[PATCH] extensions: libipt_CHECKSUM extension

[PATCH] extensions: libipt_CHECKSUM extension

[Michael S. Tsirkin]

This adds a `CHECKSUM' target, which can be used in the iptables mangle
table.

You can use this target to compute and fill in the checksum in
an IP packet that lacks a checksum.  This is particularly useful,
if you need to work around old applications such as dhcp clients,
that do not work well with checksum offloads, but don't want to disable
checksum offload in your device.

The problem happens in the field with virtualized applications.
For reference, see Red Hat bz 605555, as well as
http://www.spinics.net/lists/kvm/msg37660.html

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

This patch adds userspace support for the CHECKSUM kernel module I sent previously.

 extensions/libipt_CHECKSUM.c   |  100 ++++++++++++++++++++++++++++++++++++++++
 extensions/libipt_CHECKSUM.man |    8 +++
 2 files changed, 108 insertions(+), 0 deletions(-)
 create mode 100644 extensions/libipt_CHECKSUM.c
 create mode 100644 extensions/libipt_CHECKSUM.man

diff --git a/extensions/libipt_CHECKSUM.c b/extensions/libipt_CHECKSUM.c
new file mode 100644
index 0000000..93d3e3f
--- /dev/null
+++ b/extensions/libipt_CHECKSUM.c
@@ -0,0 +1,100 @@
+/* Shared library add-on to iptables for CHECKSUM
+ *
+ * (C) 2002 by Harald Welte <laforge@gnumonks.org>
+ * (C) 2010 by Red Hat, Inc
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This program is distributed under the terms of GNU GPL v2, 1991
+ *
+ * libipt_CHECKSUM.c borrowed heavily from libipt_ECN.c
+ *
+ * $Id$
+ */
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <getopt.h>
+
+#include <xtables.h>
+#include <linux/netfilter_ipv4/ipt_CHECKSUM.h>
+
+static void CHECKSUM_help(void)
+{
+	printf(
+"CHECKSUM target options\n"
+"  --checksum-fill			Fill in packet checksum.\n");
+}
+
+static const struct option CHECKSUM_opts[] = {
+	{ "checksum-fill", 0, NULL, 'F' },
+	{ .name = NULL }
+};
+
+static int CHECKSUM_parse(int c, char **argv, int invert, unsigned int *flags,
+                     const void *entry, struct xt_entry_target **target)
+{
+	unsigned int result;
+	struct ipt_CHECKSUM_info *einfo
+		= (struct ipt_CHECKSUM_info *)(*target)->data;
+
+	switch (c) {
+	case 'F':
+		if (*flags)
+			xtables_error(PARAMETER_PROBLEM,
+			        "CHECKSUM target: Only use --checksum-fill ONCE!");
+		einfo->operation = IPT_CHECKSUM_OP_FILL;
+		*flags |= IPT_CHECKSUM_OP_FILL;
+		break;
+	default:
+		return 0;
+	}
+
+	return 1;
+}
+
+static void CHECKSUM_check(unsigned int flags)
+{
+	if (!flags)
+		xtables_error(PARAMETER_PROBLEM,
+		           "CHECKSUM target: Parameter --checksum-fill is required");
+}
+
+static void CHECKSUM_print(const void *ip, const struct xt_entry_target *target,
+                      int numeric)
+{
+	const struct ipt_CHECKSUM_info *einfo =
+		(const struct ipt_CHECKSUM_info *)target->data;
+
+	printf("CHECKSUM ");
+
+	if (einfo->operation & IPT_CHECKSUM_OP_FILL)
+		printf("fill ");
+}
+
+static void CHECKSUM_save(const void *ip, const struct xt_entry_target *target)
+{
+	const struct ipt_CHECKSUM_info *einfo =
+		(const struct ipt_CHECKSUM_info *)target->data;
+
+	if (einfo->operation & IPT_CHECKSUM_OP_FILL)
+		printf("--checksum-fill ");
+}
+
+static struct xtables_target checksum_tg_reg = {
+	.name		= "CHECKSUM",
+	.version	= XTABLES_VERSION,
+	.family		= NFPROTO_IPV4,
+	.size		= XT_ALIGN(sizeof(struct ipt_CHECKSUM_info)),
+	.userspacesize	= XT_ALIGN(sizeof(struct ipt_CHECKSUM_info)),
+	.help		= CHECKSUM_help,
+	.parse		= CHECKSUM_parse,
+	.final_check	= CHECKSUM_check,
+	.print		= CHECKSUM_print,
+	.save		= CHECKSUM_save,
+	.extra_opts	= CHECKSUM_opts,
+};
+
+void _init(void)
+{
+	xtables_register_target(&checksum_tg_reg);
+}
diff --git a/extensions/libipt_CHECKSUM.man b/extensions/libipt_CHECKSUM.man
new file mode 100644
index 0000000..4f83335
--- /dev/null
+++ b/extensions/libipt_CHECKSUM.man
@@ -0,0 +1,8 @@
+This target allows to selectively work around broken/old applications.
+It can only be used in the mangle table.
+.TP
+\fB\-\-checksum\-fill\fP
+Compute and fill in the checksum in an IP packet that lacks a checksum.
+This is particularly useful, if you need to work around old applications
+such as dhcp clients, that do not work well with checksum offloads,
+but don't want to disable checksum offload in your device.
-- 
1.7.2.rc0.14.g41c1c

