Topics: Red Hat, System Admin

Linux Screen

The screen utility on Linux allows you to:

  • Use multiple shell windows from a single SSH session
  • Keep a shell active even through network disruptions
  • Disconnect and re-connect to a shell sessions from multiple locations
  • Run a long running process without maintaining an active shell session
First, let's install screen on a CentOS system:
# yum -y install screen
Once it's installed, screen can be easily started:
# screen
You are now inside of a window within screen. This functions just like a normal shell except for a special control command: "Ctrl-a".

Screen uses the command "Ctrl-a" (that's the control key and a lowercase "a") as a signal to send commands to screen instead of the shell.

For example, type "Ctrl-a", let go, and then type "?". You should now see the screen help page, showing you all the available key binding. Key bindings are the commands the screen accepts after you hit "Ctrl-a". You can reconfigure these keys to your liking using a .screenrc file, if you like.

To create a new window, you can use "Ctrl-a" and "c". This will create a new window for you with your default prompt. Your old window is still active.

For example, you can be running top and then open a new window to do other things. Top will remain running in the first window.

Screen allows you to switch between screens, by using "Ctrl-a" and "n". This command switches you to the next window. If you were to open more windows in screen, then "Ctrl-a" and "n" will allow you to cycle through all the windows, by repating the "Ctrl-a" and "n" commands. The windows work like a carousel and will loop back around to your first window. You can create several windows and toggle through them with "Ctrl-a" and "n" for the next window or "Ctrl-a" and "p" for the previous window. Each process in a window will keep running until you exit out of that window by typing "exit".

Anoter feature of screen is that you can detach from a screen, by typing "Ctrl-a" and "d". Screen allows you to detach from a window and reattach later. If your network connection fails, screen will automatically detach your session! If you detach from screen, you will drop back into your shell. All screen windows are still there and you can re-attach to them later.

If your connection drops or you have detached from a screen, you can re-attach by just running:
# screen -r
This will re-attach to your screen.

Screen will also allow you to create a log of the session, by typing "Ctrl-a" and "H". When you do that, you'll see in the Putty titlebar of your session the name of the log file being created, usually in the form of "screenlog.0". Screen will keep appending data to the file through multiple sessions. Using the log function is very useful for capturing what you have done, especially if you are making a lot of changes. If something goes awry, you can look back through your logs. Locking Your Screen Session

If you need to step away from your computer for a minute, you can lock your screen session using "Ctrl-a" and "x". This will require a password to access the session again.

When you are done with your work, you can stop screen by typing exit from your shell. This will close that screen window. You have to close all screen windows to terminate the session. You should get a message about screen being terminated once you close all windows. Alternatively, you can use "Ctrl-a" and "k". You should get a message if you want to kill the screen.

Topics: Red Hat, System Admin

Multi-user VNC setup on RHEL 7.5

Here's how to set up VNC on Red Hat 7.5, combined with the Gnome desktop, Firefox and TigerVNC.

The goal is to install a Linux desktop, Firefox and TigerVNC on a system with just a base (minimal) Red Hat 7.5 install (without a desktop), and to set up the VNC service for 2 users, in this case for user root, and for user oracle. The VNC port to use for user root will be 5901, and it will be 5092 for user oracle.

Note: This procedure will also work on older RHEL 7 versions, like RHEL 7.2 through RHEL 7.4, with a few minor changes as there are a few differences between these RHEL releases. Please see below.

Install the GUI first (based on: https://access.redhat.com/solutions/5238):

# yum -y groupinstall "Server with GUI"
# yum install xterm xorg-x11-xinit
Install TigerVNC:
# yum -y install tigervnc tigervnc-server
There is no need to specifically install Firefox - it is installed as part of the GUI installation.

If here, you are not using RHEL 7.5, but an older version of RHEL 7, then please make sure to (at least) update the following packages to the latest available versions. These latest package versions are needed to make this work:
# yum -y update xterm xorg-x1-xinit tigervnc tigervnc-server
Start the GUI:
# systemctl set-default graphical.target
# systemctl start graphical.target
Configure VNC (based on https://access.redhat.com/solutions/966063):

Configure the VNC password for both root and user oracle (repeat for both users - log in as each user, and run the following command):
# vncpasswd
You will be asked if you would like to enter a view-only password. You may answer "n" for no.

