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

  • 3
    Firewall rules are a normal course of action to take – Raman Sailopal Nov 22 '17 at 10:16
  • 2
    firewall or /etc/hosts.allow if ssh compile w/ TCP wrappers or /etc/ssh/sshd_config file rules. – Rui F Ribeiro Nov 22 '17 at 10:41
  • more than one way to do, refer to linux.die.net/man/5/sshd_config which explains everything in /etc/ssh/sshd_config – ron Dec 20 '18 at 21:54
35

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.

  • What about adding a ListenAddress directive in /etc/ssh/sshd_config ? – jerome Nov 27 '17 at 18:54
  • It is possible in specific situations (for example listening to private network address), depending on your network configuration and which hosts you want to allow. – sebasth Nov 27 '17 at 19:17
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    Additionally, ,sshd_config can set filterings with AlowUsers directive, and also, the authorized_keys can be set with 'from IP or subnet" to filter also. – tonioc Jun 11 '18 at 17:38
  • @tonioc Great solution for my use case. Please expand this suggestion into an answer. – simlev Dec 20 '18 at 11:28
  • if you have a NIC with 4 ports each going to a different network, then the default #ListenAddress :: in /etc/ssh/sshd_config will tell the SSH server to accept incoming from any of those networks. Otherwise do ListenAddress <ip address> where <ip address> is that of those NIC ports you want allowed. My eth0is 192.168.3.4 therefore ListenAddress 192.168.3.4 results in SSH only working on network 192.168.3.4 which is on eth0; and eth1 eth2 eth3 is denied. – ron Dec 20 '18 at 21:49
4

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.

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