This document was adapted from:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/PROTOCOL
$OpenBSD: PROTOCOL,v 1.29 2015/07/17 03:09:19 djm Exp $
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This documents OpenSSH's deviations and extensions to the published SSH
filexfer protocol.

Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
filexfer protocol described in:

http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt

Newer versions of the draft will not be supported, though some features
are individually implemented as extensions described below.

3. SFTP protocol changes

3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK

When OpenSSH's sftp-server was implemented, the order of the arguments
to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
the reversal was not noticed until the server was widely deployed. Since
fixing this to follow the specification would cause incompatibility, the
current order was retained. For correct operation, clients should send
SSH_FXP_SYMLINK as follows:

	uint32		id
	string		targetpath
	string		linkpath

3.2. sftp: Server extension announcement in SSH_FXP_VERSION

OpenSSH's sftp-server lists the extensions it supports using the
standard extension announcement mechanism in the SSH_FXP_VERSION server
hello packet:

	uint32		3		/* protocol version */
	string		ext1-name
	string		ext1-version
	string		ext2-name
	string		ext2-version
	...
	string		extN-name
	string		extN-version

Each extension reports its integer version number as an ASCII encoded
string, e.g. "1". The version will be incremented if the extension is
ever changed in an incompatible way. The server MAY advertise the same
extension with multiple versions (though this is unlikely). Clients MUST
check the version number before attempting to use the extension.

3.3. sftp: Extension request "posix-rename@openssh.com"

This operation provides a rename operation with POSIX semantics, which
are different to those provided by the standard SSH_FXP_RENAME in
draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
SSH_FXP_EXTENDED request with the following format:

	uint32		id
	string		"posix-rename@openssh.com"
	string		oldpath
	string		newpath

On receiving this request the server will perform the POSIX operation
rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
This extension is advertised in the SSH_FXP_VERSION hello with version
"1".

3.4. sftp: Extension requests "statvfs@openssh.com" and
         "fstatvfs@openssh.com"

These requests correspond to the statvfs and fstatvfs POSIX system
interfaces. The "statvfs@openssh.com" request operates on an explicit
pathname, and is formatted as follows:

	uint32		id
	string		"statvfs@openssh.com"
	string		path

The "fstatvfs@openssh.com" operates on an open file handle:

	uint32		id
	string		"fstatvfs@openssh.com"
	string		handle

These requests return a SSH_FXP_STATUS reply on failure. On success they
return the following SSH_FXP_EXTENDED_REPLY reply:

	uint32		id
	uint64		f_bsize		/* file system block size */
	uint64		f_frsize	/* fundamental fs block size */
	uint64		f_blocks	/* number of blocks (unit f_frsize) */
	uint64		f_bfree		/* free blocks in file system */
	uint64		f_bavail	/* free blocks for non-root */
	uint64		f_files		/* total file inodes */
	uint64		f_ffree		/* free file inodes */
	uint64		f_favail	/* free file inodes for to non-root */
	uint64		f_fsid		/* file system id */
	uint64		f_flag		/* bit mask of f_flag values */
	uint64		f_namemax	/* maximum filename length */

The values of the f_flag bitmask are as follows:

	#define SSH_FXE_STATVFS_ST_RDONLY	0x1	/* read-only */
	#define SSH_FXE_STATVFS_ST_NOSUID	0x2	/* no setuid */

Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
advertised in the SSH_FXP_VERSION hello with version "2".

10. sftp: Extension request "hardlink@openssh.com"

This request is for creating a hard link to a regular file. This
request is implemented as a SSH_FXP_EXTENDED request with the
following format:

	uint32		id
	string		"hardlink@openssh.com"
	string		oldpath
	string		newpath

On receiving this request the server will perform the operation
link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
This extension is advertised in the SSH_FXP_VERSION hello with version
"1".

10. sftp: Extension request "fsync@openssh.com"

This request asks the server to call fsync(2) on an open file handle.

	uint32		id
	string		"fsync@openssh.com"
	string		handle

One receiving this request, a server will call fsync(handle_fd) and will
respond with a SSH_FXP_STATUS message.

This extension is advertised in the SSH_FXP_VERSION hello with version
"1".