Set up the VNC service on the system:

For user root:
# cp /lib/systemd/system/vncserver@.service /etc/systemd/system/vncserver@:1.service
Edit the new file, and replace all entries in the files of "<USER>" with "root"; ensure the home directory of user root is also set to /root, not /home/root.

For user oracle:
# cp /lib/systemd/system/vncserver@.service /etc/systemd/system/vncserver@:2.service
Edit the new file, and replace all entries of "<USER>" with "oracle".

Edit the xstartup user file in ~root/.vnc/xstartup and ~oracle/.vnc/xstartup. Replace the contents of the xstartup file with this:
#!/bin/sh

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
  [ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
  vncconfig -iconic &
  dbus-launch --exit-with-session gnome-session &
If necessary, if the firewall is in use, add the ports in the firewall.

First check if the firewall daemon is running right now, and enabled at boot time:
# systemctl status firewalld
If so, then add the ports used by VNC to the firewall configuration:
# firewall-cmd --permanent --zone=public --add-port 5901/tcp
# firewall-cmd --permanent --zone=public --add-port 5902/tcp
# firewall-cmd --reload
Run the following command as changes were made to systemd files:
# systemctl daemon-reload
Enable and start the TigerVNC service:
# systemctl enable vncserver@:1.service
# systemctl enable vncserver@:2.service
# systemctl start vncserver@:1.service
# systemctl start vncserver@:2.service
If, at this point, when starting either VNC service, you get an error about not available resources, it may be that either VNC was already running, or that there are old VNC files in /tmp. In this case, first search for any running VNC processes:
# ps -ef | grep VNC
If any VNC processes are still running, then kill them, by using "kill -9". Then move over to the /tmp folder and clear out any old files used by VNC:
# cd /tmp
# rm -rf .X*
And then, try starting the VNC services again:
# systemctl start vncserver@:1.service
# systemctl start vncserver@:2.service
That should work. If so, then proceed with the next steps:

Check if the VNC services are listening on the ports 5901 and 5902:
# netstat -an | grep ::590
tcp        0      0 0.0.0.0:5901            0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:5902            0.0.0.0:*               LISTEN
tcp6       0      0 :::5901                 :::*                    LISTEN
tcp6       0      0 :::5902                 :::*                    LISTEN
Now, it's time to test the VNC connections. A good way to test, without having to install a VNC client (usually requiring admin privileges on your Windows desktop), use realVNC VNC viewer, from https://www.realvnc.com/en/connect/download/viewer/windows/. In the dropdown list on this website, make sure to select the "Standalone" version that applies to your operating system version. The regular EXE file on this site is a VNC viewer that requires admin privileges on Windows to install. This "standalone" VNC viewer can be used without having to install any software, and does not require admin-level access on Windows.

Open the screen for user root, by typing the following string, assuming the IP address of the server is 172.29.126.210:
172.29.126.210:5901
And for user oracle:
172.29.126.210:5902
And type the password provided earlier through the vncpasswd command.

That's it. You should be presented with desktop screens for both users root and oracle, and you should be able to run Firefox within those desktops.

Topics: Red Hat, Security, System Admin

Resetting the root password for a KVM guest image

Red Hat provides you the opportunity to download a KVM guest image, that you can use within virt-manager to start immediately. This saves you the trouble and time of having to install the operating system.

However, the root password is not known, and so, it may be difficult to log in as root when using the KVM guest image provided by Red Hat.

Luckily, there is an easy solution to changing the root password on a KVM guest image.

Start by installing guestfish:

# yum -y install guestfish
Guestfish is a tool that can be used from the command line to access guest virtual machine file systems.

