Notice the sshdns bit? The tunnel daemon will only answer with tunnel replies when this is set. You can change the name on the server with the -f option. Of course the access to the box behind the tunnel is protected by the usual SSH mechanisms. Notice the use of localhost? That localhost refers to the tunnel server of course, because when SSH arrives at the end of the tunnel it is localhost already. Run the dnstunnelc script without any arguments to learn about a few more options.
PS: Circumventing access restrictions with this method might be illegal depending on where and what for you use it. Use it at your own risk. I googled for implementations and came across two. I spent some hours to clean up the OzimanDNS scripts by doing the following things: fixed code indention. Here's what you need: The tarball above. Ask at dnstunnel. It can not already run an external DNS service. Perl, a bunch of Perl modules, screen , SSH.
Some Unix and DNS knowledge helps as well. How is internet use browsing, apps, chat etc so reliable then? Figure 1 below illustrates the basic process of how DNS operates: the client sends a query string for example, mail.
Once a name is resolved to an IP caching also helps: the resolved name-to-IP is typically cached on the local system and possibly on intermediate DNS servers for a period of time. Of course, once the IP address of the remote service is known, applications can use that information to enable other TCP-based protocols, such as HTTP, to do their actual work, for example ensuring internet cat GIFs can be reliably shared with your colleagues.
If you want to go deep on how DNS works — all the way from you typing keys to spell the domain name you want to browse — then please read this article. Just as when you browse the internet, whether pivoting from a search engine result or directly accessing a website URL, your DNS queries also leave a trace. How much of a trace depends on the systems and processes involved along the way, from the query leaving the operating system, to receiving the resultant IP address.
In this section we will describe how command and control C2 beacons can operate over DNS, and how data exfiltration and infiltration is possible. A C2 channel often serves two purposes for the adversary. You could consider the basic DNS operation, as shown in Figure 1 above, as an example of a heartbeat.
What becomes difficult is distinguishing between multiple victims that are infected with the implant. Queries like these still act as a heartbeat indicating to the adversary their payload is still active, however they also provide some basic meta-data about the victim and, importantly, ways to uniquely identify one victim from another.
Usernames and hostnames may not always be unique, and some IPs could be duplicated across multiple networks using Network Address Translation NAT , however systems do have Universal Unique Identifiers UUIDs or other properties, that when combined could create a unique identifier for a given host or victim.
Some of the meta-data from the compromised host could be sent as plaintext but might appear more suspicious at first glance to anyone seeing such strings in a DNS query. In many cases the data will contain characters not supported by DNS, in which case encoding will be required. In some ways, a log like this, or perhaps a small database containing the decoded records from them, could be compared to the more snazzy-looking botnet control panels that allow the botnet herder to control their zombie victim systems.
So, what else could be sent up in DNS queries? A way for getting around the latter constraint could be to send multiple A record messages and have them stitched together somehow on the server-side. Complications would arise however in dropped or missing datagrams. Depending on the amount of data to transmit — every PDF on the system, for example — may take an age, and look hugely suspicious to network administrators. In contrast, infiltration of data whether it be code, commands, or a binary file to drop to disk and execute could be much easier, especially using the DNS type of TXT as opposed to host record type A.
TXT types were designed to provide descriptive text, such as service details, contact names, phone numbers, etc in response to TXT DNS queries for domain names. Guess what looks likes text? Baseencoded non-text data!
Figure 4 below shows the identical query being sent to the malicious site as in Figure 2, however, the type is now TXT on both the request and response, and the response data contains the first or so characters of an encoded binary executable file that could be executed by the client malware. Again, using the logs, the adversary would be able to know which client asked for the payload, and that the payload was sent who knows if it actually arrived….
Accessing the internet via restricted networks can be a pain. But so can be securing a network and putting those restrictions in place.
Seeing this technique in action can help you understand how unauthorized users could get around your security measures and use less monitored channels for communication e.
I always wanted to set up something like this since I first read about the technique. Today, I finally did and want to document the process. For the whole thing to work, we need control over a domain and be able to edit the zone file. Note that due to these limitations, the domain and subdomain should be as short as possible to leave as much as possible room for the actual data.
Most of the work in this setup will be done by a tool called Iodine by kyro. It has a server and client component and it is recommended to run the same version on both ends. I will use the latest master, which currently points to commit 27e5d6f. Iodine will take care of sending client and answering server the DNS queries, plus handle fragmentation, compression, encoding, record type to use, etc.
We essentially only need to take care of starting both the server and the client tool. I used an Ubuntu 18 server and Kali Linux As mentioned above, the version of Iodine we are going to compile is from commit 27e5d6f.
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