How to modify backend content before it reach to client's browser?

[fulan Peng]

Hi,

I am looking for the functionality similar to Apache's mod_proxy_html.
It can modify backend's content before it reach to customer. I used it
to rewrite url in the links.

Usually, when people are talking about url rewriting, they mean to
rewrite the url before send out to backend. That is varnish's url
rewriting. That is not I want.

Thanks a lot!

Fulan Peng