Re: [PATCH] Configure MTU via kernel DHCP
From: Hans-Peter Jansen
Date: Fri Feb 04 2005 - 11:59:55 EST
Hi Shane,
On Thursday 03 February 2005 05:47, Shane Hathaway wrote:
> The attached patch enhances the kernel's DHCP client support (in
> net/ipv4/ipconfig.c) to set the interface MTU if provided by the
> DHCP server. Without this patch, it's difficult to netboot on a
> network that uses jumbo frames. The patch is based on 2.6.10, but
> I'll update it to the latest testing kernel if that would expedite
> its inclusion in the kernel.
Well, I've been there before, and asked for exact the same back in
June 2003, but had much less luck, nobody of kernel fame even
responded:
http://marc.theaimsgroup.com/?l=linux-kernel&m=105624464918574&w=4
Only Ken Yap of etherboot fame told me:
> I have a feeling you will not get much sympathy for 2.[56] because
> there ipconfig in the kernel is deprecated in favour of userspace
> config from the initrd.
Well that's life.
For what is worth it, I ported my patch to current 2.6, which raised
some comments compared to yours:
- Is it really necessary to protect the dev_set_mtu call, since it is
just setting up the device?
- I prefer to call dev_set_mtu only, if a change mtu request is
sent..
- Are you sure, you got the endianess right?
Here's the "cost": ipconfig.o without my patch on x86:
3 .init.data 0000005a 00000000 00000000 00000220 2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
4 .rodata.str1.1 000001a2 00000000 00000000 0000027a 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .rodata.str1.4 000003ad 00000000 00000000 0000041c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .init.text 00001a45 00000000 00000000 000007d0 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
With patch:
3 .init.data 0000005e 00000000 00000000 00000220 2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
4 .rodata.str1.1 000001ab 00000000 00000000 0000027e 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .rodata.str1.4 000003e5 00000000 00000000 0000042c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .init.text 00001ab5 00000000 00000000 00000820 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
Difference: 181 Bytes (padding ignored)
The whole module takes about 9K, compared to dhcp in initrd, which
takes a few hundred K! Hmm.
> Incidentally, ipconfig.c doesn't appear to do enough bounds
> checking on byte 1 of DHCP/BOOTP extension fields (the length
> field). It looks like a malicious DHCP server could mess with
> kernel memory that way. I could try to fix the hole, but maybe
> someone more experienced with this code would like to verify
> there's a problem first.
That's an interesting question. Please keep me informed on any new
perceptions in this respect.
May the linux gods indulge on this topic one day or remove the
ipconfig module completely.
--- linux-2.6/net/ipv4/ipconfig.c.orig 2005-02-04 15:59:42.430518242 +0100
+++ linux-2.6/net/ipv4/ipconfig.c 2005-02-04 17:07:14.526384702 +0100
@@ -29,6 +29,10 @@
*
* Multiple Nameservers in /proc/net/pnp
* -- Josef Siemes <jsiemes@xxxxxx>, Aug 2002
+ *
+ * Support for MTU selection via DHCP
+ * -- Hans-Peter Jansen <hpj@xxxxxxxxx>, June 2003
+ * redone for 2.6 in February 2005
*/
#include <linux/config.h>
@@ -151,6 +155,9 @@
/* Protocols supported by available interfaces */
static int ic_proto_have_if __initdata = 0;
+/* MTU of device (if requested) */
+static int ic_dev_mtu __initdata = 0;
+
#ifdef IPCONFIG_DYNAMIC
static DEFINE_SPINLOCK(ic_recv_lock);
static volatile int ic_got_reply __initdata = 0; /* Proto(s) that replied */
@@ -322,6 +329,12 @@
printk(KERN_ERR "IP-Config: Unable to set interface broadcast address (%d).\n", err);
return -1;
}
+ if (ic_dev_mtu) {
+ if ((err = dev_set_mtu(ic_dev, ic_dev_mtu)) < 0)
+ printk(KERN_ERR "IP-Config: Unable to set interface mtu to %d (%d).\n",
+ ic_dev_mtu, err);
+ /* Don't error out because set mtu failure, just notice the operator */
+ }
return 0;
}
@@ -609,6 +622,7 @@
12, /* Host name */
15, /* Domain name */
17, /* Boot path */
+ 26, /* MTU */
40, /* NIS domain name */
};
@@ -812,6 +826,9 @@
if (!root_server_path[0])
ic_bootp_string(root_server_path, ext+1, *ext, sizeof(root_server_path));
break;
+ case 26: /* MTU */
+ ic_dev_mtu = ntohs(*(u16 *)(ext+1));
+ break;
case 40: /* NIS Domain name (_not_ DNS) */
ic_bootp_string(system_utsname.domainname, ext+1, *ext, __NEW_UTS_LEN);
break;
@@ -1363,6 +1380,8 @@
*/
printk("IP-Config: Complete:");
printk("\n device=%s", ic_dev->name);
+ if (ic_dev_mtu)
+ printk(", mtu=%d", ic_dev_mtu);
printk(", addr=%u.%u.%u.%u", NIPQUAD(ic_myaddr));
printk(", mask=%u.%u.%u.%u", NIPQUAD(ic_netmask));
printk(", gw=%u.%u.%u.%u", NIPQUAD(ic_gateway));
Compile tested. Any takers?
Cheers,
Pete
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