Subject: Re: [PATCH] Netfilter crossover module.

From: "James R. Leu" <[email-masked]>
Date: Thu, 10 Jul 2003 09:06:43 -0500

Between you and Ben Greear the linux kernel will have every possible
scheme for sending packets to your self.

I still think my work on this (Virtual routing and forwarding:
http://linux-vrf.sf.net/) is the less perverted(*) then the work that either
you or Ben have come up with.  Plus it has other applications besides
just being able to send packets to your self.

* in terms of the concept, not necessarily the actual implementation.

On Thu, Jul 10, 2003 at 06:47:05PM +1000, Rusty Russell wrote:
> Lots of people keep asking to be able to plug a crossover cables
> between to NICs in a machine, and use it for testing.
> 
> This is a simple module which does this, by creating phantom
> machine(s) on each network with IP address 1 greater than the
> interface.  Testers welcome.
> 
> Ignore the backwards compat crap, it'll be out of the final version.
> 
> Example usage:
>   # Bring interfaces up
>   ifconfig eth0 192.168.1.1
>   ifconfig eth1 192.168.2.1
> 
>   # Add module which creates "phantom" machines 192.168.1.2, and 192.168.2.2.
>   modprobe ip_crossover dev1=eth0 dev2=eth1
> 
>   # Tell kernel that 192.168.1.2 packets go to eth1, and .2.1 to eth0.
>   arp -s 192.168.1.2 <hardware address of eth1>
>   arp -s 192.168.2.2 <hardware address of eth0>
> 
> It'd be nice to have the module hardwire the arps itself, but this was
> quickest.  Patch welcome.
> 
> Rusty.
> 
> Name: Hardware Loopback Module
> Author: Rusty Russell
> Status: Tested on 2.5.74-bk5
> 
> D: For testing it is often nice to connect two NICs with a crossover
> D: cable and have the machine route packets between them.
> D: 
> D: Since Linux steadfastly regards IP addresses as properties of the
> D: box, not the individual NICs, this requires some trickery.  A simple
> D: netfilter module makes this possible, by producing "phantom" boxes.
> 
> diff -urNp --exclude TAGS -X /home/rusty/current-dontdiff --minimal linux-2.5.74-bk5/net/ipv4/netfilter/Kconfig working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/Kconfig
> --- linux-2.5.74-bk5/net/ipv4/netfilter/Kconfig	2003-07-03 09:44:02.000000000 +1000
> +++ working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/Kconfig	2003-07-08 18:03:29.000000000 +1000
> @@ -587,5 +587,18 @@ config IP_NF_COMPAT_IPFWADM
>  	  If you want to compile it as a module, say M here and read
>  	  <file:Documentation/modules.txt>.  If unsure, say `N'.
>  
> +config IP_NF_CROSSOVER
> +	tristate "IP forced crossover support (EXPERIMENTAL)"
> +	depends on EXPERIMENTAL
> +	help
> +	  This option allows you to connect two local network cards
> +	  with a crossover cable, and then force packets to pass over
> +	  that cable (Linux will normally short-circuit such packets).
> +
> +	  If you want to compile it as a module, say M here and read
> +	  <file:Documentation/modules.txt>: the module will be called
> +	  ip_crossover.
> +
> +	  Say `N'.
>  endmenu
>  
> diff -urNp --exclude TAGS -X /home/rusty/current-dontdiff --minimal linux-2.5.74-bk5/net/ipv4/netfilter/Makefile working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/Makefile
> --- linux-2.5.74-bk5/net/ipv4/netfilter/Makefile	2003-07-03 09:44:02.000000000 +1000
> +++ working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/Makefile	2003-07-08 18:03:29.000000000 +1000
> @@ -92,3 +92,5 @@ obj-$(CONFIG_IP_NF_COMPAT_IPCHAINS) += i
>  obj-$(CONFIG_IP_NF_COMPAT_IPFWADM) += ipfwadm.o
>  
>  obj-$(CONFIG_IP_NF_QUEUE) += ip_queue.o
> +
> +obj-$(CONFIG_IP_NF_CROSSOVER) += ip_crossover.o
> diff -urNp --exclude TAGS -X /home/rusty/current-dontdiff --minimal linux-2.5.74-bk5/net/ipv4/netfilter/ip_crossover.c working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/ip_crossover.c
