[lwip-devel] [bug #47787] DHCP client quits after 256 tries

[Stephen Hersey]

URL:
  <http://savannah.nongnu.org/bugs/?47787>

                 Summary: DHCP client quits after 256 tries
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: shersey
            Submitted on: Tue 26 Apr 2016 02:46:54 PM GMT
                Category: DHCP
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 
            lwIP version: 1.4.1

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Details:

There's a weird corner case in LwIP where, if the DHCP client is unable to get
a response from a DHCP server after 256 tries, the DHCP retry counter (which
is a u8_t) will roll over to zero, and the DHCP timout will stop invoking
dhcp_timeout(). This is in a configuration where DHCP is enabled but AUTOIP is
not.

Now, most users will never see this, because it normally takes a LONG time to
reach 256 tries with the default exponential DHCP  backoff. However, I'm using
a modified dhcp.c (attached, search for "S.H." in the text) that reduces the
maximum DHCP retry timeout to 10 seconds, because my DHCP client may come up
several minutes before the DHCP server does, and it's important that the
client get an address quickly once the server finally comes up.

My target hardware is an ARM Cortex-M3 MCU, and the build environment is GCC
(Rowley Crossworks 3.5).

To reproduce the failure, attach the LwIP DHCP client system to a PC that has
no DHCP server, run Wireshark on the PC, and observe the client's DHCP
requests. After 256 tries without a DHCP response, the DHCP client will stop
requesting.

My local fix for this failure is to take each occurrence of "dhcp->tries" in
dhcp.c and wrap it in a conditional, as: "if (dhcp->tries < 255) {
dhcp->tries++; }"

This causes "tries" to saturate at 255 instead of rolling over, and the DHCP
client won't stop requesting.




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File Attachments:


-------------------------------------------------------
Date: Tue 26 Apr 2016 02:46:54 PM GMT  Name: dhcp.c  Size: 65kB   By: shersey

<http://savannah.nongnu.org/bugs/download.php?file_id=36997>

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