I tried installing ettercap in openwrt, but it failed.
Anyone knows how to use ettercap in openwrt.
I'm guessing you tried to build it on your OpenWrt box as there isn't a package for it that I can find.
In general, it is a futile task to try to compile on OpenWrt directly, rather than with the build system. The suggested approach is to create a package, build it on a build machine, then install on the OpenWrt device, either with opkg or by including in a custom ROM.