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Connecting to the server via ssh, using multiple consoles and screen command

Log in and switch users in multiuser targets

News Red Hat Certification Program Understanding and using essential tools Access a shell prompt and issue commands with correct syntax Finding Help Managing files in RHEL Working with hard and soft links Working with archives and compressed files Using the Midnight Commander as file manager
Text files processing Using redirection and pipes Use grep and extended regular expressions to analyze text files Finding files and directories; mass operations on files Connecting to the server via ssh, using multiple consoles and screen command Introduction to Unix permissions model      
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Extracted from Professor Nikolai Bezroukov unpublished lecture notes.

Copyright 2010-2018, Dr. Nikolai Bezroukov. This is a fragment of the copyrighted unpublished work. All rights reserved.

NOTE: Exercises are partially derived from:

Contents


Introduction

Typical sysadmin in enterprise environment connect to the server from windows environment using terminal emulator such as Putty and Teraterm.

On the exam you will work with the server directly. But  s multiple servers are used you need to connect to them via ssh.  To speed up the work you need to work with multiple windows and on other methods to increase you productivity in command line environment. One of such methods is to use screen -- terminal multiplexor which is included in RHEL ISO.  It allow open multiple terminal windows (one for each directory you are using on the remote server) and this save time on switching between various, sometimes deeply nested directories.

The other more complex, time consuming and risky method (for the exam) is to try to install GUI and use GUI environment, if it was not provided by default. During training you preferably should use GUI environment as it more convenient. If you connect from Windows laptop you can also use multiple terminals.

Working with Multiple Terminals in a Non-graphical Environment

In a non=graphical environment, you just have a set of six virtual consoles -- six terminal windows. To switch  use Alt+F1  through Alt+F6.

Tip: A convenient alternative to using the Alt+F1 Alt-F6 in non-graphical environment when you access the console directly (if and only if you access the console directly, ssh session does not qualify)  is offered by the chat  command which is installed by default of RHEL7. For example, type chvt 3. This brings you to a login prompt on virtual terminal 3. Switch back to the graphical environment using the chvt 1  command.

The first virtual terminal is so called the default console. the first corresponds to the device /dev/tty1. The other virtual consoles to devices /dev/tty2 through /dev/tty6.

Working in GUI is more convenient, and is preferable in training

The graphical environment is the most convenient environment to work as it allow open any number of terminal sessions (aka virtual consoles) on the screen. And  if you have large display you can position 4 terminal sessions on the screen which make working with multiple sessions very convenient.

Gnome also has multiple virtual screen, which is another big advantage as you can have several sets of terminal session on different virtual screens.

If you connect from windows you can open multiple session via your terminal emulator and also potion then side byside on the screen. Window does not have multiple virtual screens though.

NOTE:

Understanding Pseudo Terminal Devices

When you connect via ssh you connect to so called pseudo terminals.  These pseudo terminals are corresponds to devices /dev/pts directory. The first is  /dev/pts/1, the second  is /dev/pts/2,  and so on.  You can see then if you type the  command last

[0]test01@ROOT:/mnt/Packages # last
bezroun  pts/0        ool-44c79867.dyn Mon Oct 15 22:41   still logged in
reboot   system boot  3.10.0-862.14.4. Mon Oct 15 22:37 - 05:35  (06:57)
bezroun  pts/1        ool-44c79867.dyn Sun Oct 14 08:45 - 06:04  (21:18)
bezroun  pts/0        ool-44c79867.dyn Sun Oct 14 08:32 - 06:04  (21:31)
reboot   system boot  3.10.0-862.14.4. Sun Oct 14 08:23 - 06:04  (21:40)
bezroun  pts/0        ool-44c79867.dyn Sat Oct 13 22:30 - 05:03  (06:32)
reboot   system boot  3.10.0-862.14.4. Sat Oct 13 22:29 - 05:04  (06:35)
bezroun  pts/0        ool-44c79867.dyn Fri Oct 12 22:27 - 05:02  (06:35)
bezroun  pts/0        ool-44c79867.dyn Fri Oct 12 17:43 - 21:02  (03:18)
reboot   system boot  3.10.0-862.14.4. Fri Oct 12 17:43 - 05:02  (11:19)
alterego pts/1        ool-44c79867.dyn Fri Oct 12 04:16 - 05:03  (00:47)
bezroun  pts/0        ool-44c79867.dyn Fri Oct 12 02:33 - 05:03  (02:30)

Exercise:  Working with Pseudo Terminals

In this exercise, you learn how to work with pseudo terminals. You also learn how to get more information on user activity on specific pseudo interfaces.

