The following is a step-by-step roadmap to use when dealing with X.25 error codes:
1. Call the Network User Address (NUA) on the X.25 network from an X.25 or X.28 connection to your national X.25 carrier1 and get the results codes. As previously stated, the X.25 answer should be "call connected" or "COM" if the system connected to the X.25 network (and the DTE modem) is alive.
2. If the call isn't successfully connected, you may find one of the following answer codes, as detailed in Section 4.3:
You may encounter this problem when performing X.25 security testing from one continent to the other (e.g., from Europe to Africa, the Middle East, or Asia and from North America to South America) and if you are working on remote X.25 networks not well linked to the major X.25 international switches.
1 For X.25 data calls, please note that X.25 calls are usually charged by the national X.25 carrier; normally the X.25 operator may also ask for a yearly based contract to obtain a leased or dial-up X.25 public access.
X.25 spoofing may be applicable for DTE problems. • RPE
3. If you get a CLEAR DTE answer message when calling the X.25 target(s), you can scan the X.25 address by adding a one- or two-digit subaddress to the target NUA.
4. If you get a CLEAR RPE answer message when calling the X.25 target(s), you can scan the X.25 address using a brute-force attack for alphanumeric extensions with a three-character base.
5. When you find a live system, verify that vendor-default, easily guessable, or insecure accounts do not exist; brute-force the target in the case of critical systems (see Chapter 5).
6. If you find a vulnerable bridge system, exploit the weakness and map the networks used and the system links.
7. If you find a direct X.25 access available from hosts that have an X.25 trace capability (Sun Solaris, Linux, VMS, OpenVMS, Motorola Codex PAD, etc.), execute at least three different X.25 sniffing actions on various X.25 active calls to determine the data's privacy level and the presence of encryption technologies.
8. If you find a direct X.25 access available from the OS that allows checking network logs (Sun Solaris, Linux, VMS, OpenVMS, Motorola Codex PAD, etc.), find the X.25 logs to verify the calling and called addresses.
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