Setting up Tor with Freecap and Proxomitron

[Graycode]

(Feb. 05, 2009 02:30 AM)eclipse Wrote:  

Quote:Some people emphasize to appear un-proxified.

How could anyone tell I'm behind a proxy by those two headers ? ("HTTP_CONNECTION=keep-alive" - "HTTP_KEEP_ALIVE=300")
So chaining proxo > polipo is helping with that?

There's some misconception and vague references in the HTTP RFC's. Both the "Connection:" and "Keep-Alive:" headers are hop-by-hop. A proxy sits between the clients and servers, it's up to the proxy to decide how it wants to play ball with communication in either direction.

Many modern browsers do send the "Connection: Keep-Alive" header as explicit indication that they want to be able to re-use an established connection, even if it turns out that server is only HTTP/1.0. Most (not all) browsers send the "Proxy-Connection:" version to a proxy, but servers expect to see "Connection:". If a server sees "Proxy-Connection:" then that's a good indication of a (faulty) proxy in the mixture.

A browser is not likely to send the "Keep-Alive:" header, I've not seen any do that (yet). So if a server sees "Keep-Alive:" then it's probably been put there by a proxy.
Edited: Firefox seems to be sending the "Keep-Alive:" header with a value of 300 seconds of desired persistence.

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One open question would be for how long should that TCP connection remain open for possible re-use ... a few seconds? 2 minutes? 10 minutes? That's where the "Keep-Alive:" header might come into play.

The detrimental impact of keeping connections open is more on the server's end keeping track of them. Servers will sometimes respond with both a "Connection: Keep-Alive" header, and also something like "Keep-Alive: 300" trying to indicate willingness to wait 300 seconds maximum time. Unfortunately there isn't a real standard for the "Keep-Alive:" header value. RFC 2616 points back to RFC 2068 on the subject, and that one says "HTTP/1.1 does not define any parameters" for the Keep-Alive: header.

Consider this response from cnn.com:
Keep-Alive: timeout=5, max=64
Connection: Keep-Alive

It's trying to indicate willingness to remain connected for 5 seconds and we shouldn't be keeping more than 64 of them. If ours was a proxy servicing many people at a company, then that server is asking for no more than 64 connections be left open for re-use by that company's proxy. Who knows, beyond 64 the server could initiate a denial-of-service defense.

Similarly, a request to ietf.org yields:
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive

This site's server headers contain:
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive

I know how my proxy handles connection management, but know little about how Proxo does it.

The Polipo proxy seems to be intentionally aggressive about keeping server connections open for possible re-use. Doing that in a network environment like Tor might make sense because of the excessive cost of opening those connections. Othersise I'd rather be "kinder and gentler" with regard to the duration of retaining open connections and server resources.