1.  Login via ssh to your test server. Switch to root.

2.  From the terminal window, type w. This will give an overview of all users who are currently logged in.

[0]test01@ROOT:/mnt/Packages # w
 05:37:28 up  6:59,  1 user,  load average: 0.00, 0.01, 0.05
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
bezroun  pts/0    dsk-44           22:41    0.00s  0.32s  0.20s sshd: bezroun [priv]

Booting, Rebooting, and Shutting Down Servers

In RHEL7 to issue a proper reboot you need to use the systemd. The "addition" to RHEL7 make administration of RHEL7 drastically different from administration of RHEL6 and this is not a good thing.  So now you need use:

The systemctl poweroff  command should shut off power on the machine. While systemctl halt should shutdown and then hang the server without switching the power off.

When other users are login issuing systemd reboot  and similar commands is dangerous as those you might not be able to save their work. You need to warn them first.

Note: systemd is very complex and thus notoriously unreliable (and problems with it significantly slowed down adoption of RHEL7 in enterprise environment)  and might not work if the server is malfunctioning. In such cases you need to try to force a machine to reset, you can try to type echo b > /proc/sysrq-trigger. This command immediately resets the machine without saving anything. Of course, this command should be used only if there are no other options!

Using SSH to connect to remote server

Those days sysadmins typically connect to servers from corporate Windows laptop in his/her office or cubical. And often servers are in remote locations, that if are not physically accessible. To manage these servers, Secure Shell (SSH) is normally used. In this section, you learn how to work with SSH.

In RHEL7 ssh service (sshd daemon) is running  by default and offering services at port 22, and it not blocked by the firewall. You use iether terminal emulator like  PuTTY or if you are on other linux server command like ssh command to reach the other server. The ssh  command by default uses port 22. If  you ssh server was reconfigured to run on other port (which happen in some Web hosting companies), use ssh -p  followed by the port number you want to connect to.

to connect to other Linux server from a Linux session just type ssh  followed by the name of the other Linux machine.  For example

ssh test02

ssh will try to connect you as the same user ID under which you are logged on the first machine (for example root). If you need other user you need specify the name. For example

ssh alterego@test02

If, for example, you are logged as regular use on you current server and  want to connect as root to the other server you need to use the command ssh root@remoteserver.

An alternative way of specifying the user account is by using the option -l userID. So, ssh remoteserver -l root  test02 will try to establish a root session to the remote server test02

NOTE: On Windows there is reach choice of terminal emulators including  Teraterm and MobaXterm.  The latter is so called "rich" terminal emulator and also offers major Unix command in windows environment as well as the ability to transfer files.

Exercise: connect to remote server via SSH

1.  From the root shell on server1 type systemctl status sshd. This command in RHEL7 replaces server command of RHEL6 and allows to determine the status of this service.

2.  To avoid any firewall-related problems, type systemctl stop firewalld. (this product of systemd team replaced  iptables and you you learn how to configure this firewall to limit traffic).

