We'd like to implement a protocol for Git LFS that uses SSH protocol exclusively, avoiding the need to use HTTPS altogether. This will make deployment and use easier in a variety of situations where access to certain ports is limited.
This is merely a proposal, not a commitment to implement for either the client or server side. Implementers who prefer to use HTTP can continue to do so.
There are several possible approaches that could be adopted. SSH provides a native capability for the SFTP protocol, which can be used to transfer files. However, in order to implement this on the server side, each access (upload or download) must have an access control check instead of one at the beginning of the operation. This would be inefficient in some server-side implementations, and nearly impossible to implement securely for implementations that use the system OpenSSH for implementation.
Git already has some places we can look for inspiration. Its SSH protocol is based on the Git native protocol, which is based on the pkt-line scheme. Recently, Git has learned about protocol version 2, which provides better support for expressing and negotiating capabilities.
Ideally, we would allow multiple operations to occur on a single connection for efficiency's sake, especially on high-latency connections, where the cost of SSH connection setup may be high due to multiple round trips. In addition, a protocol which maps well onto HTTP may be beneficial for those server-side implementations which would like to proxy connections to an HTTP-based backend.
This design assumes a reference to Git's pkt-line and protocol v2 documentation. pkt-line headers for this document may contain values up to 65519 decimal.
To initiate a connection, Git LFS should run the following command:
$ ssh [{user}@]{server} git-lfs-transfer {path} {operation}
If authentication fails, or some other connection error occurs, errors will be
read from standard error and displayed to the user. The operation may be
upload
, download
. Other operations may be implemented in the future.
Once the connection is established, the server should send a capability advertisement:
capability-advertisement = capability-list flush-pkt
capability-list = *capability
capability = PKT-LINE(key[=value] LF)
key = 1*(ALPHA | DIGIT | "-_")
value = 1*(ALPHA | DIGIT | "-_.,?\/{}[]()<>!@#$%^&*+=:;")
Unlike the Git protocol, but like IMAP, the protocol version is specified as a
capability. This document defines protocol version 1, which is specified as
version=1
. If the server supports other protocol versions, it may enumerate
them here as well.
If the server supports locking, the locking
capability should be advertised,
and the client may then use the lock
, unlock
, and list-lock
commands.
No capabilities other than the base functionality specified here are enabled
without the client explicitly enabling them. Note that the value
production
here, unlike in Git, does not include the space character, since it is used as a
delimiter in parts of the protocol.
The client will then issue an appropriate version command:
version-request = PKT-LINE("version " number LF) flush-pkt
number = 1*DIGIT
The response from the server will look like the following:
version-response = status-command
delim-pkt
error-message
flush-pkt
status-command = PKT-LINE("status " http-status-code LF)
http-status-code = 3DIGIT
error-message = *PKT-LINE(data LF)
The http-status-code
portion of the response is an HTTP status code, identical
to those used if the request is made over HTTP.
The response code should be 200 if the version is accepted or 400 if it is not. Other values are possible if other errors occur.
When the protocol is complete, the client sends a quit request:
quit-request = PKT-LINE("quit" LF) flush-pkt
The server must respond with success and then terminate the connection:
quit-response = PKT-LINE("status 200" LF)
flush-pkt
This command exists to help distinguish expected termination from unexpected termination.
These commands may be used if the operation was upload
or download
.
The batch
command is used to specify a JSON command identical those used at
the info/lfs/object/batch
endpoint:
batch-request = batch-command
*argument
delim-pkt
*oid-line
flush-pkt
batch-command = PKT-LINE("batch" LF)
argument = PKT-LINE(key=[data] LF)
oid-line = PKT-LINE(oid size *(key=[value]) LF)
oid = 1*("a-f0-9")
size = 1*DIGIT
The transfer
argument is equivalent to the corresponding value in the HTTP
JSON API. The refname
argument is equivalent to the name
argument of the
ref
object in the HTTP JSON API. The hash-algo
argument is equivalent to
the hash_algo
argument in the HTTP JSON API.
Unknown arguments should be ignored, as should unknown key-value pairs in the
oid-line
production.
The response from the server will look like the following:
batch-response = status-command
*argument
delim-pkt
(*batch-oid-line | error-message)
flush-pkt
batch-oid-line = PKT-LINE(oid size action *(key=[value]) LF)
If the status command is successful (that is, the status is not 200-series
response), the data provided matches the *batch-oid-line
production;
otherwise, the data provided represents a user-visible error message.
The server response should contain one pkt-line per oid-size-action tuple. That
is, the same oid and size may be repeated if there are multiple actions. If the
server has no actions that are valid for an object, it should be listed once in
the response with the noop
action.
The response for an oid may include a string, id
, which is an opaque
identifier relevant only to the server to help it identify the object, and
another string, token
, which is an opaque identifier relevant only to the
server to help it manage authentication. These strings must meet the syntax for
the value
production above; if arbitrary bytes are needed, Base64 encoding is
recommended.
