Temporary code to debug and play with pass-through device.
Create device pair by
	modprobe veth
        echo 'add veth1 0:1:2:3:4:1 eth0 0:1:2:3:4:2' >/proc/net/veth_ctl
and your shell will appear into a new namespace with `eth0' device.
Configure device in this namespace
        ip l s eth0 up
        ip a a 1.2.3.4/24 dev eth0
and in the root namespace
        ip l s veth1 up
        ip a a 1.2.3.1/24 dev veth1
to establish a communication channel between root namespace and the newly
created one.

Code is done by Andrey Savochkin and ported by me over Cedric'c patchset

Signed-off-by: Dmitry Mishin <dim@openvz.org>

---
 drivers/net/veth.c       |  121 +++++++++++++++++++++++++++++++++++++++++++++++
 fs/proc/array.c          |    8 +++
 kernel/fork.c            |    1 
 kernel/nsproxy.c         |    1 
 net/core/net_namespace.c |    3 +
 5 files changed, 134 insertions(+)

--- linux-2.6.20-rc4-mm1.net_ns.orig/drivers/net/veth.c
+++ linux-2.6.20-rc4-mm1.net_ns/drivers/net/veth.c
@@ -12,6 +12,7 @@
 #include <linux/etherdevice.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
+#include <linux/syscalls.h>
 #include <net/dst.h>
 #include <net/xfrm.h>
 
@@ -245,6 +246,123 @@ void veth_entry_del_all(void)
 
 /* ------------------------------------------------------------------- *
  *
+ * Temporary interface to create veth devices
+ *
+ * ------------------------------------------------------------------- */
+
+#ifdef CONFIG_PROC_FS
+
+static int veth_debug_open(struct inode *inode, struct file *file)
+{
+	return 0;
+}
+
+static char *parse_addr(char *s, char *addr)
+{
+	int i, v;
+
+	for (i = 0; i < ETH_ALEN; i++) {
+		if (!isxdigit(*s))
+			return NULL;
+		*addr = 0;
+		v = isdigit(*s) ? *s - '0' : toupper(*s) - 'A' + 10;
+		s++;
+		if (isxdigit(*s)) {
+			*addr += v << 16;
+			v = isdigit(*s) ? *s - '0' : toupper(*s) - 'A' + 10;
+			s++;
+		}
+		*addr++ += v;
+		if (i < ETH_ALEN - 1 && ispunct(*s))
+			s++;
+	}
+	return s;
+}
+
+static ssize_t veth_debug_write(struct file *file, const char __user *user_buf,
+		size_t size, loff_t *ppos)
+{
+	char buf[128], *s, *parent_name, *child_name;
+	char parent_addr[ETH_ALEN], child_addr[ETH_ALEN];
+	struct net_namespace *parent_ns, *child_ns;
+	int err;
+
+	s = buf;
+	err = -EINVAL;
+	if (size >= sizeof(buf))
+		goto out;
+	err = -EFAULT;
+	if (copy_from_user(buf, user_buf, size))
+		goto out;
+	buf[size] = 0;
+
+	err = -EBADRQC;
+	if (!strncmp(buf, "add ", 4)) {
+		parent_name = buf + 4;
+		if ((s = strchr(parent_name, ' ')) == NULL)
+			goto out;
+		*s = 0;
+		if ((s = parse_addr(s + 1, parent_addr)) == NULL)
+			goto out;
+		if (!*s)
+			goto out;
+		child_name = s + 1;
+		if ((s = strchr(child_name, ' ')) == NULL)
+			goto out;
+		*s = 0;
+		if ((s = parse_addr(s + 1, child_addr)) == NULL)
+			goto out;
+
+		get_net_ns(current_net_ns);
+		parent_ns = current_net_ns;
+		if (*s == ' ') {
+			unsigned int id;
+			id = simple_strtoul(s + 1, &s, 0);
+			err = sys_bind_ns(id, NS_ALL);
+		} else
+			err = sys_unshare(CLONE_NEWNET2);
+		if (err)
+			goto out;
+		/* after bind_ns() or unshare_ns() namespace is changed */
+		get_net_ns(current_net_ns);
+		child_ns = current_net_ns;
+		err = veth_entry_add(parent_name, parent_addr, parent_ns,
+			child_name, child_addr, child_ns);
+		if (err) {
+			put_net_ns(child_ns);
+			put_net_ns(parent_ns);
+		} else
+			err = size;
+	}
+out:
+	return err;
+}
+
+static struct file_operations veth_debug_ops = {
+	.open	= &veth_debug_open,
+	.write	= &veth_debug_write,
+};
+
+static int veth_debug_create(void)
+{
+	proc_net_fops_create("veth_ctl", 0200, &veth_debug_ops);
+	return 0;
+}
+
+static void veth_debug_remove(void)
+{
+	proc_net_remove("veth_ctl");
+}
+
+#else
+
+static int veth_debug_create(void) { return -1; }
+static void veth_debug_remove(void) { }
+
+#endif
+
+/* ------------------------------------------------------------------- *
+ *
  * Information in proc
  *
  * ------------------------------------------------------------------- */
@@ -304,12 +422,15 @@ static inline void veth_proc_remove(void
 
