Securing Your SSH Server

Keeping your data secure is critically important. Aspera strongly recommends taking additional steps to set up and configure your SSH server to protect against common attacks. These steps include the following:

Changing the TCP port.
Configuring transfer server authentication.

Aspera also recommends restricting user access to the server. For more information, see Setting Up Users for instructions using the GUI or Setting Up Transfer Users (Terminal).

Changing and Securing the TCP Port

Generally, SSH servers listen for incoming connections on TCP Port 22. As such, Port 22 is subject to numerous unauthorized login attempts by hackers who are attempting to access unsecured servers. An effective deterrent is to close 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, Aspera recommends setting the TCP port to 33001. The OpenSSH suite included in the installer uses TCP/22 as the default port for SSH connections. Remote Aspera clients attempt to establish an SSH connection with the server on port 33001. However, if the connection fails, the client retries the connection on port 22.

Aspera recommends opening TCP/33001 and disabling TCP/22 to prevent security breaches of your SSH server.

Open TCP/33001 and keep TCP/22 open until users are notified that they should switch to TCP/33001.
Once users are notified, block TCP/22 and allow traffic only on TCP/33001.

Prerequisites:

Before changing the default port for SSH connections, verify with your network administrators that TCP/33001 is open.
Before closing port TCP/22, notify users of the change.

Notifiying Users - How to Specify TCP/33001

Aspera recognizes that disabling the default SSH connection port (TCP/22) may affect your client users. When you change the port, ensure that you advise your users on how to configure the new port number, from the GUI (if available and used) and from the command line.

GUI: To change the SSH port in Desktop Client, click Connections and select the entry for the server whose ports are changing. On the Connection tab, click Show Advanced Settings and enter the SSH port number in the SSH Port (TCP) field.
Command line: Clients running FASP transfers from the command line can specify the port by using the -P 33001 option.

Changing to TCP/33001

The following steps require root privileges.

Open the SSH configuration file.
```
/etc/ssh/sshd_config
```
Add the TCP/33001 SSH port.
SSHD can listen on multiple ports, so you can have both TCP/33001 and TCP/22 open. To enable TCP/33001, add the port to your sshd_config file, as in the following example:
```
Port 22 
Port 33001
```
Once your client users have been notified of the port change to TCP/33001, disable TCP/22 and use only TCP/33001 by commenting out "Port 22" in your sshd_config file. For example:
```
#Port 22
Port 33001 
```
Once this setting takes effect:
- Aspera clients must set the transfer port to 33001 in the GUI or specify -P 33001 for command line transfers.
- To remotely access the server, server administrators should use ssh -p 33001.
Disable non-admin SSH tunneling.
These instructions require that OpenSSH 4.4 or newer is installed on your system in order to use the Match directive. Match allows you to selectively override certain configuration options when specific criteria (based on user, group, hostname, or address) are met.
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 might have additional instances of AllowTCPForwarding that are set to the default Yes. Review your sshd_config file for other instances and disable if necessary.

Disabling TCP forwarding does not improve security unless users are also denied shell access, because they can still install their own forwarders. Review your user and file permissions, and see Setting Up Transfer Users (Terminal) for instructions on modifying user shell access.
Update authentication methods
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.
Note: Before proceeding, configure at least one non-root, non-transfer user with a public key to use to manage the server. This is because in other server-securing steps, root login is disabled and Aspera recommends that transfer users are restricted to aspshell, which does not allow interactive login. This user and public key is what you use to access and manage the server as an administrator.