Re: [PATCH] netfilter: add CHECKSUM target

[Herbert Xu]

On Fri, Jul 09, 2010 at 01:29:13AM +0300, Michael S. Tsirkin wrote:
> This adds a `CHECKSUM' target, which can be used in the iptables mangle
> table.
> 
> You can use this target to compute and fill in the checksum in
> an IP packet that lacks a checksum.  This is particularly useful,
> if you need to work around old applications such as dhcp clients,
> that do not work well with checksum offloads, but don't want to
> disable checksum offload in your device.
> 
> The problem happens in the field with virtualized applications.
> For reference, see Red Hat bz 605555, as well as
> http://www.spinics.net/lists/kvm/msg37660.html
> 
> Typical expected use (helps old dhclient binary running in a VM):
> iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM
> --checksum-fill
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

I'd think that this target would be protocol-agnostic, no?

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] netfilter: add CHECKSUM target

[Michael S. Tsirkin]

On Fri, Jul 09, 2010 at 03:18:14PM +0800, Herbert Xu wrote:
> On Fri, Jul 09, 2010 at 01:29:13AM +0300, Michael S. Tsirkin wrote:
> > This adds a `CHECKSUM' target, which can be used in the iptables mangle
> > table.
> > 
> > You can use this target to compute and fill in the checksum in
> > an IP packet that lacks a checksum.  This is particularly useful,
> > if you need to work around old applications such as dhcp clients,
> > that do not work well with checksum offloads, but don't want to
> > disable checksum offload in your device.
> > 
> > The problem happens in the field with virtualized applications.
> > For reference, see Red Hat bz 605555, as well as
> > http://www.spinics.net/lists/kvm/msg37660.html
> > 
> > Typical expected use (helps old dhclient binary running in a VM):
> > iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM
> > --checksum-fill
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> I'd think that this target would be protocol-agnostic, no?

Meaning it should go into net/netfilter/? Will do.

> Cheers,
> -- 
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] netfilter: add CHECKSUM target

[Herbert Xu]

On Sun, Jul 11, 2010 at 12:21:17PM +0300, Michael S. Tsirkin wrote:
>
> > I'd think that this target would be protocol-agnostic, no?
> 
> Meaning it should go into net/netfilter/? Will do.

Right, and you should get rid of any IPv4 dependencies.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] netfilter: add CHECKSUM target

[Jan Engelhardt]

On Friday 2010-07-09 00:29, Michael S. Tsirkin wrote:
>
> include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
> net/ipv4/netfilter/Kconfig                  |   16 ++++++
> net/ipv4/netfilter/Makefile                 |    1 +
> net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++

New modules should use xt.

>+static unsigned int
>+checksum_tg(struct sk_buff *skb, const struct xt_action_param *par)
>+{
>+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
>+		skb_checksum_help(skb);
>+	}

- {}

Re: [PATCH] netfilter: add CHECKSUM target

[Michael S. Tsirkin]

On Fri, Jul 09, 2010 at 11:56:26AM +0200, Jan Engelhardt wrote:
> 
> On Friday 2010-07-09 00:29, Michael S. Tsirkin wrote:
> >
> > include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
> > net/ipv4/netfilter/Kconfig                  |   16 ++++++
> > net/ipv4/netfilter/Makefile                 |    1 +
> > net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++
> 
> New modules should use xt.

I tried moving the module to xt_CHECKSUM but now
	iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill
does not to load the module.
It seems that xt_request_find_target in x_tables uses the prefix 'ip' for a module.
What am I missing?

> >+static unsigned int
> >+checksum_tg(struct sk_buff *skb, const struct xt_action_param *par)
> >+{
> >+	if (skb->ip_summed == CHECKSUM_PARTIAL) {
> >+		skb_checksum_help(skb);
> >+	}
> 
> - {}

Re: [PATCH] netfilter: add CHECKSUM target

[Changli Gao]

On Sun, Jul 11, 2010 at 8:27 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Fri, Jul 09, 2010 at 11:56:26AM +0200, Jan Engelhardt wrote:
>>
>> On Friday 2010-07-09 00:29, Michael S. Tsirkin wrote:
>> >
>> > include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
>> > net/ipv4/netfilter/Kconfig                  |   16 ++++++
>> > net/ipv4/netfilter/Makefile                 |    1 +
>> > net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++
>>
>> New modules should use xt.
>
> I tried moving the module to xt_CHECKSUM but now
>        iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill
> does not to load the module.
> It seems that xt_request_find_target in x_tables uses the prefix 'ip' for a module.
> What am I missing?
>

You should add MODULE_ALIAS clause, i.e.

