udhcpc: Wrong lease time is passed to the --script (PATCH)

[Karel Dolezal]

Hi,

I've noticed udhcpc was not attempting to renew the address in time,
in case the server offered
only 30s lease time. The reason ended up being the "--script" which I
use to install the address.
It was still getting the 30s value, whereas udhcpc internally corrects
the 30s value into 122s.

Please see the proposed patch below.

Best regards
Karel



Fixes DHCP lease time passed from udhcpc into the "--script"

  For unusually short leases, udhcpc enforces a minimum lease time to avoid
  excessive network traffic.

  The original, unadjusted value, was however still being passed to the dhcp
  event script, which was making it install the obtained IP address with a
  lifetime shorter than the udhcpc's renewal period. As a result, the address
  aged out and was removed without udhcpc attempting a renewal for some time.

  This patch writes the corrected value back to the packet variable, from which
  the script's environment is populated.

Signed-off-by: Karel Dolezal <karel.dolezal at ui.com>

--- a/networking/udhcp/dhcpc.c	2020-07-10 11:11:43.229706806 +0200
+++ b/networking/udhcp/dhcpc.c	2020-07-10 11:17:30.366254882 +0200
@@ -1867,16 +1867,21 @@ int udhcpc_main(int argc UNUSED_PARAM, c
 					lease_seconds = 60 * 60;
 				} else {
 					/* it IS unaligned sometimes, don't "optimize" */
 					move_from_unaligned32(lease_seconds, temp);
 					lease_seconds = ntohl(lease_seconds);
 					/* paranoia: must not be too small and not prone to overflows */
 					/* timeout > 60 - ensures at least one unicast renew attempt */
-					if (lease_seconds < 2 * 61)
+					if (lease_seconds < 2 * 61) {
 						lease_seconds = 2 * 61;
+						/* The --script reads from the &packet variable itself. */
+						/* Write the adjusted value back there, to give script */
+						/* the same lease time as we'll use internally. */
+						move_to_unaligned32(temp, htonl(lease_seconds));
+					}
 					//if (lease_seconds > 0x7fffffff)
 					//	lease_seconds = 0x7fffffff;
 					//^^^not necessary since "timeout = lease_seconds / 2"
 					//does not overflow even for 0xffffffff.
 				}
 #if ENABLE_FEATURE_UDHCPC_ARPING
 				if (opt & OPT_a) {