Limit SSH access by IP address

This article describes how to secure the SSH access to your server by performing the following actions:

• Prevent login attempts for specific IP addresses in the permissive configuration
• Allow access for specific IP addresses in the restrictive configuration

Permissive configuration

The permissive configuration allows access to everyone except the denied IP address. You can deny access to specific
to IP addresses in the following ways:

TCP wrappers

Make this configuration in the file /etc/hosts.deny by adding the follwing rule:

# vi /etc/host.deny

Set a denying rule in one of the following ways:

sshd: Bad_IP_address
sshd: 192.168.0.8

sshd: Bad_Segment/CIDR
sshd: 192.168.0.0/24

sshd: Bad_IP_address, Bad_IP_address
sshd: 192.168.0.3, 192.168.0.4

iptables

This configuration resides in the OS firewall rules configuration.

# iptables -A INPUT -s "BAD_IP" -p tcp --dport SSH_Port -j DROP

Block an IP address:

# iptables -A INPUT -s 192.168.0.3 -p tcp --dport 22 -j DROP

Block a segment:

# iptables -A INPUT -s 192.168.0.0/24 -p tcp --dport 22 -j DROP

Restrictive Configuration

This configuration allows access to only specific IP addresses and drops other incoming traffic.

TCP wrappers

First, deny all traffic incoming to SSH in the /etc/hosts.deny file:

# vi /etc/hosts.deny
sshd: ALL

Then, allow access for trusted IP addresses:

# vi /etc/hosts.allow
sshd: Trusted_IP_address
sshd: 192.168.0.8

sshd: Trusted_Segment/CIDR
sshd: 192.168.0.0/24

sshd: Trusted_IP_address, Trusted_IP_address
sshd: 192.168.0.3, 192.168.0.4

iptables

For IP Tables, the order in which you place the rules is very important. That's why you must configure first the
allowing rules and then the denying rule.

# iptables -A INPUT --source Trusted_IP -p tcp --dport SSH_PORT  -j ACCEPT

Accepting traffic from an IP:

# iptables -A INPUT --source 192.168.0.8 -p tcp --dport 22 -j ACCEPT

Accepting traffic from a segment:

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

Setting the blocking rule:

iptables -A INPUT -p tcp --dport 22 -j DROP

How to improve the changes

You can review your changes by executing the following commands depending on the restrictive or permissive configuration:

# cat /etc/hosts.deny
# cat /etc/hosts.allow
# iptables -L

Note: Some distributions include default firewall software, such as UFW for Debian® and the Ubuntu® operating system and derivated, Firewalld for Red Hat® Enterprise Linux® 7 and derivated.

Configure ufw to deny specific IP, port number, and protocol

For blocking an IP address to TCP port 22:

# ufw deny proto tcp from 192.168.0.8 to any port 22

For blocking a subnet:

# ufw deny proto tcp from 192.168.0.0/24 to any port 22

For listing the configured rules:

# ufw status numbered

Configure firewalld to deny a specific IP address, port number, and protocol

On firewalld, you can ban an IP address or a segment, but it won't allow any kind of connection:

Block an IP address:

# firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='192.168.0.8' reject"

Block a segment:

# firewall-cmd --permanent --add-rich-rule="rule family='ipv4' source address='192.168.0.0/24' reject"

That's why you might choose to whitelist trusted IP addresses to connect to SSH port:

# firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.0.20" port protocol="tcp" port="22" accept'

You have to reload the firewalld configuration:

# firewall-cmd --reload

List the rules configured on the firewall:

# firewall-cmd --list-all