Here's how I did it:
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follow the guide on how to set up a build environment for pfsense (on the wiki) but before you do a ./cvsup_current, you need to modify the pfsense.6 kernel config file to enable linux compatability in the kernel. pfSense.6 is in tools/builder_scripts/conf.
add an options line for COMPAT_LINUX and while you are at it, add any other stuff you want like NETSMB, SMBFS etc
go ahead with the cvsup_current step now and the freesbie scripts will build you your pfSense kernel with compat support.
You still need to add the linux libraries to pfsense.
For this, you have to go to /usr/ports/emulators/linux_base-fc4 and do a make install, then move the /compat folder that is created into your pfSense file system for inclusion on the CD
Hope that helps,
Craig
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