MODULE_ALIAS("ipt_CHECKSUM");

-- 
Regards,
Changli Gao(xiaosuo@gmail.com)

Re: [PATCH] netfilter: add CHECKSUM target

[Michael S. Tsirkin]

On Sun, Jul 11, 2010 at 08:45:01PM +0800, Changli Gao wrote:
> On Sun, Jul 11, 2010 at 8:27 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Fri, Jul 09, 2010 at 11:56:26AM +0200, Jan Engelhardt wrote:
> >>
> >> On Friday 2010-07-09 00:29, Michael S. Tsirkin wrote:
> >> >
> >> > include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
> >> > net/ipv4/netfilter/Kconfig                  |   16 ++++++
> >> > net/ipv4/netfilter/Makefile                 |    1 +
> >> > net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++
> >>
> >> New modules should use xt.
> >
> > I tried moving the module to xt_CHECKSUM but now
> >        iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill
> > does not to load the module.
> > It seems that xt_request_find_target in x_tables uses the prefix 'ip' for a module.
> > What am I missing?
> >
> 
> You should add MODULE_ALIAS clause, i.e.
> 
> MODULE_ALIAS("ipt_CHECKSUM");

This worked, thanks!

> -- 
> Regards,
> Changli Gao(xiaosuo@gmail.com)

Re: [PATCH] netfilter: add CHECKSUM target

[Jan Engelhardt]

On Sunday 2010-07-11 14:27, Michael S. Tsirkin wrote:
>> >
>> > include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
>> > net/ipv4/netfilter/Kconfig                  |   16 ++++++
>> > net/ipv4/netfilter/Makefile                 |    1 +
>> > net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++
>> 
>> New modules should use xt.
>
>I tried moving the module to xt_CHECKSUM but now
>	iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill
>does not to load the module.
>It seems that xt_request_find_target in x_tables uses the prefix 'ip' for a module.
>What am I missing?

MODULE_ALIAS like the others have.

Re: [PATCH] netfilter: add CHECKSUM target

[Michael S. Tsirkin]

On Sun, Jul 11, 2010 at 02:45:49PM +0200, Jan Engelhardt wrote:
> 
> On Sunday 2010-07-11 14:27, Michael S. Tsirkin wrote:
> >> >
> >> > include/linux/netfilter_ipv4/ipt_CHECKSUM.h |   18 +++++++
> >> > net/ipv4/netfilter/Kconfig                  |   16 ++++++
> >> > net/ipv4/netfilter/Makefile                 |    1 +
> >> > net/ipv4/netfilter/ipt_CHECKSUM.c           |   72 +++++++++++++++++++++++++++
> >> 
> >> New modules should use xt.
> >
> >I tried moving the module to xt_CHECKSUM but now
> >	iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM --checksum-fill
> >does not to load the module.
> >It seems that xt_request_find_target in x_tables uses the prefix 'ip' for a module.
> >What am I missing?
> 
> MODULE_ALIAS like the others have.

Worked for me, thanks!

Re: [PATCH] netfilter: add CHECKSUM target

[Patrick McHardy]

Am 09.07.2010 00:29, schrieb Michael S. Tsirkin:
> This adds a `CHECKSUM' target, which can be used in the iptables mangle
> table.
> 
> You can use this target to compute and fill in the checksum in
> an IP packet that lacks a checksum.  This is particularly useful,
> if you need to work around old applications such as dhcp clients,
> that do not work well with checksum offloads, but don't want to
> disable checksum offload in your device.
> 
> The problem happens in the field with virtualized applications.
> For reference, see Red Hat bz 605555, as well as
> http://www.spinics.net/lists/kvm/msg37660.html
> 
> Typical expected use (helps old dhclient binary running in a VM):
> iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM
> --checksum-fill

I'm not sure this is something we want to merge upstream and
support indefinitely. Dave suggested this as a temporary
out-of-tree workaround until the majority of guest dhcp clients
are fixed. Has anything changed that makes this course of
action impractical?

Re: [PATCH] netfilter: add CHECKSUM target

[Jan Engelhardt]

On Friday 2010-07-09 17:17, Patrick McHardy wrote:
>
>> This adds a `CHECKSUM' target, which can be used in the iptables mangle
>> table.
>> 
>> You can use this target to compute and fill in the checksum in
>> an IP packet that lacks a checksum.  This is particularly useful,
>> if you need to work around old applications such as dhcp clients,
>> that do not work well with checksum offloads, but don't want to
>> disable checksum offload in your device.
>
>I'm not sure this is something we want to merge upstream and
>support indefinitely.

