rustfs/docs/operations/sftp.md

SFTP Operations Guide

The guide covers enabling and operating the SFTP server in RustFS. It is written for operators who need to expose buckets over SFTP, manage host keys across platforms, size the server for large transfers, and diagnose session cleanup behaviour.

Enabling SFTP: Recommended Configuration

Two settings are required: the enable flag and the host-key directory. The listen address has a default of 0.0.0.0:2222 and setting it explicitly is recommended practice.

RUSTFS_SFTP_ENABLE=true
RUSTFS_SFTP_ADDRESS=0.0.0.0:2222
RUSTFS_SFTP_HOST_KEY_DIR=/etc/rustfs/sftp-keys

• RUSTFS_SFTP_ENABLE starts the SFTP listener at server startup. Off by default.
• RUSTFS_SFTP_ADDRESS is the listen address and port. 0.0.0.0 accepts connections on every interface. Port 2222 avoids the privileged port 22.
• RUSTFS_SFTP_HOST_KEY_DIR is the directory the SSH host keys are loaded from. No default, and startup fails when SFTP is enabled without it.

Generate a host key before first start. On Unix, ssh-keygen also writes a world-readable .pub file into the directory, and the server requires every file it considers in the host-key directory (regular, non-empty, at most 1 MiB) to be owner-only, so restrict or remove it:

ssh-keygen -t ed25519 -f /etc/rustfs/sftp-keys/ssh_host_ed25519_key -N ""
chmod 600 /etc/rustfs/sftp-keys/ssh_host_ed25519_key.pub

The private key itself is already written with owner-only permissions. The server does not read the .pub file, so it can be removed instead. Keys must be unencrypted (no passphrase).

On success the server logs SFTP server listening with the bound address. A port of 0 in RUSTFS_SFTP_ADDRESS is resolved to a free port at startup and the resolved port appears in that log line.

Every other setting has a tested default and should be left alone unless a section below gives a concrete reason to change it.

Path Model

The SFTP root directory is the account's bucket list. The first path component names the bucket and the remainder is the object key:

/reports/2026/q1.pdf

bucket:     reports
object key: 2026/q1.pdf

Files cannot be created at the root level. Creating or removing a top-level directory creates or removes a bucket. The root listing shows at most 10000 buckets and logs a warning when truncated.

Host Keys

RUSTFS_SFTP_HOST_KEY_DIR must name an existing directory containing at least one decodable private key. Startup fails otherwise. There is no generated fallback key.

• Any private key that russh can decode is accepted. Ed25519, ECDSA, and RSA are the expected formats. Passphrase-protected keys cannot be decoded and do not count. Keys are offered to clients in the order Ed25519, ECDSA, RSA, then anything else. Multiple keys of one algorithm all load, so during key rotation clients can be offered the new key as soon as it is added, not when the old one is removed.
• Empty files and files larger than 1 MiB are skipped entirely.
• On Unix, every other regular file in the directory must have no group or other permission bits set (owner-only, for example mode 0600, 0400, or 0700). The check covers non-key files too: a world-readable README or .pub file fails startup with host key file has insecure permissions. Keep only owner-only files in the directory.
• On Windows there is no permission-bit check. The server logs a one-time warning at startup whose alertable first sentence reads exactly SFTP host key file permission enforcement is not active on Windows. Restrict the NTFS ACL on the host-key directory to the running rustfs service account, NT AUTHORITY\SYSTEM, and BUILTIN\Administrators. Default ProgramData inheritance grants BUILTIN\Users read access. Remove that grant on the host-key directory.
• Targets that are neither Unix nor Windows do not support SFTP and fail startup.

Host keys can be hot-reloaded without a restart by setting RUSTFS_SFTP_HOST_KEY_RELOAD_ENABLE=true. The directory is rescanned every RUSTFS_SFTP_HOST_KEY_RELOAD_INTERVAL seconds (default 30, silently raised to 5 if set lower). A failed rescan, including a permission violation that would be fatal at startup, keeps the previous keys and logs SFTP host key reload failed; keeping previous keys. Reloaded keys affect new connections only.

Configuration Reference

Changing values beyond the recommended section is rarely necessary. The defaults are the configuration the test suites and stress runs exercise. Tuning values interact (for example part size multiplies against the handle cap in worst-case memory), so change them deliberately and one at a time.

Invalid values fall into four classes:

1. Fatal at startup. The server refuses to start and names the problem.
2. Warn and use the default. The server starts and logs a warning naming the variable, the rejected value, and the bounds.
3. Silently clamped. The reload interval is the one variable raised to its floor without a warning.
4. Silent fallback. Non-numeric text in any numeric variable behaves as if the variable were unset. Invalid boolean text logs a one-time warning and uses the default.

Variable	Default	Valid values	On invalid
RUSTFS_SFTP_ENABLE	false	boolean	warn, default
RUSTFS_SFTP_ADDRESS	0.0.0.0:2222	see binding note below	fatal
RUSTFS_SFTP_HOST_KEY_DIR	none	existing directory, required	fatal
RUSTFS_SFTP_IDLE_TIMEOUT	600	seconds, greater than zero	fatal
RUSTFS_SFTP_PART_SIZE	16777216	bytes, 5 MiB to 5 GiB	fatal
RUSTFS_SFTP_READ_ONLY	false	boolean	warn, default
RUSTFS_SFTP_BANNER	SSH-2.0-RustFS	must start with SSH-2.0-	fatal
RUSTFS_SFTP_HANDLES_PER_SESSION	64	8 to 1024	warn, default
RUSTFS_SFTP_BACKEND_OP_TIMEOUT_SECS	60	5 to 600	warn, default
RUSTFS_SFTP_READ_CACHE_WINDOW_BYTES	4194304	0 disables, else 256 KiB to 64 MiB	warn, default
RUSTFS_SFTP_READ_CACHE_TOTAL_MEM_BYTES	268435456	at least 16 MiB	warn, default
RUSTFS_SFTP_HOST_KEY_RELOAD_ENABLE	false	boolean	warn, default
RUSTFS_SFTP_HOST_KEY_RELOAD_INTERVAL	30	seconds, minimum 5	clamped to 5

Notes on individual variables:

• RUSTFS_SFTP_ADDRESS: the host part must be a wildcard (0.0.0.0 or [::]) or an address or hostname assigned to the host, otherwise startup fails. Use [::]:2222 to listen on IPv6, which on Linux usually accepts IPv4 as well via dual-stack. Port 0 auto-assigns a free port.
• RUSTFS_SFTP_BANNER is the SSH protocol identification string sent on connect, not a free-text login banner. Values that do not start with SSH-2.0- fail startup.
• RUSTFS_SFTP_IDLE_TIMEOUT cannot be set to 0 to disable idle disconnects. See the sessions section for what closes idle sessions.
• RUSTFS_SFTP_PART_SIZE bounds follow S3 multipart limits. See the large files section before changing it.
• RUSTFS_SFTP_READ_CACHE_WINDOW_BYTES set to exactly 0 disables read caching, which turns every client read request into one backend call.
• Worst-case buffered write memory per session is the handle cap times the part size: 64 handles at 16 MiB is 1 GiB per session. The server imposes no limit on concurrent sessions, so total worst case is that figure times however many clients connect. Only the read cache has a global cap. Enforce connection limits externally if that matters.

Sessions and Cleanup

Several mechanisms close sessions:

• The server sends an SSH keepalive every 15 seconds and closes the connection after 3 consecutive unanswered keepalives, about 60 seconds after a client stops responding. Not configurable. The keepalive check cleans up clients that vanish without closing TCP.
• RUSTFS_SFTP_IDLE_TIMEOUT (default 600 seconds) sets the SSH inactivity timeout, but keepalive replies count as SSH traffic and reset it, so a client that answers keepalives is never disconnected by it.
• The session watchdog closes sessions that are silent at the SFTP request layer for 30 minutes, on every platform. Keepalives do not count as SFTP-layer activity. The watchdog is the mechanism that ends a healthy but idle session. On Linux the close logs wedge watchdog cancelling session with reason fallback_silence. On other platforms it logs fallback watchdog cancelling session.
• On Linux the watchdog additionally reads kernel TCP state for fast cleanup of wedged sessions: a socket sitting in CLOSE_WAIT across two consecutive 15-second checks while the SFTP layer has been silent for 30 seconds is cancelled, typically 45 to 60 seconds after the client vanished. Two consecutive failed TCP-state probes are treated the same way, so a container that blocks /proc/net/tcp can see sessions cancelled on that schedule. The reason field of the log line names the trigger.
• If duplicating the connection socket fails at accept time (rare, usually file-descriptor exhaustion) the session runs with no watchdog at all and only the keepalive and idle mechanisms apply. Log line: wedge watchdog: dup_socket failed.
• Server shutdown cancels every live session immediately. Clients are disconnected mid-transfer. The server then waits up to 30 seconds for session cleanup, including aborting in-flight multipart uploads, before remaining tasks are dropped.

A session ended by any cancellation path logs SFTP session cancelled (watchdog or server shutdown).

Authentication and Authorization

Authentication is by password only, verified against RustFS IAM users. Public-key authentication is rejected. Anonymous access is not available. Failed logins are rejected without delay and there is no lockout, so apply rate limiting externally when the listener is exposed to untrusted networks. Accepted logins log SFTP auth accepted at info level, rejections log SFTP auth rejected at warn level.

Every SFTP operation is authorized against IAM policy before it reaches storage. Policy condition keys (for example aws:SourceIp) are not evaluated on the SFTP path. Only unconditional Allow and Deny statements take effect.

The S3 actions a user needs:

SFTP activity	Required S3 actions
List the root directory (the bucket list)	s3:ListAllMyBuckets
List inside a bucket	s3:ListBucket
Download a file	s3:GetObject
Upload a file	s3:PutObject
Stat a file	s3:GetObject
Stat a bucket	s3:ListBucket
Delete a file	s3:DeleteObject
Rename (implemented as copy then delete)	s3:GetObject on the source, s3:PutObject on the destination, s3:DeleteObject on the source
Create or remove a top-level directory	s3:CreateBucket or s3:DeleteBucket, removal also needs s3:ListBucket for the emptiness check
Upload cleanup on disconnect	s3:AbortMultipartUpload (see the large files section)

Upload-only users also need s3:GetObject, because SFTP clients stat files as part of normal transfers.

With RUSTFS_SFTP_READ_ONLY=true the server rejects all mutating packets at the protocol layer: opening a file for write, WRITE, REMOVE, MKDIR, RMDIR, RENAME, SETSTAT, and FSETSTAT.

Client Compatibility

The server speaks SFTP version 3 and maps onto object storage. Differences from a filesystem-backed SFTP server:

• Uploads must be a single sequential stream from offset zero. Transfer resume, append mode, in-place edits (read-write opens), and segmented or multi-connection uploads of one file are rejected. Configure clients for whole-file, single-connection transfers.
• In normal (read-write) mode, SETSTAT and FSETSTAT are accepted and ignored: chmod, timestamp preservation, and ownership changes silently have no effect, and listed permissions are fixed server-generated values.
• Symlink operations (SYMLINK, READLINK) are not supported and return an unsupported-operation error. Object storage has no symlink equivalent.
• Renaming copies the object server-side and then deletes the source, so large-file renames are slow and not atomic: a failure after the copy can leave the file at both paths. Renaming a top-level directory (a bucket) is not supported.

Large Files and Multipart Uploads

An upload smaller than RUSTFS_SFTP_PART_SIZE bytes is buffered in memory and written with a single PutObject when the file is closed. At part size or larger the handle switches to S3 multipart, flushing a part each time a full part accumulates.

The server enforces the S3 limit of 10000 parts per upload, so the largest single upload is part size times 10000: 156.25 GiB at the default 16 MiB part size. Raise RUSTFS_SFTP_PART_SIZE until that product covers the largest expected file. A larger part size raises per-session memory. A write that would exceed the cap is rejected with log line SFTP write would exceed the S3 multipart parts limit. The same cap can reject at file close, logging SFTP close rejected: trailing part would exceed S3 multipart parts limit.

When a session ends mid-upload the server aborts the in-flight multipart upload. Four situations leave an orphaned upload behind that the session itself cannot clean up:

• the user lacks s3:AbortMultipartUpload,
• the abort itself fails or times out,
• a burst of simultaneous disconnects exhausts the global abort task pool (sized at twice the CPU parallelism, between 8 and 128),
• the server process is killed outright.

Orphaned uploads are not permanent. RustFS runs a background cleanup that aborts stale incomplete multipart uploads (by default, uploads older than 24 hours, checked every 6 hours). An AbortIncompleteMultipartUpload bucket lifecycle rule reclaims them as well and gives per-bucket control of the window. The log lines when a session-drop abort is skipped or fails:

skipped abort of orphaned multipart upload on session drop, principal lacks s3:AbortMultipartUpload, bucket lifecycle rules must reclaim parts
abort permit pool exhausted on session drop, bucket lifecycle rule must reclaim parts
failed to abort orphaned multipart upload
Drop abort of orphaned multipart upload timed out; bucket lifecycle rule must reclaim parts

Abort failures at file close log with different wording and are retried at session drop, where the lines above appear.

Log Lines Worth Alerting On

Every line below logs at warn level except SFTP server listening and SFTP auth accepted, which log at info. The server's default log level is error, so none of them are visible until the log level is raised to info (for example RUSTFS_OBS_LOGGER_LEVEL=info, or warn to capture alerts only).

Message	Meaning
SFTP server listening	Startup complete, address bound
SFTP auth rejected	Failed login, no built-in lockout exists
SFTP host key file permission enforcement is not active on Windows	Expected once per start on Windows, verify the ACL guidance once
host key file has insecure permissions	Fatal at startup on Unix, fix file modes. During hot reload the same text appears inside the reload-failed warning and previous keys stay active
SFTP host key reload failed; keeping previous keys	Hot reload rescan failed, service unaffected, investigate the directory
wedge watchdog cancelling session	Linux only. reason field: tcp_state_close_wait_confirmed or probe_failed_confirmed mean a wedged or unprobeable socket, fallback_silence means routine 30-minute idle cleanup
fallback watchdog cancelling session	Non-Linux platforms: session silent at the SFTP layer for 30 minutes
wedge watchdog: dup_socket failed	Rare, that session runs without any watchdog
SFTP session cancelled (watchdog or server shutdown)	Session ended by cancellation rather than client close
SFTP write would exceed the S3 multipart parts limit	Client hit the per-upload size cap, raise the part size if legitimate. Also check the close-time variant below
SFTP close rejected: trailing part would exceed S3 multipart parts limit	Same cap hit at file close
root READDIR truncated	A principal can see more than 10000 buckets, listing was cut off
skipped abort of orphaned multipart upload	Orphaned parts on storage until background cleanup or lifecycle rule reclaims them
abort permit pool exhausted on session drop	Mass-disconnect burst, orphaned parts on storage until reclaimed
RUSTFS_SFTP_ prefix in a warning	A tuning variable was rejected and its default applied