DHCP: Improve from/to address selection

While here add a comment about broadcasting over UDP,
why we don't do it and why we need BPF.

diff --git a/src/dhcp.c b/src/dhcp.c
index 0a87e036eb7b6450f9f8762b8560912159a1d12c..5e549171050b168abdd6e70e116c9ab0956c178a 100644 (file)
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -1738,15 +1738,32 @@ send_message(struct interface *ifp, uint8_t type,
        if (r == -1)
                goto fail;
        len = (size_t)r;
-       from.s_addr = bootp->ciaddr;
-       if (from.s_addr != INADDR_ANY)
+
+       if (ipv4_iffindaddr(ifp, &state->lease.addr, NULL) != NULL)
+               from.s_addr = state->lease.addr.s_addr;
+       else
+               from.s_addr = INADDR_ANY;
+       if (from.s_addr != INADDR_ANY &&
+           state->lease.server.s_addr != INADDR_ANY)
                to.s_addr = state->lease.server.s_addr;
        else
-               to.s_addr = INADDR_ANY;
+               to.s_addr = INADDR_BROADCAST;
 
-       /* If unicasting, try and avoid sending by BPF so we don't
-        * use a L2 broadcast. */
-       if (to.s_addr != INADDR_ANY && to.s_addr != INADDR_BROADCAST) {
+       /*
+        * If not listening on the unspecified address we can
+        * only receive broadcast messages via BPF.
+        * Sockets bound to an address cannot receive broadcast messages
+        * even if they are setup to send them.
+        * Broadcasting from UDP is only an optimisation for rebinding
+        * and on BSD, at least, is reliant on the subnet route being
+        * correctly configured to recieve the unicast reply.
+        * As such, we always broadcast and receive the reply to it via BPF.
+        * This also guarantees we have a DHCP server attached to the
+        * interface we want to configure because we can't dictate the
+        * interface via IP_PKTINFO unlike for IPv6.
+        */
+       if (to.s_addr != INADDR_BROADCAST)
+       {
                if (dhcp_sendudp(ifp, &to, bootp, len) != -1)
                        goto out;
                logerr("%s: dhcp_sendudp", ifp->name);