fix

README.md

@@ -37,6 +37,14 @@ sudo ip link set eip0 up
 sudo ip addr add 192.0.2.1/30 dev eip0
 ```
 
+Overlay MTUのデフォルトは1500です。Underlayの経路MTUも1500の場合、
+カプセル化後の外側IPv6パケットはMTUを超えるため、送信元で経路MTU以下の
+IPv6フラグメントに分割します。
+
+```sh
+sudo ip link set eip0 mtu 1500
+```
+
 ## トンネルの削除
 
 ```sh

etherip6.c

@@ -32,6 +32,7 @@
 #define ETHERIP6_HLEN 2
 #define ETHERIP6_DEFAULT_HOP_LIMIT 64
 #define ETHERIP6_MAX_MTU 9000
+#define ETHERIP6_ENCAP_HLEN (sizeof(struct ipv6hdr) + ETHERIP6_HLEN)
 
 struct etherip6_tunnel {
 	struct list_head list;
@@ -191,6 +192,14 @@ static netdev_tx_t etherip6_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	skb->protocol = htons(ETH_P_IPV6);
 	skb->dev = dst->dev;
+	/*
+	 * This skb used to contain an inner Ethernet frame.  Clear its control
+	 * block before handing it to IPv6, then pin source fragmentation to the
+	 * current underlay PMTU.  ip6_local_out() adds the Fragment Header and
+	 * splits oversized packets in ip6_finish_output().
+	 */
+	memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
+	IP6CB(skb)->frag_max_size = dst_mtu(dst);
 	skb->ignore_df = 1;
 	skb_dst_set(skb, dst);
 
@@ -252,6 +261,7 @@ static void etherip6_setup(struct net_device *dev)
 	dev->vlan_features = 0;
 	dev->min_mtu = ETH_MIN_MTU;
 	dev->max_mtu = ETHERIP6_MAX_MTU;
+	dev->needed_headroom = ETHERIP6_ENCAP_HLEN;
 	eth_hw_addr_random(dev);
 }