SSH Hardening Guides
Below are guides to hardening SSH on various systems. Note that following them may not result in a perfect auditing score, as not all packaged SSH server versions support the required options. However, these instructions will result in the best possible score.
These guides were inspired by this document (which is now out-dated).
Server Guides:
- Amazon Linux 2023
- Debian 11 (Bullseye)
- Debian 12 (Bookworm)
- Rocky Linux 9
- Ubuntu 20.04 LTS
- Ubuntu 22.04 LTS
- Ubuntu 24.04 LTS
Client Guides:
- Amazon Linux 2023
- Debian 12 (Bookworm)
- Rocky Linux 9
- Ubuntu 20.04 LTS / Linux Mint 20
- Ubuntu 22.04 LTS / Linux Mint 21
- Ubuntu 24.04 LTS / Linux Mint 22
Community-Developed Guides:
The following guides have been written by the community. They have not been officially tested, and are not officially supported:
- ArubaOS Switch (AOS S) 16.11
- Dropbear 2022.83
- Fortinet FortiOS
- FreeBSD (various versions)
- Mac OS 13 (Ventura) & 14 (Sonoma)
- Mikrotik RouterOS
- OPNsense 20.7 and newer
- Proxmox-VE-7.3-6
- Synology DSM
- Void Linux
Instructions for submitting a hardening guide can be found here.
Legacy Guides:
- Debian Buster (Debian 10)
- OpenBSD 6.2
- pfSense 2.4
- RedHat Enterprise Linux 7 / CentOS 7
- RedHat Enterprise Linux 8 / CentOS 8
- Ubuntu 14.04 LTS
- Ubuntu 16.04 LTS (Server)
- Ubuntu 16.04 LTS / Linux Mint 18 (Client)
- Ubuntu 18.04 LTS
- Ubuntu 18.04 LTS / Linux Mint 19 (Client)
- Ubuntu Core 16
- Ubuntu Core 18
Ubuntu 24.04 LTS Server Last modified: October 1, 2024
Change Log:
- October 1, 2024: Added RequiredRSASize directive to enforce a minimum of 3072-bit user and host-based authentication keys.
- April 29, 2024: Initial revision. In comparison to Ubuntu 22.04 LTS guide, the following changes were made: 1.) For key exchanges, diffie-hellman-group18-sha512 and diffie-hellman-group-exchange-sha256 were prioritized over diffie-hellman-group16-sha512 due to greater security strength; GSS algorithms were prioritized over their non-GSS equivalents in order to match the client guide, 2.) For ciphers, 256-bit AES ciphers were prioritized over 192 and 128-bit AES ciphers due to their increased resistence against quantum computing attacks (previously, weaker GCM ciphers had priority over CTR ciphers), 3.) The HostbasedAcceptedAlgorithms and PubkeyAcceptedAlgorithms settings are now the same as HostKeyAlgorithms setting, 4.) The hmac-sha2-512-etm@openssh.com MAC was increased in priority due to its increased resistence against quantum computing attacks, and 5.) The ED25519 host keys were given priority over RSA host keys due to their greater efficiency.
Note: all commands below are to be executed as the root user.
