File System Operations

All file operations are served by the AirLink daemon over HTTP. Every request is HMAC-authenticated and validated against a volume root. The daemon exposes a flat namespace rooted at the container’s mounted volume.

Directory Listing

GET /fs/list

Returns entries in a directory. Results are cached per path for a short window to avoid hammering the filesystem on repeated calls.

Rate Limit

5 requests per second per container. Exceeding this returns 429.

fileSpecifier

The daemon reads fileSpecifier.json from the container’s config directory. This file maps category names to glob patterns. Clients can request a category to filter listings without downloading the full tree.

{
  "logs": ["*.log", "logs/**"],
  "config": ["*.json", "*.yaml", "*.toml"]
}

Response

{
  "entries": [
    {
      "name": "data.csv",
      "type": "file",
      "size": 1048576,
      "modified": 1690000000
    },
    { "name": "subdir", "type": "directory" }
  ]
}

Directories are listed first, then files, sorted alphabetically within each group.

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File Reading

GET /fs/file/content

Reads and returns a file’s contents. The response is the raw file bytes with appropriate Content-Type.

Limits

• Maximum file size: 10 MB
• Path is validated against the volume root (path jail)
• If start or end exceed file bounds, the response is clamped to valid range

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File Writing

POST /fs/file/content

Writes data to a file. Creates the file if it does not exist, overwrites if it does. Parent directories are created automatically.

Security

• Path is validated against the volume root
• Null bytes in the path are rejected
• Symlink chains are followed with depth limit 10
• On Linux, openat2 with RESOLVE_BENEATH prevents TOCTOU races

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File Operations

DELETE /fs/rm

Deletes a file or directory. The root directory itself is protected and cannot be deleted.

Protected paths:

• / (volume root)
• . (current directory)
• Any path that resolves outside the volume

POST /fs/rename

Renames or moves a file/directory. Both source and destination must resolve within the same volume. Moving across volume boundaries is not allowed.

POST /fs/copy

Copies a file. If the destination already exists, a -copy suffix is appended before the extension (e.g., data.csv becomes data-copy.csv). If that also exists, a numeric suffix is tried (data-copy-2.csv, etc.).

POST /fs/create-empty-file

Creates a zero-byte file. Fails if the file already exists.

POST /fs/mkdir

Creates a directory. Parent directories are created recursively if needed.

POST /fs/append-file

Appends data to a file. Uses chunked upload with session tracking for large appends. The connection is kept alive across chunks.

Timeout

60 seconds of inactivity closes the upload session. Any partially appended data is flushed to disk.

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Archive Operations

POST /fs/zip

Creates a zip archive from the specified files. Uses the system zip binary (not a library implementation).

The resulting archive is stored at the given path within the volume.

POST /fs/unzip

Extracts an archive. Supports tar, zip, rar, and 7z formats.

Security Checks

Archive entries are validated during extraction:

After extraction, the daemon walks the extracted tree and checks for any symlinks that point outside the volume root. Suspicious entries are flagged.

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Download System

GET /fs/download

Serves a file directly. Sets Content-Disposition: attachment so the browser prompts a save dialog.

POST /fs/download-token

Generates a one-time token for downloading a file without re-authenticating. Useful for sharing links or handing off to external tools.

Token Properties

The token is consumed on first use. Expired tokens are cleaned up automatically.

POST /fs/pull

Downloads a file from a public URL and saves it into the volume. The daemon acts as a proxy, so the container never touches the network directly.

Limits

• Maximum download size: 512 MB
• SSRF protection: internal IPs (loopback, link-local, private, CGNAT, ULA, multicast) are blocked
• DNS resolution is checked before connecting
• HTTP redirects are followed up to 5 hops, each re-checked

$ALVKT() Variable Substitution

URLs may contain $ALVKT() tokens that are resolved at request time. These reference daemon-injected environment variables. The substitution happens before the HTTP request is made.

Example:

https://storage.example.com/data/$ALVKT(AIRLINK_DATA_TOKEN)/export.csv

ALC (Airlink Local Cache)

The pull operation checks the local cache before making a network request. If the URL has been fetched recently and the response included cache headers, the cached copy is used. Cache entries expire based on the Cache-Control or Expires headers from the original response.

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Upload System

POST /fs/upload

Uploads a file to the volume. Accepts either base64-encoded data or raw binary body.

When sending raw binary, set Content-Type to the appropriate MIME type and omit the data parameter. The daemon reads the body stream directly.

For large files, use chunked upload via POST /fs/append-file (see above). The append endpoint tracks upload sessions by ID so you can resume interrupted uploads.

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Path Safety

Every file operation goes through path validation before execution. This is not optional and cannot be disabled.

Validation Pipeline

1. Null byte check — any \0 in the path is an immediate reject
2. Resolve against volume root — the path is joined to the volume root and resolved
3. Symlink walk — symlinks are followed manually up to depth 10; anything deeper is rejected
4. Boundary check — the resolved path must start with the volume root
5. openat2 (Linux) — if available, openat2 is called with RESOLVE_BENEATH | RESOLVE_NO_SYMLINKS to catch TOCTOU races that the walk might miss

Reject Criteria

openat2 Details

On Linux 5.6+, the daemon uses openat2 via FFI to open files. This provides atomic path resolution with RESOLVE_BENEATH (prevents escaping the root) and RESOLVE_NO_SYMLINKS (blocks symlink follow). This eliminates TOCTOU vulnerabilities where a symlink could be swapped between the walk step and the open step.

On non-Linux platforms, the symlink walk with depth limit is the primary defense.