perhaps know how to do, can be useful for others:
looking inside: /etc/pure-ftpd/pureftpd-mysql.conf
it seems configuration can be "enhanced" in order to take in consideration
the source IP connected to the ftp server.
# In the following directives, parts of the strings are replaced at
# run-time before performing queries :
# \L is replaced by the login of the user trying to authenticate.
# \I is replaced by the IP address the user connected to.
# \P is replaced by the port number the user connected to.
# \R is replaced by the IP address the user connected from.
# \D is replaced by the remote IP address, as a long decimal number.
# Very complex queries can be performed using these substitution strings,
# especially for virtual hosting.
i think changing the actual queries configured adding the following statement:
"AND Status="1" AND (Ipaddress = "*" OR Ipaddress LIKE "\R")"
can achieve the result..
but in the ispconfig mysql db should be added an extra column called "ipaddress" inside the ftp_user table..and put there the desired IP who can connect from to that specified user..