From aa62f3c3efb07ea3c9cc39a42a4856356277c454 Mon Sep 17 00:00:00 2001
From: florian <florian@openbsd.org>
Date: Sun, 5 Oct 2025 07:25:16 +0000
Subject: [PATCH] Lower bound for how long to prefer IPv6 at least.

RFC 8925 suggests 5 minutes, so that's what we are going with.

kn noticed that if the IPv6-Only preferred option is send from the
server with a value of 0, dhcpleased(8) would still request a lease
while apple devices would not.

There were two problems with that:
1. A value of 0 would be treated as turning the feature of, which is
not correct.
2. The value would be used for a timeout, so a very small value would
create a timeout that would constantly fire.

OK kn
---
 sbin/dhcpleased/engine.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/sbin/dhcpleased/engine.c b/sbin/dhcpleased/engine.c
index 0b5f3e6f370..6a79539cb5b 100644
--- a/sbin/dhcpleased/engine.c
+++ b/sbin/dhcpleased/engine.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: engine.c,v 1.59 2025/09/18 11:37:01 florian Exp $	*/
+/*	$OpenBSD: engine.c,v 1.60 2025/10/05 07:25:16 florian Exp $	*/
 
 /*
  * Copyright (c) 2017, 2021 Florian Obser <florian@openbsd.org>
@@ -61,6 +61,9 @@
 #define	MAX_EXP_BACKOFF_FAST	 2
 #define	MINIMUM(a, b)		(((a) < (b)) ? (a) : (b))
 
+/* RFC 8925 3.4 disable IPv4 leases for at least this long */
+#define	MIN_V6ONLY_WAIT		 300
+
 enum if_state {
 	IF_DOWN,
 	IF_INIT,
@@ -1200,6 +1203,14 @@ parse_dhcp(struct dhcpleased_iface *iface, struct imsg_dhcp *dhcp)
 				log_debug("DHO_IPV6_ONLY_PREFERRED %us",
 				    ipv6_only_time);
 			}
+			if (ipv6_only_time < MIN_V6ONLY_WAIT) {
+				ipv6_only_time = MIN_V6ONLY_WAIT;
+				if (log_getverbose() > 1) {
+					log_debug("DHO_IPV6_ONLY_PREFERRED too "
+					    "small, setting to %us",
+					    ipv6_only_time);
+				}
+			}
 			p += dho_len;
 			rem -= dho_len;
 			break;