Linux Remote Shell Notes
Secure Shell (SSH) and Telnet are the two most common remote management protocols. The configuration of Telnet is a common practice, but it is advisable to use SSH. The reason for this is that Telnet data is sent in plain text in contrast to SSH, which encrypts the data. Tools like wireshark and snort can be used to intercept unencrypted data over a LAN or extra network connection.
telnet - You can install the old telnet server on your linux system and use it to connect sending your password as plain text and then work in a remote shell with everything being sent as plain text, in the event you wish to share everything you are doing with anyone listening with a packet sniffer at any point between you and the remote host.
ssh - SSH works by connecting a client program to an ssh server. Use ssh as the modern secure alternative to telnet. Connect to your linux host from another linux host or a host with a different operating system such as Microsoft Windows. On Microsoft Windows you can use the free ssh client Putty to connect via ssh.
Netcat - netcat has been around a long time and should be harmless to have on your system unless you really open things up like a fool.
Contents
- 1 Want to install telnet?
- 2 Enable TELNET as ROOT on GUEST CentOS system
- 3 telnet
- 4 ssh - The Secure Shell
- 5 Use telnet on Debian / Ubuntu / Mint
- 6 use SSH to execute a remote command
- 7 auto login ssh
- 8 shells that don't die when connection is lost
- 9 Remote File Copy via SSH
- 10 login and non-login shell
- 11 Related Pages
Want to install telnet?
telnet server is not recommended for security reasons, plain text over tcp/ip
# yum install telnet telnet-server -y
How about telnet client? It is not installed by default.
#yum intall telnet -y
The package is Package telnet.i686
Enable TELNET as ROOT on GUEST CentOS system
Simply edit the file /etc/securetty and add the following to the end of the file:
pts/0 pts/1 pts/2 pts/3 pts/4 pts/5 pts/6 pts/7 pts/8 pts/9
This will allow up to 10 telnet sessions to the server as root. note: Just one is probably fine
ref: http://www.idevelopment.info/data/Unix/Linux/LINUX_TelnetFTPAsRoot.shtml
telnet
There are still safe and legitimate uses for this over four decade old connection protocol.
We've already mentioned the lack of security. Quote, "The telnet daemons (the process that sits on the server and processes your login) have had numerous vunerabilities and will continue to have them. Running telnetd is just one more old daemon waiting to be used to crack your machine!" from the post Why you should use ssh instead of telnet.
There are times when telnet may be useful. On a private LAN (no route to the Internet) or a connection between two machines only (ultra private LAN) the use of telnet is acceptable, quick, and convenient.
Furthermore, exploration on exactly how ssh is more secure, and where it is not can find validation in the use of telnet. One of the key advantages of SSH over telnet is that the server authenticates itself to the client before it collects credentials from the client. Most sniffing occurs within the network itself and not outside of the router. Common ssh installations ignore precautions that protect against external threats, making those networks no more secure when using ssh versus telnet.
If there is no threat within the network itself, all traffic tunnelled to a remote network within a secure VPN, and concluding there is no internal threat within that remote network, telnet would offer no lessor security as compared to ssh in the described scenario.
ssh - The Secure Shell
The developer folks hated the "service" command so now we use the new systemctl (System Control) method to start the ssh server daemon.
sudo systemctl start ssh
While Telnet was once widely used by administrators for remote management, it does not offer the security mechanisms like SSH, which establishes a secure connection from the host to the remote host.
Secure Shell Support for Remote Login
If the ssh server is not already installed, we will use apt-get to install. As root or using sudo do the following:
apt-get install openssh-server openssh-client service ssh start
Note: Added 10/17/2016 old cyphers no longer configured, may cause connection problems with legacy ssh clients such as SecureCRT. ref: https://blog.hqcodeshop.fi/archives/245-SSH-connection-fails.html
Use telnet on Debian / Ubuntu / Mint
You don't give a rats rear about security and you wish to use good old fashioned telnet (For Debian/Ubuntu/Mint). Ok... Install the Telnet server
sudo apt install xinetd telnetd
Assign port
sudo vi /etc/services
Look for the line or add a line like
telnet 23/tcp
Restart xinetd
use SSH to execute a remote command
You'll need to be able to ssh with automated password or enter the password each time you run the command.
Execute a remote command on a host over SSH:
ssh nicolep@192.168.100.10 'reboot'
The example above will reboot the remote computer.
