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Thread: disable send mail from telnet on port 25

  1. #1
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    Default disable send mail from telnet on port 25

    Hi Community,

    I'm altering the file main.cf to define the correct set of rules in order to block telnet on port 25 or at least require authentication when someone uses that command.

    I've googled for a while and after reading some guides I've understood that there are 3 kind of postfix restriction classes.

    If I try to do a telnet on port 465 or 587 it directly cuts my connection when i digit the HELO command. I'm trying to replicate the same behaviour when I use telnet on port 25. If I close port 25 I don't receive emails anymore.
    In our infrastructure is not possible configure a external smtp relay.

    The server now works perfectly on sending/receiving mails.

    Thanks in advance,

    elpteam.

  2. #2
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    All that you are doing when you connect via telnet is replicating an SMTP connection on port 25. The only way to stop this would be to block port 25, or require SSL/TLS on port 25 (which will mean many mail servers will not be able to send mail to you).

    Why are you concerned about this?
    ---
    Paul Chauvet
    State University of New York at New Paltz

  3. #3
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    So you are trying to block email delivery to your users from outside sources? That doesn't seem particularly wise unless your service is entirely self-contained.
    Quanah Gibson-Mount
    Server Architect
    Zimbra, Inc
    --------------------
    Zimbra :: the leader in open source messaging and collaboration

  4. #4
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    Thanks for your help.
    We have to block any chance that a user can send spam emails through telnet on port 25 towards our domain.
    Right now we are blocking the chance to send or receive emails to a non-existing domain through our smtp server.
    At the same time a user that telnets on port 25 and uses an existing domain on mailfrom command (like gmail.com) can't send mails to any other domain but ours. This means that he can use our server to spam our mailboxes.

    Our goal is to inhibit any non authenticated attempt to send emails using port 25 on telnet without enable tls on given port or completly block it.

    Thanks in advance,
    elpteam

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