From b3e1db42b4dbb5f99705e4d1057ca49b44f4f5ee Mon Sep 17 00:00:00 2001
From: Jonas Lochmann <git@inkompetenz.org>
Date: Mon, 27 Oct 2025 01:00:00 +0100
Subject: [PATCH] odhcp6c: fix safe interval processing to follow RFC 4862
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This resolves an issue occuring in combination with Fritz! upstream
routers that send old prefixes with a valid time of zero for a long
time. Before this change, they were with each received RA extended
to two hours if the prefix was already known before.

Signed-off-by: Jonas Lochmann <git@inkompetenz.org>
Link: https://github.com/openwrt/odhcp6c/pull/108
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
---
 src/odhcp6c.c | 55 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 48 insertions(+), 7 deletions(-)

diff --git a/src/odhcp6c.c b/src/odhcp6c.c
index b66c5c3..cf786e4 100644
--- a/src/odhcp6c.c
+++ b/src/odhcp6c.c
@@ -942,20 +942,61 @@ bool odhcp6c_update_entry(enum odhcp6c_state state, struct odhcp6c_entry *new,
 		uint32_t safe, unsigned int holdoff_interval)
 {
 	struct odhcp6c_entry *x = odhcp6c_find_entry(state, new);
-
-	if (x && x->valid > new->valid && new->valid < safe)
-		new->valid = safe;
+	uint32_t new_valid;
+
+	/*
+	 * "safe" refers to https://www.rfc-editor.org/rfc/rfc4862#section-5.5.3
+	 * section e; it is either the two hours in seconds or zero when (e)
+	 * does not apply.
+	 *
+	 * The base condition for applying safe is that there is already a
+	 * matching prefix (and safe itself must be set).
+	 */
+	if (safe && x) {
+		if (new->valid > safe || new->valid > x->valid) {
+			/*
+			 * 1: If the received Valid Lifetime is greater than 2 hours or
+			 * greater than RemainingLifetime, set the valid lifetime of the
+			 * corresponding address to the advertised Valid Lifetime.
+			 */
+			new_valid = new->valid;
+		} else if (x->valid <= safe) {
+			/*
+			 * 2: If RemainingLifetime is less than or equal
+			 * to 2 hours, ignore the Prefix Information option with
+			 * regards to the valid lifetime, unless the Router
+			 * Advertisement from which this option was obtained has
+			 * been authenticated (e.g., via Secure Neighbor
+			 * Discovery [RFC3971]).  If the Router Advertisement
+			 * was authenticated, the valid lifetime of the
+			 * corresponding address should be set to the Valid
+			 * Lifetime in the received option.
+			 *
+			 * Since authenticated advertisements aren't supported we
+			 * always keep the old value.
+			 */
+			new_valid = x->valid;
+		} else {
+			/*
+			 * 3: Otherwise, reset the valid lifetime of the corresponding
+			 * address to 2 hours.
+			 */
+			new_valid = safe;
+		}
+	} else {
+		new_valid = new->valid;
+	}
 
 	if (x) {
-		if (holdoff_interval && new->valid >= x->valid &&
-				new->valid != UINT32_MAX &&
-				new->valid - x->valid < holdoff_interval &&
+		if (holdoff_interval && new_valid >= x->valid &&
+				new_valid != UINT32_MAX &&
+				new_valid - x->valid < holdoff_interval &&
 				new->preferred >= x->preferred &&
 				new->preferred != UINT32_MAX &&
 				new->preferred - x->preferred < holdoff_interval)
 			return false;
 
-		x->valid = new->valid;
+		x->valid = new_valid;
 		x->preferred = new->preferred;
 		x->t1 = new->t1;
 		x->t2 = new->t2;
-- 
2.30.2