This patch comes from OpenEmbedded.
The original patch is from Debian / SuSE to implement replacedefaultroute
Rebased it to fit ppp-2.4.5. Dongxiao Xu <dongxiao.xu@intel.com>

Upstream-Status: Inappropriate [debian/suse patches]

diff -urN ppp-2.4.5-orig/pppd/ipcp.c ppp-2.4.5/pppd/ipcp.c
--- ppp-2.4.5-orig/pppd/ipcp.c	2010-06-30 15:51:12.050166398 +0800
+++ ppp-2.4.5/pppd/ipcp.c	2010-06-30 16:40:00.478716855 +0800
@@ -198,6 +198,16 @@
       "disable defaultroute option", OPT_ALIAS | OPT_A2CLR,
       &ipcp_wantoptions[0].default_route },
 
+#ifdef __linux__
+    { "replacedefaultroute", o_bool,
+                               &ipcp_wantoptions[0].replace_default_route,
+      "Replace default route", 1
+    },
+    { "noreplacedefaultroute", o_bool,
+                               &ipcp_allowoptions[0].replace_default_route,
+      "Never replace default route", OPT_A2COPY,
+                               &ipcp_wantoptions[0].replace_default_route },
+#endif
     { "proxyarp", o_bool, &ipcp_wantoptions[0].proxy_arp,
       "Add proxy ARP entry", OPT_ENABLE|1, &ipcp_allowoptions[0].proxy_arp },
     { "noproxyarp", o_bool, &ipcp_allowoptions[0].proxy_arp,
@@ -271,7 +281,7 @@
     ip_active_pkt
 };
 
-static void ipcp_clear_addrs __P((int, u_int32_t, u_int32_t));
+static void ipcp_clear_addrs __P((int, u_int32_t, u_int32_t, bool));
 static void ipcp_script __P((char *, int));	/* Run an up/down script */
 static void ipcp_script_done __P((void *));
 
@@ -1742,7 +1752,12 @@
     if (!sifnpmode(u, PPP_IP, NPMODE_QUEUE))
 	return 0;
     if (wo->default_route)
+#ifndef __linux__
 	if (sifdefaultroute(u, wo->ouraddr, wo->hisaddr))
+#else
+	if (sifdefaultroute(u, wo->ouraddr, wo->hisaddr,
+                                            wo->replace_default_route))
+#endif
 	    default_route_set[u] = 1;
     if (wo->proxy_arp)
 	if (sifproxyarp(u, wo->hisaddr))
@@ -1830,7 +1845,8 @@
      */
     if (demand) {
 	if (go->ouraddr != wo->ouraddr || ho->hisaddr != wo->hisaddr) {
-	    ipcp_clear_addrs(f->unit, wo->ouraddr, wo->hisaddr);
+	    ipcp_clear_addrs(f->unit, wo->ouraddr, wo->hisaddr, 
+				      wo->replace_default_route);
 	    if (go->ouraddr != wo->ouraddr) {
 		warn("Local IP address changed to %I", go->ouraddr);
 		script_setenv("OLDIPLOCAL", ip_ntoa(wo->ouraddr), 0);
@@ -1855,7 +1871,12 @@
 
 	    /* assign a default route through the interface if required */
 	    if (ipcp_wantoptions[f->unit].default_route) 
+#ifndef __linux__
 		if (sifdefaultroute(f->unit, go->ouraddr, ho->hisaddr))
+#else
+		if (sifdefaultroute(f->unit, go->ouraddr, ho->hisaddr,
+					     wo->replace_default_route))
+#endif
 		    default_route_set[f->unit] = 1;
 
 	    /* Make a proxy ARP entry if requested. */
@@ -1905,7 +1926,12 @@
 
 	/* assign a default route through the interface if required */
 	if (ipcp_wantoptions[f->unit].default_route) 
+#ifndef __linux__
 	    if (sifdefaultroute(f->unit, go->ouraddr, ho->hisaddr))
+#else
+	    if (sifdefaultroute(f->unit, go->ouraddr, ho->hisaddr,
+					 wo->replace_default_route))
+#endif
 		default_route_set[f->unit] = 1;
 
 	/* Make a proxy ARP entry if requested. */
@@ -1983,7 +2009,7 @@
 	sifnpmode(f->unit, PPP_IP, NPMODE_DROP);
 	sifdown(f->unit);
 	ipcp_clear_addrs(f->unit, ipcp_gotoptions[f->unit].ouraddr,
-			 ipcp_hisoptions[f->unit].hisaddr);
+			 ipcp_hisoptions[f->unit].hisaddr, 0);
     }
 
