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I'm sure some of you have been on free wifi networks where they inject a small ad onto every page you visit. Basically, how do they do that?

How does one append html to every request that passes through a specific router?

I assume the router itself will have to make the actual request to get the content to then be able to append the html. This would be costly on the server side. I'm curious if there's another way to do it via caching or some other method.

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This is usually handled with a transparent proxy that re-writes the server's response.

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