7.7 KiB
Namespaces
To get the namespace of a container you can do:
docker run -dt --rm denial sleep 1234 #Run a large sleep inside a Debian container
ps -ef | grep 1234 #Get info about the sleep process
ls -l /proc/<PID>/ns #Get the Group and the namespaces (some may be uniq to the hosts and some may be shred with it)
To illustrate the five following namespaces, let’s create two Ubuntu containers:
docker run -ti --name ubuntu1 -v /usr:/ubuntu1 ubuntu bash
docker run -ti --name ubuntu2 -v /usr:/ubuntu2 ubuntu bash
PID namespace
Let’s look at processes running in Container ubuntu1:
root@3a1bf12161c9:/# ps
PID TTY TIME CMD
1 ? 00:00:00 bash
15 ? 00:00:00 ps
Let’s look at processes running in Container ubuntu2:
root@8beb85abe6a5:/# ps
PID TTY TIME CMD
1 ? 00:00:00 bash
14 ? 00:00:00 ps
Let’s look at the 2 “bash” process in host machine:
$ ps -eaf|grep root | grep bash
root 5413 1697 0 05:54 pts/28 00:00:00 bash
root 5516 1697 0 05:54 pts/31 00:00:00 bash
bash process in Container1 and Container2 have the same PID 1 since they have their own process namespace. The same bash process shows up in host machine as a different pid.
Mount namespace
Let’s look at the root directory content in Container ubuntu1:
root@3a1bf12161c9:/# ls /
bin dev home lib64 mnt proc run srv tmp usr
boot etc lib media opt root sbin sys ubuntu1 var
Let’s look at the root directory content in Container ubuntu2:
root@8beb85abe6a5:/# ls /
bin dev home lib64 mnt proc run srv tmp usr
boot etc lib media opt root sbin sys ubuntu2 var
As we can see above, each Container has its own filesystem and we can see “/usr” from host machine mounted as “/ubuntu1” in Container1 and as “/ubuntu2” in Container2.
Network namespace
Let’s look at ifconfig output in Container ubuntu1:
root@3a1bf12161c9:/# ifconfig
eth0 Link encap:Ethernet HWaddr 02:42:ac:15:00:02
inet addr:172.21.0.2 Bcast:0.0.0.0 Mask:255.255.0.0
inet6 addr: fe80::42:acff:fe15:2/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:36 errors:0 dropped:0 overruns:0 frame:0
TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:4940 (4.9 KB) TX bytes:648 (648.0 B)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Let’s look at ifconfig output in Container ubuntu2:
root@8beb85abe6a5:/# ifconfig
eth0 Link encap:Ethernet HWaddr 02:42:ac:15:00:03
inet addr:172.21.0.3 Bcast:0.0.0.0 Mask:255.255.0.0
inet6 addr: fe80::42:acff:fe15:3/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:28 errors:0 dropped:0 overruns:0 frame:0
TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:4292 (4.2 KB) TX bytes:648 (648.0 B)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
As we can see above, each Container has their own IP address.
IPC Namespace
Let’s create shared memory in Container ubuntu1:
root@3a1bf12161c9:/# ipcmk -M 100
Shared memory id: 0
root@3a1bf12161c9:/# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x2fba9021 0 root 644 100 0
Let’s create shared memory in Container ubuntu2:
root@8beb85abe6a5:/# ipcmk -M 100
Shared memory id: 0
root@8beb85abe6a5:/# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x1f91e62c 0 root 644 100 0
As we can see above, each Container has its own IPC namespace and shared memory created in Container 1 is not visible in Container 2.
UTS namespace
Let’s look at hostname of Container ubuntu1:
root@3a1bf12161c9:/# hostname
3a1bf12161c9
Let’s look at hostname of Container ubuntu2:
root@8beb85abe6a5:/# hostname
8beb85abe6a5
As we can see above, each Container has its own hostname and domainname.
User namespace
User namespaces are available from Linux kernel versions > 3.8. With User namespace, userid and groupid in a namespace is different from host machine’s userid and groupid for the same user and group. When Docker Containers use User namespace, each container gets their own userid and groupid. For example, root user inside Container is not root inside host machine. This provides greater security. In case the Container gets compromised and the hacker gets root access inside Container, the hacker still cannot break inside the host machine since the root user inside the Container is not root inside the host machine. Docker introduced support for user namespace in version 1.10.
To use user namespace, Docker daemon needs to be started with --userns-remap=default(In ubuntu 14.04, this can be done by modifying /etc/default/docker and then executing sudo service docker restart)
Following output shows Docker daemon running with user namespace turned on:
root 8207 1 0 20:03 ? 00:00:09 /usr/bin/docker daemon --userns-remap=default
Let’s start a ubuntu Container and look at its UID and GID:
root@3a1bf12161c9:/# id
uid=0(root) gid=0(root) groups=0(root)
To find the UID associated with the root UID inside Container, we need to first find the PID in host machine for the Container process and get the associated UID.
Following output shows the “bash” PID in host machine for the Container:
231072 8955 8207 0 21:23 pts/14 00:00:00 bash
Let’s look at the associated UID for PID 8955:
smakam14@jungle1:/usr$ cat /proc/8955/uid_map
0 231072 65536
As we can see above, userid 0(root) in container 1 is mapped to userid 231072 in host machine.
In the current Docker user namespace implementation, UID and GID mapping happens at Docker daemon level. There is work ongoing to allow the mappings to be done at Container level so that multi-tenant support is possible.
CGroup Namespace
Each cgroup namespace has its own set of cgroup root directories. These root directories are the base points for the relative locations displayed in the corresponding records in the /proc/[pid]/cgroup file. When a process creates a new cgroup namespace using clone(2) or unshare(2) with the CLONE_NEWCGROUP flag, its current cgroups directories become the cgroup root directories of the new namespace. (This applies both for the cgroups version 1 hierarchies and the cgroups version 2 unified hierarchy.)