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Blacklisting DNS Server on Amazon EC2

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.

  • Creating the instance
    • For the first year, for the instance type, you might as well use a t2.micro, as it is free. After that, a t2.nano (which is a new lower level) currently at $56.94/year ($0.0065/Hour), should be fine.
    • After you select your instance type, click “Review and Launch” to launch the instance with all of the defaults.
    • After the confirmation screen, it will ask you to create a key pair. You can see other tutorials about this and how it enables you to log into your instance.
  • Edit the security group
    • Next, you need to edit the security group for your instance to allow incoming connections.
    • Go to “Instances” under the “Instances” group on the left menu, and click your instance.
    • In the bottom of the window, in the “Descriptions” tab, click the link next to “Security Groups”, which will bring you to the proper group in the security groups tab.
    • Right click it and “Edit inbound Rules”.
    • Make sure it has the following rules with Source=Anywhere: ALL ICMP [For pinging], SSH, HTTP, DNS (UDP), DNS (TCP)
  • Assign a permanent IP to your instance
    • To do this, click the “Elastic IPs” under “Network & Security” in the left menu.
    • Click “Allocate New Address”.
    • After creating it, right click the new address, then “Associate Address”, and assign it to your new instance.
  • You should probably set this IP up as an A record somewhere. I will refer to this IP as dns.yourdomain.com from now on.
  • Login to root
    • SSH into your instance as the ec2-user via “ssh ec2-user@dns.yourdomain.com”. If in windows, you could also use putty.
    • Sudo into root via “sudo su”.
  • Allow root login
    • At this point, I recommend setting it up so you can directly root into the server. Warning: some people consider this a security risk.
    • Copy your key pair(s) to the root user via “cat /home/ec2-user/.ssh/authorized_keys > /root/.ssh/authorized_keys
    • Set SSHD to permit root logins by changing the PermitRootLogin variable to “yes” in /etc/ssh/sshd_config. A quick command to do this is “perl -pi -e 's/^\s*#?\s*PermitRootLogin.*$/PermitRootLogin yes/igm' /etc/ssh/sshd_config”, and then reload the SSHD config with “service sshd reload”. Make sure to attempt to directly log into SSH as root before exiting your current session to make sure you haven’t locked yourself out.
  • Install apache (the web server), bind/named (the DNS server), and PHP (a scripting language)
    • yum -y install bind httpd php
  • Start and set services to run at boot
    • service httpd start; service named start; chkconfig httpd on; chkconfig named on;
  • Set the DNS server to be usable by other computers
    • Edit /etc/named.conf and change the 2 following lines to have the value “any”: “listen-on port 53” and “allow-query”
    • perl -pi -e 's/^(\s*(?:listen-on port 53|allow-query)\s*{).*$/$1 any; };/igm' /etc/named.conf; service named reload;
  • Point the DNS server to the blacklist files
    • This is done by adding “include "/var/named/blacklisted.conf";” to /etc/named.conf
    • echo -ne '\ninclude "/var/named/blacklisted.conf";' >> /etc/named.conf
  • Create the blacklist domain list file
    • touch /var/named/blacklisted.conf
  • Create the blacklist zone file
    • Put the following into /var/named/blacklisted.db . Make sure to change dns.yourdomain.com to your domain (or otherwise, “localhost”), and to dns.yourdomain.com’s (your server’s) IP address. Make sure to keep all periods intact.
      $TTL 14400
      @       IN SOA dns.yourdomain.com. dns.yourdomain.com ( 2003052800  86400  300  604800  3600 )
      @       IN      NS   dns.yourdomain.com.
      @       IN      A
      *       IN      A
    • The first 2 lines tell the server the domains belong to it. The 3rd line sets the base blacklisted domain to your server’s IP. The 4th line sets all subdomains of the blacklisted domain to your server’s IP.
    • This can be done via (Update the first line with your values)
      YOURDOMAIN="dns.yourdomain.com"; YOURIP="";
      echo -ne "\$TTL 14400\n@       IN SOA $YOURDOMAIN. $YOURDOMAIN ( 2003052800  86400  300  604800  3600 )\n@       IN      NS   $YOURDOMAIN.\n@       IN      A    $YOURIP\n*       IN      A    $YOURIP" > /var/named/blacklisted.db;
  • Fix the permissions on the blacklist files
    • chgrp named /var/named/blacklisted.*; chmod 660 /var/named/blacklisted.*;
  • Set the server’s domain resolution name servers
    • The server always needs to look at itself before other DNS servers. To do this, comment out everything in /etc/resolv.conf and add to it “nameserver localhost”. This is not the best solution. I’ll find something better later.
