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IP Networking
Proxy Server
iptables as "proxy" and a filter
Post 302927078 by Smiling Dragon on Sunday 30th of November 2014 07:02:17 PM
11-30-2014
One thing I note in your friend's script is that it sets the firewall to drop all incoming connections before allowing port 22 (ssh) connections. If something goes wrong along the way on this, you'll need console access to get back in :/ I'd recommend setting the -P INPUT DROP last tbh 
As you've noted, this is just INPUT filters, your one is just NAT/IPMASQ. So it's like comparing apples and oranges really.
Looking just at your rules, I don't see anything untoward there, it appears that you are causing the proxy to accept traffic arriving on port 80 and redirecting it to 5.196.130.245.
An external customer connecting to the address would not be able to detect the redirect and would see all their connections as going to and coming back from the address of your proxy.
The webserver would see all the connections as coming from the proxy server address, unless the customer is setting "X-Forwarded-For" headers in their requests (not all that uncommon to find) which would be passed along to the webserver and quite possibly included in it's logs.
If you have a little more background of what your end goal is, we might be able to offer more advice
I would recommend also adding a FORWARD rule to DROP anything not for that port arriving from the external interface, as you've turned on IP forwarding and at present your setup would happily forward anything anyone asks it to. It's note really serious but could exacerbate any existing security issue into a full exploit.
As you've noted, this is just INPUT filters, your one is just NAT/IPMASQ. So it's like comparing apples and oranges really.
Looking just at your rules, I don't see anything untoward there, it appears that you are causing the proxy to accept traffic arriving on port 80 and redirecting it to 5.196.130.245.
An external customer connecting to the address would not be able to detect the redirect and would see all their connections as going to and coming back from the address of your proxy.
The webserver would see all the connections as coming from the proxy server address, unless the customer is setting "X-Forwarded-For" headers in their requests (not all that uncommon to find) which would be passed along to the webserver and quite possibly included in it's logs.
If you have a little more background of what your end goal is, we might be able to offer more advice
I would recommend also adding a FORWARD rule to DROP anything not for that port arriving from the external interface, as you've turned on IP forwarding and at present your setup would happily forward anything anyone asks it to. It's note really serious but could exacerbate any existing security issue into a full exploit.
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curlopt_pre_proxy
CURLOPT_PRE_PROXY(3) curl_easy_setopt options CURLOPT_PRE_PROXY(3) NAME
CURLOPT_PRE_PROXY - set pre-proxy to use SYNOPSIS
#include <curl/curl.h> CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PRE_PROXY, char *preproxy); DESCRIPTION
Set the preproxy to use for the upcoming request. The parameter should be a char * to a zero terminated string holding the host name or dotted numerical IP address. A numerical IPv6 address must be written within [brackets]. To specify port number in this string, append :[port] to the end of the host name. The proxy's port number may optionally be specified with the separate option CURLOPT_PROXYPORT(3). If not specified, libcurl will default to using port 1080 for proxies. A pre proxy is a SOCKS proxy that curl connects to before it connects to the HTTP(S) proxy specified in the CURLOPT_PROXY option. The pre proxy can only be a SOCKS proxy. The pre proxy string should be prefixed with [scheme]:// to specify which kind of socks is used. Use socks4://, socks4a://, socks5:// or socks5h:// (the last one to enable socks5 and asking the proxy to do the resolving, also known as CURLPROXY_SOCKS5_HOSTNAME type) to request the specific SOCKS version to be used. Otherwise SOCKS4 is used as default. Setting the pre proxy string to "" (an empty string) will explicitly disable the use of a pre proxy. The application does not have to keep the string around after setting this option. DEFAULT
Default is NULL, meaning no pre proxy is used. When you set a host name to use, do not assume that there's any particular single port number used widely for proxies. Specify it! PROTOCOLS
All except file://. Note that some protocols don't do very well over proxy. EXAMPLE
TODO AVAILABILITY
Added in 7.52.0 RETURN VALUE
Returns CURLE_OK if proxies are supported, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space. SEE ALSO
CURLOPT_PROXY(3), CURLOPT_HTTPPROXYTUNNEL(3), libcurl 7.54.0 December 21, 2016 CURLOPT_PRE_PROXY(3)