Re-generate the ED25519 and RSA keys
rm /etc/ssh/ssh_host_*
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
ssh-keygen -t rsa -b 4096 -f /etc/ssh/ssh_host_rsa_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliEnable the ED25519 and RSA keys
Enable the ED25519 and RSA HostKey directives in the /etc/ssh/sshd_config file:
echo -e "\nHostKey /etc/ssh/ssh_host_ed25519_key\nHostKey /etc/ssh/ssh_host_rsa_key" >> /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,gss-group16-sha512-,diffie-hellman-group16-sha512\n\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com\n\nRequiredRSASize 3072\n\nHostKeyAlgorithms 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\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nGSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\nHostbasedAcceptedAlgorithms 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\n\nPubkeyAcceptedAlgorithms 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" > /etc/ssh/sshd_config.d/ssh-audit_hardening.confRestart OpenSSH server
service ssh restartImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPEnable persistence of the iptables rules across server reboots:
DEBIAN_FRONTEND=noninteractive apt install -q -y netfilter-persistent iptables-persistent
service netfilter-persistent save
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu 22.04 LTS Server Last modified: April 22, 2024
Change Log:
- October 1, 2024: Re-ordered host keys to prioritize ED25519 due to efficiency. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- April 22, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Enable the ED25519 and RSA keys
Enable the ED25519 and RSA HostKey directives in the /etc/ssh/sshd_config file:
echo -e "\nHostKey /etc/ssh/ssh_host_ed25519_key\nHostKey /etc/ssh/ssh_host_rsa_key" >> /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms 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\n\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\nHostKeyAlgorithms 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\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nGSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\nHostbasedAcceptedAlgorithms 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\n\nPubkeyAcceptedAlgorithms 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" > /etc/ssh/sshd_config.d/ssh-audit_hardening.confImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPEnable persistence of the iptables rules across server reboots:
DEBIAN_FRONTEND=noninteractive apt install -q -y netfilter-persistent iptables-persistent
service netfilter-persistent saveRestart OpenSSH server
service ssh restart
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu 20.04 LTS Server Last modified: April 24, 2024
Change Log:
- April 24, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliEnable the RSA and ED25519 keys
Enable the RSA and ED25519 HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com" > /etc/ssh/sshd_config.d/ssh-audit_hardening.confRestart OpenSSH server
service ssh restartImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPEnable persistence of the iptables rules across server reboots:
DEBIAN_FRONTEND=noninteractive apt install -q -y netfilter-persistent iptables-persistent
service netfilter-persistent save
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu 18.04 LTS Server Last modified: February 8, 2020
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliDisable the DSA and ECDSA host keys
Comment out the DSA and ECDSA HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^HostKey \/etc\/ssh\/ssh_host_\(dsa\|ecdsa\)_key$/\#HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh restart
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu 16.04 LTS Server Last modified: July 6, 2020
Note: all commands below are to be executed as the root user.
Re-generate ED25519 key
rm /etc/ssh/ssh_host_*
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliDisable the RSA, DSA, and ECDSA host keys
Comment out the RSA, DSA and ECDSA HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^HostKey \/etc\/ssh\/ssh_host_\(rsa\|dsa\|ecdsa\)_key$/\#HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh restart
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu 14.04 LTS Server Last modified: October 17, 2017
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliDisable the DSA and ECDSA host keys
Comment out the DSA and ECDSA HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^HostKey \/etc\/ssh\/ssh_host_\(dsa\|ecdsa\)_key$/\#HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh restart
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu Core 18 Server Last modified: October 6, 2019
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
Note: It is highly recommended that you run the ssh-keygen commands below on another host. Some IoT devices do not have good entropy sources to generate sufficient keys with!
ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N ""
Be sure to upload the following 4 files to the target device's /etc/ssh directory:- ssh_host_ed25519_key
- ssh_host_ed25519_key.pub
- ssh_host_rsa_key
- ssh_host_rsa_key.pub
Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Only enable RSA and ED25519 host keys.\nHostKey /etc/ssh/ssh_host_rsa_key\nHostKey /etc/ssh/ssh_host_ed25519_key\n\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh reload
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Ubuntu Core 16 Server Last modified: October 17, 2017
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
Note: It is highly recommended that you run the ssh-keygen commands below on another host. Some IoT devices do not have good entropy sources to generate sufficient keys with!
