[PATCH v4] Send an initial ARP and NDP request to resolve the guest IP address

[Volker Diels-Grabsch <v@njh.eu>]

When restarting passt while QEMU keeps running with a configured
"reconnect-ms" setting, the port forwardings will stop working until
the guest sends some outgoing network traffic.

Reason: Although QEMU reconnects successfully to the unix domain
socket of the new passt process, that one no longer knows the guest's
MAC address and uses instead the broadcast MAC address.  However, this
is ignored by the guest, at least if the guest runs Linux.  Only after
the guest sends some network package on its own initiative, passt will
know the MAC address and will be able to establish forwarded
connections.

This change fixes this issue by sending an ARP and an NDP request to
resolve the guest's MAC address via its IPv4 and IPv6 address, which
we do know, right after the unix domain socket (re)connection.

The only case where the IP is "wrong" would be if the configuration
changed, or on the very first start right after qemu started.  But in
those cases, we just wouldn't get an ARP/NDP response, and can't do
anything until we receive the guest's DHCP request - just as before.
In other words, in the worst case the ARP/NDP requests would be
harmless.

Signed-off-by: Volker Diels-Grabsch <v@njh.eu>
---
 arp.c   | 35 +++++++++++++++++++++++++++++++++++
 arp.h   |  1 +
 ndp.c   | 21 +++++++++++++++++++++
 ndp.h   |  1 +
 passt.1 |  9 +++++----
 tap.c   | 15 +++++++++++----
 util.h  |  2 ++
 7 files changed, 76 insertions(+), 8 deletions(-)

diff --git a/arp.c b/arp.c
index 44677ad..b263419 100644
--- a/arp.c
+++ b/arp.c
@@ -112,3 +112,38 @@ int arp(const struct ctx *c, struct iov_tail *data)
 
 	return 1;
 }
+
+/**
+ * arp_send_init_req() - Send initial ARP request to retrieve guest MAC address
+ * @c:		Execution context
+ */
+void arp_send_init_req(const struct ctx *c)
+{
+	struct {
+		struct ethhdr eh;
+		struct arphdr ah;
+		struct arpmsg am;
+	} __attribute__((__packed__)) req;
+
+	/* Ethernet header */
+	req.eh.h_proto = htons(ETH_P_ARP);
+	memcpy(req.eh.h_dest, MAC_BROADCAST, sizeof(req.eh.h_dest));
+	memcpy(req.eh.h_source, c->our_tap_mac, sizeof(req.eh.h_source));
+
+	/* ARP header */
+	req.ah.ar_op = htons(ARPOP_REQUEST);
+	req.ah.ar_hrd = htons(ARPHRD_ETHER);
+	req.ah.ar_pro = htons(ETH_P_IP);
+	req.ah.ar_hln = ETH_ALEN;
+	req.ah.ar_pln = 4;
+
+	/* ARP message */
+	memcpy(req.am.sha,	c->our_tap_mac,		sizeof(req.am.sha));
+	memcpy(req.am.sip,	&c->ip4.our_tap_addr,	sizeof(req.am.sip));
+	memcpy(req.am.tha,	MAC_BROADCAST,		sizeof(req.am.tha));
+	memcpy(req.am.tip,	&c->ip4.addr,		sizeof(req.am.tip));
+
+	debug("Sending initial ARP request to retrieve"
+	      " guest MAC address after reconnect");
+	tap_send_single(c, &req, sizeof(req));
+}
diff --git a/arp.h b/arp.h
index 86bcbf8..d5ad0e1 100644
--- a/arp.h
+++ b/arp.h
@@ -21,5 +21,6 @@ struct arpmsg {
 } __attribute__((__packed__));
 
 int arp(const struct ctx *c, struct iov_tail *data);
+void arp_send_init_req(const struct ctx *c);
 
 #endif /* ARP_H */
diff --git a/ndp.c b/ndp.c
index eb090cd..a47fe42 100644
--- a/ndp.c
+++ b/ndp.c
@@ -438,3 +438,24 @@ void ndp_timer(const struct ctx *c, const struct timespec *now)
 first:
 	next_ra = now->tv_sec + interval;
 }
+
+/**
+ * ndp_send_init_req() - Send initial NDP NS to retrieve guest MAC address
+ * @c:		Execution context
+ */
+void ndp_send_init_req(const struct ctx *c)
+{
+	struct ndp_ns ns = {
+		.ih = {
+			.icmp6_type		= NS,
+			.icmp6_code		= 0,
+			.icmp6_router		= 0, /* Reserved */
+			.icmp6_solicited	= 0, /* Reserved */
+			.icmp6_override		= 0, /* Reserved */
+		},
+		.target_addr = c->ip6.addr
+	};
+	debug("Sending initial NDP NS request to retrieve"
+	      " guest MAC address after reconnect");
+	ndp_send(c, &c->ip6.addr, &ns, sizeof(ns));
+}
diff --git a/ndp.h b/ndp.h
index b1dd5e8..781ea86 100644
--- a/ndp.h
+++ b/ndp.h
@@ -11,5 +11,6 @@ struct icmp6hdr;
 int ndp(const struct ctx *c, const struct in6_addr *saddr,
 	struct iov_tail *data);
 void ndp_timer(const struct ctx *c, const struct timespec *now);
+void ndp_send_init_req(const struct ctx *c);
 