We could put it into Xtables-addons. That would also be consistent
with Dave's suggestion.

>Dave suggested this as a temporary
>out-of-tree workaround until the majority of guest dhcp clients
>are fixed. Has anything changed that makes this course of
>action impractical?

Re: [PATCH] netfilter: add CHECKSUM target

[Patrick McHardy]

Am 09.07.2010 18:26, schrieb Jan Engelhardt:
> 
> On Friday 2010-07-09 17:17, Patrick McHardy wrote:
>>
>>> This adds a `CHECKSUM' target, which can be used in the iptables mangle
>>> table.
>>>
>>> You can use this target to compute and fill in the checksum in
>>> an IP packet that lacks a checksum.  This is particularly useful,
>>> if you need to work around old applications such as dhcp clients,
>>> that do not work well with checksum offloads, but don't want to
>>> disable checksum offload in your device.
>>
>> I'm not sure this is something we want to merge upstream and
>> support indefinitely.
> 
> We could put it into Xtables-addons. That would also be consistent
> with Dave's suggestion.

Sure, that would be fine with me.

Re: [PATCH] netfilter: add CHECKSUM target

[Michael S. Tsirkin]

On Fri, Jul 09, 2010 at 05:17:36PM +0200, Patrick McHardy wrote:
> Am 09.07.2010 00:29, schrieb Michael S. Tsirkin:
> > This adds a `CHECKSUM' target, which can be used in the iptables mangle
> > table.
> > 
> > You can use this target to compute and fill in the checksum in
> > an IP packet that lacks a checksum.  This is particularly useful,
> > if you need to work around old applications such as dhcp clients,
> > that do not work well with checksum offloads, but don't want to
> > disable checksum offload in your device.
> > 
> > The problem happens in the field with virtualized applications.
> > For reference, see Red Hat bz 605555, as well as
> > http://www.spinics.net/lists/kvm/msg37660.html
> > 
> > Typical expected use (helps old dhclient binary running in a VM):
> > iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM
> > --checksum-fill
> 
> I'm not sure this is something we want to merge upstream and
> support indefinitely. Dave suggested this as a temporary
> out-of-tree workaround until the majority of guest dhcp clients
> are fixed. Has anything changed that makes this course of
> action impractical?

If I understand what Dave said correctly, it's up to you ...

The arguments for putting this upstream are:

Given the track record, I wouldn't hope for quick fix in the majority of
guest dhcp clients, unfortunately :(.  We are talking years here.
Even after that, one of the uses of virtualization is
to keep old guests running. So yes, I think we'll
keep using work-arounds for this for a very long time.

Further, since we have to add the module and we have to teach management
to program it, it will be much less painful for everyone
involved if we can put the code upstream, rather than forking
management code.

-- 
MST

Re: [PATCH] netfilter: add CHECKSUM target

[Patrick McHardy]

Am 11.07.2010 12:47, schrieb Michael S. Tsirkin:
> On Fri, Jul 09, 2010 at 05:17:36PM +0200, Patrick McHardy wrote:
>> Am 09.07.2010 00:29, schrieb Michael S. Tsirkin:
>>> This adds a `CHECKSUM' target, which can be used in the iptables mangle
>>> table.
>>>
>>> You can use this target to compute and fill in the checksum in
>>> an IP packet that lacks a checksum.  This is particularly useful,
>>> if you need to work around old applications such as dhcp clients,
>>> that do not work well with checksum offloads, but don't want to
>>> disable checksum offload in your device.
>>>
>>> The problem happens in the field with virtualized applications.
>>> For reference, see Red Hat bz 605555, as well as
>>> http://www.spinics.net/lists/kvm/msg37660.html
>>>
>>> Typical expected use (helps old dhclient binary running in a VM):
>>> iptables -A POSTROUTING -t mangle -p udp --dport 68 -j CHECKSUM
>>> --checksum-fill
>>
>> I'm not sure this is something we want to merge upstream and
>> support indefinitely. Dave suggested this as a temporary
>> out-of-tree workaround until the majority of guest dhcp clients
>> are fixed. Has anything changed that makes this course of
>> action impractical?
> 
> If I understand what Dave said correctly, it's up to you ...
> 
> The arguments for putting this upstream are:
> 
> Given the track record, I wouldn't hope for quick fix in the majority of
> guest dhcp clients, unfortunately :(.  We are talking years here.
> Even after that, one of the uses of virtualization is
> to keep old guests running. So yes, I think we'll
> keep using work-arounds for this for a very long time.
> 
> Further, since we have to add the module and we have to teach management
> to program it, it will be much less painful for everyone
> involved if we can put the code upstream, rather than forking
> management code.

Fair enough, its simple enough that I don't expect much maintenance
overhead.