Remote  traffic  sniffing  through  tunnels and route mangling: You can play with
      linux cooked interfaces or use the integrated plugin to sniff tunneled or  route-
      mangled remote connections and perform mitm attacks on them.

      gre_relay

              This plugin can be used to sniff GRE-redirected remote traffic.  The basic
              idea  is  to  create  a  GRE tunnel that sends all the traffic on a router
              interface to the ettercap machine. The plugin will send back the GRE pack-
              ets  to  the  router,  after ettercap "manipulation" (you can use "active"
              plugins such as smb_down, ssh decryption, filters,  etc...  on  redirected
              traffic)  It needs a "fake" host where the traffic has to be redirected to
              (to avoid kernel's responses). The "fake" IP will be the tunnel  endpoint.
              Gre_relay  plugin  will impersonate the "fake" host.  To find an unused IP
              address for the "fake" host you can use  find_ip  plugin.  Based  on  the
              original  Tunnelx  technique  by  Anthony  C.  Zboralski  published  in
              http://www.phrack.org/show.php?p=56&a=10 by HERT.

http://www.phrack.org/show.php?p=56&a=10 <- Check that link, quiet a good how-to, was of interest to me...