 #endif /* NDP_H */
diff --git a/passt.1 b/passt.1
index cef98b2..7000377 100644
--- a/passt.1
+++ b/passt.1
@@ -319,8 +319,9 @@ silently dropped.
 
 .TP
 .BR \-\-no-icmp
-Disable the ICMP/ICMPv6 echo handler. ICMP and ICMPv6 echo requests coming from
-guest or target namespace will be silently dropped.
+Disable the ICMP/ICMPv6 handler, and hence also NDP. ICMP and ICMPv6 requests
+coming from guest or target namespace will be silently dropped. No initial NDP
+message will be sent.
 
 .TP
 .BR \-\-no-dhcp
@@ -330,8 +331,8 @@ selected IPv4 default route.
 
 .TP
 .BR \-\-no-ndp
-Disable NDP responses. NDP messages coming from guest or target namespace will
-be ignored.
+Disable Neighbor Discovery. NDP messages coming from guest or target
+namespace will be ignored. No initial NDP message will be sent.
 
 .TP
 .BR \-\-no-dhcpv6
diff --git a/tap.c b/tap.c
index 7ba6399..25c32f9 100644
--- a/tap.c
+++ b/tap.c
@@ -1096,7 +1096,9 @@ void tap_add_packet(struct ctx *c, struct iov_tail *data,
 		return;
 
 	if (memcmp(c->guest_mac, eh->h_source, ETH_ALEN)) {
+		char bufmac[ETH_ADDRSTRLEN];
 		memcpy(c->guest_mac, eh->h_source, ETH_ALEN);
+		info("Guest MAC address: %s", eth_ntop(c->guest_mac, bufmac, sizeof(bufmac)));
 		proto_update_l2_buf(c->guest_mac, NULL);
 	}
 
@@ -1355,6 +1357,11 @@ static void tap_start_connection(const struct ctx *c)
 	ev.events = EPOLLIN | EPOLLRDHUP;
 	ev.data.u64 = ref.u64;
 	epoll_ctl(c->epollfd, EPOLL_CTL_ADD, c->fd_tap, &ev);
+
+	if (c->ifi4)
+		arp_send_init_req(c);
+	if (c->ifi6 && !c->no_ndp && !c->no_icmp)
+		ndp_send_init_req(c);
 }
 
 /**
@@ -1503,11 +1510,11 @@ void tap_backend_init(struct ctx *c)
 	case MODE_PASST:
 		tap_sock_unix_init(c);
 
-		/* In passt mode, we don't know the guest's MAC address until it
-		 * sends us packets.  Use the broadcast address so that our
-		 * first packets will reach it.
+		/* In passt mode, we don't know the guest's MAC address until
+		 * it sends us packets.  Until then, use the broadcast address
+		 * so that our first packets will have a chance to reach it.
 		 */
-		memset(&c->guest_mac, 0xff, sizeof(c->guest_mac));
+		memcpy(&c->guest_mac, MAC_BROADCAST, sizeof(c->guest_mac));
 		break;
 	}
 
diff --git a/util.h b/util.h
index 2a8c38f..22eaac5 100644
--- a/util.h
+++ b/util.h
@@ -97,6 +97,8 @@ void abort_with_msg(const char *fmt, ...)
 #define FD_PROTO(x, proto)						\
 	(IN_INTERVAL(c->proto.fd_min, c->proto.fd_max, (x)))
 
+#define MAC_BROADCAST							\
+	((uint8_t [ETH_ALEN]){ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff })
 #define MAC_ZERO		((uint8_t [ETH_ALEN]){ 0 })
 #define MAC_IS_ZERO(addr)	(!memcmp((addr), MAC_ZERO, ETH_ALEN))
 
-- 
@@ -97,6 +97,8 @@ void abort_with_msg(const char *fmt, ...)
 #define FD_PROTO(x, proto)						\
 	(IN_INTERVAL(c->proto.fd_min, c->proto.fd_max, (x)))
 
+#define MAC_BROADCAST							\
+	((uint8_t [ETH_ALEN]){ 0xff, 0xff, 0xff, 0xff, 0xff, 0xff })
 #define MAC_ZERO		((uint8_t [ETH_ALEN]){ 0 })
 #define MAC_IS_ZERO(addr)	(!memcmp((addr), MAC_ZERO, ETH_ALEN))
 
-- 
2.47.3