> --- linux-2.5.74-bk5/net/ipv4/netfilter/ip_crossover.c	1970-01-01 10:00:00.000000000 +1000
> +++ working-2.5.74-bk5-hardware_loopback/net/ipv4/netfilter/ip_crossover.c	2003-07-10 18:04:59.000000000 +1000
> @@ -0,0 +1,257 @@
> +/* Copyright 2003 Rusty Russell, IBM Corporation.
> + * 
> + * Simple packet mangling.  The idea is to use a crossover between two
> + * local NICs for testing, then this module creates "phantom" boxes on
> + * each network at the interface address + 1.
> + *
> + * Packets sent to one phantom will come in like they came from the other.
> + *
> + * Usage:
> + *   ifconfig eth0 192.168.1.1
> + *   ifconfig eth1 192.168.2.1
> + *   arp -s 192.168.1.2 <hardware address of eth1>
> + *   arp -s 192.168.2.2 <hardware address of eth2>
> + *   modprobe ip_crossover dev1=eth0 dev2=eth1
> + *
> + *   Then doing ping 192.168.1.2, ICMP ping goes out eth0 and comes
> + *   back in eth1.  Reply goes out eth1 and comes back in eth0.  */
> +#include <linux/config.h>
> +#include <linux/netfilter_ipv4.h>
> +#include <linux/ip.h>
> +#include <linux/skbuff.h>
> +#include <linux/moduleparam.h>
> +#include <linux/inetdevice.h>
> +#include <linux/tcp.h>
> +#include <linux/udp.h>
> +#include <linux/icmp.h>
> +#include <linux/version.h>
> +#include <net/ip.h>
> +#include <asm/checksum.h>
> +
> +struct ifinfo
> +{
> +	/* Keep track of name so we can drop reference. */
> +	char name[IFNAMSIZ];
> +
> +	/* Cached interface addr. */
> +	u32 ifaddr;
> +
> +	/* "Phantom" box which gets mapped. */
> +	u32 phantom;
> +};
> +
> +static struct ifinfo devinfo1, devinfo2;
> +
> +/* Stolen from Alexey's ip_nat_dumb. */
> +static int nat_header(struct sk_buff *skb, u32 saddr, u32 daddr)
> +{
> +	struct iphdr *iph = skb->nh.iph;
> +
> +	u32 odaddr = iph->daddr;
> +	u32 osaddr = iph->saddr;
> +	u16 check;
> +
> +	/* Rewrite IP header */
> +	iph->saddr = saddr;
> +	iph->daddr = daddr;
> +	iph->check = 0;
> +	iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
> +
> +	/* If it is the first fragment, rewrite protocol headers */
> +	if (!(iph->frag_off & htons(IP_OFFSET))) {
> +		u16 *cksum;
> +
> +		switch(iph->protocol) {
> +		case IPPROTO_TCP:
> +			cksum  = (u16*)&((struct tcphdr*)
> +					 (((char*)iph)+(iph->ihl<<2)))->check;
> +			if ((u8*)(cksum+1) > skb->tail)
> +				return 0;
> +			check = *cksum;
> +			if (skb->ip_summed != CHECKSUM_HW)
> +				check = ~check;
> +			check = csum_tcpudp_magic(iph->saddr, iph->daddr,
> +						  0, 0, check);
> +			check = csum_tcpudp_magic(~osaddr, ~odaddr, 0, 0,
> +						  ~check);
> +			if (skb->ip_summed == CHECKSUM_HW)
> +				check = ~check;
> +			*cksum = check;
> +			break;
> +		case IPPROTO_UDP:
> +			cksum  = (u16*)&((struct udphdr*)
> +					 (((char*)iph)+(iph->ihl<<2)))->check;
> +			if ((u8*)(cksum+1) > skb->tail)
> +				return 0;
> +			if ((check = *cksum) != 0) {
> +				check = csum_tcpudp_magic(iph->saddr, 
> +							  iph->daddr, 0, 0,
> +							  ~check);
> +				check = csum_tcpudp_magic(~osaddr, ~odaddr,
> +							  0, 0, ~check);
> +				*cksum = check ? : 0xFFFF;
> +			}
> +			break;
> +		case IPPROTO_ICMP:
> +		{
> +			struct icmphdr *icmph
> +				= (struct icmphdr*)((char*)iph+(iph->ihl<<2));
> +			struct iphdr *ciph;
> +			u32 idaddr, isaddr;
> +
> +			if ((icmph->type != ICMP_DEST_UNREACH) &&
> +			    (icmph->type != ICMP_TIME_EXCEEDED) &&
> +			    (icmph->type != ICMP_PARAMETERPROB))
> +				break;
> +
> +			ciph = (struct iphdr *) (icmph + 1);
> +
> +			if ((u8*)(ciph+1) > skb->tail)
> +				return 0;
> +
> +			isaddr = ciph->saddr;
> +			idaddr = ciph->daddr;
> +
> +			/* Change addresses inside ICMP packet. */
> +			ciph->daddr = iph->saddr;
> +			ciph->saddr = iph->daddr;
> +			cksum  = &icmph->checksum;
> +			/* Using tcpudp primitive. Why not? */
> +			check  = csum_tcpudp_magic(ciph->saddr, ciph->daddr,
> +						   0, 0, ~(*cksum));
