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Fax not working behind Sophos XG

Decently new to Sophos XG, I'm mostly familiar with pfsense/opnsense firewalls.

I have a Grandstream ATA that connects to a remote PBX to send fax (t.38). Fax does not work behind a Sophos XG. Fax machine says no reply from destination. Fax does work when bypassing the Sophos XG connected directly to the back of a Comcast Business modem/FW. I'm using the same devices and destination fax # in both scenarios, only the IP schemes are different https://19216801.onl/ https://192168ll.link/.

I've gone through all the types of logs in the Log Viewer of the XG and don't see any log of blocking fax traffic. When I filter for my PBX IP in the firewall logs as the destination there are plenty of results. There are no results when I search for my PBX IP as the source. I'm expecting to see traffic in both directions.

I don't see anything in the IPS or web filter logs related to voip or fax. I'm not sure where else to look but I feel like the Sophos XG is the issue.



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  • When I have issues like that, where nothing obvious is in the logs, it's often an SSL/TLS decryption issue. I create web exceptions to see if one or more fix the issue and I'm usually successful. And if that doesn't work, I uncheck the 'Use web proxy instead of DPI engine' box in the firewall rule--this fixed the Apple Mac store for me as I had already used web exceptions to exclude all Apple domains.

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  • When I have issues like that, where nothing obvious is in the logs, it's often an SSL/TLS decryption issue. I create web exceptions to see if one or more fix the issue and I'm usually successful. And if that doesn't work, I uncheck the 'Use web proxy instead of DPI engine' box in the firewall rule--this fixed the Apple Mac store for me as I had already used web exceptions to exclude all Apple domains.

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