    • perl -pi -e 's/^(?!;)/;/gm' /etc/resolv.conf; echo -ne '\nnameserver localhost' >> /etc/resolv.conf
  • Run a test
    • At this point, it’s a good idea to make sure the DNS server is working as intended. So first, we’ll add an example domain to the DNS server.
    • Add the following to /var/named/blacklisted.conf and restart named to get the server going with example.com: “zone "example.com" { type master; file "blacklisted.db"; };
    • echo 'zone "example.com" { type master; file "blacklisted.db"; };' >> /var/named/blacklisted.conf; service named reload;
    • Ping “test.example.com” and make sure it’s IP is your server’s IP
    • Set your computer’s DNS to your server’s IP in your computer’s network settings, ping “test.example.com” from your computer, and make sure the returned IP is your server’s IP. If it works, you can restore your computer’s DNS settings.
  • Have the server return a message when a blacklisted domain is accessed
    • Add your message to /var/www/html
    • echo 'Domain is blocked' > /var/www/html/index.html
    • Set all URL paths to show the message by adding the following to the /var/www/html/.htaccess file
      RewriteEngine on
      RewriteCond %{REQUEST_URI} !index.html
      RewriteCond %{REQUEST_URI} !AddRules/
      RewriteRule ^(.*)$ /index.html [L]
    • echo -ne 'RewriteEngine on\nRewriteCond %{REQUEST_URI} !index.html\nRewriteCond %{REQUEST_URI} !AddRules/\nRewriteRule ^(.*)$ /index.html [L]' > /var/www/html/.htaccess
    • Turn on AllowOverride in the /etc/httpd/conf/httpd.conf for the document directory (/var/www/html/) via “ perl -0777 -pi -e 's~(<Directory "/var/www/html">.*?\n\s*AllowOverride).*?\n~$1 All~s' /etc/httpd/conf/httpd.conf
    • Start the server via “service httpd graceful
  • Create a script that allows apache to refresh the name server’s settings
    • Create a script at /var/www/html/AddRules/restart_named with “/sbin/service named reload” and set it to executable
    • mkdir /var/www/html/AddRules; echo '/sbin/service named reload' > /var/www/html/AddRules/restart_named; chmod 755 /var/www/html/AddRules/restart_named
    • Allow the user to run the script as root by adding to /etc/sudoers “apache ALL=(root) NOPASSWD: /var/www/html/AddRules/restart_named” and “Defaults!/var/www/html/AddRules/restart_named !requiretty
    • echo -e 'apache ALL=(root) NOPASSWD:/var/www/html/AddRules/restart_named\nDefaults!/var/www/html/AddRules/restart_named !requiretty' >> /etc/sudoers
  • Create a script that allows the user to add, remove, and list the blacklisted domains
    • Add the following to /var/www/html/AddRules/index.php (one line command not given. You can use “nano” to create it)
      //Get old domains
      foreach(explode("\n", file_get_contents($BlockedFile)) as $Line)
              if(preg_match('/^zone "([\w\._-]+)"/', $Line, $Results))
      //List domains
              return print implode('<br>', $CurrentZones);
      //Get new domains
              return print 'Missing Domains';
      if(!preg_match('/^[\w\._-]+(,[\w\._-]+)*$/uD', $Domains))
              return print 'Invalid domains string';
      $Domains=explode(',', $Domains);
      //Remove domains
              foreach($Domains as $Domain)
      else //Combine domains
              $FinalDomainList=array_unique(array_merge($Domains, $CurrentZones));
      //Output to the file
      foreach($FinalDomainList as $Domain)
                      "zone \"$Domain\" { type master; file \"blacklisted.db\"; };";
      file_put_contents($BlockedFile, implode("\n", $FinalDomainData));
      //Reload named
      print `sudo /var/www/html/AddRules/restart_named`;
    • Add the “apache” user to the “named” group so the script can update the list of domains in /var/named/blacklisted.conf via “usermod -a -G named apache; service httpd graceful;
  • Run the domain update script
    • To add a domain (separate by commas): http://dns.yourdomain.com/AddRules/?Domains=domain1.com,domain2.com
    • To remove a domain (add “Remove&” after the “?”): http://dns.yourdomain.com/AddRules/?Remove&Domains=domain1.com,domain2.com
    • To list the domains: http://dns.yourdomain.com/AddRules/?List
  • Password protect the domain update script
    • Add to AddRules/.htaccess the following
      AuthType Basic
      AuthName "Admins Only"
      AuthUserFile "/var/www/html/AddRules/.htpasswd"
      require valid-user
    • echo -ne 'AuthType Basic\nAuthName "Admins Only"\nAuthUserFile "/var/www/html/AddRules/.htpasswd"\nrequire valid-user' > /var/www/html/AddRules/.htaccess
    • Warning: Putting the password file in an http accessible directory is a security risk. I just did this for sake of organization.
    • Create the user+password via “htpasswd -bc /var/www/html/AddRules/.htpasswd USERNAME” and then entering the password

