From: florian <florian@openbsd.org>
Date: Mon, 13 Dec 2021 16:12:10 +0000 (+0000)
Subject: Only generate a new xid on state change.
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=9e257558e2b1e0746659862ae054f6c62c79f1f5;p=openbsd

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 7c7cb458c4f..39eca76924e 100644
--- 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;