You could program in a custom message on each host, or grab a variable from the environment, or you could also have SBD add the “tag.” The only other consideration to be aware of concerns SBD's behavior after a file transfer is completed. You want each system to log a status message so you know the process completed successfully. Take for example, a scenario where you have two or more systems running a custom script you created. The original intent seems to be to facilitate SBD as a primitive chat client however, it could also be useful to provide a sort of data “tag” for the server to log.

Another interesting option is –P, which is used to specify a prefix for incoming data. Similar to cryptcat, the –k option is used to specify the shared secret key. SBD takes the same approach except encryption can be disabled completely via the –c option (– c off disables encryption, –c on is the default). With cryptcat, encryption is enabled by default, even if you do not specify a shared secret key. The delay can be set to zero seconds which provides identical behavior to the Windows Netcat –L option. The Windows version of Netcat provides the same functionality via the –L option, while the original *nix version offers no such option. You can also configure a delay in seconds before re-spawning, this delay could be useful to help slow down any attempts to brute force your listening process.

The –r option allows you to tell SBD to re-spawn after a client disconnect instead of the default Netcat behavior, which is to shut down the server. Of course, this trick is not going to fool a statefull firewall, but having this increased control never hurts. This feature could allow you to get through a firewall by making the SBD session look like a reply to a friendly protocol such as DNS. SBD allows you to specify the source port via the –p option if you are running in client mode (in server mode, –p specifies the listening port as usual). While the basic operation of SBD remains much the same as the other Netcat-like tools, there are a couple of additional options that could be useful for transferring files.

You can put a wrapper script around any version of Netcat to cause it to restart on *nix, but with SBD providing that functionality built-in, there is little need for such effort. If that is not enough reason, it offers encryption like cryptcat, and a respawn option, providing the –L functionality for both Windows and *nix hosts. This feature alone makes SBD a good candidate to settle on as your Netcat-like utility of choice. SBD is available for both Windows and *nix, with pre-compiled binaries included in the single g-zipped file from. Shadowinteger's Backdoor (SBD) is another Netcat variant, with all of the features of the original Netcat plus some new ones.

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