UTS Namespace - HackTricks

# 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:

1. 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.
2. 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.
3. 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

When `unshare` is executed without the `-f` option, an error is encountered due to the way Linux handles new PID (Process ID) namespaces. The key details and the solution are outlined below:

1. Problem Explanation:

   - The Linux kernel allows a process to create new namespaces using the `unshare` system call. However, the process that initiates the creation of a new PID namespace (referred to as the "unshare" process) does not enter the new namespace; only its child processes do.
   - Running `%unshare -p /bin/bash%` starts `/bin/bash` in the same process as `unshare`. Consequently, `/bin/bash` and its child processes are in the original PID namespace.
   - The first child process of `/bin/bash` in the new namespace becomes PID 1. When this process exits, it triggers the cleanup of the namespace if there are no other processes, as PID 1 has the special role of adopting orphan processes. The Linux kernel will then disable PID allocation in that namespace.

2. Consequence:

   - The exit of PID 1 in a new namespace leads to the cleaning of the `PIDNS_HASH_ADDING` flag. This results in the `alloc_pid` function failing to allocate a new PID when creating a new process, producing the "Cannot allocate memory" error.

3. Solution:

   - The issue can be resolved by using the `-f` option with `unshare`. This option makes `unshare` fork a new process after creating the new PID namespace.
   - Executing `%unshare -fp /bin/bash%` ensures that the `unshare` command itself becomes PID 1 in the new namespace. `/bin/bash` and its child processes are then safely contained within this new namespace, preventing the premature exit of PID 1 and allowing normal PID allocation.

By ensuring that `unshare` runs with the `-f` flag, the new PID namespace is correctly maintained, allowing `/bin/bash` and its sub-processes to operate without encountering the memory allocation error.

#### 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

    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>

### Enter inside an UTS namespace

    nsenter -u TARGET_PID --pid /bin/bash

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