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Classify the disconnect before changing anything
Write down the exact elapsed time, whether the session was idle or transferring data, and what the client displayed. This single distinction prevents the most common wrong fix: adding keepalives to a connection that is actually failing because of a server crash, packet loss, authentication policy, or an exhausted resource.
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| Observed pattern | Most useful first hypothesis | What to check next |
|---|---|---|
| Drop after a repeatable idle period | A packet filter, NAT device, VPN, or load balancer expired inactive TCP state. | Path idle timers, keepalive traffic, and firewall/NAT logs. |
| Drop while bytes are moving | Endpoint, path, server policy, resource, or SFTP-subsystem problem. | Client debug output, sshd/SFTP logs, resets, disk and process health. |
| Drop at the same file or operation every time | An application-level limit, permission problem, subsystem error, or server-side resource issue. | Server logs for that account, path, file, and operation. |
| Only one network fails | The failing network’s NAT, firewall, VPN, or wireless path. | Repeat from a wired connection and a mobile hotspot, then compare timestamps. |
| Every network and client fails | A common server, account, host, or SFTP subsystem fault. | Server-side logs, limits, service health, and account restrictions. |
The OpenSSH FAQ describes an idle-only failure as usually caused by “a packet filter or NAT device timing out your TCP connection due to inactivity.” That diagnosis is plausible, not automatic: confirm it with timestamps and device logs.
Collect evidence from all three layers
Client-side trace
Reproduce the problem with the OpenSSH client at high verbosity and save the output with UTC timestamps:
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sftp -vvv user@hostFor a repeatable test, redirect output to a file and record when the transfer starts, when it becomes idle, and when the failure appears. The final event may be an SSH disconnect, a TCP reset, an authentication failure, or a local timeout. Those outcomes lead to different fixes.
Server-side logs
At the same UTC time, inspect the SSH daemon and SFTP-subsystem logs. Look for session termination, account restrictions, forced commands, authentication events, subsystem crashes, resource errors, and administrator-initiated disconnects. Log locations and service names vary by operating system and packaging, so use the platform’s configured sshd journal or log files rather than assuming one path.
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Check the firewall, NAT gateway, VPN concentrator, reverse proxy, and load balancer carrying the connection. Correlate connection tracking expiry, policy drops, TCP resets, idle-timeout events, and tunnel renegotiations with the client and server timestamps. A network device that silently expires state may leave no SSH-level reason code; its own event log is often the only confirmation.
What an SSH disconnect code tells you
SSH runs over a transport such as TCP and defines machine-readable disconnect reason codes. RFC 4253 assigns SSH_DISCONNECT_CONNECTION_LOST decimal reason code 10; the IANA registry represents the same value as 0x0000000A. The code confirms that the SSH transport was lost, but it does not identify whether a client process, server process, firewall, NAT device, VPN, or physical path caused the loss. Treat it as a symptom to correlate with the other logs, not as a diagnosis.
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How OpenSSH keepalives work
OpenSSH has two separate SSH-layer mechanisms. The client sends probes with ServerAliveInterval; the server sends probes with ClientAliveInterval. Each uses a companion count setting to decide how many unanswered probes are tolerated. These messages travel inside the encrypted SSH channel, unlike ordinary TCP keepalives.
| Setting | Configured in | Meaning | When disconnect occurs |
|---|---|---|---|
ServerAliveInterval |
OpenSSH client configuration or sftp -o option |
After this many seconds with no data received from the server, the client sends an encrypted request. | Only after the client has sent the number of unanswered probes allowed by ServerAliveCountMax. |
ServerAliveCountMax |
OpenSSH client configuration or command line | Maximum unanswered server-alive requests the client accepts. | After the limit is reached, the client closes the connection. |
ClientAliveInterval |
Server sshd_config |
After this many seconds with no data received from the client, sshd sends an encrypted channel request. | Only after the number of unanswered requests allowed by ClientAliveCountMax. |
ClientAliveCountMax |
Server sshd_config |
Maximum unanswered client-alive requests the server accepts. | sshd terminates the session when the count is exceeded. |
OpenSSH’s documented example uses a 15-second ServerAliveInterval and the default count of 3. If the peer is unresponsive, the client disconnects after approximately 45 seconds. The exact elapsed time can vary with scheduling and network behavior, so use the product of interval and count as an planning estimate, not a stopwatch guarantee.
