Still it's not very clear
Anyway you have tons of options, from VFP side, from SQL server side, from a 3rd side (asp, asp.net, VBA, .Net etc).
-From VFP side you can connect to SQL server in a variety of ways (RV,SPT,CA,ADO) and insert - insert bulk/update/delete/select directly or call an SP. You could as well use XML and web services as a publisher or consumer.
-From SQL server side, you can add VFP tables as linked servers or use OpenRowSet(), OpenDataSource() functions to directly read from VFP tables (in SQL2005, require surface area configuration either using configuration tool or in code). Or more elegantly you could create a DTS or SSIS job to do that daily.
-3rd option is straightforward also. An asp/VBA code could connect to both (to VFP only use OLEDB connection, ODBC option is out) and transfer as needed. With .Net it's even much more easier with the SqlBulkCopy class.
Details would differ depending on from where you'd do it, how you'd do it (ie: simply add all records, or check against existing keys etc).
PS: As I understand you do not want them to directly use SQL server as backend (and even if you do they need to do modifications to write into SQL server instead of VFP tables).
>> My concern is security, as the foxpro tables are behind a firewall and sql server is in hosting center.
Is it possible to add VFP tables as linked server outside the firewall
No you cannot add VFP Tables to SQL Server as a linked server. However you can use an OLEDB connection to mimic a "linked server" and then access it using OPENQUERY.
>>Now, in terms of performance and concurrency is it feasible to access the fox pro tables in "real time" from a .net web site (production)
Sure its feasible. There are loads of sites using VFP Tables as their back end - more than you would realize, or even expect. Again, use OLEDB to access the data from the page.
You can add it as a linked server. But adding as a linked server woudl only let you select (doesn't support modification - I think that is why Andy says you can't add as a linked server. Not a usefull link really and firewall adds obstacles).
Yes it is feasible but since tou have SQL server you would want to use that instead.
OLE DB provider "SQLNCLI" for linked server "APOLLO01" returned message "No transaction is active.".
Msg 7391, Level 16, State 2, Line 5
The operation could not be performed because OLE DB provider "SQLNCLI" for linked server "APOLO01" was unable to begin a distributed transaction.