These tools allow you to add packet filtering to a PC or UNIX system.
ftp://ftp.vix.com/pub/vixie/
ftp://coast.cs.purdue.edu/pub/tools/unix/screend
screend , which was originally written by Jeff Mogul at Digital Equipment Corporation and is now maintained by Paul Vixie, is a package that lets you add packet filtering capabilities to the kernel of BSD -based UNIX systems.
ftp://net.tamu.edu/pub/security/TAMU/
ftp://coast.cs.purdue.edu/pub/tools/unix/TAMU/
Drawbridge, by Texas A&M University, is a package that lets you turn a PC (one that is running MS-DOS and has two Ethernet or two FDDI boards) into an IP packet filter. There are three programs: Filter, Filter Compiler, and Filter Manager. Filter is the program that runs on the PC itself. Filter Compiler and Filter Manager are support programs that run on a UNIX box, and allow you to compile the filter lists into the form needed by the PC , and then download them over the Internet to the PC . (Alternatively, you can transfer them to the PC on floppy, if you can write an MS-DOS floppy disk from your UNIX box.)
The KarlBridge package, by Doug Karl, is a program that runs on a PC with two Ethernet boards. It turns the PC into a packet-filtering bridge.