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
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
Option 2: Using TCP wrappers
Note: this might not be an option on modern distributions, as support for tcpwrappers was removed from OpenSSH 6.7
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.
sshd : ALL
Then list allowed hosts in hosts.allow. For example to allow network 192.168.0.0/24 and localhost.
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.
Here some additional configuration for SSH daemon to extend previous answer:
Add user filtering with
AllowUsers email@example.com.* firstname.lastname@example.org.* 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_keysfile 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.
If you don't mind installing UFW:
sudo ufw allow from 192.168.1.0/24 to any port 22
As previous mentioned it's a good practice to only authenticate using keys instead of passwords which can be done by editing
PubkeyAuthentication yes PasswordAuthentication no ChallengeResponseAuthentication no UsePAM no
If you're using SSH CA for client authentication, you can specify the
source-address option when signing certificates:
ssh-keygen -s ca_privkey -O source-address=172.16.0.0/16 id_rsa.pub ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
id_rsa-cert.pub can be used to log in to hosts only from
172.16.0.0/16 addresses (not even
127.0.0.1 unless you specify that as well).
man 1 ssh-keygen is a good document if you want more details.
Another way you can limit access to sshd on a GNU/Linux system at the socket level with a built-in (assuming init is systemd 235+ and kernel 4.11+) is by utilizing systemd with cgroup/eBPF access lists
Modify the base sshd systemd stanza
sudo systemctl edit sshd
Append the sshd [Service] stanza to your liking
[Service] #requires systemd 235+ and kernel 4.11+ IPAccounting=yes IPAddressDeny=any IPAddressAllow=126.96.36.199 IPAddressAllow=10.161.0.0/16 IPAddressAllow=100.64.0.0/24
reload for immediate effect
sudo systemctl daemon-reload