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82 lines
4.4 KiB
Markdown
82 lines
4.4 KiB
Markdown
# UTS Namespace
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{{#include ../../../../banners/hacktricks-training.md}}
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## Basic Information
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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.
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### How it works:
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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.
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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.
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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.
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## Lab:
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### Create different Namespaces
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#### CLI
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```bash
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sudo unshare -u [--mount-proc] /bin/bash
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```
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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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<details>
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<summary>Error: bash: fork: Cannot allocate memory</summary>
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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:
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1. **Problem Explanation**:
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- 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.
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- 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.
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- 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.
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2. **Consequence**:
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- 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.
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3. **Solution**:
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- 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.
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- 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.
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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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</details>
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#### Docker
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```bash
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docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash
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```
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### Check which namespace is your process in
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```bash
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ls -l /proc/self/ns/uts
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lrwxrwxrwx 1 root root 0 Apr 4 20:49 /proc/self/ns/uts -> 'uts:[4026531838]'
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```
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### Find all UTS namespaces
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```bash
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sudo find /proc -maxdepth 3 -type l -name uts -exec readlink {} \; 2>/dev/null | sort -u
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# Find the processes with an specific namespace
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sudo find /proc -maxdepth 3 -type l -name uts -exec ls -l {} \; 2>/dev/null | grep <ns-number>
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```
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### Enter inside an UTS namespace
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```bash
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nsenter -u TARGET_PID --pid /bin/bash
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```
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{{#include ../../../../banners/hacktricks-training.md}}
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