ssh-keygen -t rsa -b 4096 -f ssh_host_rsa_key -N ""
ssh-keygen -t ed25519 -f ssh_host_ed25519_key -N ""
Be sure to upload the following 4 files to the target device's /etc/ssh directory:- ssh_host_ed25519_key
- ssh_host_ed25519_key.pub
- ssh_host_rsa_key
- ssh_host_rsa_key.pub
Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
sed -i 's/^MACs \(.*\)$/\#MACs \1/g' /etc/ssh/sshd_config
echo -e "\n# Restrict MAC algorithms, as per sshaudit.com hardening guide.\nMACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh reload
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Debian Bookworm (Debian 12) Last modified: October 1, 2024
Change Log:
- October 1, 2024: Re-ordered host keys to prioritize ED25519 due to efficiency. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- April 24, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Enable the ED25519 and RSA keys
Enable the ED25519 and RSA HostKey directives in the /etc/ssh/sshd_config file:
echo -e "\nHostKey /etc/ssh/ssh_host_ed25519_key\nHostKey /etc/ssh/ssh_host_rsa_key" >> /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\n 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\n\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\nHostKeyAlgorithms 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\n\nRequiredRSASize 3072\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nGSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\nHostbasedAcceptedAlgorithms 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\n\nPubkeyAcceptedAlgorithms 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\n\n" > /etc/ssh/sshd_config.d/ssh-audit_hardening.confRestart OpenSSH server
service ssh restartImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
DEBIAN_FRONTEND=noninteractive apt install -q -y iptables netfilter-persistent iptables-persistent
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPEnable persistence of the iptables rules across server reboots:
service netfilter-persistent save
Debian Bullseye (Debian 11) Last modified: September 17, 2021
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
rm -f /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_ed25519_key -N ""
Enable the RSA and ED25519 keys
Enable the RSA and ED25519 HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com" > /etc/ssh/sshd_config.d/ssh-audit_hardening.confRestart OpenSSH server
service ssh restart
Debian Buster (Debian 10) Last modified: September 17, 2021
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
rm -f /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_ed25519_key -N ""
Enable the RSA and ED25519 keys
Enable the RSA and ED25519 HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^\#HostKey \/etc\/ssh\/ssh_host_\(rsa\|ed25519\)_key$/HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\nHostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512-cert-v01@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service ssh restart
Rocky Linux 9 Last modified: October 1, 2024
Change Log:
- October 1, 2024: Re-ordered host keys to prioritize ED25519 due to efficiency. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- April 24, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
rm -f /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_ed25519_key -N ""Enable the ED25519 and RSA keys
Enable the ED25519 and RSA HostKey directives in the /etc/ssh/sshd_config file:
echo -e "\nHostKey /etc/ssh/ssh_host_ed25519_key\nHostKey /etc/ssh/ssh_host_rsa_key" >> /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms 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\n\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\nHostKeyAlgorithms 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\n\nRequiredRSASize 3072\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nGSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\nHostbasedAcceptedAlgorithms 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\n\nPubkeyAcceptedAlgorithms 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\n\n" > /etc/crypto-policies/back-ends/opensshserver.configRestart OpenSSH server
systemctl restart sshdImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables/firewalld to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
firewall-cmd --permanent --direct --add-rule ipv4 filter INPUT 0 -p tcp --dport 22 -m state --state NEW -m recent --set
firewall-cmd --permanent --direct --add-rule ipv4 filter INPUT 1 -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
firewall-cmd --permanent --direct --add-rule ipv6 filter INPUT 0 -p tcp --dport 22 -m state --state NEW -m recent --set
firewall-cmd --permanent --direct --add-rule ipv6 filter INPUT 1 -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPReload firewalld to enable new rules:
systemctl reload firewalld
RedHat Enterprise Linux 8 Server / CentOS 8 Server Last modified: October 20, 2020
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
rm -f /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_ed25519_key -N ""
chgrp ssh_keys /etc/ssh/ssh_host_ed25519_key /etc/ssh/ssh_host_rsa_key
chmod g+r /etc/ssh/ssh_host_ed25519_key /etc/ssh/ssh_host_rsa_keyRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliDisable ECDSA host key
Comment out the ECDSA HostKey directive in the /etc/ssh/sshd_config file:
sed -i 's/^HostKey \/etc\/ssh\/ssh_host_ecdsa_key$/\#HostKey \/etc\/ssh\/ssh_host_ecdsa_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
cp /etc/crypto-policies/back-ends/opensshserver.config /etc/crypto-policies/back-ends/opensshserver.config.orig
echo -e "CRYPTO_POLICY='-oCiphers=chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr -oMACs=hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com -oGSSAPIKexAlgorithms=gss-curve25519-sha256- -oKexAlgorithms=curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256 -oHostKeyAlgorithms=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512 -oPubkeyAcceptedKeyTypes=ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512'" > /etc/crypto-policies/back-ends/opensshserver.config
Restart OpenSSH server
systemctl restart sshd.service
RedHat Enterprise Linux 7 Server / CentOS 7 Server Last modified: February 18, 2024
Note: all commands below are to be executed as the root user.
