While compiling a small program playing with NIS, I got:

$ cc -O -o ypc ypc.o -lrpcsvc -lnsl
ypc.o: In function `get_all':
ypc.o(.text+0x204): undefined reference to `xdr_ypall'

And indeed I didn't find xdr_ypall in libnsl.so or in any
other library. OTOH, this function is defined in an include
file:

$ grep xdr_ypall /usr/include/rpcsvc/yp_prot.h
extern bool_t xdr_ypall __P ((XDR *__xdrs, struct
ypall_callback * __objp));

(I see the same problem on Red Hat 5.1 BTW)