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EC-Council

312-50V11 · Question #82

While examining audit logs, you discover that people are able to telnet into the SMTP server on port 25. You would like to block this, though you do not see any evidence of an attack or other wrong…

The correct answer is E. None of the above. None of the listed options can block telnet access to port 25 while preserving normal SMTP functionality, because SMTP itself is a plaintext TCP protocol on port 25 that is indistinguishable from a telnet connection at the network level.

Scanning Networks

Question

While examining audit logs, you discover that people are able to telnet into the SMTP server on port 25. You would like to block this, though you do not see any evidence of an attack or other wrong doing. However, you are concerned about affecting the normal functionality of the email server. From the following options choose how best you can achieve this objective?

Options

  • ABlock port 25 at the firewall.
  • BShut off the SMTP service on the server.
  • CForce all connections to use a username and password.
  • DSwitch from Windows Exchange to UNIX Sendmail.
  • ENone of the above.

How the community answered

(33 responses)
  • A
    3% (1)
  • C
    15% (5)
  • D
    6% (2)
  • E
    76% (25)

Why each option

None of the listed options can block telnet access to port 25 while preserving normal SMTP functionality, because SMTP itself is a plaintext TCP protocol on port 25 that is indistinguishable from a telnet connection at the network level.

ABlock port 25 at the firewall.

Blocking port 25 at the firewall would also block all legitimate inbound SMTP email traffic, breaking the mail server's core delivery function.

BShut off the SMTP service on the server.

Shutting off the SMTP service entirely would prevent all email from being received or relayed, which contradicts the requirement to preserve normal email functionality.

CForce all connections to use a username and password.

Requiring SMTP AUTH credentials restricts who can relay mail through the server but does not prevent an unauthenticated user from telnetting to port 25 and interacting with the SMTP banner and commands.

DSwitch from Windows Exchange to UNIX Sendmail.

Switching mail server software from Exchange to Sendmail changes the application but does not resolve the underlying protocol issue that SMTP and telnet share the same plaintext TCP connection on port 25.

ENone of the above.Correct

Telnet is simply a raw TCP connection tool, and since SMTP operates over plaintext TCP on port 25, there is no way to block a telnet session to that port without also disrupting legitimate email traffic. None of the listed options achieves the specific goal of blocking telnet while keeping email functional - each option either breaks email delivery entirely or fails to prevent the telnet connection itself.

Concept tested: SMTP port 25 telnet access and protocol limitations

Source: https://www.rfc-editor.org/rfc/rfc5321

Topics

#SMTP#port security#firewall rules#email server

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