3.  Type ip a | grep 'inet '. (Notice the space between inet  and the closing quote. Command ip is also a "new brave world", although unlike farewald it has some advantages over ifconfig command ( unlike firewalld it also was available in RHEL6 ) 

[0]test01@ROOT:/mnt/Packages # ip a | grep 'inet '
    inet 127.0.0.1/8 scope host lo
    inet 10.0.0.4/24 brd 10.0.0.255 scope global eth0

That is a new way to determine the IPv4 address on RHEL7 server tha tyou now is supposed to know. But old good command ifconfig -a is still availbe too:

[0]test01@ROOT:/mnt/Packages # ifconfig -a
eth0: flags=4163  mtu 1500
        inet 10.0.0.4  netmask 255.255.255.0  broadcast 10.0.0.255
        inet6 fe80::20d:3aff:fe14:13ca  prefixlen 64  scopeid 0x20
        ether 00:0d:3a:14:13:ca  txqueuelen 1000  (Ethernet)
        RX packets 3483378  bytes 4885873205 (4.5 GiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 812773  bytes 105224318 (100.3 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

4.  Now type cat /etc/hosts and determine ip of your second server. Let's assume that it is 10.0.0.5

5.  On server1, type ssh  -l root 10.0.0.5  This connects to the sshd process on test02 server and opens a root shell.

6.  Before being prompted for a password, you see a message indicating that the authenticity of host  10.0.0.5 cannot be established. This message is shown because the host you are connecting to is not yet known on your current host, which might involve a security risk. that also is normal message if the remote server has never been contacted before and therefore there is no way to verify the identity of the remote server. After connecting to the remote server, a public key fingerprint is stored in the file ~/.ssh/known_hosts. Type yes  to continue. You should not be prompted if you connect the second time.

7.  When prompted, enter the root password. After entering it, you now are logged in to test02 and can investigate it.  To terminate this  session type exit

NOTES:

  1. In some cases, for example if you reinstalled OS on the second server of changed its IP address the remote host key fingerprint does not match the key fingerprint that is stored locally. This situation also arise if you ssh session is decoded in the middle (attack in the middle)  which is a potentially dangerous attack. If you made changes mention before then you can  remove the key fingerprint from the ~/.ssh/known_hosts file. Otherwise you need to investigate why this situation arose.
  2. If you connect to other computer form terminal window in GUI environment you might wish to see X11 screen of the terget computer too.  It is possible if X server is running of target computer too. If this is the case you can add option  -X  to the ssh  command. for example  ssh -X alterego@test02   you can also create a systemwide configuration that allows you to use “X forwarding,” which is starting graphical applications through an SSH session. As root, open the configuration file /etc/ssh/ssh_config  and make sure it includes the following line:

    ForwardX11 yes

    The next time you use the ssh  command, X forwarding will be available by default.

Configuring key-based authentication for SSH

Instead of using password for authentication ssh provide more secure method based on you a public/private key pair. The private key needs to be kept private and will never be distributed outside server on which it was generated. The public key is stored in the home directory of the target user on each og the servers you want to communicate with

When authenticating using key pairs, the ssh client generates a hash derived from the private key. This hash is sent to the server, then the server sends something bask to complete authentication. This is called ssh handshake.

To create a key pair, use the ssh-keygen  command. The ssh-copy-id  command is used to copy the public key to the target server.

When creating a public/private key pair, you are prompted for a passphrase. If you want maximal security, you should enter a passphrase.  If not you can skip this step and get so called passwordless public/private paie which is still more secure then any password based authentication and is enough for most purposes outside financial and military applications.  You should never transfer your private key using unencrypted channel, or send it by email, though. 

Creating Public/Private Keys and distribution private key to the target server

  1. On the server test01, open a root shell.
  2. Switch to ~/.ssh directory and type ssh-keygen. When asked for the filename in which to store the (private) key, accept the default filename ~/.ssh/id_rsa. When asked whether you want to use a passphrase, press Enter  twice to creates passwordless public/private pair
  3. Check if the private key will now be written to the ~/.ssh/id_rsa file and the public key is written to the ~/.ssh/id_rsa.pub file.
  4. If yes execute  the utility ssh-copy-id test02  to copy the public key you have just created over to test02. it will be added to the file ~/.ssh/authorized_keys file on the remote host. You are then asked for the password on the remote server. After that you can enjoy passwordless login.
  5. To check the setup, type ssh test02. You should now authenticate without having to enter the password for the remote server test02.

