udhcpc: paranoia when using kernel UDP mode for sending renew: server ID may be bogus

With new code, we request that target IP (server ID) must be directly reachable.
If it's not, this happens:

udhcpc: waiting 2000 seconds
udhcpc: entering listen mode: kernel
udhcpc: opening listen socket on *:68 wlan0
udhcpc: entering renew state
udhcpc: sending renew to 1.1.1.1
udhcpc: send: Network is unreachable
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 1.1.1.1 needs routing, this is fishy!
udhcpc: entering rebinding state
udhcpc: entering listen mode: raw
udhcpc: created raw socket
udhcpc: sending renew to 0.0.0.0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ going to use broadcast

which is the desired behavior. Before the patch, packet to 1.1.1.1 was routed
over eth0 (!) and maybe even into Internet (!!!).

function                                             old     new   delta
udhcpc_main                                         2752    2763     +11
udhcp_send_kernel_packet                             295     301      +6
send_renew                                            82      84      +2
send_packet                                          166     168      +2
bcast_or_ucast                                        23      25      +2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 5/0 up/down: 23/0)               Total: 23 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 24 insertions, 10 deletions

diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 6c74996ef..2ae8bcc4d 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -694,10 +694,16 @@ static int raw_bcast_from_client_config_ifindex(struct dhcp_packet *packet, uint
 
 static int bcast_or_ucast(struct dhcp_packet *packet, uint32_t ciaddr, uint32_t server)
 {
-        if (server)
+        if (server) {
+                /* Without MSG_DONTROUTE, the packet was seen routed over
+                 * _other interface_ if server ID is bogus (example: 1.1.1.1).
+                 */
                 return udhcp_send_kernel_packet(packet,
                         ciaddr, CLIENT_PORT,
-                        server, SERVER_PORT);
+                        server, SERVER_PORT,
+                        /*send_flags: "to hosts only on directly connected networks" */ MSG_DONTROUTE
+                );
+        }
         return raw_bcast_from_client_config_ifindex(packet, ciaddr);
 }
 
@@ -735,7 +741,7 @@ static NOINLINE int send_discover(uint32_t xid, uint32_t requested)
 static NOINLINE int send_select(uint32_t xid, uint32_t server, uint32_t requested)
 {
         struct dhcp_packet packet;
-        struct in_addr addr;
+        struct in_addr temp_addr;
 
 /*
  * RFC 2131 4.3.2 DHCPREQUEST message
@@ -766,8 +772,8 @@ static NOINLINE int send_select(uint32_t xid, uint32_t server, uint32_t requeste
          */
         add_client_options(&packet);
 
-        addr.s_addr = requested;
-        bb_error_msg("sending select for %s", inet_ntoa(addr));
+        temp_addr.s_addr = requested;
+        bb_error_msg("sending select for %s", inet_ntoa(temp_addr));
         return raw_bcast_from_client_config_ifindex(&packet, INADDR_ANY);
 }
 
@@ -776,6 +782,7 @@ static NOINLINE int send_select(uint32_t xid, uint32_t server, uint32_t requeste
 static NOINLINE int send_renew(uint32_t xid, uint32_t server, uint32_t ciaddr)
 {
         struct dhcp_packet packet;
+        struct in_addr temp_addr;
 
 /*
  * RFC 2131 4.3.2 DHCPREQUEST message
@@ -806,7 +813,8 @@ static NOINLINE int send_renew(uint32_t xid, uint32_t server, uint32_t ciaddr)
          */
         add_client_options(&packet);
 
-        bb_error_msg("sending %s", "renew");
+        temp_addr.s_addr = server;
+        bb_error_msg("sending renew to %s", inet_ntoa(temp_addr));
         return bcast_or_ucast(&packet, ciaddr, server);
 }
 
@@ -1524,11 +1532,17 @@ int udhcpc_main(int argc UNUSED_PARAM, char **argv)
                          * Anyway, it does recover by eventually failing through
                          * into INIT_SELECTING state.
                          */
-                                        send_renew(xid, server_addr, requested_ip);
-                                        timeout >>= 1;
-                                        continue;
+                                        if (send_renew(xid, server_addr, requested_ip) >= 0) {
+                                                timeout >>= 1;
+                                                continue;
+                                        }
+                                        /* else: error sending.
+                                         * example: ENETUNREACH seen with server
+                                         * which gave us bogus server ID 1.1.1.1
+                                         * which wasn't reachable (and probably did not exist).
+                                         */
                                 }
-                                /* Timed out, enter rebinding state */
+                                /* Timed out or error, enter rebinding state */
                                 log1("entering rebinding state");
                                 state = REBINDING;
                                 /* fall right through */