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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsStart by checking when the SFTP session drops. A connection that fails only after sitting idle usually indicates an expiring NAT or firewall state. A session that disconnects while files are actively transferring points instead to packet loss, a server or client fault, resource limits, or an SFTP policy problem. Record the timing, collect client and server logs, and then tune SSH keepalives to a value shorter than the shortest idle timeout on the network path.
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Classify the disconnect before changing settings
Do not begin by increasing every timeout. First record the exact elapsed time, whether the session was idle, and what operation was running when it failed. This single distinction determines which evidence is useful.
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Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →| What you observe | Most likely direction | First action |
|---|---|---|
| The session drops after a repeatable period with no commands or file traffic | NAT, firewall, VPN, load-balancer, or other packet-filter state expired due to inactivity | Measure the idle period and configure an encrypted SSH keepalive below the shortest known idle timeout |
| The session drops while bytes are moving | Packet loss, path instability, endpoint failure, resource exhaustion, account policy, or an unhealthy SFTP subsystem | Correlate verbose client output, sshd/SFTP logs, and network-device events at the same UTC time |
| Only one network or VPN path fails | A middlebox or route specific to that path | Repeat from a wired connection, mobile hotspot, or other independent network |
| Every path fails at the same file, duration, or operation | Server-side policy, storage, limits, process failure, or a reproducible client issue | Inspect server logs, account limits, disk and memory capacity, and subsystem health |
The OpenSSH FAQ describes an idle-only failure as usually resulting from “a packet filter or NAT device timing out your TCP connection due to inactivity.” That explanation fits a repeatable idle interval, but it does not explain a transfer that fails while data is continuously flowing.
What SSH keepalives do—and what they cannot do
Client-side keepalives: ServerAliveInterval and ServerAliveCountMax
These are OpenSSH client settings. After the specified number of seconds with no data received from the server, the client sends an encrypted request through the SSH channel. ServerAliveCountMax sets how many consecutive requests may go unanswered before the client terminates the connection.
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For example, OpenSSH documents a 15-second interval with the default count of 3 as approximately 45 seconds before the client disconnects when the peer is unresponsive. The arithmetic is interval multiplied by the allowed unanswered requests; real timing can vary slightly because of scheduling and network delay.
Server-side keepalives: ClientAliveInterval and ClientAliveCountMax
These are sshd settings. When no data has been received from a client for the configured interval, the server sends an encrypted channel request. If the client does not respond for the configured number of probes, sshd closes the session. They are useful when the server must detect a client or path that has become unresponsive.
SSH keepalives versus TCP keepalives
SSH-level keepalive requests are carried inside the encrypted SSH channel, so an intermediary cannot spoof their contents. TCP keepalive is a separate transport mechanism. It probes the TCP connection and is generally considered spoofable; it also depends on operating-system settings and may not be enabled or frequent enough for a particular firewall. Do not assume that enabling TCP keepalive has the same effect as configuring OpenSSH’s ServerAlive* or ClientAlive* options.
Keepalives preserve an otherwise idle channel and make an unresponsive peer detectable. They cannot repair a crashed server, sustained packet loss, exhausted disk or memory, failed authentication, an account session limit, or a broken SFTP subsystem.
Collect evidence at matching UTC timestamps
Capture high-verbosity client output
Reproduce the failure with the OpenSSH client at maximum diagnostic verbosity:
sftp -vvv user@hostSave the output with timestamps. The final lines can show whether the client received an SSH disconnect, a TCP reset, an authentication error, or simply stopped receiving responses. Avoid sharing private keys, passwords, or unredacted host information when distributing logs.
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Check server and SFTP subsystem logs
At the same UTC time, inspect the server’s sshd authentication and session logs plus the SFTP subsystem or transfer-service logs. Look for forced command or chroot failures, account restrictions, process termination, storage errors, resource-limit messages, and deliberate administrative disconnects. A client-side “connection lost” line is not a substitute for the server’s reason.
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Check every network device in the path
Collect events from firewalls, NAT gateways, VPN concentrators, load balancers, and managed security appliances. A device may log an idle-state expiration, TCP reset, policy denial, tunnel renegotiation, or health-check failure. Make sure clocks are synchronized or normalize all records to UTC before comparing them.
RFC 4253 defines SSH as running over a transport such as TCP and specifies machine-readable disconnect reason codes. The logs become useful when you align the protocol message with the transport event and the server process that handled the session.
Configure the OpenSSH client for idle disconnects
Use a host-specific configuration
Put the settings in ~/.ssh/config so they apply only to the affected destination. A host-specific block avoids unintentionally changing behavior for unrelated SSH connections:
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HostName sftp.example.com
User transfer-user
ServerAliveInterval 30
ServerAliveCountMax 3Connect with sftp example-sftp. You can test a temporary override without editing the file:
sftp -o ServerAliveInterval=30 -o ServerAliveCountMax=3 [email protected]Choose the interval from the path’s shortest idle timeout
Find the smallest documented idle timeout among the office firewall, VPN, cloud load balancer, carrier-grade NAT, and server-side policy. Set ServerAliveInterval below that value, leaving margin for scheduling and jitter. If the shortest known timeout is 60 seconds, a 30-second probe is a reasonable starting point; it is an example, not a universal setting.
If no timeout is documented, measure the failure experimentally: leave a session idle, record when it drops, and repeat from another path. Do not set a one-second interval merely to keep a connection open; frequent probes create needless traffic and can increase load on large fleets.
Set the outage-detection window deliberately
ServerAliveCountMax controls how long the client waits after probes go unanswered. With an interval of 30 seconds and a count of 3, the client generally declares the peer dead after about 90 seconds of unanswered probes. A lower count fails faster but can react to a brief outage; a higher count tolerates transient loss but leaves automation waiting longer. Choose the window that matches the job’s retry and alerting policy.
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These options mainly address idle sessions. If the transfer is continuously exchanging data and still fails, keepalive probes may not be sent because the channel is not idle, and changing the interval will not fix the underlying fault.
Review server-side sshd settings
On the server, inspect the effective configuration rather than only a template file. Include any Match blocks that apply to the account or source address. The relevant directives are:
ClientAliveInterval 60
ClientAliveCountMax 3After editing the server configuration, validate it with the platform’s sshd configuration test before reloading the service. Use your operating system’s service manager to perform a reload that preserves existing sessions where supported, and confirm the new values in the daemon’s effective configuration.
The server’s interval should also be shorter than the path’s idle expiry if the server must keep inactive client sessions alive. Coordinate it with provider policies, load-balancer timers, VPN limits, and account-level session ceilings. A server that probes too aggressively can disconnect clients on a high-latency or intermittently congested link; one that probes too slowly may leave dead sessions consuming slots.
Do not confuse ClientAlive* with an application-level transfer timeout. These directives test whether the SSH client responds to an encrypted channel request; they do not guarantee that a stalled disk, remote filesystem, or SFTP operation will complete.
Separate a middlebox problem from an endpoint problem
Repeat from an independent network
- Run the same command and, if possible, the same file transfer from the original network.
- Repeat from a materially different path, such as a wired office connection versus a mobile hotspot. Avoid merely switching Wi-Fi access points behind the same gateway.
- Compare the elapsed idle time, client log ending, and server log entry for each attempt.
If only one path fails after a consistent idle period, focus on that path’s NAT or firewall state timeout, VPN behavior, or load-balancer policy. Ask the network administrator for the TCP idle timer and whether the device sends resets or silently discards expired flows.
Try a second SFTP implementation
A second client helps identify a client-specific defect or configuration mismatch. Keep host-key verification enabled in every client. Do not disable authentication checks, strict host-key checking, or encryption as a troubleshooting shortcut; doing so can hide a configuration problem and expose credentials.
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Interpret the result matrix
| Original path | Alternate path | Likely conclusion |
|---|---|---|
| Fails | Works | Investigate the original firewall, NAT, VPN, route, or carrier path |
| Fails | Fails at the same point | Investigate server policy, storage, resources, account limits, or a reproducible client operation |
| Works with one client only | Works with one client only | Compare SSH versions, cipher and MAC negotiation, proxy settings, and client timeout behavior |
When the drop happens during active transfer
An active transfer should normally generate traffic, so an idle keepalive cannot be treated as the cure. Use the timestamped logs to test these branches:
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- Server process failure:
sshdor the SFTP subsystem exits, is killed, or hits a process, file-descriptor, CPU, or memory limit. Correct the service or host resource condition. - Storage or filesystem error: a full volume, quota, stale mount, permission change, or remote filesystem error interrupts the operation. Resolve the storage condition and retry safely.
- Account or policy enforcement: transfer-size, duration, concurrent-session, chroot, or forced-command rules terminate the session. Have the administrator verify the account’s effective policy.
- Client-side failure: the local process, laptop sleep, network-interface change, or automation wrapper closes the socket. Check operating-system events and the job runner around the disconnect time.
Increasing ServerAliveInterval or ClientAliveInterval in these cases can add noise without addressing the failure. Preserve the original logs before making changes so that a new test can be compared with the baseline.
Understand “connection lost” reason code 10
SSH can send a protocol disconnect message with a numeric reason. RFC 4253 assigns SSH_DISCONNECT_CONNECTION_LOST decimal code 10; the IANA registry represents the same value as 0x0000000A. This identifies a transport connection loss at the SSH protocol layer. It does not, by itself, identify which device or process caused the loss. A firewall expiry, a server process, a client crash, or a broken route can all result in a similar final symptom.
Use the reason code as one clue, then correlate it with the TCP event, server log, and network-device record. Avoid claiming that code 10 proves a NAT timeout or any other single cause.
Common configuration pitfalls
- Changing the wrong side:
ServerAlive*belongs to the OpenSSH client, whileClientAlive*belongs tosshd. Editing one does not automatically change the other. - Setting the interval equal to the timeout: a probe sent at the exact expiry boundary can arrive too late. Leave margin below the shortest timer and account for jitter.
- Using a global setting unnecessarily: put experimental values in a host-specific block or command-line override so unrelated SSH sessions are not affected.
- Assuming a probe means a healthy transfer: a keepalive can show that the SSH peer answers while the filesystem or SFTP operation is still stalled.
- Ignoring the count: the interval controls probe frequency; the count controls how long unanswered probes are tolerated. Both determine the failure-detection window.
- Disabling security checks: never turn off host-key verification or authentication safeguards just to make a test pass.
Build a keepalive policy for automation
- Document the shortest idle timeout for every managed path, including VPN and load-balancer layers.
- Set client and server probe intervals below that timeout, with enough margin to account for jitter.
- Choose count values from the maximum outage your transfer jobs may tolerate.
- Alert when a session is closed after unanswered probes, and make retries bounded and idempotent.
- Log the host, account, start time, last successful byte or command, disconnect reason, and retry result in UTC.
- Test long idle periods as well as sustained transfers after any firewall, VPN, OpenSSH, or provider change.
For unattended jobs, design the transfer so it can resume or safely restart rather than assuming a single SSH session will survive every network event. Keepalive settings improve session persistence and failure detection; they are not a substitute for resumable workflows, monitoring, and server capacity planning.
A practical decision sequence
- Record whether the failure is idle-only or occurs during active bytes.
- Reproduce with
sftp -vvvand save the timestamped output. - Collect matching
sshd, SFTP, firewall, NAT, VPN, and load-balancer events. - Test from a second network while keeping host-key verification enabled.
- For idle-only failures, set
ServerAliveIntervalbelow the shortest idle timeout and chooseServerAliveCountMaxfor the required detection window. - Review matching
ClientAliveIntervalandClientAliveCountMaxvalues on the server. - For active-transfer failures or identical failures on every path, investigate endpoint, storage, policy, resource, and subsystem evidence instead of blindly increasing keepalives.
- Retest, compare logs, and document the final values and reason for the change.
Frequently Asked Questions
Why does SFTP disconnect after a few minutes of inactivity?
An idle NAT, firewall, VPN, load balancer, or other packet filter may expire the inactive TCP state. Measure the repeatable idle period and set an encrypted SSH keepalive interval below the shortest timeout on the path.
What is the difference between ServerAliveInterval and ClientAliveInterval?
ServerAliveInterval is an OpenSSH client setting: the client sends an encrypted request after receiving no data from the server. ClientAliveInterval is an sshd setting: the server sends an encrypted request after receiving no data from the client. Their CountMax settings determine how many unanswered probes are tolerated.
Are SSH keepalives the same as TCP keepalives?
No. SSH keepalives travel through the encrypted SSH channel and are not spoofable by an intermediary. TCP keepalive is a separate transport probe controlled by the operating system and may be too infrequent for a firewall or NAT timeout.
Will keepalives fix a transfer that disconnects while a file is moving?
Usually not. Active-transfer failures require correlation of verbose client output with sshd, SFTP, storage, endpoint, and network logs. Keepalives primarily preserve idle sessions and detect an unresponsive peer.
What does SSH connection-lost reason code 10 prove?
RFC 4253 defines SSH_DISCONNECT_CONNECTION_LOST as decimal code 10, represented by IANA as 0x0000000A. It identifies a transport loss at the SSH layer, but it does not identify the device or process that caused it.
How should I choose ServerAliveCountMax?
Choose it with the interval to create the outage-detection window your job can tolerate. A 30-second interval with a count of 3 generally waits about 90 seconds for unanswered probes. Lower counts fail faster; higher counts tolerate longer transient outages.
How can I tell whether the network or server is responsible?
Repeat the same test from an independent path, such as a wired network and a mobile hotspot, and compare UTC-aligned client, server, and network logs. A failure isolated to one path points to its middlebox or route; identical failures on every path point toward the server, account, storage, resources, or client operation.
The Bottom Line
Classify the timing first. For repeatable idle-only drops, use host-specific OpenSSH keepalives set below the shortest NAT or firewall timeout, and choose the probe count for an acceptable detection window. For disconnects during active transfers, collect synchronized client, server, and network evidence instead of increasing keepalive values blindly. Reason code 10 confirms a transport loss, not its cause.
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