Only generate a new xid on state change.

When we first request a lease (INIT or REBOOTING state) we run with
very short timeouts. If the dhcp server is slow to respond we already
have a new xid and ignore the server's response. This goes on until we
increase the timeout high enough. If we just stick to an xid this will
not happen and we accept "late" responses.

RFC 2131 has:
Selecting a new 'xid' for each retransmission is an implementation
decision.  A client may choose to reuse the same 'xid' or select a new
'xid' for each retransmitted message.

Problem seen by phessler on german train wifi.
OK phessler

diff --git a/sbin/dhcpleased/engine.c b/sbin/dhcpleased/engine.c
index 7c7cb45..39eca76 100644 (file)
--- a/sbin/dhcpleased/engine.c
+++ b/sbin/dhcpleased/engine.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: engine.c,v 1.32 2021/12/13 11:03:23 florian Exp $     */
+/*     $OpenBSD: engine.c,v 1.33 2021/12/13 16:12:10 florian Exp $     */
 
 /*
  * Copyright (c) 2017, 2021 Florian Obser <florian@openbsd.org>
@@ -616,6 +616,7 @@ engine_update_iface(struct imsg_ifinfo *imsg_ifinfo)
                if ((iface = calloc(1, sizeof(*iface))) == NULL)
                        fatal("calloc");
                iface->state = IF_DOWN;
+               iface->xid = arc4random();
                iface->timo.tv_usec = arc4random_uniform(1000000);
                evtimer_set(&iface->timer, iface_timeout, iface);
                iface->if_index = imsg_ifinfo->if_index;
@@ -1309,6 +1310,9 @@ state_transition(struct dhcpleased_iface *iface, enum if_state new_state)
        char             ifnamebuf[IF_NAMESIZE], *if_name;
 
        iface->state = new_state;
+       if (new_state != old_state)
+               iface->xid = arc4random();
+
        switch (new_state) {
        case IF_DOWN:
                if (iface->requested_ip.s_addr == INADDR_ANY) {
@@ -1468,7 +1472,6 @@ request_dhcp_discover(struct dhcpleased_iface *iface)
 
        memset(&imsg, 0, sizeof(imsg));
 
-       iface->xid = arc4random();
        imsg.if_index = iface->if_index;
        imsg.xid = iface->xid;
 
@@ -1495,7 +1498,6 @@ request_dhcp_request(struct dhcpleased_iface *iface)
 {
        struct imsg_req_dhcp     imsg;
 
-       iface->xid = arc4random();
        imsg.if_index = iface->if_index;
        imsg.xid = iface->xid;