I think I've seen this one on the netbsd-current mailing list.

 Users of netbsd-current who don't have an ethernet card will get
unresolved externals for ether_output() and arp_rtrequest() in in.o
when compiling a netbsd-current kernel. The "XXX: Another Kludge"
comment means that something better than the fix below should be
done. But for the moment this works just fine.

Onno van der Linden    c/o   vdlinden@fwi.uva.nl (Frank van der Linden)
*** /sys/netinet/in.c.orig	Sun Jun  5 13:42:02 1994
--- /sys/netinet/in.c	Sun Jun  5 13:42:36 1994
***************
*** 424,431 ****
--- 424,434 ----
  		return (error);
  	}
+ #include "ether.h"
+ #if NETHER > 0
  	if (ifp->if_output == ether_output) { /* XXX: Another Kludge */
  		ia->ia_ifa.ifa_rtrequest = arp_rtrequest;
  		ia->ia_ifa.ifa_flags |= RTF_CLONING;
  	}
+ #endif
  	splx(s);
  	if (scrub) {

------------------------------------------------------------------------------