This kind of stuff is usually in the
/sys kernel interfaces (first, keep in mind nothing in those directories is a regular disk file, they are all direct lines to the kernel).
»for x in /proc/sys/net/ipv4/conf/*/rp_filter; do echo -ne "$x "`cat $x`"\n"; done
Looks like I have rp_filter set for em1, wlan0, and "default". You can set or unset them by just writing to the file handle:
»echo 1 > rp_filter
»echo 0 > rp_filter
As mentioned, this is direct communication with the kernel, so that takes effect immediately. These are not configuration files. If you try and do something wrong:
»echo whatever > rp_filter
bash: echo: write error: Invalid argument
Which is not to say you can't screw things up this way, of course. And be sure to read the comments below.