[Edit on 2016-01-30 @ noon]

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.

Security Issue

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.

	forwarders {
	forward only;

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.

[End of edit]

Full serverside script
Make sure to run this as root (login as root or sudo it)

Download the script here. Make sure to chmod and sudo it when running. “chmod +x dnsblacklist_install.sh; sudo ./dnsblacklist_install.sh;

#User defined variables
VARIABLES_SET=0; #Set this to 1 to allow the script to run
BLOCKED_ERROR_MESSAGE="Domain is blocked";

#Confirm script is ready to run
if [ $VARIABLES_SET != 1 ]; then
    echo 'Variables need to be set in the script';
    exit 1;
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;

#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' \
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"; };' >> \
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" ]
  echo 'Success: Example domain matches your given IP' > /dev/stderr;
  echo 'Warning: Example domain does not match your given IP' > /dev/stderr;

#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~$1 All~s' \
echo -n 'RewriteEngine on
RewriteCond %{REQUEST_URI} !index.html
RewriteCond %{REQUEST_URI} !AddRules/
RewriteRule ^(.*)$ /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 $'<?php
//Get old domains
foreach(explode("\\n", file_get_contents($BlockedFile)) as $Line)
        if(preg_match(\'/^zone "([\\w\\._-]+)"/\', $Line, $Results))

//List domains
        return print implode(\'<br>\', $CurrentZones);

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

//Remove domains
        foreach($Domains as $Domain)
else //Combine domains
        $FinalDomainList=array_unique(array_merge($Domains, $CurrentZones));

//Output to the file
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';
The Cost of becoming a .COM Domain Registrar
How can this be financially feasible for anyone?!?

I’ve apparently had an incorrect view on the exact schema and costs of how domain registration works. I had always assumed that to become a registrar (the companies that normal people register domains through) of any .COM domain, you just had to get accredited by ICANN, and then pay $0.20 per domain. However, accreditation is an expensive and tough process, including (taken verbatim from the link):

  • US$2,500 non-refundable application fee, to be submitted with application.
  • US$4,000 yearly accreditation fee due upon approval and each year thereafter.
  • Variable fee (quarterly) billed once you begin registering domain names or the first full quarter following your accreditation approval, whichever occurs first. This fee represents a portion of ICANN’s operating costs and, because it is divided among all registrars, the amount varies from quarter to quarter. Recently this fee has ranged from US$1,200 to S$2,000 per quarter.
  • Transaction-based gTLD fee (quarterly). This fee is a flat fee (currently $0.20) charged for each new registration, renewal or transfer. This fee can be billed by the registrar separately on its invoice to the registrant, but is paid by the registrar to ICANN.
  • Please refer to http://www.icann.org/general/financial.html for the most recent ICANN Budget to find additional details about invoicing, including options for relief.
  • Please refer to http://www.icann.org/financials/payments.htm for instructions on how to submit payments to ICANN.

So I had thought that becoming an accredited .COM registrar would pay itself off in the first year if you had ~1,177 domains registered...
  • BASE FIRST YEAR FEE=$2500 application + $4000 yearly + ~$1500 ICANN operation fee = $8000
  • PER DOMAIN DIFFERENCE= $7.00 to register a domain at a good registrar - $0.20 ICANN FEE = $6.80 savings per domain
  • TO BREAK EVEN= BASE FIRST YEAR FEE / PER DOMAIN DIFFERENCE = $8000 / $6.80 = ~1,177 domains
but unfortunately, I was incorrect in that you ALSO have to pay Verisign (who owns the .COM TLD) a hefty fee per domain.

