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How do we allow certain set of Private IPs to enter through SSH login(RSA key pair) into Linux Server?

You can limit which hosts can connect by configuring TCP wrappers or filtering network traffic (firewalling) using iptables. If you want to use different authentication methods depending on the client IP address, configure SSH daemon instead (option 3).

Option 1: Filtering with IPTABLES

Iptables rules are evaluated in order, until first match.

For example, to allow traffic from 192.168.0.0/24 network and otherwise drop the traffic (to port 22). The DROP rule is not required if your iptables default policy is configured to DROP.

iptables -A INPUT -p tcp --dport 22 --source 192.168.0.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 22 -j DROP

You can add more rules before the drop rule to match more networks/hosts. If you have a lot of networks or host addresses, you should use ipset module. There is also iprange module which allows using any arbitrary range of IP addresses.

Iptables are not persistent across reboots. You need to configure some mechanism to restore iptables on boot.

iptables apply only to IPv4 traffic. Systems which have ssh listening to IPv6 address the necessary configuration can be done with ip6tables.

Option 2: Using TCP wrappers

You can also configure which hosts can connect using TCP wrappers. With TCP wrappers, in addition to IP addresses you can also use hostnames in rules.

By default, deny all hosts.

/etc/hosts.deny:

sshd : ALL

Then list allowed hosts in hosts.allow. For example to allow network 192.168.0.0/24 and localhost.

/etc/hosts.allow:

sshd : 192.168.0.0/24
sshd : 127.0.0.1
sshd : [::1]

Option 3: SSH daemon configuration

You can configure ssh daemon in sshd_config to use different authentication method depending on the client address/hostname. If you only want to block other hosts from connecting, you should use iptables or TCP wrappers instead.

First remove default authentication methods:

PasswordAuthentication no
PubkeyAuthentication no

Then add desired authentication methods after a Match Address in the end of the file. Placing Match in the end of the file is important, since all the configuration lines after it are placed inside the conditional block until the next Match line. For example:

Match Address 127.0.0.*
 PubkeyAuthentication yes

Other clients are still able to connect, but logins will fail because there is no available authentication methods.

Match arguments and allowed conditional configuration options are documented in sshd_config man page. Match patterns are documented in ssh_config man page.

Here some additional configuration for SSH daemon to extend previous answer:

• 
  

  Add user filtering with AllowUsers option in sshd_config file:

  
  
  

  AllowUsers johndoe@192.168.1.* admin2@192.168.1.* otherid1 otherid2
  

  
  
  

  This allows johndoe and admin2 only from 192.168.1.* addresses and otherid1, otherid2 from anywhere.

  
  


• 
  

  Restrict a ssh key or ca-based key to a set of addresses in .ssh/authorized_keys file of a given user's home directory:

  
  
  

  from="192.168.1.*,192.168.2.*" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABA...etc...mnMo7n1DD useralias
  

  
  
  

  In this example, the public key for useralias will be effective only from given addresses.