can not find installation source

my install server server(dhcp server, tftp server and nfs server) has two network interface, one for public, one for internal, the autoinstallation for public network works, but the installation for internal network doesn't work, the error messages are "can not find installation source", when I tried to do the internal network autoinstall, I change dhcp server and control file in ~pxelinux.cfg to the internal network settings(using internal IP....) the contents in the /etc/exports file is export *(ro,all_squash,async,insecure) I can mount this nfs file from another internal machine, but it can't mount for internal autointall. Any suggestion is appreciated. Lisa