301 Woe and Pain

redirecting an entire site with variables!?

[delboy1978uk]

hey guys!
ive a troublesome little 301 to write here, but not sure where to start.

i have two websites, which i'll call site A and site B.
I need to redirect site B to site A, but with a twist:

b.example.com should redirect to a.example.com/folder

that landing page will change content depending what the secondary site's url is, ie. if site-c.com or site-d.com arrives at that page, different content is displayed.

also the folder in site-a would need to be the sites root.

is this possible? i'm sure it must be!
so far i've changed the dns of site-b to the same as site-a, and am waiting on the changes to propagate.

any suggestions?

[edited by: incrediBILL at 8:03 pm (utc) on Apr 2, 2012]
[edit reason] exemplified URLs [/edit]

[lucy24]

Do you mean you're redirecting URL-for-URL, or that everyone arriving at site b will end up on the same page?

Does "site-a.com/folder" mean a physical directory or an URL?

Are you looking at something like a package of redirects pointing to

{URL on site a}?content={details depending on source}

with query string set in the originating domain before redirection?