     /* Execute the ip-down script */
@@ -1999,12 +2025,21 @@
  * proxy arp entries, etc.
  */
 static void
-ipcp_clear_addrs(unit, ouraddr, hisaddr)
+ipcp_clear_addrs(unit, ouraddr, hisaddr, replacedefaultroute)
     int unit;
     u_int32_t ouraddr;  /* local address */
     u_int32_t hisaddr;  /* remote address */
+    bool replacedefaultroute;
 {
-    if (proxy_arp_set[unit]) {
+    /* If replacedefaultroute, sifdefaultroute will be called soon
+     * with replacedefaultroute set and that will overwrite the current
+     * default route. This is the case only when doing demand, otherwise
+     * during demand, this cifdefaultroute would restore the old default
+     * route which is not what we want in this case. In the non-demand
+     * case, we'll delete the default route and restore the old if there
+     * is one saved by an sifdefaultroute with replacedefaultroute.
+     */
+    if (!replacedefaultroute && default_route_set[unit]) {
 	cifproxyarp(unit, hisaddr);
 	proxy_arp_set[unit] = 0;
     }
diff -urN ppp-2.4.5-orig/pppd/ipcp.h ppp-2.4.5/pppd/ipcp.h
--- ppp-2.4.5-orig/pppd/ipcp.h	2010-06-30 15:51:12.043682063 +0800
+++ ppp-2.4.5/pppd/ipcp.h	2010-06-30 16:40:49.586203129 +0800
@@ -70,6 +70,7 @@
     bool old_addrs;		/* Use old (IP-Addresses) option? */
     bool req_addr;		/* Ask peer to send IP address? */
     bool default_route;		/* Assign default route through interface? */
+    bool replace_default_route; /* Replace default route through interface? */
     bool proxy_arp;		/* Make proxy ARP entry for peer? */
     bool neg_vj;		/* Van Jacobson Compression? */
     bool old_vj;		/* use old (short) form of VJ option? */
diff -urN ppp-2.4.5-orig/pppd/pppd.8 ppp-2.4.5/pppd/pppd.8
--- ppp-2.4.5-orig/pppd/pppd.8	2010-06-30 15:51:12.043682063 +0800
+++ ppp-2.4.5/pppd/pppd.8	2010-06-30 16:42:47.102413859 +0800
@@ -121,6 +121,13 @@
 This entry is removed when the PPP connection is broken.  This option
 is privileged if the \fInodefaultroute\fR option has been specified.
 .TP
+.B replacedefaultroute
+This option is a flag to the defaultroute option. If defaultroute is
+set and this flag is also set, pppd replaces an existing default route
+with the new default route.
+
+
+.TP
 .B disconnect \fIscript
 Execute the command specified by \fIscript\fR, by passing it to a
 shell, after
@@ -717,7 +724,12 @@
 .TP
 .B nodefaultroute
 Disable the \fIdefaultroute\fR option.  The system administrator who
-wishes to prevent users from creating default routes with pppd
+wishes to prevent users from adding a default route with pppd
+can do so by placing this option in the /etc/ppp/options file.
+.TP
+.B noreplacedefaultroute
+Disable the \fIreplacedefaultroute\fR option. The system administrator who
+wishes to prevent users from replacing a default route with pppd
 can do so by placing this option in the /etc/ppp/options file.
 .TP
 .B nodeflate
diff -urN ppp-2.4.5-orig/pppd/pppd.h ppp-2.4.5/pppd/pppd.h
--- ppp-2.4.5-orig/pppd/pppd.h	2010-06-30 15:51:12.050166398 +0800
+++ ppp-2.4.5/pppd/pppd.h	2010-06-30 16:43:36.514148327 +0800
@@ -643,7 +643,11 @@
 int  cif6addr __P((int, eui64_t, eui64_t));
 				/* Remove an IPv6 address from i/f */
 #endif
+#ifndef __linux__
 int  sifdefaultroute __P((int, u_int32_t, u_int32_t));
+#else
+int  sifdefaultroute __P((int, u_int32_t, u_int32_t, bool replace_default_rt));
+#endif
 				/* Create default route through i/f */
 int  cifdefaultroute __P((int, u_int32_t, u_int32_t));
 				/* Delete default route through i/f */
diff -urN ppp-2.4.5-orig/pppd/sys-linux.c ppp-2.4.5/pppd/sys-linux.c
--- ppp-2.4.5-orig/pppd/sys-linux.c	2010-06-30 15:51:12.050166398 +0800
+++ ppp-2.4.5/pppd/sys-linux.c	2010-06-30 16:54:00.362716231 +0800
@@ -206,6 +206,8 @@
 
 static int	if_is_up;	/* Interface has been marked up */
 static int	have_default_route;	/* Gateway for default route added */
+static struct rtentry old_def_rt;       /* Old default route */
+static int       default_rt_repl_rest;  /* replace and restore old default rt */
 static u_int32_t proxy_arp_addr;	/* Addr for proxy arp entry added */
 static char proxy_arp_dev[16];		/* Device for proxy arp entry */
 static u_int32_t our_old_addr;		/* for detecting address changes */
@@ -1537,6 +1539,9 @@
 	p = NULL;
     }
 
