Episode 5 – Security Investigation Series – DNS Reflection Attacks


One of the most popular attacks in the Internet today is the DNS Reflection Attacks resulting in a Distributed DoS. One of the major DoS mitigation vendors, Prolexic released a Report for 2013 saying that, Distributed DoS Attacks have increased by over 20% and bandwidth utilizations have seen never before levels. Spamhaus, Network Solutions and several other companies this year have been hit by DNS Reflection attacks. The attackers specifically targeted organizations in order to hurt and humiliate them. Distributed DoS protection service providers are slowly gaining prominence. At this time, we at infosecnirvana.com feel that it is important to understand the mechanics of such attacks and how they can be detected and responded from an Enterprise Security standpoint. In this Security Investigation Series post, we talk about the usual suspects – What is DNS Reflection Attack? How do we detect them? & How do we prevent them?.

Understanding DNS Reflection Attacks: As we all know, DNS is one of the components of the internet that serves as a Directory Assistance service akin to the Yellow pages. The only difference is that DNS gives IP numbers when requested for Domain Names. People who understand how DNS Works, would definitely have heard about “Recursive DNS Querying”. Essentially a Recursive Query is like following the trail of bread crumbs till you solve the puzzle. Every DNS Server pushes up the query recursively till it gets the response for the DNS Query. This also essentially means that the Original DNS Query will be very small in size, however, the recursive query/responses will be huge in size. Since DNS uses UDP, Volume based DOS Attacks are possible by using this Recursive Querying capability. This is the basic premise of a DNS Reflection attack. By making several thousands of spoofed DNS queries that result in recursion, an amplified DNS Response can be directed to the spoofed address. To understand the attack pattern that typically a Distributed DoS Attacker would follow, lets list down the attack pattern:

  1. Attacker first compromises an Authoritative Name Server.
  2. Attacker then creates a large TXT RR (Large sized Resource Record).
  3. Attacker Spoofs the Target IP Range
  4. Attacker Sends DNS Query (with Target IP Range As Client IP) to a number of Open DNS Servers (close to 5 million Open DNS Servers allow recursive querying) in such a way that Recursive Query happens and they retrieve the TXT RR.
  5. In order to achieve Amplification, the attacker then uses Several compromised Zombies to send our DNS Requests for the larger resource record (RR).
  6. All the Responses go to the Spoofed IP – Typically the Organisation the attacker wants to flood with DNS responses thereby causing a potential Distributed DOS scenario because of Bandwidth Consumption.
  7. Typical rates of amplification achieved are – For every 100 Mb/s of request Traffic, reply traffic can be up to 10 Gb/s

Imaging several gigabytes of DNS Packets hitting your perimeter and choking the bandwidth. This is what a DNS Amplification Attack or a DNS reflection attack can do.

Detection of DNS Reflection Attacks: Now that we understand the Anatomy of the attack, lets see how we can detect them. DNS Reflection attacks have IP Spoofing as the basic premise to redirect DNS Query Responses to a Target site. However, if the target site was able to detect that it never sent a DNS Query to elicit a DNS response, these attacks can be mitigated or stopped right away. However, detecting this is easier said then done. This can be done using a combination of Network Traffic monitoring, IDS/IPS & SIEM Technologies. Using a Network Monitoring tool like IPTraf, Netwatch,Netramet, we can gather DNS Statistics. Use a custom Script or a custom parser (SIEM parlance), we can normalize the Statistics into a more simpler State table (This is similar to the Firewall State table but mainly for UDP, called a Pseudo-state table). The table should contain a minimum of the following parameters:

  • Transaction ID (This is unique DNS Query ID. The Response received from the Authoritative DNS Server will have the same Transaction ID). 
  • Source IP Address of DNS Query initiator client
  • Source Port of the initiator client
  • Destination Address to which the Query is directed to
  • Destination port the Query is directed to

