[lwip-devel] [bug #40303] DHCP xid renewed when sending a DHCPREQUEST

[Florent Matignon]

URL:
  <http://savannah.nongnu.org/bugs/?40303>

                 Summary: DHCP xid renewed when sending a DHCPREQUEST
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: fmatignon
            Submitted on: ven. 18 oct. 2013 16:48:54 CEST
                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: git head

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Details:

lwIP always renews the 'xid' field of a DHCP packet when sending a new packet:


see dhcp_create_msg() (dhcp.c line 1675)

  if (dhcp->tries == 0) {
#if DHCP_CREATE_RAND_XID && defined(LWIP_RAND)
    xid = LWIP_RAND();
#else /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
    xid++;
#endif /* DHCP_CREATE_RAND_XID && defined(LWIP_RAND) */
  }
  dhcp->xid = xid;
  (...)
  dhcp->msg_out->xid = htonl(dhcp->xid);


This is not compliant with the RFC 2131:

"The DHCPREQUEST message contains the same 'xid' as the DHCPOFFER message."
(RFC 2131, section 4.4.1 "Initialization and allocation of network address")

The RFC does not explicitely indicates if a new xid must be regenerated when a
DHCPREQUEST is sent from BOUND, RENEWING or REBINDING states; my guess would
be that a xid is specific to a given DHCP "session" and should not be
generated in those cases.

I've actually never seen a DHCP server checking that the DHCPREQUEST xid
matches the DHCPOFFER's one... But it's just to be compliant with the RFC.




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