Network Namespace

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Basic Information

A network namespace is a Linux kernel feature that provides isolation of the network stack, allowing each network namespace to have its own independent network configuration, interfaces, IP addresses, routing tables, and firewall rules. This isolation is useful in various scenarios, such as containerization, where each container should have its own network configuration, independent of other containers and the host system.

How it works:

When a new network namespace is created, it starts with a completely isolated network stack, with no network interfaces except for the loopback interface (lo). This means that processes running in the new network namespace cannot communicate with processes in other namespaces or the host system by default.
Virtual network interfaces, such as veth pairs, can be created and moved between network namespaces. This allows for establishing network connectivity between namespaces or between a namespace and the host system. For example, one end of a veth pair can be placed in a container's network namespace, and the other end can be connected to a bridge or another network interface in the host namespace, providing network connectivity to the container.
Network interfaces within a namespace can have their own IP addresses, routing tables, and firewall rules, independent of other namespaces. This allows processes in different network namespaces to have different network configurations and operate as if they are running on separate networked systems.
Processes can move between namespaces using the setns() system call, or create new namespaces using the unshare() or clone() system calls with the CLONE_NEWNET flag. When a process moves to a new namespace or creates one, it will start using the network configuration and interfaces associated with that namespace.

Lab:

Create different Namespaces

CLI

sudo unshare -n [--mount-proc] /bin/bash
# Run ifconfig or ip -a

By mounting a new instance of the /proc filesystem if you use the param --mount-proc, you ensure that the new mount namespace has an accurate and isolated view of the process information specific to that namespace.

Error: bash: fork: Cannot allocate memory

If you run the previous line without -f you will get that error.
The error is caused by the PID 1 process exits in the new namespace.

After bash start to run, bash will fork several new sub-processes to do somethings. If you run unshare without -f, bash will have the same pid as the current "unshare" process. The current "unshare" process call the unshare systemcall, create a new pid namespace, but the current "unshare" process is not in the new pid namespace. It is the desired behavior of linux kernel: process A creates a new namespace, the process A itself won't be put into the new namespace, only the sub-processes of process A will be put into the new namespace. So when you run:

unshare -p /bin/bash

The unshare process will exec /bin/bash, and /bin/bash forks several sub-processes, the first sub-process of bash will become PID 1 of the new namespace, and the subprocess will exit after it completes its job. So the PID 1 of the new namespace exits.

The PID 1 process has a special function: it should become all the orphan processes' parent process. If PID 1 process in the root namespace exits, kernel will panic. If PID 1 process in a sub namespace exits, linux kernel will call the disable_pid_allocation function, which will clean the PIDNS_HASH_ADDING flag in that namespace. When linux kernel create a new process, kernel will call alloc_pid function to allocate a PID in a namespace, and if the PIDNS_HASH_ADDING flag is not set, alloc_pid function will return a -ENOMEM error. That's why you got the "Cannot allocate memory" error.

You can resolve this issue by use the '-f' option:

unshare -fp /bin/bash

If you run unshare with '-f' option, unshare will fork a new process after it create the new pid namespace. And run /bin/bash in the new process. The new process will be the pid 1 of the new pid namespace. Then bash will also fork several sub-processes to do some jobs. As bash itself is the pid 1 of the new pid namespace, its sub-processes can exit without any problem.

Copied from https://stackoverflow.com/questions/44666700/unshare-pid-bin-bash-fork-cannot-allocate-memory

Docker

docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash
# Run ifconfig or ip -a

Check which namespace is your process in

ls -l /proc/self/ns/net
lrwxrwxrwx 1 root root 0 Apr  4 20:30 /proc/self/ns/net -> 'net:[4026531840]'

Find all Network namespaces

{% code overflow="wrap" %}

sudo find /proc -maxdepth 3 -type l -name net -exec readlink {} \; 2>/dev/null | sort -u | grep "net:"
# Find the processes with an specific namespace
sudo find /proc -maxdepth 3 -type l -name net -exec ls -l  {} \; 2>/dev/null | grep <ns-number>

{% endcode %}

Enter inside a Network namespace

nsenter -n TARGET_PID --pid /bin/bash

Also, you can only enter in another process namespace if you are root. And you cannot enter in other namespace without a descriptor pointing to it (like /proc/self/ns/net).