Redirect local traffic to a web cache with iptables

| iptables | Linux | webcache

Very occasionally I come across a Linux application that does not respect the http_proxy variable. This stops the application from working while I’m at university as they forbid traffic on port 80 unless it goes via their webcache. So today I came up with a hack of a solution that involves iptables:

# IP address and port number of the webcache

# Flush any previous rules
iptables -t nat --flush

# Delete and recreate the chain
iptables -t nat -X HTTPFORCE
iptables -t nat -N HTTPFORCE

# Don't touch local traffic (localhost and internal network)
iptables -t nat -A HTTPFORCE -o lo -j RETURN
iptables -t nat -A HTTPFORCE --dst -j RETURN
iptables -t nat -A HTTPFORCE --dst -j RETURN
# Add any other local networks here.

# Now we have two options. Please uncomment out one of them
# 1) Redirect packets on port 80 to the webcache
#    This may not work unless the webcache is generous with its input
#iptables -t nat -A HTTPFORCE -p tcp --dport 80 -j DNAT --to $WEBCACHE

# 2) Redirect packets on port 80 to localhost port 1234
#    On port 1234 you need to run a local web proxy, which forwards 
#    requests to the real webcache
#iptables -t nat -A HTTPFORCE -p tcp --dport 80 -j REDIRECT --to-port 1234

# Capture all outgoing TCP syns
iptables -t nat -A OUTPUT -p tcp --syn -j HTTPFORCE

All outgoing TCP packets on port 80 which are not destined for a local network are captured and changed in one of two ways. The first option just manipulates the IP/TCP header, and the second redirects the traffic to a proxy running on localhost. The reason for the two options was that my university’s webcache seemed unable to deal with HTTP requests without the full URL in the GET line, even though the request contains a valid Host header. I think this is a misconfiguration of their squid proxy, so instead you can redirect to a local proxy which forwards the request on to the webcache.

This all seems a hassle but sometimes it is needed when a application does not respect the http_proxy environment. On a good note all libcurl applications should respect it by default.

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