make installers: make the server listen only to "localhost" by default

This makes the build more secure, but it's a backward-incompatible change.
To adapt old site-configuration scripts, the simplest option is to set
`#:server' to "localhost" everywhere and rely on SSH tunneling to let
a client reach a server (though, unfortunately, that option doesn't
seem to work if a Windows machine uses freeSSHd). Another possibility
is to se `#:server-hosts' to the empty list.

INSTALL.txt

@@ -314,11 +314,24 @@ Roughly, the steps are
     so that it redirects to the given URL when a remote search is
     necessary.
 
+    Add `SERVER_PORT=...' to the `server' line to choose the port on
+    which the server listens to clients. The default is port 9440.
+
+    Add `SERVER_HOSTS="..."' to determine the interfaces at which the
+    server listens. The default is "localhost" which listens only on
+    the loopback device (for security). Supply the empty string to
+    listen on all interfaces. Supply multiple addresses by separating
+    them with a comma.
+
  2. On each client machine:
       make client SERVER=... PKGS="..."
     or
       nmake win32-client SERVER=... PKGS="..."
 
+    Add `SERVER_PORT=...' to the `client' line if you picked an
+    alternate port in the `server` line; it shoul dbe the same as for
+    the `server' line.
+
     Add `DOC_SEARCH="..."' to the `client' line, if needed;
     normally, it should be the same as for the `server' line.
 
@@ -356,16 +369,19 @@ In more detail:
  1a. Build "racket" on a server.
     
      The `base' target of the makefile will do that, if you haven't
-     done it already. On Windows, use `nmake win32-base'.
+     done it already. (The server only works on non-Windows platforms,
+     currently.)
 
  1b. On the server, build packages and start a catalog server.
 
-     The `server-from-base' target of the makefile will do that. (The
-     server only works on non-Windows platforms, currently.)
+     The `server-from-base' target of the makefile will do that.
 
      Alternatively, use the `server' target, which combines `base' and
      `server-from-base' (i.e., steps 1a and 1b).
 
+     The `SERVER_PORT' and `SERVER_HOSTS' variables of the makefile
+     configure the prt and interfaces of the server.
+
      The `PKGS' variable of the makefile determines which packages are
      built for potential inclusion in a distribution.
 
@@ -396,8 +412,15 @@ In more detail:
  2b. On each client, create an installer.
 
      The `client' (or `win32-client') target of the makefile will do
-     that. Provide `SERVER' as the hostname of the server machine, and
-     provide the same `PKGS' (or a subset) as in step 1b if you want a
+     that.
+
+     Provide `SERVER' as the hostname of the server machine, but a
+     "locahost"-based tunnel back to the server is more secure and
+     avoids the need to specify `SERVER_HOSTS' when starting the
+     server in step 1b. Also, provide `SERVER_PORT' if an alternate
+     port was specified in step 1b.
+
+     Provide the same `PKGS' (or a subset) as in step 1b if you want a
      different set than the ones listed in the makefile. Similarly,
      `DOC_SEARCH' normally should be the same as in step 1b, but for a
      client, it affects future documentation builds in the

Makefile

@@ -119,11 +119,12 @@ DOC_SEARCH =
 # server):
 SERVER = localhost
 SERVER_PORT = 9440
+
 # Set `SERVER_HOSTS` to a comma-delimited set of server addresses
-# that determine the interfaces on which the server listens; for
-# example, set SERVER_HOSTS to "localhost" to listen only on the
-# loopback device:
-SERVER_HOSTS = 
+# that determine the interfaces on which the server listens; the
+# default, "localhost", listens only on the loopback device, while
+# anf empty value listens on all interfaces:
+SERVER_HOSTS = localhost
 
 # Set to "--release" to create release-mode installers (as opposed to
 # snapshot installers):

pkgs/distro-build/doc.txt

@@ -24,13 +24,16 @@ client machines hierarchically, where configuration options
 propagate down the hierarchy when they are not overridden more
 locally.
 