Next, update the image file as follows, assuming the image file is located in /var/lib/libvirt/images, and the image file is called "rhel7.5.beta1.qcow2", and you want to set the password to "PASSWORD":
# cd /var/lib/libvirt/images
# virt-customize -a rhel7.5.beta1.qcow2 --root-password password:PASSWORD

Topics: Red Hat, System Admin

Keystrokes used in top

The top command is quite useful in Red Hat Enterprise Linux. This is a list of common keystrokes that can be used in top:

Key Purpose
? or h Help for interactive keystrokes.
l, t, m Toggles for load, threads, and memory header lines.
1 Toggle showing individual CPUs or a summary for all CPUs in header.
s or d Change the refresh (screen) rate, in decimal seconds (e.g., 0.5, 1, 5).
b Toggle reverse highlighting for Running processes; default is bold only.
B Enables use of bold in display, in the header, and for Running processes.
H Toggle threads; show process summary or individual threads.
u, U Filter for any user name (effective, real).
M Sorts process listing by memory usage, in descending order.
P Sorts process listing by processor utilization, in descending order.
k Kill a process. When prompted, enter PID, then signal.
r Renice a process. When prompted, enter PID, then nice_value.
W Write (save) the current display configuration for use at the next top restart.
q Quit.

Topics: Red Hat, System Admin

Processes

A process is a running instance of a launched, executable program. A process consists of:

  • an address space of allocated memory,
  • security properties including ownership credentials and privileges,
  • one or more execution threads of program code, and
  • the process state.
The environment of a process includes:
  • local and global variables,
  • a current scheduling context, and
  • allocated system resources, such as file descriptors and network ports.
An existing (parent) process duplicates its own address space (fork) to create a new (child) process structure. Every new process is assigned a unique process ID (PID) for tracking and security. The PID and the parent's process ID (PPID) are elements of the new process environment. Any process may create a child process. All processes are descendants of the first system process, which is systemd(1) on a Red Hat Enterprise Linux 7 system.


Through the fork routine, a child process inherits security identities, previous and current file descriptors, port and resource privileges, environment variables, and program code. A child process may then exec its own program code. Normally, a parent process sleeps while the child process runs, setting a request (wait) to be signaled when the child completes. Upon exit, the child process has already closed or discarded its resources and environment; the remainder is referred to as a zombie. The parent, signaled awake when the child exited, cleans the remaining structure, then continues with its own program code execution.

In a multitasking operating system, each CPU (or CPU core) can be working on one process at a single point in time. As a process runs, its immediate requirements for CPU time and resource allocation change. Processes are assigned a state, which changes as circumstances require.


The Linux process states are illustrated in the previous diagram and described in the following table.

Name Flag Kernel-defined state name and description
Running R

TASK_RUNNING: The process is either executing on a CPU or waiting to run. Process can be executing user routines or kernel routines (system calls), or be queued and ready when in the Running (or Runnable) state.

Sleeping S

TASK_INTERRUPTIBLE: The process is waiting for some condition: a hardware request, system resource access, or signal. When an event or signal satisfies the condition, the process returns to Running.

D

TASK_UNINTERRUPTIBLE: This process is also Sleeping, but unlike S state, will not respond to delivered signals. Used only under specific conditions in which process interruption may cause an unpredictable device state.

K

TASK_KILLABLE: Identical to the uninterruptible D state, but modified to allow the waiting task to respond to a signal to be killed (exited completely). Utilities frequently display Killable processes as D state.

Stopped T

TASK_STOPPED: The process has been Stopped (suspended), usually by being signaled by a user or another process. The process can be continued (resumed) by another signal to return to Running.

T

TASK_TRACED: A process that is being debugged is also temporarily Stopped and shares the same T state flag.

Zombie Z

EXIT_ZOMBIE: A child process signals its parent as it exits. All resources except for the process identity (PID) are released.

X

EXIT_DEAD: When the parent cleans up (reaps) the remaining child process structure, the process is now released completely. This state will never be observed in process-listing utilities.

Topics: Red Hat, System Admin

How to tie a system to a specific update of Red Hat Enterprise Linux

Please see the following page: https://access.redhat.com/solutions/238533, if you need to update a Red Hat Enterprise Linux system, but need to ensure at the same time that the system isn't upgraded to a new minor release (e.g. from version 7.3 to version 7.4).

The trick is to use the "releasever" option with the yum commmand. For example, if you have a Red Hat Enterprise Linux system running version 7.3 (check with "cat /etc/redhat-release), and you need to keep it at this version (e.g. for application specific reasons), then run the following command to only update the system pacakages relevant to version 7.3:

# yum --releasever=7.3 update
Once the update has been completed, check file /etc/redhat-release to ensure the system is still at version 7.3, and has not been upgraded to version 7.4.