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SSH-level probes are carried through the encrypted channel and are not spoofable by an intermediary that cannot authenticate as an SSH endpoint. TCP keepalives are a separate transport feature and can be spoofed; they also depend on operating-system TCP settings. For an SFTP session, configure the OpenSSH options when you need an encrypted application-level liveness check. Do not assume that enabling a kernel TCP keepalive changes OpenSSH’s ServerAliveInterval behavior.
Configure the OpenSSH client
Use a one-time command-line test
Apply options to one reproduction before changing a user’s permanent configuration:
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sftp -o ServerAliveInterval=30 -o ServerAliveCountMax=3 [email protected]This example sends an encrypted probe after 30 seconds without received data and gives the client three unanswered probes. It is a starting point only; the interval must be shorter than the shortest idle timeout on the route, and the count must match how quickly your operation should declare the peer dead.
Set a host-specific profile
Put the settings in the user’s SSH configuration so they apply only to the affected endpoint:
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Host example-sftp
HostName sftp.example.com
User transfer-user
ServerAliveInterval 30
ServerAliveCountMax 3Keep the profile host-specific instead of applying an aggressive value to every SSH connection. A very short interval creates extra traffic and can cause a healthy but slow peer to be declared dead sooner than intended. A very large count keeps a dead session open longer, which may delay job retry or failover.
Confirm which values OpenSSH will use
Use the effective-configuration view before testing:
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ssh -G example-sftpCheck that the output contains the expected serveraliveinterval and serveralivecountmax. An included configuration file, a more-specific Host block, or a command-line option can override the value you edited.
Configure the server deliberately
On the server, review the corresponding directives in sshd_config:
ClientAliveInterval 30
ClientAliveCountMax 3The server sends a request after receiving no data from the client for the configured interval. If the client does not answer within the configured count, sshd ends the session. This is useful when the server must detect clients that have disappeared, but it does not repair a broken path or a stalled SFTP process.
- Edit the active sshd configuration, including any files loaded by an
Includedirective. - Validate syntax with the platform’s sshd configuration test command, commonly
sshd -t, before reloading. - Reload the SSH service using the operating system’s service manager so existing sessions are not needlessly terminated.
- Open a new test session and verify the effective server values in the daemon’s startup or diagnostic output.
- Coordinate the values with provider policies, account-level session limits, VPN timers, and load-balancer idle timers.
Do not lower counts or intervals simply to make a failure appear faster. Choose the maximum outage-detection window your transfer process can tolerate, then ensure the interval still precedes the shortest middlebox idle timeout.
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Choose values from the path, not from a template
Find the shortest idle timeout among every device between client and server. Set the relevant SSH interval below that threshold, with enough margin for scheduling and temporary delay. For example, if a known intermediary expires a connection after 10 minutes of inactivity, a 30- or 60-second interval is comfortably below it; if the shortest timeout is unknown, begin with a conservative test value and confirm that the intermediary sees the probes.
The count controls a different decision: how long to wait before declaring an unresponsive peer dead. With an interval of 30 seconds and a count of 3, the planned detection window is about 90 seconds. A count of 1 detects failure sooner but is less tolerant of a single lost probe; a higher count tolerates transient loss but leaves dead sessions around longer. Document the chosen window so operators know when an automated retry should begin.
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Keepalives preserve an otherwise idle SSH channel and improve detection of an unresponsive peer. They cannot repair packet loss, a crashed server, a stopped SFTP subsystem, exhausted disk or memory, authentication-policy failures, account restrictions, or a path that resets active TCP traffic. They can also mask a capacity problem by keeping abandoned sessions alive longer.
Active-transfer failures
If the session drops while bytes are continuously moving, first verify whether the client saw a TCP reset, an SSH disconnect, or an application error. Inspect server resource limits, filesystem errors, account quotas, process limits, and SFTP-subsystem health. Check whether a VPN or load balancer renegotiates or reaps long-running flows. Increasing ServerAliveInterval does not address a failure that occurs while data is already crossing the channel.
Failures at one operation or file
A repeatable failure at the same path can indicate permissions, a forced command, a server-side policy, a filesystem problem, or an SFTP subsystem exception. Compare the client operation with the server log entry at the same second. Do not infer a network timeout solely from the fact that the client reports a lost connection after an application-level error.
Isolate the middlebox from the endpoint
- Repeat the same transfer from the original network and record the idle duration or transfer point at which it fails.
- Repeat from a materially different path, such as a wired office connection versus a mobile hotspot.
- If possible, use a second SFTP client while keeping the same server account and file operation.
- Compare the results. A failure confined to one path and one idle duration strongly prioritizes that path’s NAT or firewall timeout. A failure at the same point from every path prioritizes the server, account, or subsystem.
- Correlate each reproduction with client verbosity, sshd logs, and intermediary events in UTC.
Keep host-key verification enabled during every test. Disabling authentication safeguards can hide a separate security problem and does not repair transport state.
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Quick Recap
Read the common failure signatures
| Client symptom | Interpretation to test | Next action |
|---|---|---|
| No data, then a disconnect after nearly the same number of minutes | Idle state expired in a NAT, firewall, VPN, or load balancer. | Set an SSH interval below that timeout and verify keepalive packets in device logs. |
SSH_DISCONNECT_CONNECTION_LOST or reason code 10 |
The SSH transport was lost; the code does not identify the culprit. | Compare TCP events, server logs, and path changes at the same UTC timestamp. |
| Immediate TCP reset during transfer | An endpoint or intermediary actively rejected the flow. | Inspect firewall policy, server process health, VPN events, and resource exhaustion. |
| Authentication or authorization error before transfer | Not an idle keepalive problem. | Fix credentials, host-key policy, account restrictions, or server authorization. |
| Only one client implementation fails | Client-specific configuration, subsystem request, or compatibility issue. | Compare effective options and reproduce with another client without weakening host-key checks. |
A repeatable repair runbook
- Record the activity state, elapsed time, endpoint, account, network path, and exact client message.
- Run
sftp -vvvand save timestamped output. - Collect sshd/SFTP logs and firewall, NAT, VPN, and load-balancer events for the same UTC window.
- Classify the event as idle-only, active-transfer, operation-specific, or authentication-related.
- For idle-only drops, identify the shortest middlebox timeout and set
ServerAliveIntervalorClientAliveIntervalbelow it. - Choose
ServerAliveCountMaxorClientAliveCountMaxto meet the required dead-peer detection window. - Test first with command-line overrides, then place the verified values in a host-specific client block or coordinated server configuration.
- Reproduce from a second network and, if available, a second client.
- If active transfers still fail, stop tuning keepalives and investigate server resources, account policy, subsystem health, packet loss, resets, and intermediary events.
- Document the final interval, count, path, and observed failure signature so future incidents can be compared rather than guessed at.
Practical settings checklist
- Use encrypted OpenSSH keepalives when the problem is an idle SSH channel.
- Keep the interval shorter than the shortest confirmed or suspected idle timeout.
- Use the count to define how long an unresponsive peer may remain before disconnect.
- Remember that client and server directives are independent; changing one side does not automatically change the other.
- Do not treat TCP keepalive as equivalent to SSH-level keepalive.
- Do not expect keepalives to cure active-transfer failures, server crashes, authentication failures, or resource exhaustion.
- Correlate client, server, and network evidence in UTC before assigning blame.
- Retain host-key checking and normal authentication safeguards throughout testing.
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