- djm@cvs.openbsd.org 2008/06/12 05:15:41

     [PROTOCOL]
     document tun@openssh.com forwarding method

1 files changed, 66 insertions, 6 deletions

diff --git a/PROTOCOL b/PROTOCOL
index 76e6adb4b..08b16dc16 100644
--- a/PROTOCOL
+++ b/PROTOCOL
@@ -84,7 +84,68 @@ connection.
 Note that this is not a general defence against compromised clients
 (that is impossible), but it thwarts a simple attack.
 
-5. sftp: Reversal of arguments to SSH_FXP_SYMLINK
+5. connection: Tunnel forward extension "tun@openssh.com"
+
+OpenSSH supports layer 2 and layer 3 tunneling via the "tun@openssh.com"
+channel type. This channel type supports forwarding of network packets
+with datagram boundaries entact between endpoints equipped with 
+interfaces like the BSD tun(4) device. Tunnel forwarding channels are
+requested by the client with the following packet:
+
+        byte            SSH_MSG_CHANNEL_OPEN
+        string          "tun@openssh.com"
+        uint32          sender channel
+        uint32          initial window size
+        uint32          maximum packet size
+        uint32          tunnel mode
+        uint32          remote unit number
+
+The "tunnel mode" parameter specifies whether the tunnel should forward
+layer 2 frames or layer 3 packets. It may take one of the following values:
+
+        SSH_TUNMODE_POINTOPOINT  1              /* layer 3 packets */
+        SSH_TUNMODE_ETHERNET     2              /* layer 2 frames */
+
+The "tunnel unit number" specifies the remote interface number, or may
+be zero to allow the server to automatically chose an interface. A server
+that is not willing to open a client-specified unit should refuse the
+request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful open,
+the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
+
+Once established the client and server may exchange packet or frames
+over the tunnel channel by encapsulating them in SSH protocol strings
+and sending them as channel data. This ensures that packet boundaries
+are kept intact. Specifically, packets are transmitted using normal
+SSH_MSG_CHANNEL_DATA packets:
+
+        byte            SSH_MSG_CHANNEL_DATA
+        uint32          recipient channel
+        string          data
+
+The contents of the "data" field for layer 3 packets is:
+
+        uint32                  packet length
+        uint32                  address family
+        byte[packet length - 4] packet data
+
+The "address family" field identifies the type of packet in the message.
+It may be one of:
+
+        SSH_TUN_AF_INET         2               /* IPv4 */
+        SSH_TUN_AF_INET6        24              /* IPv6 */
+
+The "packet data" field consists of the IPv4/IPv6 datagram itself
+without any link layer header.
+
+The contents of the "data" field for layer 3 packets is:
+
+        uint32                  packet length
+        byte[packet length]     frame
+
+The "frame" field contains an IEEE 802.3 ethernet frame, including
+header.
+
+6. 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 inadvertendly reversed. Unfortunately,
@@ -97,7 +158,7 @@ SSH_FXP_SYMLINK as follows:
         string          targetpath
         string          linkpath
 
-6. sftp: Server extension announcement in SSH_FXP_VERSION
+7. 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
@@ -118,7 +179,7 @@ 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 attemping to use the extension.
 
-7. sftp: Extension request "posix-rename@openssh.com"
+8. 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
@@ -135,7 +196,7 @@ 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".
 
-8. sftp: Extension requests "statvfs@openssh.com" and
+9. sftp: Extension requests "statvfs@openssh.com" and
          "fstatvfs@openssh.com"
 
 These requests correspond to the statvfs and fstatvfs POSIX system
@@ -176,5 +237,4 @@ The values of the f_flag bitmask are as follows:
 This extension is advertised in the SSH_FXP_VERSION hello with version
 "2".
 
-$OpenBSD: PROTOCOL,v 1.6 2008/06/10 22:15:23 djm Exp $
-
+$OpenBSD: PROTOCOL,v 1.7 2008/06/12 05:15:41 djm Exp $