Multiple commands
ssh nicolep@192.168.100.10 'uptime; df -h'
Show the kernel version, number of CPUs and the total RAM:
ssh root@192.168.100.10 << EOF uname -a lscpu | grep "^CPU(s)" grep -i memtotal /proc/meminfo EOF
Here is how Nicole can execute her script on the remote computer
ssh nicolep@192.168.100.10 'bash -s' < nicolejob.sh
Nicole's script is local on her machine, and she executed it on the remote host.
auto login ssh
There is a key authentication system that can be used with OpenSSH. The OpenSSH ssh utility doesn't accept a password on the command line or on its standard input, except for prompting the user for password requiring manual interaction. By using the key authentication system you will not be prompted for a password.
OPTIONS FOR SSH AUTOMATED
- Use an SSH key for authentication, instead of a password.
- Use sshpass, expect, or a similar tool to automate responding to the password prompt.
- Use the SSH_ASKPASS feature to get ssh to get the password by running another program.
- Use the insecure host-based authentication, sometimes common on private networks.
- Use a custom or modified ssh client adapted from source code, or one that allows for stored password.
sshpass
Install the sshpass utility so that you can automate ssh login including password.
apt install sshpass
Now you can automate the login process
sshpass -p "mysecretpass" ssh -o StrictHostKeyChecking=no nicolep@192.168.100.10
Custom port example:
sshpass -p "mysecretpass" ssh -o StrictHostKeyChecking=no nicolep@192.168.100.10:9600
public key authentication
BEFORE MAKING NEW KEY PAIRS: Try to Find an Existing Secure Shell Key Pair. See if you already have a .ssh under your home directory. If you already have a key pair then you should not need to make a new key pair.
In the source host run this only once:
ssh-keygen -t rsa
Now you've generated the public key. It needs to be copied onto the remote host.
ssh-copy-id -i ~/.ssh/id_rsa.pub nicolep@192.168.100.10
add identities to the ssh-agent – the authentication agent on the local host.
ssh-add
now press ENTER to every field
ssh-copy-id nicolep@192.168.100.10
expect
Example script
#!/usr/bin/expect set timeout 15 set cmd [lrange $argv 1 end] set password [lindex $argv 0] eval spawn $cmd expect "assword:" send "$password\r"; interact
Another example
#!/usr/bin/expect -f # ./ssh.exp password 192.168.100.10 id set pass [lrange $argv 0 0] set server [lrange $argv 1 1] set name [lrange $argv 2 2] spawn ssh $name@$server match_max 100000 expect "*?assword:*" send -- "$pass\r" send -- "\r" interact
And finally, a more elaborate example can be found here:
shells that don't die when connection is lost
The two most common solutions are "screen" and "tmux." Screen has been around longer, tmux has some additional capabilities, and both will keep your terminal session alive even if your connection is lost.
Screen - Virtual Terminals From Console
Multiple virtual terminals from one terminal (like a remote shell), with the ability to detach processes and leave them running even when you disconnect.
tmux, the shell that doesn't die
tmux is a terminal multiplexer. It lets you switch easily between several programs in one terminal, detach them (they keep running in the background) and reattach them to a different terminal.
tmux may be detached from a screen and continue running in the background, then later reattached.
- ssh into the remote machine
- start tmux by typing tmux into the shell
- start the process you want inside the started tmux session
- leave/detach the tmux session by typing Ctrl+b and then d
The tmux and GNU screen utilities have many similarities. tmux is considered more modern.
Each session is persistent and will survive accidental disconnection (such as ssh(1) connection timeout) or intentional detaching (with the ‘C-b d’ key strokes). tmux may be reattached using:
tmux attach
Example tmux commands include:
refresh-client -t/dev/ttyp2 rename-session -tfirst newname set-window-option -t:0 monitor-activity on new-window ; split-window -d bind-key R source-file ~/.tmux.conf \; \ display-message "source-file done"
Or from sh(1):
tmux kill-window -t :1 tmux new-window \; split-window -d tmux new-session -d 'vi /etc/passwd' \; split-window -d \; attach
Remote File Copy via SSH
SCP - Secure Copy Protocol
login and non-login shell
The .bash_profile is executed for login shells, while .bashrc is executed for interactive non-login shells.
When you login (type username and password) via console, either sitting at the machine, or remotely via ssh: .bash_profile is executed prior to the initial command prompt.
When you are logged into your machine already and open xterm in your desktop environment then .bashrc is executed prior to seeing a prompt. The .bashrc is also run when you start a new bash instance by typing /bin/bash in a terminal.
