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4 Décembre 2020
What to know about SSH. Secure Shell is a secure and encrypted connection protocol allowing remote and secure sign-ins over unsecured connections. The connection works in the client-server mode, so the connection is established by the SSH client connecting to the SSH server. The following has the advantage that X11 and SSH agent socket forwardings are also disallowed, which might still be allowed in Calebs way. Another advantage is, that if the user is able to change his default shell through any other way, this will still restrict his SSH access to only TCP forwardings. SSH مخفف Secure Shell پروتکلی امن بر پایه TPC/IP برای برقراری اتصال بین کاربر و سرور است. این پروتکل ارتباطی با رمزگذاری دادههای مبادله شده بین. SSH مخفف Secure Shell پروتکلی امن بر.
Did you know that when you're using OpenSSH from the command line you have a variety of escape sequences available to you? SSH somewhere, then type '~' and '?' (tilde, then question mark) to see all the options. You should get something like:
Most commonly, I use tilde-period (~.) to close an unresponsive session, like when a firewall has closed my connection.
BREAK is useful for various things, usually getting back to a terminal server console or getting the attention of network equipment.
The command line doesn't do much, but you can alter forwards from it. I've never used it but it's probably handy for troubleshooting if your tunnels aren't working right:
I've also never had to rekey a session for any reason, as SSH protocol version 2 does it automatically after a certain amount of data has been transferred. You can mess with it via the RekeyLimit configuration directives, or read more about it in RFC 4344.
Suspending SSH via tilde-Ctrl-Z is handy from time to time, especially when you're on the console of a machine that doesn't have screen or some other multiplexor on it (or you forgot to start one). Of course, you have to remember that when you need it, but now that you've read it maybe you will.
List forwarded connections is handy for managing the forwards you might have created with the command line.
Backgrounding SSH attempts to close all the connections, and will wait patiently for them to die. I have never needed this, because I'm the impatient bastard that just tilde-periods them if they don't close right away.
You can use the EscapeChar configuration directive to change the tilde, if that conflicts with something. Or you can just type it twice to send it.
Secure Shell or with its most know name SSH is a protocol developed to connect IT systems remotely and securely. SSH works as expected client-server architecture. In this post, we will look at various security-related configuration options of the SSH daemon service or sshd. The tutorial about the client-side ssh configuration can be found in the following link.
Ssh generally works as a service or daemon. This service starts by reading some configure file about the service. This configuration file is located at /etc/ssh/sshd_config . In most Linux distributions the file is used as startup-config. In order to modify the configuration file, we need root privileges.
Some rules require comments about them. Comments are created with # lines. Comments have no effect on the SSH configuration.
After ssh configuration file changes ssh service or daemon should be restart to take effect new configuration. There are different ways to restart ssh service but the most global way to restart ssh daemon is using systemctl command like below.
If the ssh is not used and we have direct access to the system stopping ssh service is a more secure choice. We will stop ssh service with systemctl command.
After configuration changes we restart the ssh service but how can be sure it is working. There are different ways but the most appropriate way is using systemctl to get the status of the service. This command also provides last logs about the service that can provide hints about configurations errors or similar things.
SSH has two versions. As we expect version 1 is the old and nonsecure version. It is abandoned a long time ago but some ssh configurations may contain this version enabled. We will only enable version 2 with the following line.
By default, all users created on the system can log in remotely. Enabling all users remote login opportunity is not a good security practice. We can limit login for specified users like below. In this example, we only allow user ismail to login remotely by using ssh.
Another way to limit users' login is by specifying a user account that does not have the right to log in remotely by using ssh. All other users than specified ones will have remote access. In this example, we deny remote access for user root . Denying access for root is a good security practice.
By default when ssh service is started it runs on all interfaces and IP addresses. This may create some security problems if the system has more than one network interface where some of them are insecure side. We can restrict the ssh service interface to run. Ssh service will not accept connections from other interfaces.
After the connection is established the connection is stayed in the open state forever if not closed explicitly. This is not a wanted situation for resource usage and security. We should define a timeout value that will close the session after this time of inactivity. In this example, we set these values as 120 seconds.
One of the best practices is disabling root account remote login. If there are other high privileged well-known accounts these accounts must be disabled too.
While connecting systems remotely with ssh providing information warning information about the system is a good way to prevent some attacks. Even this is not a technical way to protect or prevent attacks it may convenience the attacker physiologically.
By default, ssh uses TCP port 22 as port number. Most of the users and attackers assume this default and takes action to the TCP port 22. If there is no operations cost changing the ssh server port is the best way. In the example we use port 1234 as ssh daemon port.
Password authentication is a simple method for the user to authenticate themselves. But it is simpler than other methods for attackers to crack too. Users generally prefer simple and easy to remember passwords which make attackers work easy. We can disable password-based authentication.
But the users should be already setup Public key-based authentication in order to resume using the SSH server. More information about how to set up public key-based authentication can be found in the following link.
By default public key authentication is enabled but enabling it explicitly will make it more reliable. Apple laptop storage capacity.
Another great risk for ssh daemon is empty passwords. Modern Linux distributions generally prohibit empty passwords but disabling empty passwords will make us sure.
StrictMode checks some cases before the ssh server starts. Ssh key, configuration files ownership, permission checks are performed before ssh daemon starts. If one of them fails the ssh server daemon does not starts. Strict mode is enabled by default but generally closed by system administrators. For security reasons, it should be enabled.
One of the best features for ssh is forwarding X11 over remote connections. This is a very useful feature for some system administrators and users. But this can create some security holes in the system. If X11 forwarding is not needed disable it.
Now the last but one of the most important rules to make ssh servers and daemons secure. Updating is a magical way to make ssh more secure.
Did you know that when you're using OpenSSH from the command line you have a variety of escape sequences available to you? SSH somewhere, then type '~' and '?' (tilde, then question mark) to see all the options. You should get something like:
Most commonly, I use tilde-period (~.) to close an unresponsive session, like when a firewall has closed my connection.
BREAK is useful for various things, usually getting back to a terminal server console or getting the attention of network equipment.
The command line doesn't do much, but you can alter forwards from it. I've never used it but it's probably handy for troubleshooting if your tunnels aren't working right:
I've also never had to rekey a session for any reason, as SSH protocol version 2 does it automatically after a certain amount of data has been transferred. You can mess with it via the RekeyLimit configuration directives, or read more about it in RFC 4344.
Suspending SSH via tilde-Ctrl-Z is handy from time to time, especially when you're on the console of a machine that doesn't have screen or some other multiplexor on it (or you forgot to start one). Of course, you have to remember that when you need it, but now that you've read it maybe you will.
List forwarded connections is handy for managing the forwards you might have created with the command line.
Backgrounding SSH attempts to close all the connections, and will wait patiently for them to die. I have never needed this, because I'm the impatient bastard that just tilde-periods them if they don't close right away.
You can use the EscapeChar configuration directive to change the tilde, if that conflicts with something. Or you can just type it twice to send it.
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