So once you become an accredited ICANN register, you have to hook your system up to Verisign, who charges an additional (to the $0.20 ICANN fee) $6.42 per domain. Even worse is that they require you to pay all of their fees up front for the number of domains you plan to register on a yearly basis!!!!

Taking into account these new findings, it would actually take ~21,053 domains (with PER DOMAIN DIFFERENCE being adjusted to $7.00-$0.20-$6.42=$0.38) to break even the first year when becoming your own registrar (as opposed to going through another registrar), YIKES!

I've always personally recommend gkg.net as a registrar, but their registration prices recently took a major hike, like most registrars, due to Verisign raising their per domain fee. I may have to reevaluate registrars at some point because of this.

Core Dump Files
Not all OSs crash in the same way :-)

If you ever find a file named “core.#” when running Linux, where # is replaced by a number, it means something crashed at some point. Most of the time, you will probably just want to delete the file, but sometimes you may wonder what crashed. To do this, you use gdb (The GNU debugger), a very power tool, to analyze the core dump file.


Near the very bottom of the blob of outputted text after running this command, you should see a line that says “Core was generated by `...'.”. This tells you the command line of what crashed. To exit gdb, enter “quit”. You can also use gdb to find out what actually happened and troubleshoot/debug the problem, but that’s a very long and complex topic.

Recently, I started seeing hundreds of core dump files taking up gigabytes of space showing up in “/usr/local/cpanel/whostmgr/docroot/” on multiple of our web servers. According to several online sources, it seems cPanel (web hosting made easy!) likes to dump many, if not all, of its programs' core files into this directory. In our case, it has been “dnsadmin” doing the crashing. We’ve been having some pretty major DNS problems lately, this kind on the name server level, so I may have to rebuild our DNS cluster in the next few days. Joy.

Regular Expression Examples
Finding multiple domain’s name servers

Today I thought I’d give a demonstration on the use of regular expressions [reference page here]. Regular expressions are basically a simplified scripting language for finding and replacing complex text strings, and are implemented into much of today’s software which involve a lot of text editing. They are a fabulously handy tool for computer users and are especially useful for programmers. I believe RegExps actually originally gained their notoriety through the Perl programming language. I also recently heard that it is definite that the new version of C++ (C++0x) will have native library support for regular expressions, yay!

Since I posted yesterday on DNS stuff, and have the examples from it handy, I figured I’d use those :-).

Let’s say you had a group of .com domains and wanted to find out their name servers (I’ve had to do this when switching to new name servers to make sure all the domains we did not control at the registrar level had their name servers set to the new ones). For this example, we will use the following domains “castledragmire.com”, “riaboy.com”, “NonExistantDomainA.com”, and “dakusan.com”.

  • First, we’d need to have the list of the domains, for this example, one domain per line is used.
  • Next, we need to turn them into a bash (Linux) script to grab all the information we need.
    Replace: “^(.*)$
    With: “echo '!?$1?!'; host -t ns $1 a.gtld-servers.net | grep ' name server ';”
    Sample output: (The !? ?! stuff are markers for easier viewing and parsing)
    echo '!?castledragmire.com?!'; host -t ns castledragmire.com a.gtld-servers.net | grep ' name server ';
    echo '!?riaboy.com?!'; host -t ns riaboy.com a.gtld-servers.net | grep ' name server ';
    echo '!?NonExistantDomainA.com?!'; host -t ns NonExistantDomainA.com a.gtld-servers.net | grep ' name server ';
    echo '!?dakusan.com?!'; host -t ns dakusan.com a.gtld-servers.net | grep ' name server ';
  • Next, we run the script, and it would output the following:
    castledragmire.com name server ns3.deltaarc.com.
    castledragmire.com name server ns4.deltaarc.com.
    riaboy.com name server ns3.deltaarc.com.
    riaboy.com name server ns4.deltaarc.com.
    dakusan.com name server ns3.deltaarc.com.
    dakusan.com name server ns4.deltaarc.com.
  • Next, we would keep running the following regular expression until no more replacements are found.
    This would combine all domains with multiple name servers onto one line with name servers separated by spaces.
    Replace: “(.*?) name server (.*)\n\1 name server (.*)
    With: “$1 name server $2 $3
    It would output the following:
    castledragmire.com name server ns3.deltaarc.com. ns4.deltaarc.com.
    riaboy.com name server ns3.deltaarc.com. ns4.deltaarc.com.
    dakusan.com name server ns3.deltaarc.com. ns4.deltaarc.com.
  • The final regular expression would turn the output into a single line per domain, followed by its domain servers. The current extra line before the list of name servers is to help spot any domains that did not provide us with name servers.
    Replace: “!\?(.*?)\?!\n\1 name server (.*)
    With: “#$1 \t $2
    Which would output the final following data:
    #castledragmire.com ns3.deltaarc.com. ns4.deltaarc.com.
    #riaboy.com ns3.deltaarc.com. ns4.deltaarc.com.
    #dakusan.com ns3.deltaarc.com. ns4.deltaarc.com.
    This data could be directly pasted into Excel, which would put the first column as domains and second column as name servers).
Diagnosing DNS Problems
Digging until you find the root

