nsenter

root@pc-01:~# docker ps | grep busybox
807e1730775a        busybox                "sh"                     5 hours ago         Up 5 hours                              optimistic_hermann
root@pcl-01:~# docker inspect 807e1730775a -f '{{.State.Pid}}'
2781180
root@pc-01:~# nsenter --target 2781180 --mount --uts --ipc --net --pid
root@pc-01:/# ls
bin  boot  dev  etc  home  lib  lost+found  media  mnt  opt  proc  root  run  sbin  snap  srv  sys  tmp  usr  var
root@pcl-01:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
2: tap0_kata: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UNKNOWN group default qlen 1000
    link/ether c2:03:51:46:a8:8b brd ff:ff:ff:ff:ff:ff
136: eth0@if137: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever
root@pc-01:/# exit
logout
root@pc-01:~# docker exec -it  807e1730775a  sh
/ # ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel qlen 1000
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::42:acff:fe11:2/64 scope link 
       valid_lft forever preferred_lft forever
/ # 

Linux containers and namespaces

To demonstrate the namespaces managing a container, I'll use Podman to create a container from the UBI8:8.2 image from Red Hat Container Catalog. The container resides on a Red Hat Enterprise Linux (RHEL) 8.2 host. For the purpose of this article, the procps-ng package is installed inside the container, which provides top and ps commands.

[root@workshop ~]# podman run --name namespace-demo -it registry.access.redhat.com/ubi8/ubi /bin/bash

In another terminal, use the runc command to determine the process id associated with the new container.

[root@workshop ~]# runc list
ID                                                                 PID         STATUS      BUNDLE                                                                                                                     CREATED                          OWNER
92585ccfd2d20c4bc7f03863d9b0a999eea18c91fb76f6333ef21171138beb83   7172        running     /var/lib/containers/storage/overlay-containers/92585ccfd2d20c4bc7f03863d9b0a999eea18c91fb76f6333ef21171138beb83/userdata   2020-06-24T14:56:37.895046979Z   root

The process id is 7172.

Use the lsns command to list the namespaces associated with a given process.

[root@workshop ~]# lsns -p 7172
        NS TYPE   NPROCS   PID USER COMMAND
4026531835 cgroup    137     1 root /usr/lib/systemd/systemd --switched-root --system --deserialize 16
4026531837 user      137     1 root /usr/lib/systemd/systemd --switched-root --system --deserialize 16
4026532438 net         1  7172 root /bin/bash
4026532516 mnt         1  7172 root /bin/bash
4026532517 uts         1  7172 root /bin/bash
4026532518 ipc         1  7172 root /bin/bash
4026532519 pid         1  7172 root /bin/bash

Inspect the namespaces with nsenter

The nsenter command expands to namespace enter. It accepts different options to only enter the specified namespace.

Let's enter the network namespace to check the IP address and route table.

[root@workshop ~]# nsenter -t 7172 -n ip a s

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
3: eth0@if9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
    link/ether ea:95:99:52:13:56 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.88.0.6/16 brd 10.88.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::e895:99ff:fe52:1356/64 scope link
       valid_lft forever preferred_lft forever

Here, -t is the target process id, and -n refers to the network namespace.

[root@workshop ~]# nsenter -t 7172 -n ip route

default via 10.88.0.1 dev eth0
10.88.0.0/16 dev eth0 proto kernel scope link src 10.88.0.6

Next, I enter the process namespace to check the process details.

[root@workshop ~]# nsenter -t 7172 -p -r ps -ef
UID          PID    PPID  C STIME TTY          TIME CMD
root           1       0  0 14:56 pts/0    00:00:00 /bin/bash
root         135       0  0 16:55 ?        00:00:00 ps -ef

The -r option sets the root directory to the top-level directory within the namespace so that the commands run in the context of the namespace.

[root@workshop ~]# nsenter -t 7172 -p -r top

The bash command, which executes during podman run, is the first process inside the namespace.

Image

Enter the UTC namespace to check the hostname.

[root@workshop ~]# nsenter -t 7172 -u hostname
92585ccfd2d2

Modify the hostname within the namespace and verify the new name.

[root@workshop ~]# nsenter -t 7172 -u hostname namespace.enable.sysadmin

[root@workshop ~]# nsenter -t 7172 -u hostname
namespace.enable.sysadmin

Finally, enter all namespaces by using the -a option.

[root@workshop ~]# nsenter -t 7172 -a

[root@namespace /]# df -h
Filesystem      Size  Used Avail Use% Mounted on
overlay         7.9G  6.3G  1.6G  80% /
tmpfs            64M     0   64M   0% /dev
tmpfs           1.9G  1.4M  1.9G   1% /etc/hosts
shm              63M     0   63M   0% /dev/shm
tmpfs           1.9G     0  1.9G   0% /sys/fs/cgroup
tmpfs           1.9G     0  1.9G   0% /proc/acpi
tmpfs           1.9G     0  1.9G   0% /proc/scsi
tmpfs           1.9G     0  1.9G   0% /sys/firmware
tmpfs           1.9G     0  1.9G   0% /sys/fs/selinux

Summary

Linux containers help with rapid application delivery and the use of DevOps practices. It is essential to make sure that those applications are secure, isolated, and resource-restricted. The nsenter tool helps you understand the low-level details of a container. It also helps with troubleshooting issues with container orchestration and deployment.