What's blocking Port 25?

From: Griffin Palmer (griffjo_at_pacbell.net)
Date: 07/30/04

  • Next message: Luke Kearney: "Re: What's blocking Port 25?" 
    Date: Thu, 29 Jul 2004 21:39:44 -0700 (PDT)
To: debian-user@lists.debian.org

    This may be one for the firewalls list, but I'm not
    entirely sure this is a firewall issue, or, at least,
    *entirely* a firewall issue. I'm pretty sure it's not
    an exim issue.

    I thought I'd see if there are any troubleshooting
    suggestions from this obviously very experienced user
    base, before trying more specialized lists.

    I've just installed testing/unstable on my machine.

    I'm running behind a NAT DSL router, with iptables on
    my machine.

    I have my own domain, running its own DNS, smtp host,
    http host, etc.

    I'm using exim 4.41, compiled from source, as my MTA.

    I was earlier running exim 3.36-11 on another box,
    behind the same router, using identical port
    forwarding. But in the fresh install on more robust
    hardware, something has gone amiss.

    I'm pretty sure it's not exim. All the diagnostics
    check out for exim. I can send from and reply to
    local-domain addresses, using either mutt or mail.

    I can also send messages to the outside world. But
    replies (or original messages) from the outside world
    fall on deaf ears. Eventually, I get an "I've given up
    trying" message from my ISP's MTA -- except in one
    instance (more on that below).

    Here's where it gets confusing. Running nmap against
    my machine from outside the domain shows it is
    answering that Port 25 is closed. This even though,
    using FireStarter, I've explicitly opened Port 25.

    I'm *certain* I've properly forwarded Port 25 to the
    correct IP address for my new machine.

    Here's another oddity: Even though my machine replies
    'closed' to Port 25 scans, I got one error reply with
    one message attempt from my ISP account that suggests
    the port isn't *entirely* closed.

    Thinking I might have DNS configuration problems, I
    tried sending a message to myself@123.457.789.012 (my
    user name and IP address substituted for this dummy
    address). I didn't realize at the time you had to
    configure exim to accept mail addressed in this
    fashion.

    I got an error message back informing me that exim had
    refused to forward the dotted-quad-addressed message
    to my account on my host.

    So it seems as if *something* is getting through on
    Port 25.

    It doesn't seem, though, that it's a matter of
    messages reaching my host and then exim failing to
    properly forward them locally: There are no
    unforwarded messages languishing on exim's input
    queue.

    Any troubleshooting ideas would be appreciated.

                                  Griff Palmer 

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