fragment

README.md

@@ -53,6 +53,18 @@ The transmit path permits local IPv6 fragmentation for encapsulated EtherIP
 packets that exceed the outer path MTU.  This lets a larger inner Ethernet
 frame be split into multiple outer IPv6 fragments by the local host.
 
+The device default MTU is 1500, but the module allows an overlay MTU up to
+9000.  For example, with a 1500-byte underlay and a 9000-byte overlay:
+
+```sh
+sudo ip link set eip0 mtu 9000
+```
+
+An inner 9000-byte payload is carried as a 9014-byte Ethernet frame, plus the
+2-byte EtherIP header and 40-byte outer IPv6 header.  The local IPv6 stack
+fragments that outer packet before transmission, and the peer must reassemble
+the IPv6 fragments before EtherIP decapsulation.
+
 You can still set a smaller tunnel MTU when you want to avoid IPv6 fragments:
 
 ```sh

build

@@ -0,0 +1,13 @@
+#!/bin/sh
+set -eu
+
+KVER="$(uname -r)"
+KDIR="/lib/modules/$KVER/build"
+
+if [ ! -d "$KDIR" ]; then
+	echo "error: kernel headers for running kernel $KVER were not found at $KDIR" >&2
+	echo "install matching Arch headers or reboot into the kernel that matches installed headers" >&2
+	exit 1
+fi
+
+make KDIR="$KDIR"

etherip6.c

@@ -31,6 +31,7 @@
 #define ETHERIP6_HDR htons(ETHERIP6_VERSION << 12)
 #define ETHERIP6_HLEN 2
 #define ETHERIP6_DEFAULT_HOP_LIMIT 64
+#define ETHERIP6_MAX_MTU 9000
 
 struct etherip6_tunnel {
 	struct list_head list;
@@ -250,7 +251,7 @@ static void etherip6_setup(struct net_device *dev)
 	dev->hw_features = 0;
 	dev->vlan_features = 0;
 	dev->min_mtu = ETH_MIN_MTU;
-	dev->max_mtu = ETH_DATA_LEN;
+	dev->max_mtu = ETHERIP6_MAX_MTU;
 	eth_hw_addr_random(dev);
 }