Harden SSH for AlmaLinux 9 (RHEL, Fedora)

Posted on March 6, 2024 by admin

These steps are taken from the following link. They have other guides for hardening Ubuntu, Debian etc.

https://www.sshaudit.com/hardening_guides.html#rocky9

You will need to become the root user, use either su – or sudo -i

First we need to regenerate the RSA and ED25519 keys

rm /etc/ssh/ssh_host_*
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_rsa_key -N ""

Next, remove the small Diffie-Hellman moduli. The moduli file contains prime numbers and generators. Removing the smaller numbers should help increase security as it makes attempting to factor the private keys harder.

awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduli

We can now specify which key exchange, ciphers, and algorithms to use.

Add the following to “/etc/crypto-policies/back-ends/opensshserver.config”

# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com
# hardening guide.
KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,gss-curve25519-sha256-,diffie-hellman-group16-sha512,gss-group16-sha512-,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com

HostKeyAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

RequiredRSASize 3072

CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-

HostbasedAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-256

PubkeyAcceptedAlgorithms sk-ssh-ed25519-cert-v01@openssh.com,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256

Finally, restart the ssh server

systemctl restart sshd

Install and Configure Fail2ban on Fedora/CentOS/RedHat

Posted on August 23, 2022 by admin

The following is a very basic guide for setting up Fail2ban for SSH.

Install and basic config

Install Fail2ban

sudo dnf install fail2ban

You may need to install the epel repo

sudo yum install epel-release

Configure to run on system boot

sudo systemctl enable fail2ban

Start Fail2ban service

sudo systemctl start fail2ban

Copy config file with

sudo cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.local

Modify the config file

nano /etc/fail2ban/jail.local

Uncomment the following line and add any IPs that need to be whitelisted

ignoreip = 127.0.0.1/8 ::1 192.168.1.20

Save the file and restart Fail2Ban

sudo systemctl restart fail2ban

Configuring Fail2Ban for SSH

Create a new jail file in /etc/fail2ban/jail.d/ called sshd.local

nano /etc/fail2ban/fail.d/sshd.local

Add the following. Note: if you are using a custom ssh port, change “port = ssh” to “port = portnumber”

[sshd]
enabled = true
port = ssh
action = iptables-multiport
logpath = /var/log/secure
maxretry = 5
bantime = 300

Restart Fail2ban

sudo systemctl restart Fail2ban

You can list the firewall rules to verify that an IP gets banned.

iptables -S | grep ipaddress

Unbanning an IP Address

You can unban an IP address with the following command.

sudo fail2ban-client set sshd unbanip 192.168.1.100

You can check out the following link for more information

https://www.redhat.com/sysadmin/protect-systems-fail2ban

Install SSH Server on Linux (Debian, Ubuntu, Fedora, CentOS, RedHat)

Posted on November 17, 2017 by admin

Debian / Ubuntu

sudo apt-get install -y openssh-server

RPM based Distros, Fedora / CentOS / RedHat

sudo dnf install -y openssh-server

or use yum

sudo yum install -y openssh-server

Start ssh service

sudo systemctl start sshd

By default the SSH service should start when the system starts, but if not try the following command to enable the service on boot up.

Debian / Ubuntu

systemctl enable ssh

Fedora, CentOS, RedHat

systemctl enable sshd

Change SSH port

Not necessary, but it is a good idea to change the default ssh port. To change the port edit the sshd file.

vi /etc/ssh/sshd_config

If you change the port, you’ll need to allow it in the firewall (firewalld, iptables) and if SELinux is enabled, semanage.

Allow SSH access from a specific host using hosts.allow and hosts.deny on Linux

Posted on July 14, 2016 by admin

This is just a quick write on the hosts.allow and deny files. You can lookup “spawn” and/or “twist” for some advanced usage.

So to limit an IP address, or a IP range access to SSH, do the following

Deny all incoming request for SSH

Edit the “hosts.deny” file

vi /etc/hosts.deny

add the following line

sshd : ALL

Now edit “hosts.allow” and allow the client IP, or IP range to access SSH

vi /etc/hosts.allow

add the following line to allow a single IP

sshd : 192.168.1.182

If you want to allow the whole subnet, then replace the above line with this one

sshd : 192.168.1.

hosts.allow overrides hosts.deny. So you deny everything and then allow exceptions.

Incredigeek

Tag Archives: sshd