upstream: sftp-server: implement limits@openssh.com extension

This is a simple extension that allows the server to clearly
communicate transfer limits it is imposing so the client doesn't
have to guess, or force the user to manually tune.  This is
particularly useful when an attempt to use too large of a value
causes the server to abort the connection.

Patch from Mike Frysinger; ok dtucker@

OpenBSD-Commit-ID: f96293221e5aa24102d9bf30e4f4ef04d5f4fb51

1 files changed, 44 insertions, 1 deletions

diff --git a/PROTOCOL b/PROTOCOL
index 5c5e042e..88c0c409 100644
--- a/PROTOCOL
+++ b/PROTOCOL
@@ -481,6 +481,49 @@ See the "setstat" command for more details.
 This extension is advertised in the SSH_FXP_VERSION hello with version
 "1".
 
+3.8. sftp: Extension request "limits@openssh.com"
+
+This request is used to determine various limits the server might impose.
+Clients should not attempt to exceed these limits as the server might sever
+the connection immediately.
+
+	uint32		id
+	string		"limits@openssh.com"
+
+The server will respond with a SSH_FXP_EXTENDED_REPLY reply:
+
+	uint32		id
+	uint64		max-packet-length
+	uint64		max-read-length
+	uint64		max-write-length
+	uint64		max-open-handles
+
+The 'max-packet-length' applies to the total number of bytes in a
+single SFTP packet.  Servers SHOULD set this at least to 34000.
+
+The 'max-read-length' is the largest length in a SSH_FXP_READ packet.
+Even if the client requests a larger size, servers will usually respond
+with a shorter SSH_FXP_DATA packet.  Servers SHOULD set this at least to
+32768.
+
+The 'max-write-length' is the largest length in a SSH_FXP_WRITE packet
+the server will accept.  Servers SHOULD set this at least to 32768.
+
+The 'max-open-handles' is the maximum number of active handles that the
+server allows (e.g. handles created by SSH_FXP_OPEN and SSH_FXP_OPENDIR
+packets).  Servers MAY count internal file handles against this limit
+(e.g. system logging or stdout/stderr), so clients SHOULD NOT expect to
+open this many handles in practice.
+
+If the server doesn't enforce a specific limit, then the field may be
+set to 0.  This implies the server relies on the OS to enforce limits
+(e.g. available memory or file handles), and such limits might be
+dynamic.  The client SHOULD take care to not try to exceed reasonable
+limits.
+
+This extension is advertised in the SSH_FXP_VERSION hello with version
+"1".
+
 4. Miscellaneous changes
 
 4.1 Public key format
@@ -512,4 +555,4 @@ OpenSSH's connection multiplexing uses messages as described in
 PROTOCOL.mux over a Unix domain socket for communications between a
 master instance and later clients.
 
-$OpenBSD: PROTOCOL,v 1.39 2021/02/12 03:49:09 djm Exp $
+$OpenBSD: PROTOCOL,v 1.40 2021/02/18 00:30:17 djm Exp $