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I'm working on an InfoSec exercise, specifically email spoofing. I started by executing the following commands:

C:\Users\student>nslookup
6.0.0.10.in-addr.arpa
        primary name server = localhost
        responsible mail addr = nobody.invalid
        serial  = 1
        refresh = 600 (10 mins)
        retry   = 1200 (20 mins)
        expire  = 604800 (7 days)
        default TTL = 10800 (3 hours)
Default Server:  UnKnown
Address:  10.0.0.6

> set type=mx
> gmail.com     //the name of the mail domain of the victim
Server:  UnKnown
Address:  10.0.0.6

Non-authoritative answer:
gmail.com       MX preference = 20, mail exchanger = alt2.gmail-smtp-in.l.google
.com
gmail.com       MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google
.com
gmail.com       MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google
.com
gmail.com       MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com       MX preference = 30, mail exchanger = alt3.gmail-smtp-in.l.google
.com
> exit

After these initial commands have been executed, I'm now gonna accessed telnet:

C:\Users\student>telnet alt1.gmail-smtp-in.l.google 25
Connecting To alt1.gmail-smtp-in.l.google...Could not open connection to the host,
on port 25: Connect failed

Can someone please tell me why I can't connect to port 25? Thanks!

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  • 4
    I'm voting to close this question as off-topic because this question was caused by a simple typographical error.
    – kenorb
    Commented Oct 23, 2017 at 9:45

2 Answers 2

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You are trying to connect to the wrong host name.

The MX records cite alt1.gmail-smtp-in.l.google.com, but you are trying to connect to alt1.gmail-smtp-in.l.google. Hence, the failure in name resolution leading to a failure to connect. This is an artefact seemingly of the line wrapping in the console window. If you use the correct host name, it works fine at least for me.

That said, there is another thing to keep in mind. These days, some (many) ISPs block direct connections to port 25 on hosts other than their own outgoing mail servers, to help combat spam. We cannot know whether your ISP does this or not, but in the general case, this is something that can cause trouble with exercises like this.

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1

Looks like a simple cut-and-paste failure. You missed the .com from the address in the telnet command.

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