SK0-004 · Question #608
A server administrator suspects that an unauthorized email relay agent is running on a web server. The web server has a hostname of web1.example.com and an IP address of 199.90.89.51. Which of the…
The correct answer is A. telnet web1.example.com 25. Telnetting to port 25 on the web server tests whether an SMTP service is listening, which would indicate an unauthorized email relay.
Question
A server administrator suspects that an unauthorized email relay agent is running on a web server. The web server has a hostname of web1.example.com and an IP address of 199.90.89.51. Which of the following commands should the server administrator execute to verify these suspicions?
Options
- Atelnet web1.example.com 25
- Btelnet web1.example.com 23
- Ctelnet 199.90.89.51
- Dtelnet 199.90.89.51 80
How the community answered
(35 responses)- A83% (29)
- B6% (2)
- C9% (3)
- D3% (1)
Why each option
Telnetting to port 25 on the web server tests whether an SMTP service is listening, which would indicate an unauthorized email relay.
SMTP operates on TCP port 25, so connecting via telnet to web1.example.com on port 25 directly tests whether an SMTP daemon is accepting connections on that host. If a banner or SMTP response codes are returned, an email relay agent is confirmed to be running. Using the hostname also confirms DNS resolution is working correctly for that host.
Port 23 is the standard Telnet service port, not SMTP, so this tests for a Telnet daemon rather than an email relay.
Omitting a port number in telnet defaults to port 23 (Telnet), which tests for a Telnet service, not an email relay.
Port 80 is HTTP/web traffic, so this command tests for a web server, not an email relay agent.
Concept tested: SMTP port 25 service verification with Telnet
Source: https://learn.microsoft.com/en-us/exchange/mail-flow/connectors/allow-anonymous-relay
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