sesman config: Add MaxDisplayNumber

When allocating a display number, we should be aware that
IANA only allow TCP displays up to :63. This PR adds that restriction in
to sesman.ini as a default, to prevent us allocating unavailable TCP
ports.

By default TCP ports are not enabled for X servers, but users can easily
change this if they wish to access X displays directly over the network.

This restriction is in addition to the MaxSessions limit already present
in sesman.ini

docs/man/sesman.ini.5.in

@@ -155,6 +155,13 @@ defaults to \fI10\fR.
 Sets the maximum number of simultaneous sessions. If not set or set to
 \fI0\fR, unlimited session are allowed.
 
+.TP
+\fBMaxDisplayNumber\fR=\fInumber\fR
+Sets the maximum number which can be assigned to an X11 $DISPLAY. The
+default is compatible with IANA TCP port allocations. If you are not
+allowing TCP connections to your X servers you may safely increase this
+number.
+
 .TP
 \fBKillDisconnected\fR=\fI[true|false]\fR
 If set to \fB1\fR, \fBtrue\fR or \fByes\fR, every session will be killed

sesman/libsesman/sesman_config.c

@@ -77,6 +77,7 @@
 #define SESMAN_CFG_SESS_IDLE_LIMIT   "IdleTimeLimit"
 #define SESMAN_CFG_SESS_DISC_LIMIT   "DisconnectedTimeLimit"
 #define SESMAN_CFG_SESS_X11DISPLAYOFFSET "X11DisplayOffset"
+#define SESMAN_CFG_SESS_MAX_DISPLAY  "MaxDisplayNumber"
 
 #define SESMAN_CFG_SESS_POLICY_S "Policy"
 #define SESMAN_CFG_SESS_POLICY_DFLT_S "Default"
@@ -410,6 +411,8 @@ config_read_sessions(int file, struct config_sessions *se, struct list *param_n,
 
     /* setting defaults */
     se->x11_display_offset = 10;
+    // https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml`
+    se->max_display_number = 63;
     se->max_sessions = 0;
     se->max_idle_time = 0;
     se->max_disc_time = 0;
@@ -428,6 +431,15 @@ config_read_sessions(int file, struct config_sessions *se, struct list *param_n,
             se->x11_display_offset = g_atoi(value);
         }
 
+        else if (0 == g_strcasecmp(buf, SESMAN_CFG_SESS_MAX_DISPLAY))
+        {
+            int mdn = g_atoi(value);
+            if (mdn > 0)
+            {
+                se->max_display_number = mdn;
+            }
+        }
+
         else if (0 == g_strcasecmp(buf, SESMAN_CFG_SESS_MAX))
         {
             se->max_sessions = g_atoi(value);

sesman/libsesman/sesman_config.h

@@ -118,6 +118,11 @@ struct config_sessions
      * @brief X11 TCP port offset. default value: 10
      */
     int x11_display_offset;
+    /**
+     * @var max_display_number
+     * @brief Highest X11 display number considered for allocation
+     */
+    unsigned int max_display_number;
     /**
      * @var max_sessions
      * @brief maximum number of allowed sessions. 0 for unlimited

sesman/sesman.ini.in

@@ -51,6 +51,15 @@ X11DisplayOffset=10
 ; Default: 0
 MaxSessions=50
 
+;; MaxDisplayNumer - maximum number considered for an X display
+; Type: integer
+; Default: 63
+;
+; IANA only allocates TCP ports up to 6063 for X servers. If you are not
+; allowing TCP connections to your X servers you may safely increase this
+; number.
+#MaxDisplayNumber=63
+
 ;; KillDisconnected - kill disconnected sessions
 ; Type: boolean
 ; Default: false