Cisco has already said that its VPN software is incompatible with Tiger and has since been joined by Thursby, Lobotomo, Equinux and, surprise surprise, Microsoft.
The main problem is that Apple seems to have created a new set of kernel interfaces that have removed some longstanding Unix networking program methods, known as Berkeley protocol stacks. Thursby has said that Apple has sent it back to the drawing board to redesign kernel extensions to work with Tiger. It should have the work done by August.
Microsoft said that the Virtual Switch networking feature of its Virtual PC software is also buggered by the upgrade, along with zero-configuration printing and ability to launch PC programs via the dock. Vole said it should have a patch out in a couple of months.
According to News.com, Equinux can't solve the problem, which it says is a bug in the Tiger kernel although it is working on it
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Mac Tiger OS reviewed