And don't forget to reboot the system after doing updates. The following command will indicate if the command needs to be rebooted, to ensure that all installed updates are properly activated:
# needs-restarting -r
If the needs-restarting command is not avaialable on your system, then please ensure to install RPM yum-utils:
# yum -y install yum-utils

Topics: Red Hat, System Admin

Read PDF files in Gnome

There are 2 easy ways to read PDF files in Gnone (the default desktop for Red Hat Enterprise Linux): Use Firefox or Evince.

Evince is the Gnome document viewer, and can be easily opened as follows:

# evince /usr/share/doc/libtasn1-4.10/libtasn1.pdf
You can also use Firefox. Firefox has built-in PDF support. You can open it as follows:
# firefox /usr/share/doc/libtasn1-4.10/libtasn1.pdf

Topics: Red Hat, System Admin

Resizing a Red Hat swap space

The general procedure for resizing a swap space is as follows (assuming the swap space is set up as a logical volume within the root volume group called vg_root), for example to resize a swap space to 8 GB:

# swapoff -v /dev/mapper/vg_root-lv_swap
# lvm lvresize /dev/mapper/vg_root-lv_swap -L 8G 
# mkswap /dev/mapper/vg_root-lv_swap
# swapon -va

Topics: Red Hat, System Admin

Subscribing a Red Hat system

Here's how to register and un-register a Red Hat system through subscription-manager. You'll need to do this, for example, if you wish to do operating system updates on a Red Hat system.

First, here's how to unregister a system. This might come in handy if you do not have enough subscriptions in your Red Hat account, and temporarily want to move a valid subscription over to another system):

# subscription-manager unregister
System has been unregistered.
And here's how you register:
# subscription-manager register
Registering to: subscription.rhsm.redhat.com:443/subscription
Username: [type your Red Hat username here]
Password: [type your Red Hat password here]
The system has been registered with ID: 3db39bee-bd48-46e8-9abc-9ba9
If you have issues registering a server, try removing all Red Hat subscription information first, and then register again, using the "auto-attach" option:
# subscription-manager clean
All local data removed
# subscription-manager list

+-------------------------------------------+
    Installed Product Status
+-------------------------------------------+
Product Name:   Red Hat Enterprise Linux Server
Product ID:     69
Version:        7.4
Arch:           x86_64
Status:         Unknown
Status Details:
Starts:
Ends:

# subscription-manager register --auto-attach
Registering to: subscription.rhsm.redhat.com:443/subscription
Username: [type your Red Hat username here]
Password: [type your Red Hat password here]
The system has been registered with ID: 3db39bee-bd48-46e8-9abc-9ba9

Installed Product Current Status:
Product Name: Red Hat Enterprise Linux Server
Status:       Subscribed

# subscription-manager list

+-------------------------------------------+
    Installed Product Status
+-------------------------------------------+
Product Name:   Red Hat Enterprise Linux Server
Product ID:     69
Version:        7.4
Arch:           x86_64
Status:         Subscribed
Status Details:
Starts:         12/27/2017
Ends:           12/26/2020
If you wish to use a specific Red Hat subscription, then you may first check for the available Red Hat subscriptions, by running:
# subscription-manager list --available --all
In the output of the command above, you will see, if any subscriptions are available, a Pool ID. You can use that Pool ID to attach a specific subscription to the system, for example, by running:
# subscription-manager attach pool=8a85f98c6267d2d90162734a700467b2

Topics: Security, System Admin

Automatically accept new SSH keys

Whenever you have to connect through SSH to a lot of different servers, and you create a command for it like this:

# for h in $SERVER_LIST; do ssh $h "uptime"; done
You may run into an error that stops your command, especially when a new server is added to $SERVER_LIST, like this:
The authenticity of host 'myserver (1.2.3.4)' can't be established.
RSA key fingerprint is .....
Are you sure you want to continue connecting (yes/no)?
And you'll have to type "yes" every time this error is encountered.

So, how do you automate this, and not have to type "yes" with every new host?

The answer is to disable strict host key checking with the ssh command like this:
ssh -oStrictHostKeyChecking=no $h uptime
Please note that you should only do this with hosts that you're familiar with, and/or are in trusted networks, as it bypasses a security question.

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