[Dnsmasq-discuss] [PATCH] radv: explicit lease time as a floor for constructed-prefix RA lifetimes
Giuseppe Piscitelli
ooonea at gmail.com
Sun Aug 9 21:44:32 UTC 2026
Thanks Simon.
Sure — attached is v2 with the man-page update. It documents both the
existing ceiling behaviour for explicitly configured lease times in
router advertisements and the new floor behaviour for constructed
ranges. No code changes from v1.
Patch inline below as well.
ooonea
radv: let an explicit lease time floor constructed-prefix RA lifetimes
For constructor: ranges dnsmasq advertises the residual kernel lifetimes
of the interface address, and an explicitly configured lease time only
caps them. Behind an upstream that delegates with very short lifetimes
(Starlink PD: ~300s valid / 150s preferred) every client inherits the
churn verbatim, and any device that sleeps through a few RAs (every
phone) loses its GUA mid-flow, killing established connections.
With this patch an explicitly configured lease time on a constructed
range acts as a floor as well. Prefix deprecation is untouched: a
deprecated context or kernel-deprecated address still advertises
preferred=0, and a vanished prefix still goes through the CONTEXT_OLD
path, so renumbering behaves exactly as stock.
Document both the existing RA lifetime ceiling and the new floor
behaviour for constructed ranges in dnsmasq.8.
diff --git a/src/radv.c b/src/radv.c
--- a/src/radv.c
+++ b/src/radv.c
@@ -608,6 +608,7 @@
int do_slaac = 0;
int deprecate = 0;
int constructed = 0;
+ int have_lease = 0;
int adv_router = 0;
int off_link = 0;
unsigned int time = 0xffffffff;
@@ -660,6 +661,8 @@
if (time < ((unsigned int)(3 * param->adv_interval)))
time = 3 * param->adv_interval;
}
+ if (context->flags & CONTEXT_SETLEASE)
+ have_lease = 1;
if (context->flags & CONTEXT_DEPRECATE)
deprecate = 1;
@@ -697,7 +700,19 @@
/* configured time is ceiling */
if (!constructed || valid > time)
valid = time;
-
+
+ /* For CONSTRUCTED prefixes an explicitly configured lease time is
+ also a floor: an upstream delegating with very short lifetimes
+ (mobile links) would otherwise churn the addresses of any client
+ that sleeps through a few RAs. Deprecation still wins below. */
+ if (constructed && have_lease && !deprecate && !(flags & IFACE_DEPRECATED))
+ {
+ if (valid < time)
+ valid = time;
+ if (preferred < time)
+ preferred = time;
+ }
+
if (flags & IFACE_DEPRECATED)
preferred = 0;
diff --git a/man/dnsmasq.8 b/man/dnsmasq.8
--- a/man/dnsmasq.8
+++ b/man/dnsmasq.8
@@ -1061,1 +1061,5 @@
The minimum lease time is two minutes.
+An explicitly configured lease time also affects IPv6 router advertisements:
+it acts as a ceiling on the advertised prefix lifetimes. For constructed
+ranges it also acts as a floor, unless the prefix is being deprecated.
+
On Sun, 9 Aug 2026 21:47:07 +0100, Simon Kelley <simon at thekelleys.org.uk> wrote:
> This looks sensible to me.
>
> Can I ask for one more contribution? It would really save me time if the
> patch included an update to the man page, describing an explicitly set
> lease time affects RA, both the existing ceiling behaviour, which is
> inexplicably missing :) and the new floor behaviour for constructed ranges.
>
> This is posted to the list partly because it's generally applicable.
> Your patch is more likely to get accepted if you don't make me write the
> documentation, people.
>
> Cheers,
>
> Simon.
>
> On 07.08.2026 01:16, Giuseppe Piscitelli wrote:
> > Hi,
> >
> > on constructed prefixes (--dhcp-range=::,constructor:eth0,...) the RA
> > carries the residual lifetimes of the interface address, and an
> > explicitly configured lease time only caps them ("configured time is
> > ceiling" in radv.c).
> >
> > Behind an upstream that delegates with very short lifetimes this
> > propagates the churn to every client. Starlink delegates a /56 with
> > about 300s valid / 150s preferred: the router refreshes forever, but a
> > client that sleeps through a few RAs -- every phone -- loses its
> > global address, and its established connections die with it. Seen live
> > on my LAN: phones answering on v4 and link-local while their GUA sat
> > FAILED in the router's neighbour cache, apps stalling mid-stream on
> > WiFi only.
> >
> > With this patch an explicitly configured lease time on a constructed
> > range acts as a floor as well. Ranges without an explicit lease time
> > are unchanged. Deprecation still wins: a kernel-deprecated address or
> > a deprecated context still advertises preferred=0, and a vanished
> > prefix still goes through the old-prefix path, so renumbering behaves
> > as before.
> >
> > Running in production here on 2.93 with a 12h lease on the constructed
> > range: clients hold 12h instead of the 300s residue and the
> > sleep-induced address loss is gone. The patch applies cleanly to
> > current master.
> >
> > If changing the meaning of the existing lease-time field on
> > constructed ranges is not wanted, I can rework this behind a dedicated
> > keyword instead.
> >
> > Patch inline below and attached too, in case my mailer mangles the
> > tabs.
> >
> > ooonea
> >
> > --- a/src/radv.c
> > +++ b/src/radv.c
> > @@ -608,6 +608,7 @@
> > int do_slaac = 0;
> > int deprecate = 0;
> > int constructed = 0;
> > + int have_lease = 0;
> > int adv_router = 0;
> > int off_link = 0;
> > unsigned int time = 0xffffffff;
> > @@ -660,6 +661,8 @@
> > if (time < ((unsigned int)(3 * param->adv_interval)))
> > time = 3 * param->adv_interval;
> > }
> > + if (context->flags & CONTEXT_SETLEASE)
> > + have_lease = 1;
> >
> > if (context->flags & CONTEXT_DEPRECATE)
> > deprecate = 1;
> > @@ -697,7 +700,19 @@
> > /* configured time is ceiling */
> > if (!constructed || valid > time)
> > valid = time;
> > -
> > +
> > + /* For CONSTRUCTED prefixes an explicitly configured lease time is
> > + also a floor: an upstream delegating with very short lifetimes
> > + (mobile links) would otherwise churn the addresses of any client
> > + that sleeps through a few RAs. Deprecation still wins below. */
> > + if (constructed && have_lease && !deprecate && !(flags & IFACE_DEPRECATED))
> > + {
> > + if (valid < time)
> > + valid = time;
> > + if (preferred < time)
> > + preferred = time;
> > + }
> > +
> > if (flags & IFACE_DEPRECATED)
> > preferred = 0;
> >
> >
> > _______________________________________________
> > Dnsmasq-discuss mailing list
> > Dnsmasq-discuss at lists.thekelleys.org.uk
> > https://lists.thekelleys.org.uk/cgi-bin/mailman/listinfo/dnsmasq-discuss
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