Yesterday I wrote a bit about the DNS system being rather fussy, so I thought today I’d go a bit more into how DNS works, and some good tools for problem solving in this area.

First, some technical background on the subject is required.
  • A network is simply a group of computers hooked together to communicate with each other. In the old days, all networking was done through physical wires (called the medium), but nowadays much of it is done through wireless connections. Wired networking is still required for the fastest communications, and is especially important for major backbones (the super highly utilized lines that connect networks together across the world).
  • A LAN is a local network of all computers connected together in one physical location, whether it be a single room, a building, or a city. Technically, a LAN doesn’t have to be localized in one area, but it is preferred, and we will just assume it is so for arguments sake :-).
  • A WAN is a Wide (Area) Network that connects multiple LANs together. This is what the Internet is.
  • The way one computer finds another computer on a network is through its IP Address [hereby referred to as IPs in this post only]. There are other protocols, but this (TCP/IP) is by far the most widely utilized and is the true backbone of the Internet. IPs are like a house’s address (123 Fake Street, Theoretical City, Made Up Country). To explain it in a very simplified manner (this isn’t even remotely accurate, as networking is a complicated topic, but this is a good generalization), IPs have 4 sections of numbers ranging from 0-255 (1 byte). For example, is a (class 4) IP. Each number in that address is a broader location, so the “28” is like a street address, “32” is the street, “45” is the city, and “67” is the country. When you send a packet from your computer, it goes to your local (street) router which then passes it to the city router and so on until it reaches its destination. If you are in the same city as the final destination of the packet, then it wouldn’t have to go to the country level.
  • The final important part of networking (for this post) is the domain system (DNS) itself. A domain is a label for an IP Address, like calling “1600 Pennsylvania Avenue” as “The White House”. As an example, “www.castledragmire.com” just maps to my web server at “” (this is the current IP, it will change if the site is ever moved to a new server).

Next is a brief lesson on how DNS itself works:
  • The root DNS servers (a.root-servers.net through m.root-servers.net) point to the servers that hold top-level-domain information (.com, .org., .net, .jp, etc)
    Examples of these servers are as follows:
  • Next, these root name servers (like A.GTLD-SERVERS.NET through M.GTLD-SERVERS.NET for .com) hold two main pieces of information for ALL domains under their top-level-domain jurisdiction:
    • The registrar where the domain was registered
    • The name server(s) that are responsible for the domain
    Only registrars can talk to these root servers, so you have to go through the registrar to change the name server information.
  • The final lowest rung in the DNS hierarchy is name servers. Name servers hold all the actual addressing information for a domain and can be run by anyone. The 2 most important (or maybe relevant is a better word...) types of DNS records are:
    • A: There should be many of these, each pointing a domain or subdomain (castledragmire.com, www.castledragmire.com, info.castledragmire.com, ...) to a specific IP address (version 4)
    • SOA: Start of Authority - There is only one of these records per domain, and it specifies authoritative information including the primary name server, the domain administrator’s email, the domain serial number, and several timeout values relating to refreshing domain information.

Now that we have all the basics down, on to the actual reason for this post. It’s really a nuisance trying to explain to people why their domain isn’t working, or is pointing to the wrong place. So here’s why it happens!

