Tag: restorecon

Change the LXC container root folder under CentOS with SELinux

The default LXC container folder in CentOS (all versions – 7,8, Stream 8 and Stream 9) is /var/lib/lxc, which may resides in the root partition. When changing the lxc.rootfs or (the main directory /var/lib/lxc) to another place, the containers may still work without any additional SELinux permissions. Some tools like lxc-attach would definitely stop working with permission errors – lxc_attach_run_shell: 1333 Permission denied – failed to exec shell. This article will show how to use lxc-create and SELinux commands to properly change the LXC container’s rootfs.
For detailed information how to create a LXC container check out – Run LXC CentOS Stream 9 container with bridged network under CentOS Stream 9 or Run LXC Ubuntu 22.04 LTS container with bridged network under CentOS Stream 9.

Create LXC container with not default path

  • Change the rootfs only. To change only the LXC container root filesystem location use “–dir=” lxc-create option: 
    lxc-create --template download -n mycontainer2 --dir=/mnt/storage/servers/mycontainer2 -- --dist centos --release 9-Stream --arch amd64

    It will place the files under /mnt/storage/servers/mycontainer2, but the configuration will still be located in /var/lib/lxc/mycontainer2/.

    [root@srv ~]# ls -altr /var/lib/lxc/mycontainer2/
total 16
drwxr-xr-x. 3 root root 4096 Oct 14 13:42 ..
drwxr-xr-x. 2 root root 4096 Oct 14 13:42 rootfs
-rw-r-----. 1 root root  775 Oct 14 13:42 config
drwxrwx---. 3 root root 4096 Oct 14 13:42 .
[root@srv ~]# ls -altr /var/lib/lxc/mycontainer2/rootfs/
total 8
drwxr-xr-x. 2 root root 4096 Oct 14 13:42 .
drwxrwx---. 3 root root 4096 Oct 14 13:42 ..
[root@srv ~]# ls -altr /mnt/storage/servers/mycontainer2/
total 76
drwxrwxrwt.  2 root root 4096 Aug  9  2021 tmp
drwxr-xr-x.  2 root root 4096 Aug  9  2021 srv
lrwxrwxrwx.  1 root root    8 Aug  9  2021 sbin -> usr/sbin
drwxr-xr-x.  2 root root 4096 Aug  9  2021 opt
drwxr-xr-x.  2 root root 4096 Aug  9  2021 mnt
drwxr-xr-x.  2 root root 4096 Aug  9  2021 media
lrwxrwxrwx.  1 root root    9 Aug  9  2021 lib64 -> usr/lib64
lrwxrwxrwx.  1 root root    7 Aug  9  2021 lib -> usr/lib
drwxr-xr-x.  2 root root 4096 Aug  9  2021 home
dr-xr-xr-x.  2 root root 4096 Aug  9  2021 boot
lrwxrwxrwx.  1 root root    7 Aug  9  2021 bin -> usr/bin
dr-xr-xr-x.  2 root root 4096 Aug  9  2021 afs
dr-xr-xr-x.  2 root root 4096 Oct 14 07:11 sys
dr-xr-xr-x.  2 root root 4096 Oct 14 07:11 proc
drwxr-xr-x. 12 root root 4096 Oct 14 07:11 usr
drwxr-xr-x.  8 root root 4096 Oct 14 07:11 run
drwxr-xr-x. 18 root root 4096 Oct 14 07:11 var
dr-xr-x---.  2 root root 4096 Oct 14 07:12 root
drwxr-xr-x.  2 root root 4096 Oct 14 07:12 selinux
drwxr-xr-x. 19 root root 4096 Oct 14 07:15 .
drwxr-xr-x.  4 root root 4096 Oct 14 13:41 ..
drwxr-xr-x.  3 root root 4096 Oct 14 13:42 dev
drwxr-xr-x. 63 root root 4096 Oct 14 13:42 etc
  • Change the LXC container path – the folder containing the configuration and the container’s root filesystems use “-P” 
    lxc-create -P /mnt/storage/servers/ --template download -n mycontainer -- --dist centos --release 9-Stream --arch amd64

    All the LXC container configuration and root filesystem will be placed under /mnt/storage/servers/[container_name], which in the example above is /mnt/storage/servers/mycontainer

    [root@srv ~]# ls -al /mnt/storage/servers/mycontainer
total 16
drwxrwx---.  3 root root 4096 Oct 14 13:38 .
drwxr-xr-x.  4 root root 4096 Oct 14 13:41 ..
-rw-r-----.  1 root root  780 Oct 14 13:38 config
drwxr-xr-x. 19 root root 4096 Oct 14 07:15 rootfs

It is better to use the “-P” and to change the LXC container location than only the filesystem path. In this case, a good practice is to make a symbolic link in /var/lib/lxc/[container-name] to the new location:

ln -s /mnt/storage/servers/mycontainer /var/lib/lxc/mycontainer

So all LXC tools will continue to work without explicitly adding an option for the new path of this container.

