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Accepted
John Bachir
Asked: 2010-08-21 14:38:50 +0800 CST

How can I figure out if port 25 is blocked on my network?

  • 772

telnet mydomain.tld 25 tells me "connection refused"

that's the extent of my network hax0r skillz.

how can i tell if it's my network i'm on or the server itself?

firewall networking

6 Answers

  1. Best Answer

    The tool you're looking for is "tcptraceroute". Specify the destination port, and look for at what point in the trace it fails.

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  2. Does it show up in netstat as listening on that server? Can you telnet from that server to itself on port 25? If so, the server is listening on port 25.

    Then try from another machine in the same network. If that doesn't work, then a firewall is most likely to blame, and it's on the SMTP server. I'm assuming it's iptables or the Windows firewall. Check here for disabling it on Windows, or use the GUI.

    If you can't reach it from outside your network, it may either be a firewall issue (again) or a NAT issue. If you have a private address on your server (like 192.168.x.x, 172.16-32.x.x, or 10.x.x.x), then you'll need to forward port 25 from your gateway to the server's internal IP.

    This should give you a good start. Let us know!

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  3. Probably the simplest way to check is go to a computer outside the network and telnet/nmap to the port.

    http://www.canyouseeme.org/ offers you to select your port number and they test it from online.

    Good luck!

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  4. telnet port 25 to any other server?

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  5. Take DNS out of the equation for a minute. Can you telnet to port 25 of the ip address of your email server?

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  6. use nmap to check if the host port is open. If it is, you 'd better change it to ssh :> Unfortunately telnet is dead and should be used only for backwards compatibility (Sends everything plaintext)

    But in your case, depending on what nmap returns, either the port isn't open or it's open but access is restricted only to certain group and not to anyone.

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