The DHCP-client in RENEWING state does not reply to broadcast messages

[Krzysztof Charusta]

I upgraded busybox from version 1.29.2 to 1.30.1 and noticed that the
client behaves differently after commit "udhcpc: ensure at least one
unicast renew attempt" (c05aa6a776ab2420a42c041a3b5d45db587fd9ef).

I'm testing a setup:
- dhcp server (dnsmasq-2.78) lease time 122 sec.
- dhcp client (busybox 1.30.1)
with a dhcp relay in between (also busybox but this should not matter).

Actual result:
In the test scenario the server restarts/reconfigures and when the
client sends a renew DHCPREQUEST the server replies a broadcast
DHCPNAK ("address not available"). However, when in RENEWING state (T1
reached) the client does not reply to broadcast messages due to
change_listen_mode(LISTEN_KERNEL). The client then waits until the
REBINDING state (T2) and only then listen to broadcast.

Expected result:
According to [https://www.ietf.org/rfc/rfc2131.txt, Page 22, Sec 4.1]:
"In all cases, when 'giaddr' is zero, the server broadcasts any
DHCPNAK messages to 0xffffffff."
Would it be reasonable to think that the client should listen to
broadcast in RENEWING state as well?

In the test scenario the lease time is short (122 sec) so the time
between T1 and T2 is just 30sec but if time between the RENEW and
REBIND state is large then, I guess, this could be a problem.

Changing listening mode in RENEWING state fixes the problem (see
below) but I am not sure if this is the right way to go. Let me know
what you think.

Index: busybox-1.30.1/networking/udhcp/dhcpc.c
===================================================================
--- busybox-1.30.1.orig/networking/udhcp/dhcpc.c
+++ busybox-1.30.1/networking/udhcp/dhcpc.c
@@ -1517,7 +1517,7 @@ int udhcpc_main(int argc UNUSED_PARAM, c
                                /* 1/2 lease passed, enter renewing state */
                                state = RENEWING;
                                client_config.first_secs = 0; /* make
secs field count from 0 */
-                               change_listen_mode(LISTEN_KERNEL);
+                               change_listen_mode(LISTEN_RAW);
                                log1("entering renew state");
                                /* fall right through */
                        case RENEW_REQUESTED: /* manual (SIGUSR1) renew */

Best regards,
Krzysztof Charusta
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