones);
}
else //Combine domains
        $FinalDomainList=array_unique(array_merge($Domains, $CurrentZones));

//Output to the file
$FinalDomainData=Array();
foreach($FinalDomainList as $Domain)
    $FinalDomainData[]="zone \\"$Domain\\" { type master; file \\"blacklisted.db\\"; };";
file_put_contents($BlockedFile, implode("\\n", $FinalDomainData));

//Reload named
print `sudo /var/www/html/AddRules/restart_named`;
?>' > /var/www/html/AddRules/index.php;

usermod -a -G named apache;
service httpd graceful;

#Password protect the domain update script
echo -n 'AuthType Basic
AuthName "Admins Only"
AuthUserFile "/var/www/html/AddRules/.htpasswd"
require valid-user' > /var/www/html/AddRules/.htaccess;

htpasswd -bc /var/www/html/AddRules/.htpasswd "$ADDRULES_USERNAME" "$ADDRULES_PASSWORD";

echo 'Script complete';