Back in the old days, it often took days for DNS propagation to happen after you made changes at your registrar or elsewhere, but fortunately, this problem is of the past. The reason for this is that ISPs and/or routers cached domain lookups and only refreshed them according to the metrics in the SOA record mentioned above, as they were supposed to. This was done for network speed reasons, as I believe older OSs might not have cached domains (wild speculation), and ISPs didn’t want to look up the address for a domain every time it was requested. Now, though, I rarely see caching on any level except at the local computer; not only on the OS level, but even some programs cache domains, like FireFox.

So the answer for when a person is getting the wrong address for a domain, and you know it is set correctly, is usually to just reboot. Clearing the DNS cache works too (for the OS level), but explaining how to do that is harder than saying “just reboot” ^_^;.

To clear the DNS cache in XP, enter the following into your “run” menu or in the command prompt: “ipconfig /flushdns”. This does not ALWAYS work, but it should work.

If your domain is still resolving to the wrong address when you ping it after your DNS cache is cleared, the next step is to see what name servers are being used for the information. You can do a whois on your domain to get the information directly form the registrar who controls the domain, but be careful where you do this as you never know what people are doing with the information. For a quick and secure whois, you can use “whois” from your linux command line, which I have patched through to a web script here. This script gives both normal and extended information, FYI.

Whois just tells you the name servers that you SHOULD be contacting, it doesn’t mean these are the ones you are asking, as the root DNS servers may not have updated the information yet. This is where our command line programs come into play.

In XP, you can use “nslookup -query=hinfo DOMAINNAME” and “nslookup -query=soa DOMAINNAME” to get a domain’s name servers, and then “nslookup NAMESERVER DOMAINNAME” to get the IP the name server points too. For example: (Important information in the following examples are bolded and in white)

C:\>nslookup -query=hinfo castledragmire.com
Server:  dns-redirect-lb-01.texas.rr.com

        primary name server = ns3.deltaarc.com
        responsible mail addr = admins.deltaarc.net
        serial  = 2007022713
        refresh = 14400 (4 hours)
        retry   = 7200 (2 hours)
        expire  = 3600000 (41 days 16 hours)
        default TTL = 86400 (1 day)

C:\>nslookup -query=soa castledragmire.com
Server:  dns-redirect-lb-01.texas.rr.com

Non-authoritative answer:
        primary name server = ns3.deltaarc.com
        responsible mail addr = admins.deltaarc.net
        serial  = 2007022713
        refresh = 14400 (4 hours)
        retry   = 7200 (2 hours)
        expire  = 3600000 (41 days 16 hours)
        default TTL = 86400 (1 day)

castledragmire.com      nameserver = ns4.deltaarc.com
castledragmire.com      nameserver = ns3.deltaarc.com
ns3.deltaarc.com        internet address =

C:\>nslookup ns3.deltaarc.com castledragmire.com
Server:  ev1s-209-85-115-128.theplanet.com

Name:    ns3.deltaarc.com

Nslookup is also available in Linux, but Linux has a better tool for this, as nslookup itself doesn’t always seem to give the correct answers, for some reason. So I recommend you use dig if you have it or Linux available to you. So with dig, we just start at the root name servers and work our way up to the SOA name server to get the real information of where the domain is resolving to and why.

root@www [~]# dig @a.root-servers.net castledragmire.com

; <<>> DiG 9.2.4 <<>> @a.root-servers.net castledragmire.com
; (2 servers found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5587
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 14

;castledragmire.com.            IN      A

com.                    172800  IN      NS      H.GTLD-SERVERS.NET.
com.                    172800  IN      NS      I.GTLD-SERVERS.NET.
com.                    172800  IN      NS      J.GTLD-SERVERS.NET.
com.                    172800  IN      NS      K.GTLD-SERVERS.NET.
com.                    172800  IN      NS      L.GTLD-SERVERS.NET.
com.                    172800  IN      NS      M.GTLD-SERVERS.NET.
com.                    172800  IN      NS      A.GTLD-SERVERS.NET.
com.                    172800  IN      NS      B.GTLD-SERVERS.NET.
com.                    172800  IN      NS      C.GTLD-SERVERS.NET.
com.                    172800  IN      NS      D.GTLD-SERVERS.NET.
com.                    172800  IN      NS      E.GTLD-SERVERS.NET.
com.                    172800  IN      NS      F.GTLD-SERVERS.NET.
com.                    172800  IN      NS      G.GTLD-SERVERS.NET.

