xt_CHECKSUM: initial import

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 a 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>

doc/changelog.txt

@@ -2,6 +2,7 @@
 HEAD
 ====
 - RAWNAT: IPv6 variants erroneously rejected masks /33-/128
+- new target xt_CHECKSUM
 
 
 Xtables-addons 1.27 (May 16 2010)

extensions/Kbuild

@@ -7,6 +7,7 @@ obj-m                    += compat_xtables.o
 
 obj-${build_ACCOUNT}     += ACCOUNT/
 obj-${build_CHAOS}       += xt_CHAOS.o
+obj-${build_CHECKSUM}    += xt_CHECKSUM.o
 obj-${build_DELUDE}      += xt_DELUDE.o
 obj-${build_DHCPMAC}     += xt_DHCPMAC.o
 obj-${build_ECHO}        += xt_ECHO.o

extensions/Mbuild

@@ -2,6 +2,7 @@
 
 obj-${build_ACCOUNT}     += ACCOUNT/
 obj-${build_CHAOS}       += libxt_CHAOS.so
+obj-${build_CHECKSUM}    += libxt_CHECKSUM.so
 obj-${build_DELUDE}      += libxt_DELUDE.so
 obj-${build_DHCPMAC}     += libxt_DHCPMAC.so libxt_dhcpmac.so
 obj-${build_ECHO}        += libxt_ECHO.so

extensions/libxt_CHECKSUM.c

@@ -0,0 +1,99 @@
+/* Shared library add-on to xtables 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
+ *
+ * libxt_CHECKSUM.c borrowed some bits from libipt_ECN.c
+ *
+ * $Id$
+ */
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <getopt.h>
+
+#include <xtables.h>
+#include "xt_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)
+{
+	struct xt_CHECKSUM_info *einfo
+		= (struct xt_CHECKSUM_info *)(*target)->data;
+
+	switch (c) {
+	case 'F':
+		if (*flags)
+			xtables_error(PARAMETER_PROBLEM,
+			        "CHECKSUM target: Only use --checksum-fill ONCE!");
+		einfo->operation = XT_CHECKSUM_OP_FILL;
+		*flags |= XT_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 xt_CHECKSUM_info *einfo =
+		(const struct xt_CHECKSUM_info *)target->data;
+
+	printf("CHECKSUM ");
+
+	if (einfo->operation & XT_CHECKSUM_OP_FILL)
+		printf("fill ");
+}
+
+static void CHECKSUM_save(const void *ip, const struct xt_entry_target *target)
+{
+	const struct xt_CHECKSUM_info *einfo =
+		(const struct xt_CHECKSUM_info *)target->data;
+
+	if (einfo->operation & XT_CHECKSUM_OP_FILL)
+		printf("--checksum-fill ");
+}
+
+static struct xtables_target checksum_tg_reg = {
+	.name          = "CHECKSUM",
+	.version       = XTABLES_VERSION,
+	.family        = NFPROTO_UNSPEC,
+	.size          = XT_ALIGN(sizeof(struct xt_CHECKSUM_info)),
+	.userspacesize = XT_ALIGN(sizeof(struct xt_CHECKSUM_info)),
+	.help          = CHECKSUM_help,
+	.parse         = CHECKSUM_parse,
+	.final_check   = CHECKSUM_check,
+	.print         = CHECKSUM_print,
+	.save          = CHECKSUM_save,
+	.extra_opts    = CHECKSUM_opts,
+};
+
+static __attribute__((constructor)) void _init(void)
+{
+	xtables_register_target(&checksum_tg_reg);
+}

extensions/libxt_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 a 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.

extensions/xt_CHECKSUM.c

@@ -0,0 +1,75 @@
+/* iptables module for the packet checksum mangling
+ *
+ * (C) 2002 by Harald Welte <laforge@netfilter.org>
+ * (C) 2010 Red Hat, Inc.
+ *
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+*/
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/in.h>
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/netdevice.h>
+
+#include <linux/netfilter/x_tables.h>
+#include "xt_CHECKSUM.h"
+#include "compat_xtables.h"
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Michael S. Tsirkin <mst@redhat.com>");
+MODULE_DESCRIPTION("Xtables: checksum modification");
+MODULE_ALIAS("ipt_CHECKSUM");
+MODULE_ALIAS("ip6t_CHECKSUM");
+
+static unsigned int
+checksum_tg(struct sk_buff **pskb, const struct xt_action_param *par)
+{
+	struct sk_buff *skb = *pskb;
+
+	if (skb->ip_summed == CHECKSUM_PARTIAL)
+		skb_checksum_help(skb);
+
+	return XT_CONTINUE;
+}
+
+static int checksum_tg_check(const struct xt_tgchk_param *par)
+{
+	const struct xt_CHECKSUM_info *einfo = par->targinfo;
+
+	if (einfo->operation & ~XT_CHECKSUM_OP_FILL) {
+		pr_info("unsupported CHECKSUM operation %x\n", einfo->operation);
+		return -EINVAL;
+	}
+	if (!einfo->operation) {
+		pr_info("no CHECKSUM operation enabled\n");
+		return -EINVAL;
+	}
+	return 0;
+}
+
+static struct xt_target checksum_tg_reg __read_mostly = {
+	.name       = "CHECKSUM",
+	.family     = NFPROTO_UNSPEC,
+	.target     = checksum_tg,
+	.targetsize = sizeof(struct xt_CHECKSUM_info),
+	.table      = "mangle",
+	.checkentry = checksum_tg_check,
+	.me         = THIS_MODULE,
+};
+
+static int __init checksum_tg_init(void)
+{
+	return xt_register_target(&checksum_tg_reg);
+}
+
+static void __exit checksum_tg_exit(void)
+{
+	xt_unregister_target(&checksum_tg_reg);
+}
+
+module_init(checksum_tg_init);
+module_exit(checksum_tg_exit);

extensions/xt_CHECKSUM.h

@@ -0,0 +1,18 @@
+/* Header file for iptables ipt_CHECKSUM target
+ *
+ * (C) 2002 by Harald Welte <laforge@gnumonks.org>
+ * (C) 2010 Red Hat Inc
+ * Author: Michael S. Tsirkin <mst@redhat.com>
+ *
+ * This software is distributed under GNU GPL v2, 1991
+*/
+#ifndef _IPT_CHECKSUM_TARGET_H
+#define _IPT_CHECKSUM_TARGET_H
+
+#define XT_CHECKSUM_OP_FILL	0x01	/* fill in checksum in IP header */
+
+struct xt_CHECKSUM_info {
+	u_int8_t operation;	/* bitset of operations */
+};
+
+#endif /* _IPT_CHECKSUM_TARGET_H */

mconfig

@@ -2,6 +2,7 @@
 #
 build_ACCOUNT=m
 build_CHAOS=m
+build_CHECKSUM=m
 build_DELUDE=m
 build_DHCPMAC=m
 build_ECHO=