By Anish Nath, Alibaba Cloud Tech Share Author. Tech Share is Alibaba Cloud's incentive program to encourage the sharing of technical knowledge and best practices within the cloud community.
In this article, we will go through all necessary SSH configurations that can be used to secure our Alibaba Cloud Elastic Compute Service (ECS) instances in a production environment.
When you log in to Alibaba Cloud Linux ECS instance using SSH
ssh root@[EIP address of the instance].You will see the following message, you have successfully connected to an instance.
Welcome to Alibaba Cloud Elastic Compute Service !Note: The default user is root which needs to be disabled (not the preferred way of accessing production system) so let's put a privilege escalation method such as sudo and then be used to execute commands as root.
However, this is not sufficient for most production environment. For better SSH security, we will cover various SSH secure parameter that needs to be audited/configured for the production environment.
The OpenSSH server reads a configuration file from /etc/ssh/sshd_config when it's started. The default values for /etc/ssh/sshd_config in OpenSSH are quite restrictive and need to be further tuned to meet the demand of the current security need for the production environment and being compliance with governance requirement like PCI/DSS, HIPPA etc.
You will need to be root or use sudo to edit and control the SSH server.
Usually this is done by editing the default configuration file to change and more harden configuration for example.
It is always a good idea to make a backup of any configuration files before editing them.
cp /etc/ssh/sshd_config /etc/ssh/backup.sshd_configTo disable passwords for root, but still allow key-based access without forced command, use:
PermitRootLogin prohibit-passwordTo disable passwords and only allow key-based access with a forced command, use:
PermitRootLogin forced-commands-onlyTo disable root login for the key-based access also and prompting the message
no-port-forwarding,no-agent-forwarding,no-X11-forwarding,command="echo 'Please login as the user \"alibabacloud\" rather than the user \"root\".';echo;sleep 10" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDePRIy/ ECS The /etc/ssh/moduli file usually contains several different entries called groups and sshd picks one randomly for each session. As shown in the below diagram the 1024 bits simply don't offer sufficient security margin.
OpenSSH supports 13 key exchange methods
| SL No | Key Exchange Method Name | Implement |
| 1 | curve25519-sha256 | SHOULD |
| 2 | diffie-hellman-group-exchange-sha1 | MUST NOT |
| 3 | diffie-hellman-group1-sha1 | MUST NOT |
| 4 | diffie-hellman-group14-sha1 | SHOULD- |
| 5 | diffie-hellman-group14-sha256 | MUST |
| 6 | diffie-hellman-group16-sha512 | SHOULD |
| 7 | ecdh-sha2-nistp256 | SHOULD |
| 8 | ecdh-sha2-nistp384 | SHOULD |
| 9 | gss-gex-sha1-* | MUST NOT |
| 10 | gss-group1-sha1-* | MUST NOT |
| 11 | gss-group14-sha1-* | SHOULD |
| 12 | gss-group14-sha256-* | SHOULD |
| 13 | gss-group16-sha512-* | SHOULD |
| 14 | rsa1024-sha1 | MUST NOT |
If option 4 is selected then delete the lines from the 5th column from the file /etc/ssh/moduli where bit size is less than 2000
awk '$5 > 2000' /etc/ssh/moduli > "${HOME}/moduli"
wc -l "${HOME}/moduli" # make sure there is something left
mv "${HOME}/moduli" /etc/ssh/moduliIf this file doesn't exist then generate a strong DH key size, higher bit size means more secure keys and less likely to break
ssh-keygen -G /etc/ssh/moduli.all -b 4096
ssh-keygen -T /etc/ssh/moduli.safe -f /etc/ssh/moduli.all
mv /etc/ssh/moduli.safe /etc/ssh/moduli
rm /etc/ssh/moduli.allRecommended KexAlgorithms /etc/ssh/sshd_config :
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256You should consider using Allow/Deny rules as a means of SSH access control mechanism. Once you add one AllowUsers rule, then the only users allowed to login via SSH are the listed ones:
User Based Logins
AllowUsers ecs
AllowUsers ecs2 Host based Logins
AllowUsers *@mywebserver.alibabacloud.com
AllowUsers *@myprovisioningserver. alibabacloud.com Domain Based Logins
AllowUsers *@*. alibabacloud.com pam_abl is a pam module designed to automatically block hosts who are attempting a brute force attack
# /etc/security/pam_abl.conf
debug
host_db=/var/lib/abl/hosts.db
host_purge=2d
host_rule=*:10/1h,30/1d
user_db=/var/lib/abl/users.db
user_purge=2d
user_rule=!root:10/1h,30/1d*:10/1h,30/1d: means block any user (*) if they are responsible for ten or more failed authentication attempts in the last hour or thirty or more failed attempts in the last day.
