Re: [PATCH] Configurable tap interface MTU

["Ed Swierk" <eswierk@arastra.com>]

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On 9/11/07, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> Please make it 65535 without an Ethernet header and 65521
> with an Ethernet header.

Here is a revised patch that allows MTUs up to 65535 for tap
interfaces and up to 65521 for tun interfaces.

(If I set the MTU to 65521 on a tun interface, ping complains "message
too long" when I send a 65521-byte packet; 65520 works okay, though.)

--Ed

[-- Attachment #2: tap-change-mtu.patch --]
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Index: linux-2.6.22.6/drivers/net/tun.c
===================================================================
--- linux-2.6.22.6.orig/drivers/net/tun.c
+++ linux-2.6.22.6/drivers/net/tun.c
@@ -171,6 +171,18 @@ tun_net_mclist(struct net_device *dev)
 	}
 }
 
+#define MIN_MTU 68
+#define MAX_MTU 65535
+
+static int
+tun_net_change_mtu(struct net_device *dev, int new_mtu)
+{
+	if (new_mtu < MIN_MTU || new_mtu + dev->hard_header_len > MAX_MTU)
+		return -EINVAL;
+	dev->mtu = new_mtu;
+	return 0;
+}
+
 static struct net_device_stats *tun_net_stats(struct net_device *dev)
 {
 	struct tun_struct *tun = netdev_priv(dev);
@@ -188,6 +200,7 @@ static void tun_net_init(struct net_devi
 		dev->hard_header_len = 0;
 		dev->addr_len = 0;
 		dev->mtu = 1500;
+		dev->change_mtu = tun_net_change_mtu;
 
 		/* Zero header length */
 		dev->type = ARPHRD_NONE;
@@ -200,6 +213,7 @@ static void tun_net_init(struct net_devi
 		dev->set_multicast_list = tun_net_mclist;
 
 		ether_setup(dev);
+		dev->change_mtu = tun_net_change_mtu;
 
 		/* random address already created for us by tun_set_iff, use it */
 		memcpy(dev->dev_addr, tun->dev_addr, min(sizeof(tun->dev_addr), sizeof(dev->dev_addr)) );