+    SET_SA_FAMILY (rt->rt_dst,     AF_INET);
+    SET_SA_FAMILY (rt->rt_gateway, AF_INET);
+
     SIN_ADDR(rt->rt_dst) = strtoul(cols[route_dest_col], NULL, 16);
     SIN_ADDR(rt->rt_gateway) = strtoul(cols[route_gw_col], NULL, 16);
     SIN_ADDR(rt->rt_genmask) = strtoul(cols[route_mask_col], NULL, 16);
@@ -1606,20 +1611,51 @@
 /********************************************************************
  *
  * sifdefaultroute - assign a default route through the address given.
- */
-
-int sifdefaultroute (int unit, u_int32_t ouraddr, u_int32_t gateway)
-{
-    struct rtentry rt;
-
-    if (defaultroute_exists(&rt) && strcmp(rt.rt_dev, ifname) != 0) {
-	if (rt.rt_flags & RTF_GATEWAY)
-	    error("not replacing existing default route via %I",
-		  SIN_ADDR(rt.rt_gateway));
-	else
-	    error("not replacing existing default route through %s",
-		  rt.rt_dev);
-	return 0;
+ *
+ * If the global default_rt_repl_rest flag is set, then this function
+ * already replaced the original system defaultroute with some other
+ * route and it should just replace the current defaultroute with
+ * another one, without saving the current route. Use: demand mode,
+ * when pppd sets first a defaultroute it it's temporary ppp0 addresses
+ * and then changes the temporary addresses to the addresses for the real
+ * ppp connection when it has come up.
+ */
+
+int sifdefaultroute (int unit, u_int32_t ouraddr, u_int32_t gateway, bool replace)
+{
+    struct rtentry rt, tmp_rt;
+    struct rtentry *del_rt = NULL;
+
+    if (default_rt_repl_rest) {
+	/* We have already reclaced the original defaultroute, if we
+         * are called again, we will delete the current default route
+         * and set the new default route in this function.  
+         * - this is normally only the case the doing demand: */
+	if (defaultroute_exists( &tmp_rt ))
+		del_rt = &tmp_rt;
+    } else if ( defaultroute_exists( &old_def_rt                ) &&
+	                     strcmp(  old_def_rt.rt_dev, ifname ) != 0) {
+	/* We did not yet replace an existing default route, let's
+	 * check if we should save and replace a default route:
+         */
+	u_int32_t old_gateway = SIN_ADDR(old_def_rt.rt_gateway);
+	if (old_gateway != gateway) {
+	    if (!replace) {
+	        error("not replacing default route to %s [%I]",
+			old_def_rt.rt_dev, old_gateway);
+		return 0;
+	    } else {
+		// we need to copy rt_dev because we need it permanent too:
+		char * tmp_dev = malloc(strlen(old_def_rt.rt_dev)+1);
+		strcpy(tmp_dev, old_def_rt.rt_dev);
+		old_def_rt.rt_dev = tmp_dev;
+
+		notice("replacing old default route to %s [%I]",
+			old_def_rt.rt_dev, old_gateway);
+	        default_rt_repl_rest = 1;
+		del_rt = &old_def_rt;
+	    }
+	}
     }
 
     memset (&rt, 0, sizeof (rt));
@@ -1638,6 +1674,12 @@
 	    error("default route ioctl(SIOCADDRT): %m");
 	return 0;
     }
+    if (default_rt_repl_rest && del_rt)
+        if (ioctl(sock_fd, SIOCDELRT, del_rt) < 0) {
+	    if ( ! ok_error ( errno ))
+	        error("del old default route ioctl(SIOCDELRT): %m(%d)", errno);
+	    return 0;
+        }
 
     have_default_route = 1;
     return 1;
@@ -1673,6 +1715,16 @@
 	    return 0;
 	}
     }
+    if (default_rt_repl_rest) {
+	notice("restoring old default route to %s [%I]",
+			old_def_rt.rt_dev, SIN_ADDR(old_def_rt.rt_gateway));
+        if (ioctl(sock_fd, SIOCADDRT, &old_def_rt) < 0) {
+	    if ( ! ok_error ( errno ))
+	        error("restore default route ioctl(SIOCADDRT): %m(%d)", errno);
+	    return 0;
+        }
+        default_rt_repl_rest = 0;
+    }
 
     return 1;
 }