Updated at: February 23, 2017
By default you can ssh to the server as root. It is best not to allow root to login directly to the server. Instead, you should login to the system as your account and then do ‘su -‘ to login as root.
If you have multiple sysadmins in your organization, and if they all login to the server directly as root, you might not know which sysadmin logged in as root. Instead, if you disable login as root, sysadmins are forced to login as their account first, before they can do ‘su -‘, this makes the auditing easier.
Add the following entry to sshd_config to disable root to login to the server directly.
$ vi /etc/ssh/sshd_config PermitRootLogin no
By default anybody who is authenticated successfully are allowed to login. Instead you can restrict which users (or groups) you allow to login to the system.
This is helpful when you have created several user accounts on the system, but want only few of them to login.
This is also helpful when you are using NIS, openLDAP (or some other external system) for authentication. Every user in your company might have account on NIS, OpenLDAP etc. But, on a specific server you want only few of them to login. For example, on production system you want only sysadmins to login.
Add the following entry to the sshd_config file to allow only specific users to login to the system. In the example below only ramesh, john and jason can login to this system. Usernames should be separated by space.
$ vi /etc/ssh/sshd_config AllowUsers ramesh john jason Add the following entry to the sshd_config file to allow only the users who belong to a specific group to login. In the exampe below only users who belong to sysadmin and dba group can login to the system. $ vi /etc/ssh/sshd_config AllowGroups sysadmin dba
Instead of allowing specific users (or groups), you can also deny specific users or groups.
Add the following entry to the sshd_config file to deny specific users to login to the system. In the example below cvs, apache, jane cannot login to this system. Usernames should be separated by space.
$ vi /etc/ssh/sshd_config DenyUsers cvs apache jane Add the following entry to the sshd_config file to deny users who belong to a specific group to login. In the exampe below users who belong to developers and qa group cannot login to the system. $ vi /etc/ssh/sshd_config DenyGroups developers qa
Note: You can use combination of all the Allow and Deny directivies. It is processed in this order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups
By default ssh runs on port 22. Most of the attackers will check if a server is open on port 22, and will randomly use brute force to login to the server using several username and password combination.
If you change the port # to something different, others need to know exactly what port to use to login to the server using ssh. The exampe below uses port 222 for ssh.
$ vi /etc/ssh/sshd_config Port 222
From your logs (/var/log/secure), if you see lot of invalid logins using ssh for accounts that don’t exist on your system, from the ip-address that you don’t recognize, it migth be some brute-force attack. Those kind of ssh invalid login will stop, if you change the port number.
Please note that this causes little inconvenience to your team who login to the system, as they need to know both the ip-address and the port number.
When you ssh to a server, you have 2 minutes to login. If you don’t successfully login within 2 minutes, ssh will disconnect. 2 minutes time to login successfully is too much. You should consider changing it to 30 seconds, or may be 1 minute.
Add the following entry to the sshd_config file to change the login grace time from 2 minutes to 1 minute.
$ vi /etc/ssh/sshd_config LoginGraceTime 1m
If you have multiple interfaces on the server that are configured to different ip-address, you might not want everybody to login to the server using all those ip-address.
Let us assume that you have the following 4 interfaces on the server:
By default ssh will listen on all of the above ip-addresses. If you want users to login only using ip-address 200 and 202, do the following in your sshd_config
$ vi /etc/ssh/sshd_config ListenAddress 192.168.10.200 ListenAddress 192.168.10.202
Once you’ve successfully logged in to the system, you might want to get disconnected when there are no activities after x number of minutes. This is basically idle timeout.
In Bash, you can achieve this using TMOUT variable.
In OpenSSH, this can be achieved by combining ClientAliveCountMax and ClientAliveInterval options in sshd_config file.
If you want ssh client to exit (timeout) automatically after 10 minutes (600 seconds), modify the sshd_config file and set the following two parameters as shown below.
$ vi /etc/ssh/sshd_config ClientAliveInterval 600 ClientAliveCountMax 0
You want to use Linux and OpenSSH to automate your tasks. Therefore you need an automatic login from host A / user a to Host B / user b. You don’t want to enter any passwords, because you want to call ssh from a within a shell script.
How to do it?
First log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:
a@A:~> ssh-keygen -t rsa Generating public/private rsa key pair. Enter file in which to save the key (/home/a/.ssh/id_rsa): Created directory '/home/a/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/a/.ssh/id_rsa. Your public key has been saved in /home/a/.ssh/id_rsa.pub. The key fingerprint is: 3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A
Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):
a@A:~> ssh b@B mkdir -p .ssh b@B's password:
Finally append a’s new public key to b@B:.ssh/authorized_keys and enter b’s password one last time:
a@A:~> cat .ssh/id_rsa.pub | ssh b@B 'cat >> .ssh/authorized_keys' b@B's password:
From now on you can log into B as b from A as a without password:
a@A:~> ssh b@B
A note from one of our readers: Depending on your version of SSH you might also have to do the following changes:
这里可以理解为前一个“-” pipe到后一个“-”，“-”代表stdin，后面tar/cat 读取stdin