diff --git a/stand/i386/pxeldr/pxeboot.8 b/stand/i386/pxeldr/pxeboot.8
--- a/stand/i386/pxeldr/pxeboot.8
+++ b/stand/i386/pxeldr/pxeboot.8
@@ -130,6 +130,14 @@
 acts just like
 .Xr loader 8 .
 .Pp
+.Nm
+requires NFS over UDP.
+Many recent distributions of Linux only serve NFS over TCP.
+Enable UDP in some distributions by uncommenting
+.Ql udp=y
+in
+.Pa /etc/nfs.conf .
+.Pp
 For further information on Intel's PXE specifications and Wired for
 Management (WfM) systems, see
 .Li http://www.pix.net/software/pxeboot/archive/pxespec.pdf .
diff --git a/stand/libsa/rpc.c b/stand/libsa/rpc.c
--- a/stand/libsa/rpc.c
+++ b/stand/libsa/rpc.c
@@ -415,6 +415,11 @@
 		return (-1);
 	}
 	port = (int)ntohl(res->port);
+	if (port == 0) {
+		printf("Portmapper returned 0. TCP-only NFS server?\n");
+		free(pkt);
+		return (-1);
+	}
 	free(pkt);
 
 	rpc_pmap_putcache(d->destip, prog, vers, port);