Why am I getting "No route to host"?

Question

andrew-mx-02.andrew.cmu.edu is an MTA. It is unlikely to have a web server running.

I telnet to this host on port 25 and I get the mail server's response.

On an Ubuntu or Debian machine, when I telnet to this host on port 80, I get, immediately:

telnet: Unable to connect to remote host: No route to host

On a Windows PC, I have to wait for more than 30 seconds before it says:

Could not open connection to the host, on port 80. Connect failed.

Is "No route to host" a correct description of the situation? ping responds nicely so certainly there is a route of sorts.

I am asking because I have a GoDaddy server connecting to andrew-mx-02.andrew.cmu.edu and giving this error response on port 25. From other servers it is fine. I would like to explain to the help desk accurately. If a firewall is blocking but they insist that the error message means I have not set up routing tables correctly, the problem will not be solved.

Solution

Most likely, this No route to host response is sent by the mailserver's firewall. Even iptables allows to do something like that, with --reject-with icmp-host-unreachable.

If port 80 is closed, with a host-unreachable policy, you will get a No route or similar response from any computer.

About port 25, things might be different. It needs to be open, since it is a mail server. However, your GoDaddy server may be blacklisted by IP by the mailserver. This is very common for dedicated servers that have fixed IPs. I rent an OVH/Kimsufi server that had its IP blacklisted in several places before I got it. You may also want to check what the reverse DNS of the IP is, because if it points to a blacklisted name the result will be the same.

Contact the helpdesk of the service you are trying to use, and:

• Give them the IP and domain name of your server,
• Tell them exactly what you tried on port 25 (telnet from your home works, telnet from the server doesn't)
• Ask them whether your server's IP is actually blacklisted,
• If yes, ask them why
• Tell them what you are trying to do, explain them why your server/application is harmless to them,
• Ask very politely if they can un-blacklist you
• Be very nice and polite, be precise without being verbose. Contacting helpdesks is an art that even the greatest have trouble mastering!