Among penetration testers, X.28 dialup is the most common way to access PSDN networks. They use this method because penetration testing companies may not have direct X.25 access, meaning an X.25 card connected to a system (Linux, Cisco, Sun Solaris, or other) and an active X.25 subscription to their local X.25 carrier. To perform an X.28 connection all you need is
• A machine equipped with a (good) terminal emulator
• a telephone line
• The X.28 dialup phone number
NUI stands for Network User Identifier. This is the login access for the X.25 network. The NUI will identify the X.25 calls, and every call made will be billed to the NUI, resulting in a monthly bill from your X.28 NUI subscription.
To connect to the X.28 PAD, you need to start a terminal emulator and configure it to E71 (or 8N1 with the strip high-bit function activated). Minicom will satisfy this very basic need.
Then you will dial the X.28 dialup phone number that your NUI will work on, for example, atdt0651558934 Connected 24CC/MPN5
ACP Roma Colombo 28
Usually, the X.28 PAD prompt is represented by an asterisk in most countries, even if on other X.25 networks, it varies (for example, @ on SprintNet).
Then you insert your NUI and a - followed by the NUA (or the X.121 address) you want to establish a call with, without spaces. Also, in this case, the syntax may vary. For these examples we've used the most common one among worldwide PADs.
ACP Roma Colombo 28
As you can see, the NUI will not appear on screen for obvious security reasons. ACP:COM means that the request has been satisfied and the link established.
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