NOTE: if multiple users are using keys to log in with that specific account, the authorized_keys file may contain several public keys. If you accidentally overwrite  this file, you will destroy ability of other users to login on the server using ssh private key authentication

Transferring files to remote server use scp

scp command allow to transmit files form computer to another using secure encrypted channel. For example, to transfere the /etc/hosts file to the /tmp directory on test02 server using your current user account, use the following command:

scp /etc/hosts test02:/tmp

You can also use scp  to copy an entire subdirectory structure. To do so, use the -r  command, as in the following command:

scp -r /etc test02:/tmp

that creates a backup copy of /etc of server test01 on server test01. which is useful for transfering common files to another server. It also allow you to restore files if you did something wrong in your /etc/directory on server test01.

ssh test01 mv /root/.bash_profile  /root/.bash_profile.default
ssh test01 mv /root/.bashrc  /root/.bashrc.default
scp /root/.bash_profile /root/.bashrc test02:/root

This command transfer you modified files .bash_profile and  .bashrc (with added aliases) to server test02. That makes working as root on server test02 more convenient. Old files are overwritten so we make backup of them first.

NOTES:

Using the screen command to enhance your productivity

The screen  command allows you to multiplex single session. It does not just allow you to run multiple terminals, but also allows you to share a session with other users, or to detach to remote terminal sessions and later resume it getting exactly the same environment as when you left.

Screen is not installed by default but is available from Red Hat RPM. To install it use the yum install -y screen  command. Then, type screen  to open a screen session. After installation the most important file to know about is screen configuration file which is located in etc directory:

bezroun@test01:/home/bezroun # rpm -ql screen
/etc/pam.d/screen
/etc/screenrc

You need to make at least three changes in the /etc/screenrc to make screen more user friendly:

  1. Change the line with "#autodetach off" to "autodetach on"
  2. Remove comment sign(#) from the line "#shell -$SHELL"
  3. Uncomment the first (shorter) "caption on" line
[0]test01@ROOT:/etc # egrep "^#autodetach|#shell|caption always" /etc/screenrc
#autodetach off
#shell -$SHELL
# caption always "%3n %t%? @%u%?%? [%h]%?%=%c"
# caption always "%{wk}%?%-Lw%?%{bw}%n*%f %t%?(%u)%?%{wk}%?%+Lw %=%{mk}@%H %{yk}%D %{ck}%M%{wk} %{ck}%d %{gk}%c"

After that screen is workable condition and you can start screen session  by simply typing screen on the command line. You will get terminal window with the last line reserved for screen information.  and can type any command it in it. If you close session, screen autodetach its session and it will be preserved so if you connect letter you can connect to is by typing screen -r and get the same environment which you left. Which in most cases is a big productivity booster.

NOTE: line mc (Midnight commander screen can initially intimidating if you want to use its full capabilities. In this case you need to know around a dozen of command which start with prefix Ctrl-A (see below).  Custom screenrc can help (see .screenrc examples). 

Those who  find  it difficult can find the package byobu available from EPEL repository which originated in Ubuntu and provide more user friendly screen interface and ton of bells and whistles. After installation of the package just type byobu-screen and perform configuration by pressing F9.  I did not find this package useful because it is way too complex. But it is written in  bash and as such can be a useful example of extending screen capabilities.  That's probably the main value of the package.

The screen  command is particularly useful on remote server (in our setup this test02 server). In this case when you connect to this server after disconnecting you are able to return in the same environment that you had left previous time, includes current working directory and value of env variables.

As this is command line  utility it has a set of commands. Every screen command is started with the Ctrl+A  key sequence. To get help type Ctrl+A, ?. This shows a list of all commands that are available.  An important command to remember is Ctrl+A, /, which will close the screen session. Make sure to remove screen sessions that you do not need anymore, otherwise they will stay active until the next time you reboot your server!

