I don't use freeTDS but I thought it supported integrated security by passing the domain\username in the username field http://www.freetds.org/faq.html#integratedsecurity

There are commercial SQL Server ODBC drivers which support integrated security from UNIX.

There are other ways of connecting from UNIX to MS SQL Server using ODBC and Windows authentication if you can install something on A windows machine (it does not have to be the SQL Server machine). You can use an ODBC-ODBC Bridge which installs a server process on a Windows machine which uses the MS SQL Server Driver to talk to SQL Server. An ODBC driver is installed at the client end which talks to the bridge server and on to SQL Server. Of course the only ones I know of are commercial ones from Easysoft and Openlink.