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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
56

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.

| improve this answer | |
  • 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
  • 3
    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
  • 1
    Note that sshd must be linked against libwrap for TCP wrappers to work (see e.g. here) and that support for tcpwrappers/libwrap was dropped from OpenSSH in v.6.7. On many modern systems Option 2 may no longer work. – AstroFloyd Sep 19 at 8:10
14

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.

| improve this answer | |
  • In my case, white listing in sshd was better than using iptables/ufw. The firewall was unable to keep up with traffic. We only needed to lock down ssh, so adding AllowUsers clause to /etc/ssh/sshd_config was a much lighter weight solution. Otherwise, you have to continuously tune your fire wall. You also have to stress/load test the firewall and it's hard to anticipate real network traffic patterns. – thebiggestlebowski May 13 at 16:20
1

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
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The certificate 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.

| improve this answer | |
1

If you don't mind installing UFW:

sudo ufw allow from 192.168.1.0/24 to any port 22

EDIT:

As previous mentioned it's a good practice to only authenticate using keys instead of passwords which can be done by editing /etc/ssh/sshd_config:

PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM no
| improve this answer | |
  • And how should the SSH login be limited to RSA key pair? – rickhg12hs Apr 30 at 19:24
  • @rickhg12hs by adding your public RSA key to ~/.ssh/authorized_keys – Mahmoud K. Apr 30 at 21:41
  • More is required to restrict login to only RSA key pair. – rickhg12hs May 1 at 10:26
  • In /etc/ssh/sshd_config, set PasswordAuthentication, ChallengeResponseAuthentication and UsePAM to no. – Mahmoud K. May 1 at 15:01
  • It's best if you update your answer so that future readers can look for a complete answer to the original question. I like to think about StackExchange sites as a searchable encyclopedia. Each question and answer should try to make the encyclopedia better and easy to get more correct information. – rickhg12hs May 1 at 15:14

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