Securing DNS against DDOS Amplification Attacks

When you run a DNS server on your dedicated server, it will be the target DNS amplification attacks.  To prevent these attacks from succeeding and using up your bandwidth (which you will pay for), you need to configure your DNS server not to answer recursive queries.

Check if your server is vulnerable

You can send a DNS query to your server, e.g. “thatserver.dedicated.com” using dig or nslookup.

dig @thatserver.dedicated.com www.isc.org

Alternatively:

nslookup
> server thatserver.dedicated.com
> isc.org
Non-authoritative answer:
Name:    isc.org
Address: 1.1.1.1

If you receive an answer that includes an answer of the IP address of www.isc.org, then your server is vulnerable, because it did the work of finding out the answer and presenting it to you.

Simple solutions

Often enough, if you are running a DNS server, you probably don’t need it.  Turn it off: stop the service, remove the software.

You only need a DNS server on your system for one of the following purposes:

  • Your DNS server is configured with zone files for domains that you are hosting, and you have asked a DNS registrar (e.g. enom.com) to point domains to your DNS server.  You will recognise these terms if you have done this.  You do not need DNS recursion for this function.
  • You are unhappy with the quality of the DNS resolver you are using and would rather implement this function yourself.  If this is why you have a DNS server, you do not need to answer external queries.  You can protect the server with a firewall.
  • You are providing zone files for a private domain, e.g. as some part of Active Directory.  In this case you can limit your responses to only those systems that have an interest in that private domain, i.e. members of the Active Directory system.
  • You are competing with OpenDNS and Google’s DNS recursor.  If you are doing this, you must implement appropriate rate limits, which is an exercise to the reader.

Secure named (bind) on Linux

Add this to the “options” section of /etc/named.conf :

    recursion no;
    additional-from-auth no;
    additional-from-cache no;

Then restart named so that it will use the new secure options:

    /etc/init.d/named restart

For detailed information see http://www.cymru.com/Documents/secure-bind-template.html

Secure Microsoft DNS server

If you have installed or enabled Exchange then you have implicitly turned on DNS, which by default runs as a recursive service and can be horribly attacked.  Usually you can just firewall the DNS service.

Run this command:

    dnscmd . /Config /NoRecursion 1

Or follow this procedure:

    Start | Administrative Tools | DNS (DNS manager)
    Right click DNS server | 
        Properties | 
        Advanced | 
        Server options | 
        Disable recursion -> Yes, OK

Unfortunately, it is not possible to prevent the Microsoft DNS server from replying with cached values, so your non-recursive DNS server will provide a small amount of useful traffic amplification for attackers.  Where possible, add a firewall rule that blocks incoming traffic from unauthorised clients towards port 53/UDP (and port 53/TCP for good measure).

IPtables rules for Linux

If your DNS server is used only by the machine on which you are running it, you can block external queries as follows:

iptables -A  INPUT -p udp -m udp --dport 53 -i ! lo -j DROP

These iptables firewall rules will to prohibit excessive ANY queries to a non-recursive DNS:

iptables -A  INPUT -p udp -m udp --dport 53 \
   -m string --hex-string "|0000ff0001|"  --algo bm --from 48 --to 65535 \
   -m recent --set --name dnsanyquery  --rsource
iptables -A INPUT -p udp -m udp --dport 53 \
    -m string --hex-string  "|0000ff0001|" --algo bm --from 48 --to 65535 \
   -m recent --rcheck  --seconds 60 --hitcount 5 --name dnsanyquery --rsource \
   -j DROP

If you for some reason have to run an open DNS resolver, you can limit rate limit the rate at which you will accept queries:

iptables -A INPUT -p udp --dport 53 -m hashlimit \
--hashlimit-name DNS --hashlimit-above 20/second --hashlimit-mode srcip \
--hashlimit-burst 100 --hashlimit-srcmask 28 -j DROP

If you know what the above means you can install these rules in your system.

Reference information

You can read more about this here: