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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteTo keep an SSH session from being dropped while it is idle, configure the OpenSSH client to send encrypted probes: ServerAliveInterval sets the idle interval and ServerAliveCountMax sets how many unanswered probes are tolerated. For a server you administer, the corresponding settings are ClientAliveInterval and ClientAliveCountMax. A one-time client command is:
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ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 user@hostFor a permanent client setting, put the directives in ~/.ssh/config. Server settings belong in /etc/ssh/sshd_config, followed by syntax validation and a daemon reload. These SSH-level keepalives are different from TCP keepalives and travel inside the encrypted SSH channel.
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What an SSH keepalive actually does
An idle SSH connection can be removed by a firewall, NAT gateway, VPN, load balancer, sleeping laptop, changing network, or a genuinely unreachable endpoint. SSH keepalives periodically create traffic so an otherwise quiet connection is checked instead of remaining completely idle.
OpenSSH has two directional configurations. The client-side ServerAlive* options make your SSH client send an encrypted request after a period with no incoming server data. The server-side ClientAlive* options make sshd send a request when it has received no client data. A probe is not a promise that the remote host will remain available: unanswered probes eventually close the connection, which prevents a dead session from appearing usable forever.
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These messages are SSH protocol traffic carried through the encrypted channel. OpenSSH’s server documentation states that client-alive messages are sent through the encrypted channel and therefore are not spoofable. They are not the same mechanism as TCP keepalives.
Configure the OpenSSH client
Understand the two client options
| Option | Meaning | OpenSSH default |
|---|---|---|
ServerAliveInterval |
Seconds of inactivity before the client sends an encrypted request to the server. A value of 0 disables these requests. |
0 |
ServerAliveCountMax |
Number of unanswered server-alive requests the client tolerates before disconnecting. | 3 |
The interval is not a total session timeout. It applies when no server data has arrived. With an interval of 60 seconds, an active transfer or command that is already producing server traffic does not need an additional idle probe at that moment.
Make the setting permanent for your user
- Create or open
~/.ssh/configon the computer from which you run SSH. - Add a host block. To apply it to every destination, use:
Host *
ServerAliveInterval 60
ServerAliveCountMax 3Use a narrower block such as Host production.example when only one destination needs the policy. Keep the options in the client configuration file; placing them in /etc/ssh/sshd_config will not change how your local client behaves.
Apply a setting to one connection
For a temporary test or a single maintenance session, pass the options on the command line:
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ssh -o ServerAliveInterval=60 -o ServerAliveCountMax=3 user@hostThis is useful when you cannot or do not want to alter your user configuration. It also lets you compare behavior with and without keepalives before choosing a permanent value.
How long before a failed connection is declared dead?
The approximate detection window is the interval multiplied by the number of unanswered probes. OpenSSH’s documented example uses ServerAliveInterval 15 with the default count of three; an unresponsive server is disconnected after approximately 45 seconds. The actual experience can vary with scheduling and network timing, so treat the product as an operational approximation rather than a precise stopwatch.
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Configure the OpenSSH server
Understand the server-side pair
| Option | Meaning |
|---|---|
ClientAliveInterval |
Seconds after receiving no client data before sshd sends a request through the encrypted channel. |
ClientAliveCountMax |
Number of unanswered client-alive requests allowed before the server disconnects the client. |
These settings affect connections accepted by that server. They do not replace a user’s client-side configuration, and they do not change the idle policies enforced by an intermediate firewall, VPN, NAT device, or load balancer.
Change the server configuration safely
- Back up the current
/etc/ssh/sshd_configand edit it with administrative privileges. - Add or adjust the directives, for example:
ClientAliveInterval 60
ClientAliveCountMax 3- Validate the complete configuration before applying it with
sshd -t. A syntax error must be fixed before reloading; otherwise you risk leaving the daemon unable to accept the intended configuration. - Use the operating system’s service manager to reload the SSH daemon. On a systemd installation the service may be named
sshdorssh, so use the name supplied by that operating system rather than assuming one universal command. - Start a new SSH connection and test it. Existing sessions may not inherit changed daemon settings.
Server-side keepalives are especially useful when you operate the host and want a consistent policy for many clients. Set them with awareness of the number of concurrent sessions: a short interval and high count can create more probe traffic and retain dead sessions longer.
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SSH keepalives versus TCP keepalives
ServerAliveInterval, ServerAliveCountMax, ClientAliveInterval, and ClientAliveCountMax operate at the SSH protocol layer. Their requests are encrypted SSH messages, so an intermediary cannot forge a valid response merely by seeing the packet.
TCPKeepAlive is a separate TCP mechanism. It can help the operating system detect a dead peer, but it can respond differently to transient route failures and does not let you choose an SSH-level probe schedule. The Debian server documentation describes TCPKeepAlive as enabled by default in that package documentation and warns that disabling it can leave ghost sessions consuming resources. Do not treat TCP keepalives as a substitute for selecting an SSH probe interval.
