ndp: Send unsolicited Router Advertisements

Currently, our NDP implementation only sends Router Advertisements (RA)
when it receives a Router Solicitation (RS) from the guest.  However,
RFC 4861 requires that we periodically send unsolicited RAs.

Linux as a guest also requires this: it will send an RS when a link first
comes up, but the route it gets from this will have a finite lifetime (we
set this to 65535s, the maximum allowed, around 18 hours).  When that
expires the guest will not send a new RS, but instead expects the route to
have been renewed (if still valid) by an unsolicited RA.

Implement sending unsolicited RAs on a partially randomised timer, as
required by RFC 4861.  The RFC also specifies that solicited RAs should
also be delayed, or even omitted, if the next unsolicited RA is soon
enough.  For now we don't do that, always sending an immediate RA in
response to an RS.  We can get away with this because in our use cases
we expect to just have passt itself and the guest on the link, rather than
a large broadcast domain.

Link: https://github.com/kubevirt/kubevirt/issues/13191
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

ip.h

@@ -92,4 +92,13 @@ struct ipv6_opt_hdr {
 
 char *ipv6_l4hdr(const struct pool *p, int idx, size_t offset, uint8_t *proto,
 		 size_t *dlen);
+
+/* IPv6 link-local all-nodes multicast adddress, ff02::1 */
+static const struct in6_addr in6addr_ll_all_nodes = {
+	.s6_addr = {
+		0xff, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01,
+	},
+};
+
 #endif /* IP_H */

ndp.c

@@ -372,3 +372,57 @@ int ndp(const struct ctx *c, const struct icmp6hdr *ih,
 
 	return 1;
 }
+
+/* Default interval between unsolicited RAs (seconds) */
+#define DEFAULT_MAX_RTR_ADV_INTERVAL	600	/* RFC 4861, 6.2.1 */
+
+/* Minimum required interval between RAs (seconds) */
+#define MIN_DELAY_BETWEEN_RAS		3	/* RFC 4861, 10 */
+
+static time_t next_ra;
+
+/**
+ * ndp_timer() - Send unsolicited NDP messages if necessary
+ * @c:		Execution context
+ * @now:	Current (monotonic) time
+ */
+void ndp_timer(const struct ctx *c, const struct timespec *now)
+{
+	time_t max_rtr_adv_interval = DEFAULT_MAX_RTR_ADV_INTERVAL;
+	time_t min_rtr_adv_interval, interval;
+
+	if (c->no_ra || now->tv_sec < next_ra)
+		return;
+
+	/* We must advertise before the route's lifetime expires */
+	max_rtr_adv_interval = MIN(max_rtr_adv_interval, RT_LIFETIME - 1);
+
+	/* But we must not go smaller than the minimum delay */
+	max_rtr_adv_interval = MAX(max_rtr_adv_interval, MIN_DELAY_BETWEEN_RAS);
+
+	/* RFC 4861, 6.2.1 */
+	min_rtr_adv_interval = MAX(max_rtr_adv_interval / 3,
+				   MIN_DELAY_BETWEEN_RAS);
+
+	/* As required by RFC 4861, we randomise the interval between
+	 * unsolicited RAs.  This is to prevent multiple routers on a link
+	 * getting synchronised (e.g. after booting a bunch of routers at once)
+	 * and causing flurries of RAs at the same time.
+	 *
+	 * This random doesn't need to be cryptographically strong, so random(3)
+	 * is fine.  Other routers on the link also want to avoid
+	 * synchronisation, and anything malicious has much easier ways to cause
+	 * trouble.
+	 *
+	 * The modulus also makes this not strictly a uniform distribution, but,
+	 * again, it's close enough for our purposes.
+	 */
+	interval = min_rtr_adv_interval +
+		random() % (max_rtr_adv_interval - min_rtr_adv_interval);
+
+	info("NDP: sending unsolicited RA, next in %llds", (long long)interval);
+
+	ndp_ra(c, &in6addr_ll_all_nodes);
+
+	next_ra = now->tv_sec + interval;
+}

ndp.h

@@ -6,7 +6,10 @@
 #ifndef NDP_H
 #define NDP_H
 
+struct icmp6hdr;
+
 int ndp(const struct ctx *c, const struct icmp6hdr *ih,
 	const struct in6_addr *saddr, const struct pool *p);
+void ndp_timer(const struct ctx *c, const struct timespec *now);
 
 #endif /* NDP_H */

passt.c

@@ -49,6 +49,7 @@
 #include "arch.h"
 #include "log.h"
 #include "tcp_splice.h"
+#include "ndp.h"
 
 #define EPOLL_EVENTS		8
 
@@ -107,6 +108,8 @@ static void post_handler(struct ctx *c, const struct timespec *now)
 
 	flow_defer_handler(c, now);
 #undef CALL_PROTO_HANDLER
+
+	ndp_timer(c, now);
 }
 
 /**