Firewalls can be taught to filter information by address or by content
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e.g. it could only allow inside-->outside messaging or messages to or from certain sites (this can be fooled by forging network addresses)
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e.g. it can prevent file transfer but accept email
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However email is actually a common file transfer mechanism and postscript (for instance) can easily hide rogue programs
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Often firewalls are highly inconvenient and in mysterious ways handicap legitimate traffic when in fact nobody realizes firewall exists!
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