>Hi,
>
>As far as my VFP 9 application working with the SQL Server back end, I know that TCP port 1433 has to be open (for connection to the SQL Server).
>But what about UPD? Do I need to specify the UPD for the VFP 9 application? The reason I am asking is that a new customer is asking me this question. I have never before had to specify UPD but, just in case, what should be the "rule"?

To avoid confusion it's "UDP", not "UPD": https://en.wikipedia.org/wiki/User_Datagram_Protocol

There's good documentation at https://docs.microsoft.com/en-us/sql/sql-server/install/configure-the-windows-firewall-to-allow-sql-server-access?view=sql-server-ver15 . There are some differences if you use a named instance.

Basically, it's up to you to know which SQL services/components your app uses, and the corresponding ports which need to be open. Then you give the sysadmin that list and they open ports or allow apps appropriately.

For security the rule of thumb is to minimize the number of "allow" firewall rules or open ports. If you're not sure which ones you need, start with a bare minimum (such as TCP port 1433 only) and test all the functions of your app against that. Test in your own environment, if you can - sysadmins don't like people experimenting on their production environments.

There can be a strong temptation to open all the ports in the link above "just to make sure everything works". This is almost always a terrible idea. The only time it may be acceptable is temporarily during troubleshooting. A sysadmin with SQL Server experience will question opening up a bunch of seldom-used ports (and rightfully so).