The hash-algo
argument has the same meaning as the hash_algo
field in the
HTTP JSON API.
The expires-in
and expires-at
key-value pairs have the same meaning as their
corresponding items from the HTTP JSON API.
These values, if specified, must be passed as arguments to the get-object
and
put-object
commands.
These commands may be used if the operation was download
.
If the operation was download
, the command get-object
may be used to retrieve an object:
get-object-request = get-object-command
*arguments
flush-pkt
get-object-command = PKT-LINE("get-object " oid LF)
If the id
or token
responses were provided in the response to batch
, they
must be specified as arguments here. The server may choose to ignore the oid
field specified in favor of looking up the data using the id
field.
The response looks like the following:
status-data-response = status-success-response | status-error-response
status-success-response = status-success-command
*argument
delim-pkt
binary-data
flush-pkt
status-success-command = PKT-LINE("status 200" LF)
binary-data = *PKT-LINE(data)
status-error-response = status-error-command
*argument
delim-pkt
error-message
flush-pkt
status-error-command = PKT-LINE("status " http-error-code LF)
http-error-code = ("4" | "5") 2DIGIT
The size
argument is required on status
responses to get-object
commands.
These commands may be used if the operation was upload
.
If the operation was upload
, the commands put-object
and verify-object
may be specified. put-object
is used to upload an object to the server:
put-object-request = put-object-command
*argument
delim-pkt
binary-data
flush-pkt
put-object-command = PKT-LINE("put-object " oid LF)
As above, the size
command is required and id
and token
are required if
provided by the server. The response matches the status-data-response
production. The binary-data
returned on success is not meaningful and should
be empty.
The verify-object
command is used to verify an object:
verify-object-request = verify-object-command
*argument
flush-pkt
verify-object-command = PKT-LINE("verify-object " oid LF)
A size
argument identical to the one used in put-object
is mandatory. The
id
and token
items from the batch request must also be passed as arguments
here, if specified.
The response matches the following:
generic-status-response = generic-success-response | status-error-response
generic-success-response = generic-success-command
*argument
flush-pkt
generic-success-command = PKT-LINE("status 200" LF)
The lock
command may be used to lock a file on a ref:
lock-request = lock-command
*argument
flush-pkt
lock-command = PKT-LINE("lock" LF)
The path
and refname
arguments correspond to the path
component and the
name
component of the ref
object in the HTTP JSON API.
The response is as follows:
lock-response = lock-success-response | status-error-response
lock-success-response = lock-success-command
*argument
flush-pkt
lock-success-command = PKT-LINE("status 201" LF)
If the response is either successful or a 409 response, the arguments id
,
path
, locked-at
, and ownername
are provided. In case of a successful
response, these attributes represent the created lock; if the response is a 409,
then the attributes represent the conflicting lock.
The list-lock
command may be used to list and verify locks:
list-lock-request = list-lock-command
*argument
flush-pkt
list-lock-command = PKT-LINE("list-lock" LF)
The path
, id
, cursor
, limit
, and refspec
correspond to the items in
the HTTP JSON API.
list-lock-response = list-lock-success-response | status-error-response
list-lock-success-response = list-lock-success-command
*argument
delim-pkt
*lock-spec
flush-pkt
list-lock-success-command = PKT-LINE("status 200" LF)
lock-spec = lock-decl
path-id
locked-at
ownername-id
*owner-id
lock-decl = PKT-LINE("lock " lock-id LF)
lock-id = value
path-id = PKT-LINE("path " lock-id path LF)
path = data
locked-at = PKT-LINE("locked-at " lock-id timestamp LF)
ownername-id = PKT-LINE("ownername " lock-id ownername LF)
ownername = data
owner-id = PKT-LINE("owner " lock-id who LF)
who = ("ours" | "theirs")
The lock-decl
production declares a new lock. The lock-id
production refers
to the ID provided by the server. The same ID is repeated in each line to allow
for easier parsing.
The next-cursor
argument indicates the next value of the cursor
argument to
be passed to the list-lock
command. If there is no next-cursor
argument,
this is the final response.
The owner-id
specification is optional if the user specified the command as
download
but is required if the command is upload
.
unlock-request = unlock-command
*argument
flush-pkt
unlock-command = PKT-LINE("unlock " lock-id LF)
The force
and refname
arguments have the same meaning as their corresponding
values in the HTTP JSON API. The response is as follows:
unlock-response = unlock-success-response | status-error-response
unlock-success-response = unlock-success-command
*argument
flush-pkt
unlock-success-command = PKT-LINE("status 200" LF)
The lock
and unlock
commands may be issued when the command was upload
.
If the remote side has a concept of a repository administrator, it is
recommended that unlocking a lock that the user does not own be reserved to the
administrator.
The list-lock
commands may be issued when the command was upload
or
download
.