Change the SELinux file context to be container_var_lib_t of the LXC root filesystem

Add the file context container_var_lib_t to the container’s root filesystem path and change the SELinux labels.
First, verify all the needed tools are installed:

dnf install -y policycoreutils-python-utils container-selinux

Then, add a new file context to the path /mnt/storage/servers/mycontainer and run the restorecon to change the SELinux labels to container_var_lib_t

semanage fcontext -a -t container_var_lib_t '/mnt/storage/servers/mycontainer(/.*)?'
restorecon -Rv /mnt/storage/servers/mycontainer

The file context may be shown with:

[root@srv ~]# ls -alZ /mnt/storage/servers/mycontainer
total 16
drwxrwx---.  3 root root unconfined_u:object_r:container_var_lib_t:s0 4096 Oct 14 13:38 .
drwxr-xr-x.  4 root root unconfined_u:object_r:mnt_t:s0               4096 Oct 14 13:41 ..
-rw-r-----.  1 root root unconfined_u:object_r:container_var_lib_t:s0  780 Oct 14 13:38 config
drwxr-xr-x. 19 root root unconfined_u:object_r:container_var_lib_t:s0 4096 Oct 14 07:15 rootfs

Failing to set the proper SELinux labels may result to errors such as lxc_attach_run_shell: 1333 Permission denied – failed to exec shell

lxc_attach_run_shell: 1333 Permission denied – failed to exec shell

An annoying error when using the LXC container tools like lxc-attach, which is really simple to fix.

[root@srv ~]# lxc-attach -n db-cluster-3
lxc_container: attach.c: lxc_attach_run_shell: 1333 Permission denied - failed to exec shell
[root@srv ~]#

This error just reports the bash shell in the container cannot be started and the SELinux audit file adds some errors, too:

type=AVC msg=audit(1665745824.682:24229): avc:  denied  { entrypoint } for  pid=20646 comm="lxc-attach" path="/usr/bin/bash" dev="md3" ino=111806476 scontext=system_u:system_r:unconfined_service_t:s0 tcontext=unconfined_u:object_r:var_log_t:s0 tclass=file
type=SYSCALL msg=audit(1665745824.682:24229): arch=c000003e syscall=59 success=no exit=-13 a0=24412c6 a1=7ffe87c07170 a2=2443870 a3=7ffe87c08c60 items=0 ppid=20644 pid=20646 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts12 ses=3304 comm="lxc-attach" exe="/usr/bin/lxc-attach" subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=PROCTITLE msg=audit(1665745824.682:24229): proctitle=6C78632D617474616368002D6E0064622D636C75737465722D33
type=AVC msg=audit(1665745824.682:24230): avc:  denied  { entrypoint } for  pid=20646 comm="lxc-attach" path="/usr/bin/bash" dev="md3" ino=111806476 scontext=system_u:system_r:unconfined_service_t:s0 tcontext=unconfined_u:object_r:var_log_t:s0 tclass=file
type=SYSCALL msg=audit(1665745824.682:24230): arch=c000003e syscall=59 success=no exit=-13 a0=7f08b5e579a0 a1=7ffe87c07170 a2=2443870 a3=7ffe87c08c60 items=0 ppid=20644 pid=20646 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts12 ses=3304 comm="lxc-attach" exe="/usr/bin/lxc-attach" subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)
type=PROCTITLE msg=audit(1665745824.682:24230): proctitle=6C78632D617474616368002D6E0064622D636C75737465722D33

So clearly, the problem is in SELinux, and turn it off temporarily with

setenforce 0

Turning off the SELinux is not the right thing! There are two aspects to the problem:

  • Missing SELinux rules, which are installed with a special package container-selinux
  • Wrong SELinux permissions for the LXC container’s root directory. In most cases, the user just changes the default /var/lib/lxc/[container] to something new and the LXC works, but it breaks some LXC parts.