Chroot is "an operation that changes the apparent root directory for the current running process and its children." This will give a client access to the server, but limit those users to their home directories, and it's a powerful feature and serve many secure use case like to chroot an SFTP directory.
Create an user and force root to be owner of it
cd /home
mkdir ftp
useradd -d /home/ftp -M -N -g users ftp
sudo chown root:root /home/ftp
sudo chmod 755 /home/ftpChange the subsystem location on /etc/ssh/sshd_config:
#Subsystem sftp /usr/lib/openssh/sftp-server
Subsystem sftp internal-sftpand create a user section at the end of the file
Match User john
ChrootDirectory /home/ftp
ForceCommand internal-sftp
AllowTCPForwarding no
X11Forwarding noYou can encrypt your SSH connection by running the SSHv2 Protocol.
Force SSHv2 Protocol
Protocol 2HostKeys for Protocol Version 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_keyUse Secure Ciphers and MACs
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.comDisable Unused Authentication Schemes
RhostsRSAAuthentication no
HostbasedAuthentication no
ChallengeResponseAuthentication no
KerberosAuthentication no
GSSAPIAuthentication noDisable Root SSH access
PermitRootLogin no
PermitEmptyPasswords noThe following options lets you control the types of authentications available for use. You can tune it according to your environment, such as using only key based authentication instead of password for extra security.
RSAAuthentication yes
PubkeyAuthentication yes
PasswordAuthentication yes
AuthenticationMethods publickey,passwordEnable Logging
Logging provide traceability and enable the audit for the users.
SyslogFacility AUTH
LogLevel INFOAuthentication Time
You can adjust the time it takes for authentication to happen. In this example, we will limit it to 20 seconds.
LoginGraceTime 20Reduce Timeout Intervals
# Sets a timeout interval in seconds, default is 15
ClientAliveInterval 40
# Sets the number of client alive messages, default value is 3
ClientAliveCountMax 3Deny Empty Password
You can force all logins to require a password.
# Don't allows login to accounts with empty password, The default value is no
passworPermitEmptyPasswords noFail2ban can scan logs and ban temporarily ban IPs based on possible malicious activity. You will need to install Fail2ban.
#ubutnu
sudo apt-get install fail2ban
#rhel/centos
sudo yum install fail2bancopy the fail2ban configuration file.
cp /etc/fail2ban/jail.conf /etc/fail2ban/jail.localOpen the /etc/fail2ban/jail.local files and find the spot that starts [sshd]. Edit it like so, adding enabled = true:
[sshd]
enabled = true
port = ssh
logpath = %(sshd_log)sThen restart fail2ban
service fail2ban restartFail2ban will monitor your SSH logs for possible malicious activity and then temporarily ban the source IP.
List Down Current Ciphers
$ ssh -Q cipher
3des-cbc
blowfish-cbc
cast128-cbc
arcfour
arcfour128
arcfour256
aes128-cbc
aes192-cbc
aes256-cbc
rijndael-cbc@lysator.liu.se
aes128-ctr
aes192-ctr
aes256-ctr
aes128-gcm@openssh.com
aes256-gcm@openssh.com
chacha20-poly1305@openssh.comList Down Supported MAC's
[root@localhost ~]# ssh -Q mac
hmac-sha1
hmac-sha1-96
hmac-sha2-256
hmac-sha2-512
hmac-md5
hmac-md5-96
hmac-ripemd160
hmac-ripemd160@openssh.com
umac-64@openssh.com
umac-128@openssh.com
hmac-sha1-etm@openssh.com
hmac-sha1-96-etm@openssh.com
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
hmac-md5-etm@openssh.com
hmac-md5-96-etm@openssh.com
hmac-ripemd160-etm@openssh.com
umac-64-etm@openssh.com
umac-128-etm@openssh.comList Down Supported Keys
$ ssh -Q key
ssh-ed25519
ssh-ed25519-cert-v01@openssh.com
ssh-rsa
ssh-dss
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
ssh-rsa-cert-v01@openssh.com
ssh-dss-cert-v01@openssh.com
ecdsa-sha2-nistp256-cert-v01@openssh.com
ecdsa-sha2-nistp384-cert-v01@openssh.com
ecdsa-sha2-nistp521-cert-v01@openssh.comAny changes in the file /etc/ssh/sshd_config requires restart of the SSH service.
#rhel/centos
/sbin/service sshd restart
#ubuntu
/etc/init.d/sshd restartBecause ystems will added/deleted over the time, you should consider using automation when the inventory is changing rapidly. Automation of SSH-key management include:
You should also consider other forms of automation, such as automating security group updates on Alibaba Cloud.
Additional reading materials about OpenSSH and sshd_config can be found at:
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Elastic and secure virtual cloud servers to cater all your cloud hosting needs.
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