 int __init veth_init(void)
 {
+	if (veth_debug_create())
+		return -EINVAL;
 	veth_proc_create();
 	return 0;
 }
 
 void __exit veth_exit(void)
 {
+	veth_debug_remove();
 	veth_proc_remove();
 	veth_entry_del_all();
 }
--- linux-2.6.20-rc4-mm1.net_ns.orig/fs/proc/array.c
+++ linux-2.6.20-rc4-mm1.net_ns/fs/proc/array.c
@@ -72,6 +72,7 @@
 #include <linux/highmem.h>
 #include <linux/file.h>
 #include <linux/times.h>
+#include <linux/net_namespace.h>
 #include <linux/cpuset.h>
 #include <linux/rcupdate.h>
 #include <linux/delayacct.h>
@@ -198,6 +199,13 @@ static inline char * task_state(struct t
 	put_group_info(group_info);
 
 	buffer += sprintf(buffer, "\n");
+
+#ifdef CONFIG_NET_NS
+	if (p == current)
+		buffer += sprintf(buffer, "NetContext: %p\n",
+				p->nsproxy->net_ns);
+#endif
+
 	return buffer;
 }
 
--- linux-2.6.20-rc4-mm1.net_ns.orig/kernel/fork.c
+++ linux-2.6.20-rc4-mm1.net_ns/kernel/fork.c
@@ -1770,3 +1770,4 @@ bad_unshare_cleanup_thread:
 bad_unshare_out:
 	return err;
 }
+EXPORT_SYMBOL_GPL(sys_unshare);
--- linux-2.6.20-rc4-mm1.net_ns.orig/kernel/nsproxy.c
+++ linux-2.6.20-rc4-mm1.net_ns/kernel/nsproxy.c
@@ -426,6 +426,7 @@ unlock:
 	put_nsproxy(ns);
 	return ret;
 }
+EXPORT_SYMBOL(sys_bind_ns);
 
 static int __init nshash_init(void)
 {
--- linux-2.6.20-rc4-mm1.net_ns.orig/net/core/net_namespace.c
+++ linux-2.6.20-rc4-mm1.net_ns/net/core/net_namespace.c
@@ -56,6 +56,8 @@ static struct net_namespace *clone_net_n
 	if (loopback_init())
 		goto out_loopback;
 	pop_net_ns(old_ns);
+	printk(KERN_DEBUG "NET_NS: created new netcontext %p for %s "
+		"(pid=%d)\n", ns, current->comm, current->tgid);
 	return ns;
 
 out_loopback:
@@ -109,6 +111,7 @@ void free_net_ns(struct kref *kref)
 		return;
 	}
 	ip_fib_struct_cleanup(ns);
+	printk(KERN_DEBUG "NET_NS: net namespace %p destroyed\n", ns);
 	kfree(ns);
 }
 EXPORT_SYMBOL(free_net_ns);
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