source is found at ftp://ftp.fml.org/pub/libtransparent/

Brief description is within my home page.

ipfilter_modify_dst(fd, dstp)
     int fd;
     struct sockaddr_in *dstp;

You can use this function to make application level gateway with ipfilter. For example, consider ssh proxy (ssh-gw). Firstly run proxy which listens 10022/tcp. Secondly redirect ssh connection from 22/tcp to localhost 10022/tcp at the internal interface by ipfilter (ipnat). You can get ssh-gw.

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