UTS Namespace

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Basic Information

A UTS (UNIX Time-Sharing System) namespace is a Linux kernel feature that provides isolation of two system identifiers: the hostname and the NIS (Network Information Service) domain name. This isolation allows each UTS namespace to have its own independent hostname and NIS domain name, which is particularly useful in containerization scenarios where each container should appear as a separate system with its own hostname.

How it works:

When a new UTS namespace is created, it starts with a copy of the hostname and NIS domain name from its parent namespace. This means that, at creation, the new namespace shares the same identifiers as its parent. However, any subsequent changes to the hostname or NIS domain name within the namespace will not affect other namespaces.
Processes within a UTS namespace can change the hostname and NIS domain name using the sethostname() and setdomainname() system calls, respectively. These changes are local to the namespace and do not affect other namespaces or the host system.
Processes can move between namespaces using the setns() system call or create new namespaces using the unshare() or clone() system calls with the CLONE_NEWUTS flag. When a process moves to a new namespace or creates one, it will start using the hostname and NIS domain name associated with that namespace.

Lab:

Create different Namespaces

CLI

sudo unshare -u [--mount-proc] /bin/bash

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

Check which namespace is your process in

ls -l /proc/self/ns/uts
lrwxrwxrwx 1 root root 0 Apr  4 20:49 /proc/self/ns/uts -> 'uts:[4026531838]'

Find all UTS namespaces

{% code overflow="wrap" %}

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

{% endcode %}

Enter inside an UTS namespace

nsenter -u 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/uts).

Change hostname

unshare -u /bin/bash
hostname newhostname # Hostname won't be changed inside the host UTS ns