Disable automatic re-generation of RSA & ECDSA keys
mkdir -p /etc/systemd/system/sshd-keygen.service.d
cat << EOF > /etc/systemd/system/sshd-keygen.service.d/ssh-audit.conf
[Unit]
ConditionFileNotEmpty=
ConditionFileNotEmpty=!/etc/ssh/ssh_host_ed25519_key
EOF
systemctl daemon-reloadRe-generate the ED25519 key
rm -f /etc/ssh/ssh_host_*
ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""
chgrp ssh_keys /etc/ssh/ssh_host_ed25519_key
chmod g+r /etc/ssh/ssh_host_ed25519_keyRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliDisable the RSA, DSA, and ECDSA host keys
Comment out the RSA, DSA, and ECDSA HostKey directives in the /etc/ssh/sshd_config file:
sed -i 's/^HostKey \/etc\/ssh\/ssh_host_\(rsa\|dsa\|ecdsa\)_key$/\#HostKey \/etc\/ssh\/ssh_host_\1_key/g' /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "\n# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
systemctl restart sshd.service
Amazon Linux 2023 Last modified: October 1, 2024
Change Log:
- October 1, 2024: Re-ordered host keys to prioritize ED25519 due to efficiency. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- April 22, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
- March 15, 2024: Initial revision.
Note: all commands below are to be executed as the root user.
Re-generate the RSA and ED25519 keys
rm -f /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_ed25519_key -N ""Enable the ED25519 and RSA keys
Enable the ED25519 and RSA HostKey directives in the /etc/ssh/sshd_config file:
echo -e "\nHostKey /etc/ssh/ssh_host_ed25519_key\nHostKey /etc/ssh/ssh_host_rsa_key" >> /etc/ssh/sshd_configRemove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms 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\n\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\nHostKeyAlgorithms 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\n\nCASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\nGSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\nHostbasedAcceptedAlgorithms 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\n\nPubkeyAcceptedAlgorithms 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\n\n" > /etc/crypto-policies/back-ends/opensshserver.configRestart OpenSSH server
systemctl restart sshdImplement connection rate throttling
Connection rate throttling is needed in order to protect against the DHEat denial-of-service attack. A complete and flexible solution is to use iptables to allow up to 10 connections every 10 seconds from any one source address. An alternate solution is to set OpenSSH's PerSourceMaxStartups directive to 1 (note, however, that this can cause incomplete results during ssh-audit scans, as well as other client failures when bursts of connections are made).
dnf install -y iptables
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
iptables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROP
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --set
ip6tables -I INPUT -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 10 --hitcount 10 -j DROPEnable persistence of the iptables rules across server reboots:
dnf install -y iptables-services
iptables-save > /etc/sysconfig/iptables
ip6tables-save > /etc/sysconfig/ip6tables
systemctl enable iptables
systemctl enable ip6tables
systemctl start iptables
systemctl start ip6tables
pfSense 2.4 Last modified: October 17, 2017
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Remove small Diffie-Hellman moduli
awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.safe
mv -f /etc/ssh/moduli.safe /etc/ssh/moduliRestrict supported key exchange, cipher, and MAC algorithms
sed -i.bak 's/^MACs \(.*\)$/\#MACs \1/g' /etc/ssh/sshd_config && rm /etc/ssh/sshd_config.bak
echo "" | echo "MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
service sshd onerestart
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
OpenBSD 6.2 Server Last modified: October 20, 2020
Note: all commands below are to be executed as the root user.
Re-generate 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_ed25519_key -N ""Create custom Diffie-Hellman groups
ssh-keygen -G /etc/ssh/moduli -b 3072
Note: This will likely take some time to complete.Disable the DSA and ECDSA host keys
echo -e "\n# Restrict host keys to ED25519 and RSA only.\nHostKeyAlgorithms ssh-ed25519\n" >> /etc/ssh/sshd_configRestrict supported key exchange, cipher, and MAC algorithms
echo -e "# Restrict key exchange, cipher, and MAC algorithms, as per sshaudit.com\n# hardening guide.\nKexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\nCiphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\nMACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com" >> /etc/ssh/sshd_configRestart OpenSSH server
kill -HUP `cat /var/run/sshd.pid`
Note: Because of a bug in OpenSSH, 2048-bit DH moduli will still be used in some limited circumstances. Only a maximum score of 95% is possible.