-Each client is normally built by running commands via `ssh', where
-the client's host, `#:host' (with and optional `#:port' and/or
-`#:user') indicate the SSH target. Each client machine must be set
-up with a public-key authentication, because a direct `ssh' is
-expected to work without a password prompt. An exception is when
-the host is "localhost" and user is #f, in which case a shell is
-used directly instead of `ssh'.
+Each client is normally built by running commands via `ssh', where the
+client's host, `#:host' (with and optional `#:port' and/or `#:user')
+indicate the `ssh' target. Each client machine must be set up with a
+public-key authentication, because a direct `ssh' is expected to work
+without a password prompt. An exception is when the host is
+"localhost" and user is #f, in which case a shell is used directly
+instead of `ssh'. When `ssh` is used, -R is also used to create a
+tunnel back to the server, and the client by default uses that tunnel
+for all communication, and the server by default accepts only
+connections via "localhost".
 
 On the client machine, all work is performed at a specified
 directory as specified by `#:dir'. The directory defaults to
@@ -54,7 +57,7 @@ Normally, the client directory is a git clone:
     that the server and client are in sync), which means that the
     server's directory must be a git clone.
 
-Note that neither SSH nor git turn out to be needed when the host
+Note that neither `ssh' nor `git' turn out to be needed when the host
 is "localhost", the user is #f, and the directory is not specified
 (which corresponds to the defaults in all cases).
 
@@ -124,22 +127,23 @@ Site-configuration keywords (where <string*> means no spaces, etc.):
 
   #:server <string*> --- the address of the server as accessed by the
     client; when ssh remote tunneling works, then "localhost" should
-    always work to reach the server; defaults to the `SERVER' makefile
+    work to reach the server; defaults to the `SERVER' makefile
     variable, which in turn defaults to "localhost"
 
   #:server-port <integer> --- the port of the server as accessed by
-    the client, and a port used on clients to tunnel back to the
-    server; defaults to the `SERVER_PORT' makefile variable, which in
-    turn defaults to 9440
+    the client, and also the port started on clients to tunnel back to
+    the server; defaults to the `SERVER_PORT' makefile variable, which
+    in turn defaults to 9440
 
   #:server-hosts <list-of-string*> --- addresses that determine the
     interfaces on which the server listens; an empty list means all of
-    the server's interface, while '("localhost") would listen only on
-    the loopback device; defaults to the `SERVER_HOSTS` makefile
-    variable split on comma, which in turn defaults to the empty list
+    the server's interfaces, while `(list "localhost")' listens only
+    on the loopback device; defaults to the `SERVER_HOSTS` makefile
+    variable split on commas, which in turn defaults to `(list
+    "localhost")'
 
   #:repo <string> --- the git repository for Racket; defaults to
-    "http://<server>:9440/.git"
+    "http://<server>:<server-port>/.git"
 
   #:pkgs '(<string*> ...) --- packages to install; defaults to the
     `PKGS' makefile variable
@@ -467,12 +471,12 @@ installers might look like this:
    
     (sequential
      #:pkgs '("drracket")
+     #:server-hosts '() ; Insecure? See below.
      (machine
       #:desc "Linux (32-bit, Precise Pangolin)"
       #:name "Ubuntu 32"
       #:vbox "Ubuntu 12.04"
-      #:host "192.168.56.102"
-      #:server "192.168.56.1")
+      #:host "192.168.56.102")
      (machine
       #:desc "Windows (64-bit)"
       #:name "Windows 64"
@@ -483,9 +487,22 @@ installers might look like this:
       #:bits 64))
 
 The configuration describes using the hosts "192.168.56.1" and
-"192.168.56.103" for Linux and Windows builds, respectively, which are
-run one at a time. Furthermore, the Linux machine runs in VirtualBox
-on the server machine (in a virtual machine named "Ubuntu 12.04").
+"10.0.0.7" for Linux and Windows builds, respectively, which are run
+one at a time. 
+
+The Linux machine runs in VirtualBox on the server machine (in a
+virtual machine named "Ubuntu 12.04"). It contacts the server still as
+"localhost", and that works because the ssh connection to the Linux
+machine creates a tunnel (at the same port as the server's, wjich
+defaults to 9440).
+
+The Windows machine uses freeSSHd (not a `bash'-based SSH server like
+Cygwin) and communicates back to the server as "10.0.0.1" instead of
+using an SSH tunnel. To make that work, `#:server-hosts' is specified
+as the empty list to make the server listen on all interfaces (instead
+of just "localhost") --- which is possibly less secure than the
+default restriction that allows build-server connections only via
+"localhost".
 
 With this configuration file in "site.rkt",