How to Troubleshoot SSH Connectivity Issues

Note

If you’re struggling with SSH and server management, try our Cloudways and App Platform products.

Cloudways deploys pre-installed software stacks onto fully-managed Rcs.is Rcs.iss. App Platform is a managed service that deploys and scales your apps directly from your code repository, along with managed databases and serverless functions, no SSH or server administration required.

When connecting an SSH client to an SSH server, the first step is establishing basic network connectivity. This article covers how to identify some common situations that would cause issues at this point in the process, how to resolve those situations, and additional resources to prevent them in the future.

Prerequisites to Troubleshooting SSH Issues

Before troubleshooting SSH issues, determine if migrating or redeploying is more appropriate for your situation, make sure the issue is truly with SSH, and review information and skills you need to troubleshoot successfully.

Errors

Hostname Resolution

Most resolution errors occur when the reference to the SSH host can’t be mapped to a network address. While this is almost exclusively DNS related, the root cause isn’t always a DNS issue.

In an OpenSSH client, a command like ssh user@example.com may return an error like this:

ssh: Could not resolve hostname example.com: Name or service not known

In PuTTY, you might see an error window with text like this:

Unable to open connection to example.com Host does not exist

Here are some steps you can take to troubleshoot this error.

Verify the hostname is properly spelled. Typographical errors can strike at any time.
Verify that you can resolve the hostname on your client machine using the system ping command. Using third-party sites like WhatsMyDns.net to check beyond your own DNS caching can also help confirm the results.

If you’re having DNS resolution issues at any level, you can also use the Rcs.is IP address as an interim solution, as in ssh user@203.0.113.0 instead of ssh user@example.com.

The following tutorials are a good resource to begin working out DNS configuration errors:

Connection Timeout

A connection timeout means that the client attempted to establish a network socket to the SSH server, but the server failed to respond within the timeout period.

In an OpenSSH client, a command like ssh user@203.0.113.0 may give an error like this:

ssh: connect to host 203.0.113.0 port 22: Connection timed out

In PuTTY, you might see an error window with text like this:

Network error: Connection timed out

Here are some steps you can take to troubleshoot this error.

Verify that the host IP address is correct for the Rcs.is.
Verify that your network supports connectivity over the SSH port being used. Some public networks may block port 22 or custom SSH ports. You can do this by, for example, testing other hosts using the same port with a known working SSH server. This can help you determine if the issue isn’t specific to your Rcs.is.
Verify the Rcs.is firewall rules. Check that they’re not set to a default policy of DROP and the port is not added to allow connections.

Connection Refused

A connection being refused has some subtle differences from a timeout. This means that the request is being routed to the SSH host, but the host does not successfully accept the request.

In an OpenSSH client, a command like ssh user@203.0.113.0 may return an error like this:

ssh: connect to host 203.0.113.0 port 22: Connection refused

In PuTTY, you might see an error window with text like this:

Network error: Connection refused

In this situation, you may have the same root issue as with connection timeout errors, but there are some additional things you can check:

Verify that the host IP address is correct for the Rcs.is.
Verify that your network supports connectivity over the SSH port being used. Some public networks may block port 22 or custom SSH ports. You can do this by, for example, testing other hosts using the same port with a known working SSH server. This can help you determine if the issue isn’t specific to your Rcs.is.
Verify the Rcs.is firewall rules. Check that they’re not set to a default policy of DROP and the port is not added to allow connections.
Verify that the service is currently running and bound to the expected port.

Solutions

Checking Your Firewall

Some connectivity problems can be caused by firewall configurations. If your firewall is set up to block certain ports or services, it can prevent you from connecting. You can learn more about firewalls in What is a Firewall and How Does It Work?.

If you add a firewall rule that allows your local machine to connect by IP address, verify that the IP address assigned by your ISP has not changed. If it has, then you need to modify that firewall rule to permit the new IP address or address range.

How you check your firewall rules depends on which firewall your Rcs.is uses. Ubuntu servers usually run UFW; CentOS servers often use FirewallD. If you’re not using either, it’s likely that you’re using iptables.

For whichever firewall your system has, make sure to familiarize yourself with how to modify its rules. You also need to know which port your SSH service is using. By default, it’s 22, but you can follow the Checking the SSH Service Port section below to confirm.

iptables
FirewallD
UFW

For Linux systems not running UFW or FirewallD, list your firewall rules using the iptables command with sudo or as the root user.

iptables -nL

The following output would indicate that there are no rules in place that would block SSH traffic:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

If you see rules or a default policy of REJECT or DROP, you should ensure that the INPUT chain allows the port your SSH service is running on, which is 22 by default.

