net/Kconfig: Ethernet packet buffer size cannot be permitted to go below 1294 is IPv6 is selected.

include/nuttx/net/ip.h

@@ -85,7 +85,7 @@
 /* IP Header sizes */
 
 #ifdef CONFIG_NET_IPv4
-#  define IPv4_HDRLEN     20    /* Size of IPv4 header */
+#  define IPv4_HDRLEN     20    /* Size of IPv4 header (without options) */
 #endif
 
 #ifdef CONFIG_NET_IPv6

net/Kconfig

@@ -45,8 +45,9 @@ config NET_ETH_PKTSIZE
 	int "Ethernet packet buffer size"
 	default 1294 if NET_IPv6
 	default 590 if !NET_IPv6
+	range 1294 1518 if NET_IPv6
+	range 590 1518 if !NET_IPv6
 	depends on NET_ETHERNET
-	range 590 1518
 	---help---
 		Packet buffer size.  This size includes the TCP/UDP payload plus the
 		size of TCP/UDP header, the IP header, and the Ethernet header.

net/devif/ipv4_input.c

@@ -344,6 +344,9 @@ int ipv4_input(FAR struct net_driver_s *dev)
   /* Start of IP input header processing code.
    *
    * Check validity of the IP header.
+   * REVISIT:  Does not account for varying IP header length due to the
+   * presences of IPv4 options.  The header length is encoded as a number
+   * 32-bit words in the HL nibble of the VHL.
    */
 
   if (ipv4->vhl != 0x45)