A.GTLD-SERVERS.NET.     172800  IN      A
A.GTLD-SERVERS.NET.     172800  IN      AAAA    2001:503:a83e::2:30
B.GTLD-SERVERS.NET.     172800  IN      A
B.GTLD-SERVERS.NET.     172800  IN      AAAA    2001:503:231d::2:30
C.GTLD-SERVERS.NET.     172800  IN      A
D.GTLD-SERVERS.NET.     172800  IN      A
E.GTLD-SERVERS.NET.     172800  IN      A
F.GTLD-SERVERS.NET.     172800  IN      A
G.GTLD-SERVERS.NET.     172800  IN      A
H.GTLD-SERVERS.NET.     172800  IN      A
I.GTLD-SERVERS.NET.     172800  IN      A
J.GTLD-SERVERS.NET.     172800  IN      A
K.GTLD-SERVERS.NET.     172800  IN      A
L.GTLD-SERVERS.NET.     172800  IN      A

;; Query time: 240 msec
;; WHEN: Sat Aug 23 04:15:28 2008
;; MSG SIZE  rcvd: 508

root@www [~]# dig @a.gtld-servers.net castledragmire.com

; <<>> DiG 9.2.4 <<>> @a.gtld-servers.net castledragmire.com
; (2 servers found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35586
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 2

;castledragmire.com.            IN      A

castledragmire.com.     172800  IN      NS      ns3.deltaarc.com.
castledragmire.com.     172800  IN      NS      ns4.deltaarc.com.

ns3.deltaarc.com.       172800  IN      A
ns4.deltaarc.com.       172800  IN      A

;; Query time: 58 msec
;; WHEN: Sat Aug 23 04:15:42 2008
;; MSG SIZE  rcvd: 113

root@www [~]# dig @ns3.deltaarc.com castledragmire.com

; <<>> DiG 9.2.4 <<>> @ns3.deltaarc.com castledragmire.com
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26198
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;castledragmire.com.            IN      A

castledragmire.com.     14400   IN      A

castledragmire.com.     14400   IN      NS      ns4.deltaarc.com.
castledragmire.com.     14400   IN      NS      ns3.deltaarc.com.

;; Query time: 1 msec
;; WHEN: Sat Aug 23 04:15:52 2008
;; MSG SIZE  rcvd: 97

Linux also has the “host” command, but I prefer and recommend “dig”.

And that’s how you diagnose DNS problems! :-). For reference, two common DNS configuration problems are not having your SOA and NS records properly set for the domain on your name server.

I also went ahead and added dig to the “Useful Bash commands and scripts” post.

Windows Hosts File
When DNS decides to be finicky

Another of my favorite XP hacks is modifying domain addresses through XP’s Hosts file. You can remap where a domain points on your local computer by adding an IP address followed by a domain in the “c:\windows\system32\drivers\etc\hosts” file.

Domain names are locally controlled, looked up, and cached on your computer at the OS level, so there are simple hacks like this for other OSs too.

I often utilize this solution as a server admin who controls a lot of domains (Over 100, and I control most of them at the registrar level too ^_^). The domain system itself across the web is incredibly fastidious and prone to problems if not perfectly configured, so this hack is a wonderful time saver and diagnostic tool until things resolve and work properly.

Domain system problems
Alas at insecure systems that are important to us all

I’ve been waiting to hear this kind of news for years: Domains May Disappear After Search. I’ve often told people for this kind of reason to watch where they are registering domains, as I believe some registrars like register.com are not very scrupulous and would do this exact kind of thing. I personally use GKG for all my domain registration needs, though they have recently ticked me off with a policy I recently ran into in which you can’t modify any information on a domain 6 months after renewing an expired domain with a credit card. Their tech support also isn’t very good, but they have fair prices and excellent domain management interfaces.

Another huge domain problem is “domain tasting” in which domains can be registered and then refunded within a five day grace period. Unethical people will use this to register expired domains and keep them if they get enough hits. After all, domains only really cost 25 cents to register if you are an accredited ICANN (Internet Corporation for Assigned Names and Numbers) registrar, which cost something like $3000 to obtain. This is a big problem if anyone lets their domain expire. Fortunately, some services, like GKG, give you a grace period to reregister your domain after it expires before others can try to claim it.