To configure authentication methods, add or uncomment PubkeyAuthentication yes and comment out PasswordAuthentication yes.
```
PubkeyAuthentication yes
#PasswordAuthentication yes
PasswordAuthentication no 
```
Note: If you choose to leave password authentication enabled, be sure to advise account creators to use strong passwords and set PermitEmptyPasswords to "no".
```
PermitEmptyPasswords no
```
Disable root login.
CAUTION:
This step disables root access. Make sure that you have at least one user account with sudo privileges before continuing, otherwise you may not have access to administer your server.
By default, OpenSSH allows root logins. However, disabling root access helps maintain a more secure server. Aspera recommends disabling root access by commenting out PermitRootLogin yes in the sshd_config file and adding PermitRootLogin No.
```
#PermitRootLogin yes 
PermitRootLogin no
```
Administrators can use the su command when root privileges are necessary.
Restart the SSH server to apply new settings.
Restarting your SSH server does not affect currently connected users.
```
$ sudo systemctl restart sshd.service
```
or for Linux systems that use init.d:
```
$ sudo service sshd restart
```
Review your logs periodically for attacks.
You can view the state of active TCP connections by running the netstat command:
```
# netstat -an -p tcp
```
Typical output shows multiple, different IP addresses connected to specific ports:
```
  TCP    10.0.111.200:53402     72.21.81.109:80        CLOSE_WAIT
  TCP    10.0.111.200:53865     173.194.202.188:5228   ESTABLISHED
  TCP    10.0.111.200:53876     10.0.9.16:445          TIME_WAIT
  TCP    10.0.111.200:55164     208.85.40.20:443       ESTABLISHED
  TCP    10.0.111.200:55335     207.200.35.240:443     ESTABLISHED
  TCP    10.0.111.200:55444     67.199.110.81:443      ESTABLISHED
  TCP    10.0.111.200:56278     104.24.11.90:443       ESTABLISHED
```
If your server is under attack, you might see output similar to the following, in which the same IP address attempts to connect to contiguous ports (hundreds or thousands of times) and the connection is timing out (reporting a status of TIME_WAIT):
```
  TCP    10.0.111.200:53402     72.21.81.109:60974      TIME_WAIT
  TCP    10.0.111.200:53865     72.21.81.109:60975      TIME_WAIT
  TCP    10.0.111.200:53876     72.21.81.109:60976      TIME_WAIT
  TCP    10.0.111.200:55164     72.21.81.109:60977      TIME_WAIT
  TCP    10.0.111.200:55335     72.21.81.109:60978      TIME_WAIT
  TCP    10.0.111.200:55444     72.21.81.109:60979      TIME_WAIT
  TCP    10.0.111.200:56278     72.21.81.109:60980      TIME_WAIT
```
If you see this, review your logs to determine the source and cause.

Open your syslog, which is located in /var/log/auth.log or /var/log/secure, depending on your system configuration.

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[1496]: Failed password for invalid user alex from 1.2.3.4 port 1585 ssh2
...
Mar 14 23:25:52 sku sshd[1496]: Failed password for invalid user alice from 1.2.3.4 port 1585 ssh2
...
```
If you identify attacks, take the following steps:
- Double-check the SSH security settings in this topic.
- Report attackers to your ISP's email address for abuse reports (often abuse@your_isp.com).

Configuring Transfer Server Authentication With the Host-Key Fingerprint

For transfers initiated by a web application (such as Faspex, Shares, or Console), the client browser sends the transfer request to the web application server over an HTTPS connection. The web application requests a transfer token from the target server. The transfer is executed over a UDP connection directly between the client and the target server and is authorized by the transfer token. Prior to initiating the transfer, the client can verify the server's authenticity to prevent server impersonation and man-in-the-middle (MITM) attacks.

To verify the authenticity of the transfer server, the web application passes the client the trusted SSH host key fingerprint of the transfer server. The client confirms the server's authenticity by comparing the server's fingerprint with the trusted fingerprint. In order to do this, the host key fingerprint or path must be set in the server's aspera.conf.

Set the host key fingerprint or path in the transfer server's aspera.conf file.

Note: Server SSL certificate validation (HTTPS) is enforced if a fingerprint is specified in aspera.conf and HTTP fallback is enabled. If the transfer "falls back" to HTTP and the server has a self-signed certificate, validation fails. The client requires a properly signed certificate.

If you set the host key path, the fingerprint is automatically extracted from the key file and you do not extract it manually.

Retreiving and setting the host key fingerprint:
1. Retrieve the server's SHA-1 fingerprint.
```
# cat /etc/ssh/ssh_host_rsa_key.pub | awk '{print $2}' | base64 -d | sha1sum
```
2. Set the SSH host key fingerprint in aspera.conf. (Go to the next step to set the host key path instead).
```
# asconfigurator -x "set_server_data;ssh_host_key_fingerprint,fingerprint"
```
  This command creates a line similar to the following example of the <server> section of aspera.conf:
```
<ssh_host_key_fingerprint>7qdOwebGGeDeN7Wv+2dP3HmWfP3 
</ssh_host_key_fingerprint>
```
Setting the host key path: To set the SSH host key path instead of the fingerprint, from which the fingerprint will be extracted automatically, run the following command:
```
# asconfigurator -x "set_server_data;ssh_host_key_path,ssh_key_filepath"
```
This command creates a line similar to the following in the <server> section of aspera.conf:
```
<ssh_host_key_path>/etc/ssh/ssh_host_rsa_key.pub
</ssh_host_key_path>
```
Restart the node service to activate your changes.
Run the following commands to restart asperanoded:
```
# systemctl restart asperanoded
```
or for Linux systems that use init.d:
```
# service asperanoded restart
```