Question

I am using following C function to create multiple network namespaces from a single process instance:

void create_namespace(const char *ns_name)
{
    char ns_path[100];

    snprintf(ns_path, 100, "%s/%s", "/var/run/netns", ns_name);
    close(open(ns_path, O_RDONLY|O_CREAT|O_EXCL, 0));
    unshare(CLONE_NEWNET);
    mount("/proc/self/ns/net", ns_path, "none", MS_BIND , NULL);
}

After my process creates all the namspaces and I add a tap interface to any of the one network namespace (with ip link set tap1 netns ns1 command), then I actually see this interface in all of the namespaces (presumably, this is actually a single namespace that goes under different names).

But, if I create multiple namespaces by using multiple processes, then everything is working just fine.

What could be wrong here? Do I have to pass any additional flags to the unshare() to get this working from a single process instance? Is there a limitation that a single process instance can’t create multiple network namespaces? Or is there a problem with mount() call, because /proc/self/ns/net is actually mounted multiple times?

Update:
It seems that unshare() function creates multiple network namespaces correctly, but all the mount points in /var/run/netns/ actually reference to the first network namespace that was mounted in that direcotry.

Update2:
It seems that the best approach is to fork() another process and execute create_namespace() function from there. Anyway, I would be glad to hear a better solution that does not involve fork() call or at least get a confirmation that would prove that it is impossible to create and manage multiple network namespaces from a single process.

Update3:
I am able to create multiple namespaces with unshare() by using the following code:

int  main() {
    create_namespace("a");
    system("ip tuntap add mode tap tapa");
    system("ifconfig -a");//shows lo and tapA interface
    create_namespace("b");
    system("ip tuntap add mode tap tapb");
    system("ifconfig -a");//show lo and tapB interface, but does not show tapA. So this is second namespace created.
}

But after the process terminates and I execute ip netns exec a ifconfig -a and ip netns exec b ifconfig -a it seems that both commands were suddenly executed in namespace a. So the actual problem is storing the references to the namespaces (or calling mount() the right way. But I am not sure, if this is possible).

Answer 1

You only have to bind mount /proc/*/ns/* if you need to access these namespaces from another process, or need to get handle to be able to switch back and forth between the two. It is not needed to use multiple namespaces from a single process.

• unshare does create new namespace.
• clone and fork by default do not create any new namespaces.
• there is one “current” namespace of each kind assigned to a process. It can be changed by unshare or setns. Set of namespaces (by default) is inherited by child processes.

Whenever you do open(/proc/N/ns/net), it creates inode for this file,
and all subsequent open()s will return file that is bound to the
same namespace. Details are lost in the depths of kernel dentry cache.

Also, each process has only one /proc/self/ns/net file entry, and
bind mount does not create new instances of this proc file.
Opening those mounted files are exactly the same as opening
/proc/self/ns/net file directly (which will keep pointing to the
namespace it pointed to when you first opened it).

It seems that “/proc/*/ns” is half-baked like this.

So, if you only need 2 namespaces, you can:

• open /proc/1/ns/net
• unshare
• open /proc/self/ns/net

and switch between the two.

For more that 2 you might have to clone(). There seems to be no way to create more than one /proc/N/ns/net file per process.

However, if you do not need to switch between namespaces at runtime, or to share them with other processes, you can use many namespaces like this:

• open sockets and run processes for main namespace.
• unshare
• open sockets and run processes for 2nd namespace (netlink, tcp, etc)
• unshare
• …
• unshare
• open sockets and run processes for Nth namespace (netlink, tcp, etc)

Open sockets keep reference to their network namespace, so they will not be collected until sockets are closed.

You can also use netlink to move interfaces between namespaces, by sending netlink command on source namespace, and specifying dst namespace either by PID or namespace FD (the later you don’t have).

You need to switch process namespace before accessing /proc entries that depend on that namespace. Once “proc” file is open, it keeps reference to the namespace.