In practice, SSH-level probes are the setting to tune when your goal is surviving an idle timeout imposed somewhere between the two endpoints. TCP keepalives remain a separate transport behavior with its own operating-system and network characteristics.
Choose an interval and count deliberately
Start with the timeout you need to beat
Identify the shortest idle timeout in the path: a corporate firewall, home router, cloud load balancer, VPN concentrator, or other middlebox. Choose an SSH interval shorter than that timeout so traffic is seen before the connection is classified as idle. If the timeout is unknown, begin with a moderate value such as 60 seconds and observe whether the connection remains stable, then adjust to the actual policy rather than assuming every network has the same limit.
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Balance resilience against failure-detection speed
The count determines how much silence you accept after a probe. Increasing ServerAliveCountMax or ClientAliveCountMax allows brief network interruptions to recover without immediately terminating the session, but it lengthens the period during which an unreachable endpoint appears connected. Decreasing the count shortens failure detection and releases resources sooner.
For example, an interval of 60 seconds and a count of three gives an approximate three-minute unanswered-probe window. An interval of 15 seconds and a count of three gives approximately 45 seconds, the timing described in the OpenSSH client documentation. These calculations are planning tools, not universal vendor recommendations; choose values according to the cost of a false disconnect versus the cost of waiting for a dead session to be recognized.
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Consider the workload
- Interactive shell work benefits from probes frequent enough to cross the network’s idle threshold without generating needless traffic.
- Long-running commands that produce no output still need the same idle policy if a middlebox may expire the connection.
- Very large fleets should avoid unnecessarily aggressive intervals on every account; multiply probe frequency by concurrent sessions when estimating operational load.
- If the endpoint is expected to be unavailable for an extended period, a keepalive cannot preserve a route that no longer exists. It can only detect the condition according to the chosen interval and count.
Verify what the client is really using
Configuration files can contain multiple host blocks, and a command-line option can override a file value. Ask OpenSSH to print the effective configuration for a destination:
ssh -G host | grep -i aliveCheck that the output contains the intended serveraliveinterval and serveralivecountmax values. Run the command with the same host alias you use for the real connection; testing a different alias can select a different Host block.
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When diagnosing the server side, inspect the active /etc/ssh/sshd_config, run sshd -t, reload through the service manager, and then establish a fresh connection. A successful syntax check only proves that the file parses; it does not prove that a firewall or VPN will permit the probes.
Troubleshoot disconnects that continue
The client setting has no effect
- Confirm the directives are in
~/.ssh/config, not only in the server file. - Use
ssh -G host | grep -i aliveto verify the effective values for the exact host alias. - Check spelling and capitalization. OpenSSH option names are normally written exactly as documented, such as
ServerAliveInterval. - Try the one-time
-ocommand-line form to separate a configuration-file problem from a network problem.
The server setting has no effect
- Make sure you edited
/etc/ssh/sshd_configon the server that actually accepts the connection. - Run
sshd -tbefore reloading and correct every reported error. - Reload the daemon with the host operating system’s service manager, then open a new session.
- Check server logs if the daemon rejects the configuration or closes sessions unexpectedly.
The session still drops at a predictable idle time
A middlebox may have an idle policy shorter than your interval, or it may ignore or mishandle the traffic. Reduce the SSH interval so a probe occurs before that policy expires. If the network has brief interruptions, increase the tolerated count instead of making the interval extremely short. Verify VPN behavior and firewall or NAT idle settings; SSH cannot override a device that deliberately tears down the route.
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When the client sleeps, loses connectivity, or switches between Wi-Fi and another network, probes cannot receive timely replies. Keepalives will eventually identify the connection as unresponsive, but they cannot resume a TCP path that changed underneath it. Inspect the client sleep behavior and network transitions, then reconnect after connectivity is restored.
Sessions remain as ghosts on the server
If dead sessions consume server resources, do not disable every liveness mechanism indiscriminately. The Debian server documentation warns that disabling TCPKeepAlive can leave ghost sessions consuming resources. Instead, choose an SSH interval and count that detect failure within an acceptable window, and keep the server’s resource limits and logging under review.
Quick Recap
A practical rollout checklist
- Decide whether the requirement is per-client, server-wide, or both.
- Identify the shortest relevant idle timeout in the firewall, NAT, VPN, or load-balancer path.
- Set a client interval below that timeout, then choose a count based on how long brief outages may last.
- For a client, test with
ssh -o ServerAliveInterval=...before editing~/.ssh/config. - For a server, edit
/etc/ssh/sshd_config, validate withsshd -t, reload through the service manager, and open a new session. - Confirm effective client values with
ssh -G host | grep -i alive. - If drops continue, investigate middlebox policies, VPN behavior, server logs, sleep, and network changes rather than increasing keepalive values blindly.
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