diff --git a/ports/unix/port/netif/tapif.c b/ports/unix/port/netif/tapif.c
index f78d07b..75f8954 100644
--- a/ports/unix/port/netif/tapif.c
+++ b/ports/unix/port/netif/tapif.c
@@ -223,29 +223,35 @@ static err_t
 low_level_output(struct netif *netif, struct pbuf *p)
 {
   struct tapif *tapif = (struct tapif *)netif->state;
-  char buf[1514];
+  char buf[1518]; /* max packet size including VLAN excluding CRC */
   ssize_t written;
 
 #if 0
   if (((double)rand()/(double)RAND_MAX) < 0.2) {
     printf("drop output\n");
-    return ERR_OK;
+    return ERR_OK; /* ERR_OK because we simulate packet loss on cable */
   }
 #endif
 
+  if (p->tot_len > sizeof(buf)) {
+    MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
+    perror("tapif: packet too large");
+    return ERR_IF;
+  }
+
   /* initiate transfer(); */
   pbuf_copy_partial(p, buf, p->tot_len, 0);
 
   /* signal that packet should be sent(); */
   written = write(tapif->fd, buf, p->tot_len);
-  if (written < 0) {
+  if (written < p->tot_len) {
     MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
     perror("tapif: write");
-  }
-  else {
+    return ERR_IF;
+  } else {
     MIB2_STATS_NETIF_ADD(netif, ifoutoctets, (u32_t)written);
+    return ERR_OK;
   }
-  return ERR_OK;
 }
 /*-----------------------------------------------------------------------------------*/
 /*
@@ -262,7 +268,7 @@ low_level_input(struct netif *netif)
   struct pbuf *p;
   u16_t len;
   ssize_t readlen;
-  char buf[1514];
+  char buf[1518]; /* max packet size including VLAN excluding CRC */
   struct tapif *tapif = (struct tapif *)netif->state;
 
   /* Obtain the size of the packet and put it into the "len"
diff --git a/ports/unix/port/netif/tunif.c b/ports/unix/port/netif/tunif.c
index 15fd271..c53ee87 100644
--- a/ports/unix/port/netif/tunif.c
+++ b/ports/unix/port/netif/tunif.c
@@ -125,23 +125,35 @@ static err_t
 low_level_output(struct tunif *tunif, struct pbuf *p)
 {
   char buf[1500];
-  int rnd_val;
+  ssize_t written;
 
   /* initiate transfer(); */
 
-  rnd_val = rand();
-  if (((double)rnd_val/(double)RAND_MAX) < 0.4) {
-    printf("drop\n");
-    return ERR_OK;
+#if 0
+  if (((double)rand()/(double)RAND_MAX) < 0.4) {
+    printf("drop output\n");
+    return ERR_OK; /* ERR_OK because we simulate packet loss on cable */
+  }
+#endif
+
+  if (p->tot_len > sizeof(buf)) {
+    MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
+    perror("tunif: packet too large");
+    return ERR_IF;
   }
 
   pbuf_copy_partial(p, buf, p->tot_len, 0);
 
   /* signal that packet should be sent(); */
-  if (write(tunif->fd, buf, p->tot_len) == -1) {
+  written = write(tunif->fd, buf, p->tot_len);
+  if (written < p->tot_len) {
+    MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
     perror("tunif: write");
+    return ERR_IF;
+  } else {
+    MIB2_STATS_NETIF_ADD(netif, ifoutoctets, (u32_t)written);
+    return ERR_OK;
   }
-  return ERR_OK;
 }
 /*-----------------------------------------------------------------------------------*/
 /*