connection/channel+session: lower max packet size & allow manual size.

Per section 6.1 of RFC 4253 an SSH implementation MUST support a packet
size of 32768 bytes, however, any size above this is not mandatory.
Currently, the max packet size is 64Kb and can cause a channel open
failure for hosts which only support the required 32Kb packet size.

As a result, we now default to the required size of 32768 (0x8000) in
order to avoid any errors with such hosts.

Additionally, the user can now manually define the maximum packet and
window size, should they choose to in the event that they wish to
obtain performance gains from a larger packet/window size.

3 files changed, 15 insertions, 6 deletions

diff --git a/lib/net/ssh.rb b/lib/net/ssh.rb
index ef510bc..d03b2d7 100644
--- a/lib/net/ssh.rb
+++ b/lib/net/ssh.rb
@@ -66,7 +66,8 @@ module Net
       :languages, :logger, :paranoid, :password, :port, :proxy, 
       :rekey_blocks_limit,:rekey_limit, :rekey_packet_limit, :timeout, :verbose,
       :global_known_hosts_file, :user_known_hosts_file, :host_key_alias,
-      :host_name, :user, :properties, :passphrase, :keys_only
+      :host_name, :user, :properties, :passphrase, :keys_only, :max_pkt_size,
+      :max_win_size
     ]
 
     # The standard means of starting a new SSH connection. When used with a
@@ -133,6 +134,10 @@ module Net
     #   option is intended for situations where ssh-agent offers many different
     #   identites.
     # * :logger => the logger instance to use when logging
+    # * :max_pkt_size => maximum size we tell the other side that is supported per
+    #   packet.
+    # * :max_win_size => maximum size we tell the other side that is supported for
+    #   the window.
     # * :paranoid => either false, true, :very, or :secure specifying how
     #   strict host-key verification should be (in increasing order here)
     # * :passphrase => the passphrase to use when loading a private key (default
diff --git a/lib/net/ssh/connection/channel.rb b/lib/net/ssh/connection/channel.rb
index a79419a..bf33903 100644
--- a/lib/net/ssh/connection/channel.rb
+++ b/lib/net/ssh/connection/channel.rb
@@ -107,15 +107,15 @@ module Net; module SSH; module Connection
     # that time (see #do_open_confirmation).
     #
     # This also sets the default maximum packet size and maximum window size.
-    def initialize(connection, type, local_id, &on_confirm_open)
+    def initialize(connection, type, local_id, max_pkt_size = 0x8000, max_win_size = 0x20000, &on_confirm_open)
       self.logger = connection.logger
 
       @connection = connection
       @type       = type
       @local_id   = local_id
 
-      @local_maximum_packet_size = 0x10000
-      @local_window_size = @local_maximum_window_size = 0x20000
+      @local_maximum_packet_size = max_pkt_size
+      @local_window_size = @local_maximum_window_size = max_win_size
 
       @on_confirm_open = on_confirm_open
 
diff --git a/lib/net/ssh/connection/session.rb b/lib/net/ssh/connection/session.rb
index 13270da..548b2da 100644
--- a/lib/net/ssh/connection/session.rb
+++ b/lib/net/ssh/connection/session.rb
@@ -72,6 +72,9 @@ module Net; module SSH; module Connection
       @channel_open_handlers = {}
       @on_global_request = {}
       @properties = (options[:properties] || {}).dup
+
+      @max_pkt_size = (options.has_key?(:max_pkt_size) ? options[:max_pkt_size] : 0x8000)
+      @max_win_size = (options.has_key?(:max_win_size) ? options[:max_win_size] : 0x20000)
     end
 
     # Retrieves a custom property from this instance. This can be used to
@@ -286,8 +289,8 @@ module Net; module SSH; module Connection
     #   channel.wait
     def open_channel(type="session", *extra, &on_confirm)
       local_id = get_next_channel_id
-      channel = Channel.new(self, type, local_id, &on_confirm)
 
+      channel = Channel.new(self, type, local_id, @max_pkt_size, @max_win_size, &on_confirm)
       msg = Buffer.from(:byte, CHANNEL_OPEN, :string, type, :long, local_id,
         :long, channel.local_maximum_window_size,
         :long, channel.local_maximum_packet_size, *extra)
@@ -503,7 +506,8 @@ module Net; module SSH; module Connection
         info { "channel open #{packet[:channel_type]}" }
 
         local_id = get_next_channel_id
-        channel = Channel.new(self, packet[:channel_type], local_id)
+
+        channel = Channel.new(self, packet[:channel_type], local_id, @max_pkt_size, @max_win_size)
         channel.do_open_confirmation(packet[:remote_id], packet[:window_size], packet[:packet_size])
 
         callback = channel_open_handlers[packet[:channel_type]]