All you need to know to work with screen is 12 command listed below:

  1. Help Ctrl-a ?  (you need to press Shift key to get ? on the US keyboard)
  2. Create a new windows Ctrl+a c
  3. List windows Ctrl-a w
  4. Go to a window numbered 0 9 Ctrl+a 0-9
  5. Switch to the next screen Ctrl-a space
  6. Detach Screen: Ctrl-a d
  7. Re-attach Screen: screen -x or screen -x PID
  8. Split window horizontally: Ctrl-a S Split window vertically: Ctrl-a |
  9. Move Between Windows: Ctrl-a Tab
  10. Name Session: Ctrl-a A
  11.  Detach from screen but keep shell window open Ctrl+a d. Detach and disconnect Ctrl-a D
  12. Quit screen Ctrl-a Ctrl-\

To learn how to use screen please do the following exercise:

  1. Open an SSH session to test02 server,
  2. Install screen on the test02 server, if it was not installed before. 
  3. Type screen  to open a screen session on server test02
  4. Type command tail -f /var/log/messages
  5. Type the Ctrl+A,D  key sequence to detach from the screen session  and log out from the SSH session.
  6. When you are ready to reconnect, start the SSH session again. It is essential that you are using the same user account that you used before.
  7. Attach to the screen session again using screen -r. You can see that you get into the same environment running the same command as before

If while trying to attach to a screen session you get a message that multiple screen sessions are currently running, you need to be more specific. To find out which screen sessions currently are running, use the screen -ls  command.  In this case you need to adding the number of the screen session to the screen -r  command to reconnect to the particular session.

NOTE: You can also use screen  to work together with other users in the same session. So they are able to see what you are doing in your teminal session. The second user can just connect to your pre-existing screen session using screen -x command. If multiple screen instances are running you need to provide the name of the the instance from screen -ls command.

Here are common screen command (from How do I Use Multiple Screens on One Terminal over ssh session - nixCraft

screen command Task
Ctrl+a c Create new window
Ctrl+a k Kill the current window / session
Ctrl+a w List all windows
Ctrl+a 0-9 Go to a window numbered 0 9, use Ctrl+a w to see number
Ctrl+a Ctrl+a Toggle / switch between the current and previous window
Ctrl+a S Split terminal horizontally into regions and press Ctrl+a c to create new window there
Ctrl+a :resize Resize region
Ctrl+a :fit Fit screen size to new terminal size. You can also hit Ctrl+a F for the the same task
Ctrl+a :remove Remove / delete region. You can also hit Ctrl+a X for the same taks
Ctrl+a tab Move to next region
Ctrl+a D (Shift-d) Power detach and logout
Ctrl+a d Detach but keep shell window open
Ctrl-a Ctrl-\ Quit screen
Ctrl-a ? Display help screen i.e. display a list of commands

NEWS CONTENTS

Old News ;-)

[Oct 16, 2018] How To Use Linux Screen

Working with Linux Screen Windows

When you start a new screen session by default it creates a single window with a shell in it.

You can have multiple windows inside a Screen session.

To create a new window with shell type Ctrl+a c, the first available number from the range 0...9 will be assigned to it.

Bellow are some most common commands for managing Linux Screen Windows:

Detach from Linux Screen Session

You can detach from the screen session at anytime by typing:

Ctrl+a d

The program running in the screen session will continue to run after you detach from the session.

To resume your screen session use the following command:

screen -r
Copy

In case you have multiple screen sessions running on you machine you will need to append the screen session ID after the r switch.

To find the session ID list the current running screen sessions with:

screen -ls
Copy
There are screens on:
    10835.pts-0.linuxize-desktop   (Detached)
    10366.pts-0.linuxize-desktop   (Detached)
2 Sockets in /run/screens/S-linuxize.
Copy

If you want to restore screen 10835.pts-0, then type the following command:

screen -r 10835
Copy

When screen is started it reads its configuration parameters from /etc/screenrc and ~/.screenrc if the file is present. We can modify the default Screen settings according to our own preferences using the .screenrc file.

Here is a sample ~/.screenrc configuration with customized status line and few additional options:

~/.screenrc
# Turn off the welcome message
startup_message off

# Disable visual bell
vbell off

# Set scrollback buffer to 10000
defscrollback 10000

# Customize the status line
hardstatus alwayslastline
hardstatus string '%{= kG}[ %{G}%H %{g}][%= %{= kw}%?%-Lw%?%{r}(%{W}%n*%f%t%?(%u)%?%{r})%{w}%?%+

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