diff --git a/ap/99-van-router.conf b/ap/99-van-router.conf
index 119d730..5737226 100644
--- a/ap/99-van-router.conf
+++ b/ap/99-van-router.conf
@@ -1 +1,12 @@
 net.ipv4.ip_forward=1
+
+# Ubuntu ships /usr/lib/sysctl.d/55-ipv6-privacy.conf with use_tempaddr=2
+# (prefer rotating privacy addresses) system-wide. That default fought the
+# WAN uplinks' need for one stable IPv6 source address per interface (hbc,
+# DNS, van-failover all key off it) — a long-lived flow using a source
+# address that later got deprecated mid-flight looked like random upstream
+# packet loss and cost a day of packet-capture debugging (2026-07-12/13)
+# before the cause turned out to be this file loading before ours. 99- beats
+# 55- lexically so this wins the second `sysctl --system` re-reads it.
+net.ipv6.conf.all.use_tempaddr=0
+net.ipv6.conf.default.use_tempaddr=0