Opening the Door for a Knock: Creating a Custom DShield Listener, (Thu, Dec 29th)

honey-pot
File:564-honey-pot.svg

There are a variety of services listening for connections on DShield honeypots [1]. Different systems scanning the internet can connect to these listening services due to exceptions in the firewall. Any attempted connections blocked by the firewall are logged and can be analyzed later. This can be useful to see TCP port connection attempts, but it’s usefulness is limited. Without the ability to complete the SYN, SYN-ACK, ACK handshake process other protocol data may not be sent.

There are a few easy steps to create a listener to try and collect some additional data:

  1. Modify firewall to open port
  2. Start listener with socat
  3. Create cron job to start listener script on boot

These steps will be outlined for opening a listener on TCP 6889.

 

Modify Firewall to Open Port

Before opening the firewall port, it’s probably a good idea to check what is already listening on the honeypot. If there is already a service listening on the port that will be exposed, this change may allow remote internet connections to a service that’s not desired. To check for listening services, I ran sudo netstat -tulpn | grep LISTEN.


Figure 1: Listing applications listening for network connections

TCP 6889 is already listening in my case since the socat listening command is already running.  

The firewall used on the honeypot is nftables [2]. To add the firewall rule, the nftables instructions could be followed to allow this port by running the following command:

sudo nft add rule ip filter INPUT iifname "eth0" tcp dport 6889 counter accept

This would open the port immediately, but only for a limited time until the honeypot rebooted. Upon reboot, the rules would be flushed and read from the file /etc/network/ruleset.nft. To keep this exception persistent, the ruleset must be modified. After modification of this file, the honeypot would need to be rebooted or the nftables service restarted by sudo systemctl restart nftables.

To modify the file /etc/network/ruleset.nft, you can use your text editor of choice, save the changes and then restart the honeypot or the nftables service.


Figure 2: Firewall rules for honeypot in ruleset.nft

In this case, I added the rule after other honeypot listener exceptions to keep everything organized and consistent. 

 

Start Listener with socat

Netcat is one very common utility that is used to connect to remote services and can also act as a listener [3]. I went with socat in this example based on the recommendation from Guy Bruneau. Socat can communicate over a variety of additional protocols and has some handy output options that help with data collection [4]. 

To start the listener, it’s as easy as running one command:

socat -d -d -d -v -u TCP4-LISTEN:6889,reuseaddr,fork OPEN:,creat,append

This will listen for connections on port 6889 and also redirect any input to the specified log file. In this case the logs are stored in the “logs/custom” directory. There is also some additional output for stderr (standard error). 

socat -d -d -d -v -u TCP4-LISTEN:6889,reuseaddr,fork OPEN:/logs/custom/hpot.log,creat,append 2>> /logs/custom/socat_6889_verbose.txt


Figure 3: Socat command for listening on port 6889 with local log output

Looking at example data received, here are some examples of the log data. 

Log Data


Figure 4: Socat logged transfer data

Standard Error (verbose) Log Data

2022/12/17 03:10:13 socat[1310] I accept(5, {2, AF=2 87.236.176.139:39125}, 16) -> 6
2022/12/17 03:10:13 socat[1310] N accepting connection from AF=2 87.236.176.139:39125 on AF=2 192.168.68.178:6889
2022/12/17 03:10:13 socat[1310] I permitting connection from AF=2 87.236.176.139:39125
2022/12/17 03:10:13 socat[1310] N forked off child process 5772
2022/12/17 03:10:13 socat[1310] I close(6)
2022/12/17 03:10:13 socat[1310] I still listening
2022/12/17 03:10:13 socat[1310] N listening on AF=2 0.0.0.0:6889
2022/12/17 03:10:13 socat[5772] I just born: child process 5772
2022/12/17 03:10:13 socat[5772] I close(4)
2022/12/17 03:10:13 socat[5772] I close(3)
2022/12/17 03:10:13 socat[5772] I just born: child process 5772
2022/12/17 03:10:13 socat[5772] I close(5)
2022/12/17 03:10:13 socat[5772] I setting option "o-create" to 1
2022/12/17 03:10:13 socat[5772] I setting option "append" to 1
2022/12/17 03:10:13 socat[5772] N opening regular file "/logs/custom/hpot.log" for writing
2022/12/17 03:10:13 socat[5772] I open("/logs/custom/hpot.log", 02101, 0666) -> 5
2022/12/17 03:10:13 socat[5772] I resolved and opened all sock addresses
2022/12/17 03:10:13 socat[5772] N starting data transfer loop with FDs [6,6] and [5,5]
> 2022/12/17 03:10:16.749080 length=198 from=0 to=197
GET / HTTP/1.1r
Host: :6889r
User-Agent: Mozilla/5.0 (compatible; InternetMeasurement/1.0; +https://internet-measurement.com/)r
Connection: closer
Accept: */*r
Accept-Encoding: gzipr
r
2022/12/17 03:10:16 socat[5772] I transferred 198 bytes from 6 to 5
2022/12/17 03:10:36 socat[5772] N socket 1 (fd 6) is at EOF
2022/12/17 03:10:36 socat[5772] I shutdown(6, 2)
2022/12/17 03:10:36 socat[5772] N exiting with status 0
2022/12/17 03:10:36 socat[1310] N childdied(): handling signal 17
2022/12/17 03:10:36 socat[1310] I childdied(signum=17)
2022/12/17 03:10:36 socat[1310] I childdied(17): cannot identify child 5772
2022/12/17 03:10:36 socat[1310] I waitpid(): child 5772 exited with status 0
2022/12/17 03:10:36 socat[1310] I waitpid(-1, {}, WNOHANG): No child processes
2022/12/17 03:10:36 socat[1310] I childdied() finished


