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Experiencing an ongoing issue connecting to our mail server from clients using the same ISP. Issue exists on two servers both running ClearOS 7, using separate IP, separate locations. The problem is intermittent, but ongoing. Client computers using the same ISP as the server (in this case Spectrum Business Class) will intermittently not be able to connect. The problem may last a few hours, a few days, or few weeks and then clears-up, for a while, but eventually returns. If the client computer / phone / tablet switches ISPs (using a mobile hotspot or changing locations where a different ISP is in use) the problem is eliminated. I've contacted Spectrum on multiple occasions and have been told the issue is not on their end, but it seems that it must be if a user can switch connections and the problem goes away and then returns as soon as the connection is switched back to Spectrum.

Has anyone in this group experienced a similar issue? I did have one Spectrum technician tell me he had another customer the same problem, but had never found a solution.
Wednesday, September 22 2021, 06:09 PM
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  • Accepted Answer

    Thursday, September 23 2021, 10:42 PM - #Permalink
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    Thanks, Nick. I will try your suggestions to see if I can locate the source of the issue. I'm predisposed to think it's with our ISP as changing there are never any issues when the devices are using different ISPs. As soon as I have time to run the tcpdump, I will report back. Good tips.
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  • Accepted Answer

    Thursday, September 23 2021, 08:48 AM - #Permalink
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    Difficult one. You can check if packets are arriving with something like tcpdump:
    tcpdump -nni your_wan_interface '(port (25 or 587) and host your_computer's_wan_ip)'
    For your_computer's_wan_ip, google "what is my ip" and adjust the ports. IMAP is 143 and IMAPS is 993.

    If packets are arriving, it is a ClearOS problem. If not, it is elsewhere.

    If it is elsewhere, it could be ISP routing or perhaps DNS. When you get the failures, check a DNS lookup gets the right IP. If it does not and you have a fixed IP, consider putting it in the computer's host file or switch the computer to use different DNS server - neither are great solutions. Also when you get the failures, does a traceroute (tracert in Windows) to you work? Compare it with a working trace.
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