Installing container-selinux is easy:

dnf install -y container-selinux

Or the old yum:

yum install -y container-selinux

Then check the SELinux attributes with:

[root@srv ~]# ls -altrZ /mnt/storage/servers/mycontainer/
drwxr-xr-x. root root unconfined_u:object_r:var_log_t:s0 ..
-rw-r--r--. root root unconfined_u:object_r:var_log_t:s0 config
drwxrwx---. root root unconfined_u:object_r:var_log_t:s0 .
drwxr-xr-x. root root unconfined_u:object_r:var_log_t:s0 rootfs

The problem is var_log_t, which is an SELinux file context and it should be container_var_lib_t. Stop the container and fix the permissions. If the default directory (/var/lib/lxc) were used, it would not have this problem. Adding the SELinux file context definition to the new directory is mandatory when changing the directory root of a container:

[root@srv ~]# semanage fcontext -a -t container_var_lib_t '/mnt/storage/servers/mycontainer(/.*)?'
[root@srv ~]# restorecon -Rv /mnt/storage/servers/mycontainer/
restorecon reset /mnt/storage/servers/mycontainer context unconfined_u:object_r:var_log_t:s0->unconfined_u:object_r:container_var_lib_t:s0
.....
.....
restorecon reset /mnt/storage/servers/mycontainer/config context unconfined_u:object_r:var_log_t:s0->unconfined_u:object_r:container_var_lib_t:s0

All files permissions under /mnt/storage/servers/mycontainer/ should be fixed with the restorecon. Start the LXC container and try to attach it with lxc-attach. Now, there should not be any errors:

[root@srv ~]# lxc-attach -n mycontainer
[root@mycontainer ~]#

The files’ context is the right one – container_var_lib_t:

[root@srv ~]# ls -altrZ /mnt/storage/servers/mycontainer/
drwxr-xr-x. root root unconfined_u:object_r:var_log_t:s0 ..
-rw-r--r--. root root unconfined_u:object_r:container_var_lib_t:s0 config
drwxrwx---. root root unconfined_u:object_r:container_var_lib_t:s0 .
drwxr-xr-x. root root unconfined_u:object_r:container_var_lib_t:s0 rootfs

More on LXC containers – https://ahelpme.com/category/software/lxc/.

syslog – UDP local to rsyslog and send remote with TCP and compression

This article is to show how to log Nginx’s access logs locally using UDP to the local rsyslog daemon, which will send the logs to a remote rsyslog server using TCP and compression. In general, logs could generate a lot of traffic and using UDP over distant locations could result in packet loss respectively logs’ lines loss. The idea here is to log messages locally using UDP (non-blocking way) to a local Syslog server, which will send the stream to a remote central Syslog server using TCP connections to be sure no packets are lost. In addition, we are going to enable local caching (if the remote server is temporary unreachable) and compression between the two Syslog servers.
Our goal is to use

  • UDP for our client program (Nginx in the case) for non-blocking log writes.
  • TCP between our local machine and the remote syslog server – to be sure not to lose messages on bad connectivity.
  • local caching for our client machine – not to lose messages if the remote syslog is temporary unreachable.
  • compression between the local machine and the remote syslog server.

The configuration and the commands are tested on CentOS 7, CentOS 8 and Ubuntu 18 LTS. Check out UDP remote logging here – nginx remote logging to UDP rsyslog server (CentOS 7).

STEP 1) Configure client’s local rsyslog to accept UDP log messages only if the messages’ tags are “nginx”

Couple of things should be enabled in the local client-size rsyslog daemon:

  • rsyslog to accept UDP messages. Uncomment or add the following under section “Modules” (probably the first section in the file?) in /etc/rsyslog.conf 
    $ModLoad imudp
$UDPServerRun 514

    or

    module(load="imudp")
input(type="imudp" port="514")

    The first is the old syntax, which is still supported and the second is the new syntax. For simplicity, all of the following configuration will be using the new syntax, because the old one is depricated.

  • Add a rule to catch the tag containing “nginx” and execute action to forward the messages to the remote server 
    if ($syslogtag == 'nginx:') then {
action(type="omfwd" target="10.10.10.10" port="10514" protocol="tcp" compression.Mode="single" ZipLevel="9"
queue.filename="forwarding" queue.spoolDirectory="/var/log" queue.size="1000000" queue.type="LinkedList" queue.maxFileSize="1g" queue.SaveOnShutdown="on"
action.resumeRetryCount="-1")
& stop
}

    • The options are almost self-explanatory, the important ones are there is no retry limit count of reconnecting to the server, there is in-disk cache of maximum 1G if the remote server is unavailable and the compression per message is turned on. More on actions – https://www.rsyslog.com/doc/v8-stable/configuration/actions.html, the forward module – https://www.rsyslog.com/doc/v8-stable/configuration/modules/omfwd.html and the queue – https://www.rsyslog.com/doc/v8-stable/rainerscript/queue_parameters.html

And restart the rsyslog:

systemctl restart rsyslog

Keep on reading!