proxy.pac

Dear list,
    I need some help with writing a proxy.pac that sets the correct
proxy based on the DNS name of the client. For example a client dials
into modem 20 in location foo - they get the DNS name
rack20-foo.somecompany.co.uk I want them to use the cache
webcache-foo.somecompany.co.uk.
    Regards,
        Simon

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