> +			*cksum = csum_tcpudp_magic(~isaddr, ~idaddr, 0, 0,
> +						   ~check);
> +			break;
> +		}
> +		default:
> +			break;
> +		}
> +	}
> +	return 1;
> +}
> +
> +static unsigned int xover_hook(unsigned int hook,
> +			       struct sk_buff **pskb,
> +			       const struct net_device *in,
> +			       const struct net_device *out,
> +			       int (*okfn)(struct sk_buff *))
> +{
> +	/* Going out to phantom box 1: change it to coming from
> +           phantom box 2, and vice versa. */
> +	if ((*pskb)->nh.iph->daddr == devinfo1.phantom) {
> +		printk(KERN_DEBUG "dev1: %u.%u.%u.%u->%u.%u.%u.%u"
> +		       " becomes %u.%u.%u.%u->%u.%u.%u.%u\n",
> +		       NIPQUAD((*pskb)->nh.iph->saddr),
> +		       NIPQUAD((*pskb)->nh.iph->daddr),
> +		       NIPQUAD(devinfo2.phantom),
> +		       NIPQUAD(devinfo2.ifaddr));
> +		if (!nat_header(*pskb, devinfo2.phantom, devinfo2.ifaddr))
> +			return NF_DROP;
> +	} else if ((*pskb)->nh.iph->daddr == devinfo2.phantom) {
> +		printk(KERN_DEBUG "dev1: %u.%u.%u.%u->%u.%u.%u.%u"
> +		       " becomes %u.%u.%u.%u->%u.%u.%u.%u\n",
> +		       NIPQUAD((*pskb)->nh.iph->saddr),
> +		       NIPQUAD((*pskb)->nh.iph->daddr),
> +		       NIPQUAD(devinfo1.phantom),
> +		       NIPQUAD(devinfo1.ifaddr));
> +		if (!nat_header(*pskb, devinfo1.phantom, devinfo1.ifaddr))
> +			return NF_DROP;
> +	}
> +
> +	return NF_ACCEPT;
> +}
> +
> +static struct nf_hook_ops xover_ops
> += { .hook = xover_hook,
> +    .owner = THIS_MODULE,
> +    .pf = PF_INET,
> +    .hooknum = NF_IP_POST_ROUTING, 
> +    .priority = NF_IP_PRI_MANGLE,
> +};
> +
> +static int __set_dev(const char *name, struct ifinfo *ifi)
> +{
> +	struct net_device *dev;
> +	struct in_device *indev;
> +
> +	dev = dev_get_by_name(name);
> +	if (!dev)
> +		goto fail;
> +	indev = __in_dev_get(dev);
> +	if (!indev || !indev->ifa_list)
> +		goto put_fail;
> +
> +	ifi->ifaddr = indev->ifa_list->ifa_address;
> +	ifi->phantom = htonl(ntohl(indev->ifa_list->ifa_address) + 1);
> +	if (ifi->phantom == indev->ifa_list->ifa_broadcast)
> +		goto put_fail;
> +
> +	strlcpy(ifi->name, name, sizeof(ifi->name));
> +	printk(KERN_INFO "ip_crossover: phantom for %s: %u.%u.%u.%u\n",
> +	       ifi->name, NIPQUAD(ifi->phantom));
> +	return 0;
> +
> +put_fail:
> +	dev_put(dev);
> +fail:
> +	printk(KERN_WARNING "ip_crossover: device %s is not usable.\n", name);
> +	return -ENOENT;
> +}
> +
> +#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,5,50)
> +static int set_dev(const char *val, struct kernel_param *kp)
> +{
> +	return __set_dev(val, kp->arg);
> +}
> +module_param_call(dev1, set_dev, NULL, &devinfo1, 0);
> +module_param_call(dev2, set_dev, NULL, &devinfo2, 0);
> +
> +#define compat_parse_params()
> +#else
> +static char *dev1, *dev2;
> +
> +MODULE_PARM(dev1, "s");
> +MODULE_PARM(dev2, "s");
> +
> +static void compat_parse_params(void)
> +{
> +	if (dev1)
> +		__set_dev(dev1, &devinfo1);
> +	if (dev2)
> +		__set_dev(dev2, &devinfo2);
> +}
> +#endif /* KERNEL_VERSION */
> +
> +static int __init init(void)
> +{
> +	compat_parse_params();
> +
> +	if (!devinfo1.name[0] || !devinfo2.name[0]) {
> +		printk(KERN_ERR "ip_crossover: need dev1 and dev2 args\n");
> +		return -EINVAL;
> +	}
> +
> +	return nf_register_hook(&xover_ops);
> +}
> +
> +static void __exit fini(void)
> +{
> +	struct net_device *dev;
> +
> +	nf_unregister_hook(&xover_ops);
> +
> +	/* Release devices. */
> +	dev = dev_get_by_name(devinfo1.name);
> +	dev_put(dev);
> +	dev_put(dev);
> +
> +	dev = dev_get_by_name(devinfo2.name);
> +	dev_put(dev);
> +	dev_put(dev);
> +}
> +
> +module_init(init);
> +module_exit(fini);
> +MODULE_LICENSE("GPL");
> +MODULE_PARM_DESC(dev1, "First device for crossover (required)");
> +MODULE_PARM_DESC(dev2, "Second device for crossover (required)");
> 
> --
>   Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

-- 
James R. Leu