If there is a Query with a Transaction ID (say 0xcefd) & a corresponding response with the same transaction ID, we can safely say that the DNS Query & Response pair is legitimate. However, if there is only a Query or a Response, they are categorized as an “Orphan entry”. Orphan entries can be two types: 1. Only Query packet seen but no response is seen. & 2. Only Response packet is seen but no query packet is seen. Only Query & no response in my opinion is less harmful and can be safely ignored. But keep in mind, if this number is too high, it means something is wrong with your organizations DNS client or server and could even potential indicate a compromised asset. Only Response & No Query is the most likely candidate for DNS Reflection attacks. However, we need to also keep in mind that for smaller volumes these can lead to false positives as well. Some of the reasons for this is infrastructure logging fidelity (since DNS is UDP), bad routing of outbound traffic etc. In essence, our focus is on DNS Responses without preceding Queries. Now that we know what to look for, we need to start filtering all the remaining noise. This is where Range Thresholds are important. A small volume may not warrant attention, however, if there is an exponential increase in volume, it is really important to take actions aimed at mitigation. This is where an SIEM system comes in handy, Giving you a trend analysis based on the data collected over a time period. Let me just show you how this can be done in ArcSight SIEM. Data from the Traffic monitors can be parsed using a File Reader Custom Parser (using ArcSight Flex Connector). This parser can parse the required data fields as mentioned above from the Network logs and can map them to a native Event field schema. Then we use a Rule to populate all the Queries in an (First) Active list called DNS Query List. Similarly we use a Rule to populate all the Responses in another (Second) Active List called DNS Response List. You can then have a separate rule to populate a Third Active List for Orphan entries. This list will be a count based list giving you how many entries are there without Responses. A monitoring Dashboard can be then created for detecting Trend patterns  due to an increase in the Orphan entry list which would typically indicate a DNS Reflection Attack. I am sure there are couple of other ways we can get this done in ArcSight, but I am not going to go into those details in this post. But basically you should be able to get an idea of how this can be done. As far as other SIEM vendors are concerned, QRadar SIEM has some capabilities to do this, however I don’t think McAfee Nitro & Symantec SIM have this capability (Readers, let me know what you think about these SIEM Tools and whether this logic can be implemented or not).

Mitigation Methods for DNS Reflection Attacks: Once the suspicious list shows an increased percentage of Orphaned Response entries, there is a high likelihood that the organization is targeted. Unless you have partnered with a Distributed  DoS protection service provider, fending off such attacks would be a challenging prospect. However, there are ways and means to mitigate the attack from getting bigger.To start of with, lets look at the perimeter defences and how we can leverage them to mitigate the attack. Organizations generally will have a core router, an IPS/IDS, an Authoritative name server and a firewall.

On the core router we can enable URPF (Unicast Reverse Path Forwarding) to ensure that spoofing based attacks are controlled.

On the IDS side, we can enable rate limiting signatures for DNS packets to detect and probably drop packets thereby limiting the success of DNS Amplification based attacks. We can also enable Geography based filtering to ensure that the attacks remain controlled within a region.

On the DNS server side, we can limit the recursion so that our DNS servers don’t become part of the amplification attack. There are some experimental features in DNS on Rate Limiting on DNS Responses, however it is not commercialized and not many people have this feature tested. There is a great paper on the technical details of this feature. Please visit http://ss.vix.su/~vixie/isc-tn-2012-1.txt to take a look at it.

Finally, if you have money, spend on Distributed DoS protection services from Cloudflare, Imperva, Akamai, Prolexic etc. who can provide you with some of the rate limiting and geo based filtering based protection.

A combination of all these controls will ensure that the attacks are mitigated to a great extent. However, if you bandwidth is choked, you would still face a service disruption and slow website loading, but considering the defences, this would a good start.

What do you think would be your strategy to combat DNS Reflector attacks? Would you do it yourself or would you play with the big boys shelling big bucks? Chime on.

Until then….Detect & Respond.