Amazon Linux 2023 ClientLast modified: October 1, 2024
Change Log:
- October 1, 2024: Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- April 22, 2024: added connection throttling instructions to counteract the DHEat denial-of-service attack.
- March 15, 2024: Initial revision.
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\n KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,gss-group16-sha512-,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\n 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\n\n CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\n 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\n\n PubkeyAcceptedAlgorithms 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\n\n" >> ~/.ssh/config
Debian 12 ClientLast modified: October 1, 2024
Change Log:
- October 1, 2024: Added RequiredRSASize directive to enforce a minimum of 3072-bit user and host-based authentication keys. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- March 15, 2024: Initial revision.
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\n KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,gss-group16-sha512-,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\n RequiredRSASize 3072\n\n 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\n\n CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\n 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\n\n PubkeyAcceptedAlgorithms 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\n\n" >> ~/.ssh/config
Rocky Linux 9 ClientLast modified: October 1, 2024
Change Log:
- October 1, 2024: Added RequiredRSASize directive to enforce a minimum of 3072-bit user and host-based authentication keys. Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
- March 15, 2024: Initial revision.
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\n KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,gss-group16-sha512-,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\n RequiredRSASize 3072\n\n 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\n\n CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\n 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\n\n PubkeyAcceptedAlgorithms 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\n\n" >> ~/.ssh/config
Ubuntu 24.04 LTS Client / 
Linux Mint 22 ClientLast modified: October 1, 2024
Change Log:
- October 1, 2024: Added RequiredRSASize directive to enforce a minimum of 3072-bit user and host-based authentication keys.
- April 29, 2024: Initial revision. In comparison to Ubuntu 22.04 LTS Client guide, the following changes were made: 1.) For key exchanges, diffie-hellman-group18-sha512 and diffie-hellman-group-exchange-sha256 were prioritized over diffie-hellman-group16-sha512 due to greater security strength, 2.) For ciphers, 256-bit AES ciphers were prioritized over 192 and 128-bit AES ciphers due to their increased resistence against quantum computing attacks (previously, weaker GCM ciphers had priority over CTR ciphers), 3.) The HostbasedAcceptedAlgorithms and PubkeyAcceptedAlgorithms settings are now the same as HostKeyAlgorithms setting, and 4.) The hmac-sha2-512-etm@openssh.com MAC was increased in priority due to its increased resistence against quantum computing attacks.
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\n KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256,gss-group16-sha512-,diffie-hellman-group16-sha512\n\n MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com\n\n RequiredRSASize 3072\n\n 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\n\n CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\n HostbasedAcceptedAlgorithms 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\n\n 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\n\n" >> ~/.ssh/config
Ubuntu 22.04 LTS Client / Linux Mint 21 ClientLast modified: October 1, 2024
Change Log:
- October 1, 2024: Re-ordered cipher list to prioritize larger key sizes as a countermeasure to quantum attacks.
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-gcm@openssh.com,aes128-ctr\n\n KexAlgorithms sntrup761x25519-sha512@openssh.com,gss-curve25519-sha256-,curve25519-sha256,curve25519-sha256@libssh.org,gss-group16-sha512-,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n\n 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\n\n CASignatureAlgorithms sk-ssh-ed25519@openssh.com,ssh-ed25519,rsa-sha2-512,rsa-sha2-256\n\n GSSAPIKexAlgorithms gss-curve25519-sha256-,gss-group16-sha512-\n\n 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\n\n PubkeyAcceptedAlgorithms 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\n\n" >> ~/.ssh/config
Ubuntu 20.04 LTS Client / Linux Mint 20 ClientLast modified: October 20, 2020
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\n KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,sk-ssh-ed25519@openssh.com,sk-ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-256-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-512-cert-v01@openssh.com\n" >> ~/.ssh/config
Ubuntu 18.04 LTS Client / Linux Mint 19 ClientLast modified: October 20, 2020
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\n KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512\n" >> ~/.ssh/config
Ubuntu 16.04 LTS Client / Linux Mint 18 ClientLast modified: October 20, 2020
Run the following in a terminal to harden the SSH client for the local user:
mkdir -p -m 0700 ~/.ssh; echo -e "\nHost *\n Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr\n KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256\n MACs hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,umac-128-etm@openssh.com\n HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-256,rsa-sha2-512\n" >> ~/.ssh/config