How to maintain iptables on a DD-WRT system manually

Normally the iptable enries on a dd-wrt system are maintained by the dd-wrt gui. I want to manage them directly in a file. There are many ways how not to do this and some ways how you can. Here I describe the way I sticked to after a lot of trial and error.

• Enable SSHd on the device using the gui (Services/Secure Shell)
  This allows us to start a terminal session, using any ssh client (including e.g. putty from Windows). 
• Enable jffs on the device using the gui (Administration/JJFS2 supprt)
  Jffs offers a way to store our own files on the device surviving a reboot. 
• Start a ssh session from your PC and login to the device
  (username: root, password as specified in the gui). 
• Create a new directory "firewall" under /jffs
     mkdir /jffs/firewall 
• Copy, as a starting point, the iptables file generated by the gui to this directory.
     cp /tmp/.ipt /jffs/firewall/myipt
  Notice that, depending of the version of dd-wrt you are using, this ipt file might be located somewhere else. If it isn't in the location indicated, find it using:
     find / -name *ipt*
  Also notice that the iptables-save command is not supported on dd-wrt systems (at least on the systems I have been using). 
• Make sure our own iptables file is loaded when starting the devive instead of /tmp.ipt
     nvram set rc_firewall="iptables-restore < /jffs/firewall/myipt"
     nvram commit 
• Now we can edit the myipt file with a remote editor, using scp (from Windows I use WinSCP).
• To test a configuration before rebooting the device you could make a copy of the myipt file
     cp /jffs/firewall/myipt /jffs/firewall/myipt_test
  Make your changes on that copy first and load it when you are ready to test:
     iptables-restore < /jffs/firewall/myipt_test
  After a reboot the myipt version will be used again. To make the test version the permanent one, just copy it to myipt.
     cp /jffs/firewall/myipt_test /jffs/firewall/myipt 

Happy iptabling! 

This has been tested using version dd-wrt.v24-26138_NEWD-2_K2.6_big-nv64k.bin on a linksys E1200 V2 device.

How to keep only some text with a regular expression, while removing complete lines

Regular expressions are extremely powerful in replacing one text with another. If however you want to keep some text, while removing all other text, including removing complete lines, things become a bit less obvious.

Here is a regular expression that does the tric:

(?:^.*(the text that should be kept).*$)|^.*\r?\n

The text that needs to be kept is available as the (first) replacement string (typically \1).

Here is an example; the text to be processed is:

<table>
<tr>
<td>row1</td>
<td>row2</td>
</tr>
</table>

Using the regular expression:

(?:^.*<td>(.*)</td>.*$)|^.*\r?\n

with \1 as the replacement string will result in:

row1

row2

Notice that there are many implementation of regular expression using slightly different implementations. I use this type of regular expressions frequently with EditPadPro.