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Stuck behind a restrictive firewall at work, but really jonesing to putty home to your linux box for some colossal cave? Goodness knows I was...but the firewall at work blocked all outbound connections except for ports 80 and 443. (Those were wide open for outbound connections.) So now I putty over port 443 and have my linux box redirect it to port 22 (the SSH port) before it routes it internally. So, my specific command would be:
iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-ports 22
Note that I use -A to append this command to the end of the chain. You could replace that with -I to insert it at the beginning (or at a specific rulenum).
My linux box is running slackware, with a kernel from circa 2001. Hopefully the mechanics of iptables haven't changed since then. The command is untested under any other distros or less outdated kernels.
Of course, the command should be easy enough to adapt to whatever service on your linux box you're trying to reach by changing the numbers (and possibly changing tcp to udp, or whatever). Between putty and psftp, however, I'm good to go for hours of time-killing.
This turns your iptables packet filter to a "Allow any from any to any" filter, so you can rule out any filtering issues when you have a problem to enable a connection from or to your host.
To re-enable it, run /etc/init.d/iptables restart
This is just one method of checking to see if an IP is blocked via IP tables or CSF. Simple and to the point. Replace xx.xx.xx.xx with the IP you wish to check.
Replace 10.0.0.0/8 with your largest local subnet. replace 10.1.1.123:3128 with your proxy information.. Note this only works with a proxy server configured for passive setup..
Now your firefox transparently proxy's stuff destined outside your network.. and Doesn't proxy stuff inside your network. as well as all your other favorite web applications. curl, wget, aria2 ect.
This appends (-A) a new rule to the INPUT chain, which specifies to drop all packets from a source (-s) IP address.