11/6/2023 0 Comments Filter dns query wireshark![]() ![]() You are encouraged to play around with the form of this command to both understand it better and see what else you can get from the data. sort -nr | head - Output the entries with the most duplicates.sort | uniq -c - Remove and count duplicates.rev - Reverse the string again to bring it back to normal.You may have to try a few different domains before you find one that isn’t cached. It’s highly likely that your name server of choice already has an answer cached for. We’ll make heavy use of dig’s +norecurse option to prevent the background requests that name servers will typically make on your behalf. The client asks the recursive name server for ’s A record.They use the DNS results to both direct you to the closest geographical server and load balance so that no one server gets overloaded. This is because large websites like Google will host their content on many different servers. Note: You may receive different IP addresses when you run these same commands. We will use the dig command to simulate what is going on behind the scenes. Let’s go through the above sequence diagram and understand each step that is happening. Exercise: Follow the Sequence of a DNS Query Defenders can use this behavior to detect C2 traffic over DNS. ![]() In practice, this means the more requests an attacker sends out the more unique subdomains they need to use. To get around this, attackers ensure that they never make identical requests. A name server will use its cache if it has answered an identical request recently. Second, since a name server will cache results for subsequent requests attackers need to prevent caching in order to get the NS to communicate out with their C2 server. Attackers exploit this trust to communicate out of restrictive networks. Since DNS is so critical to normal network operations most networks will implicitly trust whichever recursive NS is configured with DHCP. These two points are both very important when considering DNS as a C2 channel.įirst, the Recursive NS is effectively acting to proxy traffic between the client and the remote name servers. Note right of Recursive NS: answer in cache So the next time you or any other client makes the same DNS query the NS will answer from its local cache instead of querying other name servers. Once it has performed this work once it will cache the answer for a period of time. All these requests would quickly overwhelm servers if they were done every time so the above sequence of events only happens if your Recursive NS doesn’t already know an answer. For instance, you may choose to use one of the servers provided by large companies such as Cloudflare (1.1.1.1) or Google (8.8.8.8).īehind the scenes, your Recursive NS is doing a lot of work for you. Though you can also set your DNS servers manually. This name server is usually given to your computer when it connects to a network through DHCP. The Client (your computer) only talks with its configured Recursive Name Server (NS). Recursive NS->Google NS: A for ?Įverything going on might not yet be clear, but there are two things to notice first. DNS based C2 is different as the communication utilizes the DNS infrastructure to communicate instead.Ī normal DNS request for goes like this: This makes it easier to detect and track down. Many command & control (C2) channels communicate directly with an attacker-controlled system. Leverage frequency analysis to identify systems using DNS for C2. ![]()
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