Description: the Linux TAP driver can send packets that the DOS PC/TCP
 stack thinks are invalid (e.g. ARP replies). This pads the packet to
 avoid the problem.
Author: Mauro Flores <almauri.x@gmail.com>
Origin: http://www.spinics.net/lists/linux-msdos/msg00821.html
Bug-Upstream: http://sourceforge.net/p/dosemu/bugs/550/

Index: dosemu-1.4.0.7+20130105+b028d3f/src/dosext/net/net/pktnew.c
===================================================================
--- dosemu-1.4.0.7+20130105+b028d3f.orig/src/dosext/net/net/pktnew.c	2013-01-05 09:08:12.000000000 -0800
+++ dosemu-1.4.0.7+20130105+b028d3f/src/dosext/net/net/pktnew.c	2015-06-18 11:16:17.233137549 -0700
@@ -706,6 +706,18 @@
 		*--p = (char)(ETH_P_IPX >> 8);
 	    }
 #endif
+
+	    /*
+	     * The TAP driver can send ethernet frames shorter than 60 bytes
+	     * that may be dropped for the DOS stack implementation. This
+	     * will fix the packet size by adding padding at the end.
+	     */
+	    if( size < ETH_ZLEN ) {
+		pd_printf("Fixing packet padding. Actual length: %d\n", size);
+		memset(pkt_buf + size, '0', ETH_ZLEN - size );
+		size = ETH_ZLEN;
+	    }
+
 	    p_stats->packets_in++;
 	    p_stats->bytes_in += size;