2022/12/17 03:11:25 socat[1310] I accept(5, {2, AF=2 87.236.176.220:33827}, 16) -> 6
2022/12/17 03:11:25 socat[1310] N accepting connection from AF=2 87.236.176.220:33827 on AF=2 192.168.68.178:6889
2022/12/17 03:11:25 socat[1310] I permitting connection from AF=2 87.236.176.220:33827
2022/12/17 03:11:25 socat[1310] N forked off child process 5804
2022/12/17 03:11:25 socat[1310] I close(6)
2022/12/17 03:11:25 socat[1310] I still listening
2022/12/17 03:11:25 socat[1310] N listening on AF=2 0.0.0.0:6889
2022/12/17 03:11:25 socat[5804] I just born: child process 5804
2022/12/17 03:11:25 socat[5804] I close(4)
2022/12/17 03:11:25 socat[5804] I close(3)
2022/12/17 03:11:25 socat[5804] I just born: child process 5804
2022/12/17 03:11:25 socat[5804] I close(5)
2022/12/17 03:11:25 socat[5804] I setting option "o-create" to 1
2022/12/17 03:11:25 socat[5804] I setting option "append" to 1
2022/12/17 03:11:25 socat[5804] N opening regular file "/logs/custom/hpot.log" for writing
2022/12/17 03:11:25 socat[5804] I open("/logs/custom/hpot.log", 02101, 0666) -> 5
2022/12/17 03:11:25 socat[5804] I resolved and opened all sock addresses
2022/12/17 03:11:25 socat[5804] N starting data transfer loop with FDs [6,6] and [5,5]
> 2022/12/17 03:11:28.944065 length=44 from=0 to=43
...,'......Cookie: mstshash=eltonsr
..b.....2022/12/17 03:11:28 socat[5804] I transferred 44 bytes from 6 to 5
2022/12/17 03:11:31 socat[5804] N socket 1 (fd 6) is at EOF
2022/12/17 03:11:31 socat[5804] I shutdown(6, 2)
2022/12/17 03:11:31 socat[5804] N exiting with status 0
2022/12/17 03:11:31 socat[1310] N childdied(): handling signal 17
2022/12/17 03:11:31 socat[1310] I childdied(signum=17)
2022/12/17 03:11:31 socat[1310] I childdied(17): cannot identify child 5804
2022/12/17 03:11:31 socat[1310] I waitpid(): child 5804 exited with status 0
2022/12/17 03:11:31 socat[1310] I waitpid(-1, {}, WNOHANG): No child processes
2022/12/17 03:11:31 socat[1310] I childdied() finished

This data could still be improved to help with analysis, but it’s much more than we had before.

 

Create Cron Job to Start Listener Script on Boot

If we wanted this listener to always be active, another addition is required so that it starts on boot. To do this, crontab can be modified to include this socat command. For this, I modified the crontab for the primary user account and added the following:

@reboot sleep 80 && socat -d -d -d -v -u TCP4-LISTEN:6889,reuseaddr,fork OPEN:/logs/custom/hpot.log,creat,append 2>> /logs/custom/socat_6889_verbose.txt

This will start the process 80 seconds after reboot. This pause is often needed while other system processes get started. 

This is one of many ways to gather some additional data from connection attempts. A great example of improving on this is already on the honeypot itself: Cowrie [5]. Emulating responses to incoming connections can bring things to the next level. 

[1] https://isc.sans.edu/honeypot.html
[2] https://wiki.nftables.org/wiki-nftables/index.php/Simple_rule_management
[3] https://linux.die.net/man/1/nc
[4] https://linux.die.net/man/1/socat
[5] https://github.com/cowrie/cowrie


Jesse La Grew
Handler

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

Reposted from SANS. View original.

Adam Smith