From user-land up to the stack

In the previous story we seen what the network stack really represent and how is built up in the kernel , the interesting question would be to see how does this work from user-land.

The real question would be:

How is it that i see only the namespace’s interfaces when i do ifconfig from within a namespace???

Let me illustrate it

Root Namespace

(We get the loopback to eths and a veth pair)

From “blue” namespace:

So this all look very good , so lets drill into it.

1. We want to intercept the call that actually returns this info (strace)

Ok so strace tells you that this is reading info from /proc (we kind of knew this , or supposed it)

2. Let’s look in the kernel for the proc implementation

I ended up finding interesting stuff in fs/proc/proc_net.c

I’ve highlighted the important stuff in orange, let’s go by parts:

• The init function takes a struct net , this is the whole network stack of a namespace (devs , routes, etc) .
• netd is allocated
• netd seems to be a struct so some attributes ate filled up (data , namelen(3 ==net ) etc
• we set the attribute proc_net on stuct net to be netd

This suggest that the actual network stack implementation (struct net ) holds the information the proc fs entry?

let’s verify this in struct net , include/net/net_namespace.h

There it is , so when a network stack is allocated for a namespace the dentry for the proc fs lies inside the struct net itself , hence it can display information only related to its own struct net (namespace).

Next time let’s look into those structs and who populates the “dev” leaf inside proc/net.

Thanks!