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wrt610n “port overlap occurred” – working around and fixing it

There is a very nasty bug in “Linksys by Cisco” WRT610N wireless-N router. When you try to set up port forwarding from two different external ports to two ports with the same number but on different intranet machines, you get a nasty javascript popup error message “Port overlap occurred! Please change your entry!”. This prevents system administrators from setting up a perfectly valid scenario (for example mapping several IP web cams on intranet to different extranet ports).

Luckily the check is done only in javascript and you can still force settings on router bypassing this check.

If you fire up Fiddler dubugging proxy or a similar HTTP sniffer you would be able to see that upon submit the following post request is sent to the router:

submit_button=SingleForward&action=Apply&forward_single=15&wait_time=3

&name0=None&name1=None&name2=None&name3=None&name4=None&name5=uTorrent-srv

&name6=http-server&name7=https-server&name8=http-server01&name9=http-server02&name10=http-server03

&name11=http-server04&name12=&name13=&name14=

&ip0=0&ip1=0&ip2=0&ip3=0&ip4=0

&from5=63032&to5=63032&pro5=both&ip5=2&enable5=on

&from6=80&to6=80&pro6=both&ip6=2&enable6=on

&from7=443&to7=443&pro7=both&ip7=2&enable7=on

&from8=81&to8=81&pro8=tcp&ip8=70&enable8=on

&from9=82&to9=82&pro9=both&ip9=71&enable9=on

&from10=83&to10=83&pro10=both&ip10=72&enable10=on

&from11=84&to11=84&pro11=both&ip11=73&enable11=on

&from12=0&to12=0&pro12=both&ip12=0&from13=0

&to13=0&pro13=both&ip13=0&from14=0&to14=0&pro14=both&ip14=0

Take a look at bolded stuff. It has the port mapping information. I’ve deliberately set wrong port numbers (81, 82…) to bypass the javascript check. I then changed the port numbers to 80 and manually sent the request to the router. It doesn’t distinguish between GET and POST requests so you can just take the modified string and append it to the URL.

Worked for me like a charm!

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