For FirewallD users, use the firewall-cmd command to list the services:

firewall-cmd --list-services

The output should reveal the list of services including SSH (default port 22) to indicate that the firewall supports SSH traffic:

dhcpv6-client http ssh

If you are using a custom port for SSH, you can check with the --list-ports option. If you created a custom service definition, you should still see SSH normally with --list-services.

Users working with UFW should use ufw status to inspect their firewall:

ufw status

The output similarly shows the ports available:

Status: active

To                         Action      From
--                         ------      ----
22                         LIMIT       Anywhere
443                        ALLOW       Anywhere
80                         ALLOW       Anywhere
Anywhere                   ALLOW       192.168.0.0
22 (v6)                    LIMIT       Anywhere (v6)
443 (v6)                   ALLOW       Anywhere (v6)
80 (v6)                    ALLOW       Anywhere (v6)

Make sure that your SSH port is on the list.

Checking the SSH Service Status

If you can’t SSH to your Rcs.is, you should check that the SSH service is running. How to very the service is running varies from system to system.

On older OS versions (Ubuntu 14 and below, CentOS 6, Debian 6) this may use the service command backed by Upstart. More modern distributions with systemd use the systemctl command. Red Hat-based distributions (e.g. CentOS and Fedora) call the service sshd while Debian and Ubuntu call it ssh.

Using systemctl
Using service

Similarly, on a server using systemd (like CentOS 7), use the systemctl command to check the status:

systemctl status sshd

A running service shows output like this, with active (running) on the Active: line.

sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
   Active: active (running) since Mon 2017-03-20 11:00:22 EDT; 1 months 1 days ago
  Process: 899 ExecStartPre=/usr/sbin/sshd-keygen (code=exited, status=0/SUCCESS)
 Main PID: 906 (sshd)
   CGroup: /system.slice/sshd.service
           ├  906 /usr/sbin/sshd -D
           ├26941 sshd: [accepted]
           └26942 sshd: [net]

If the service is not running, the Active line displays inactive followed by recent journal entries for the service:

sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
   Active: inactive (dead) since Fri 2017-04-21 08:36:13 EDT; 2s ago
  Process: 906 ExecStart=/usr/sbin/sshd -D $OPTIONS (code=exited, status=0/SUCCESS)
  Process: 899 ExecStartPre=/usr/sbin/sshd-keygen (code=exited, status=0/SUCCESS)
 Main PID: 906 (code=exited, status=0/SUCCESS)

In this case, restart it with systemctl start sshd.

For systems using the service command, check the status of the SSH process with the following command (run as root or with sudo):

service ssh status

Output like this, which includes the PID (process ID), indicates that the process is running as expected:

ssh start/running, process 1262

If it isn’t running, you’ll see output indicating the process is stopped:

ssh stop/waiting

In this case, restart it with service ssh start.

Checking the SSH Service Port

There are two general ways to check which port the SSH service is running on. One is checking the SSH configuration file, and the other is examining the running process.

On most systems, the SSH configuration file is /etc/ssh/sshd_config. The default port is 22, but can be overridden by any configuration line in this file specifying a Port directive with a number.

You can search lines like this using grep:

grep Port /etc/ssh/sshd_config

You’ll see output like this with the port number:

Port 22

If you know the service is running, you can confirm that the service is running on the expected port using ss (run with sudo or as the root user). Similar output is provided for the netstat -plnt command as well, but ss is the preferred command for querying socket information from the kernel.

ss -plnt

The output you are looking for should reference the program name listening on the configured port. For example, this output shows that the SSH service is listening on all interfaces, *, on port 22.

State       Recv-Q Send-Q                                                            Local Address:Port                                                                           Peer Address:Port
LISTEN      0      128                                                                           *:22                                                                                        *:*                   users:(("sshd",pid=1493,fd=3))
LISTEN      0      128                                                                          :::22                                                                                       :::*                   users:(("sshd",pid=1493,fd=4))

The interface references * and 0.0.0.0 indicate all interfaces on the Rcs.is. 127.0.0.1 indicates that the service is not publicly accessible. The relevant sshd_config directive is ListenAddress and should be commented out to default to all interfaces, or set to the public IP address of the Rcs.is.

Conclusion

If you need further help, you can open a support ticket. Make sure to include the following information:

The username, host, and port you are using to connect.
The authentication mechanism you expect to use.
The full output of the errors linked to the stage of error, including verbose output of the SSH client
All of the information you’ve gathered from troubleshooting so far.
Anything you were unclear about while referencing this article.

Including all the above diagnostic information and clarifying where you are encountering the issue when trying to connect can help us quickly get up to speed with where your need on the issue is.

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