[Bug 11891] New: The DHCP-client in RENEWING state does not reply to broadcast messages

bugzilla at busybox.net bugzilla at busybox.net
Wed May 22 21:54:22 UTC 2019


            Bug ID: 11891
           Summary: The DHCP-client in RENEWING state does not reply to
                    broadcast messages
           Product: Busybox
           Version: 1.30.x
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Networking
          Assignee: unassigned at busybox.net
          Reporter: charustak at gmail.com
                CC: busybox-cvs at busybox.net
  Target Milestone: ---

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

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
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 */

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