Most automated robots try to log into your SSH server on Port 22 as root with various brute force and dictionary combinations in order to gain access to your data. Furthermore, automated robots can put enormous loads on your server as they perform thousands of retries to break into your system. This topic addresses steps to secure your SSH server against potential threats, including changing the default port for SSH connections from TCP/22 to TCP/33001.
It is well known that SSH servers listen for incoming connections on TCP Port 22. As such, Port 22 is subject to countless, unauthorized login attempts by hackers who are attempting to access unsecured servers. A highly effective deterrent is to simply turn off Port 22 and run the service on a seemingly random port above 1024 (and up to 65535). To standardize the port for use in Aspera transfers, we recommend using TCP/33001.
The following explains how to change the SSH port to 33001 and take additional steps to secure your SSH server. The steps all require root access privileges.
The SSH configuration file can be found in the following location:
The OpenSSH suite included in the installer uses TCP/22 as the default port for SSH connections. Aspera recommends opening TCP/33001 and disabling TCP/22 to prevent security breaches of your SSH server.
To enable TCP/33001 while your organization is migrating from TCP/22, open port 33001 from your sshd_config file (where SSHD is listening on both ports). As demonstrated by this exercise, SSHD is capable of listening on multiple ports.
... Port 22 Port 33001 ...
Once your client users have been notified of the port change (from TCP/22 to TCP/33001), you can disable port 22 in your sshd_config file. To disable TCP/22 and use only TCP/33001, comment out "Port 22" in your sshd_config file.
... #Port 22 Port 33001 ...
In OpenSSH versions 4.4 and newer, disable SSH tunneling to avoid potential attacks; thereby only allowing tunneling from root users. To disable non-admin SSH tunneling, open your SSH Server configuration file, sshd_config, with a text editor.
Add the following lines to the end of the file (or modify them if they already exist):
... AllowTcpForwarding no Match Group root AllowTcpForwarding yes
Depending on your sshd_config file, you may have additional instances of AllowTCPForwarding that are set to the default Yes. Review your sshd_config file for other instances and disable as appropriate.
Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. Review your user and file permissions, and see the instructions below on modifying shell access.
Public key authentication can prevent brute-force SSH attacks if all password-based authentication methods are disabled. For this reason, Aspera recommends disabling password authentication in the sshd_config file and enabling private/public key authentication. To do so, add or uncomment PubkeyAuthentication yes and comment out PasswordAuthentication yes.
... PubkeyAuthentication yes #PasswordAuthentication yes PasswordAuthentication no ...
OpenSSH defaults to allowing root logins; however disabling root access helps you to maintain a more secure server. Aspera recommends commenting out PermitRootLogin yes in the sshd_config file and adding PermitRootLogin No.
... #PermitRootLogin yes PermitRootLogin no ...
Administrators can then utilize the su command if root privileges are needed.
When you have finished updating your SSH server configuration, you must restart or reload the SSH service to apply your new settings. Note that restarting or reloading SSH does not impact currently connected users.
To restart or reload your SSH server, run the following commands:
|RedHat, zLinux (restart)||
$ sudo service sshd restart
$ sudo service sshd reload
$ sudo /etc/init.d/ssh restart
$ sudo /etc/init.d/ssh reload
Aspera recommends reviewing your SSH log periodically for signs of a potential attack. Locate and open your syslog—for example, /var/log/auth.log or /var/log/secure. Depending on your system configuration, syslog's path and file name may vary.
Look for invalid users in the log, especially a series of login attempts with common user names from the same address, usually in alphabetical order. For example:
... Mar 10 18:48:02 sku sshd: Failed password for invalid user alex from 18.104.22.168 port 1585 ssh2 ... Mar 14 23:25:52 sku sshd: Failed password for invalid user alice from 22.214.171.124 port 1585 ssh2 ...
If you identify attacks, do the following:
Restricting user access is a critical component of securing your server. By default, all user accounts are allowed to browse and read all files on the server. To limit a user's access to a portion of the system, set the account's shell to the Aspera secured shell (aspshell) and create a document root (docroot) for that user. The aspshell permits only the following operations:
Open the following file with a text editor:
Add or replace the user's shell with /bin/aspshell. For example, to apply aspshell to the user aspera_user_1, use the following settings in the passwd file:
... aspera_user_1:x:501:501:...:/home/aspera_user_1:/bin/aspshell ...
Edit the aspera.conf file (/opt/aspera/etc/aspera.conf). The following template displays access options:
<file_system> <access> <paths> <path> <absolute>/sandbox/aspera_user_1</absolute> <!-- Absolute Path --> <read_allowed>true</read_allowed> <!-- Read Allowed --> <write_allowed>true</write_allowed> <!-- Write Allowed --> <dir_allowed>true</dir_allowed> <!-- Browse Allowed --> </path> </paths> </access> ... </file_system>
Once you have set the user's shell and docroot, you can further restrict access by disabling read, write, and/or browse using <path> settings in aspera.conf, as shown in the example above.
|Absolute Path||The area of the file system (path) that is accessible to the Aspera user. The default empty value gives a user access to the entire file system.||Path or blank|
|Read Allowed||Setting this to true allows users to transfer from the designated area of the file system as specified by the Absolute Path value.||
|Write Allowed||Setting this to true allows users to transfer to the designated area of the file system as specified by the Absolute Path value.||
|Browse Allowed||Setting this to true allows users to browse the directory.||
The asp-check tool performs the following secure checks:
To run the asp-check tool, run the following on the command line:
$ sudo /opt/aspera/bin/asp-check.sh
Search results are displayed as in the following example. If potential issues are identified, review your users' settings before proceeding.
Users with full access: 22 (not considered insecure) Restricted users: 0 Insecure users: 0 - no restricted shell (aspshell): 0 - docroot above home directory: 0 - ssh tunneling enabled: 0