# UTS Namespace

{% hint style="success" %}
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## Basic Information

A UTS (UNIX Time-Sharing System) namespace is a Linux kernel feature that provides i**solation 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 s**hares 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

```bash
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**.

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<summary>Error: bash: fork: Cannot allocate memory</summary>

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.

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#### Docker

```bash
docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash
```

### Check which namespace is your process in

```bash
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" %}

```bash
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

```bash

<div data-gb-custom-block data-tag="hint" data-style='success'>

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<details>

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```
