[PATCH] Send an initial ARP 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 a 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 request to resolve the
guest's MAC address via its IP 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, and/or on the very first start right after qemu started.  But
in those cases, we just wouldn't get an ARP response, and can't do
anything receive the guest's DHCP request - just as before.  In other
words, in the worst case an ARP request would be harmless.

Signed-off-by: Volker Diels-Grabsch <v@njh.eu>
---
 arp.c | 34 ++++++++++++++++++++++++++++++++++
 arp.h |  1 +
 tap.c | 15 +++++++++++++--
 3 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/arp.c b/arp.c
index 44677ad..561581a 100644
--- a/arp.c
+++ b/arp.c
@@ -112,3 +112,37 @@ int arp(const struct ctx *c, struct iov_tail *data)
 
 	return 1;
 }
+
+/**
+ * send_initial_arp_req() - Send initial ARP request to retrieve guest MAC address
+ * @c:		Execution context
+ */
+void send_initial_arp_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, c->guest_mac, 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,	c->guest_mac,		sizeof(req.am.tha));
+	memcpy(req.am.tip,	&c->ip4.addr,		sizeof(req.am.tip));
+
+	info("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..5490144 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 send_initial_arp_req(const struct ctx *c);
 
 #endif /* ARP_H */
diff --git a/tap.c b/tap.c
index 7ba6399..47dedd5 100644
--- a/tap.c
+++ b/tap.c
@@ -1355,6 +1355,16 @@ 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);
+
+	switch (c->mode) {
+	case MODE_PASST:
+		send_initial_arp_req(c);
+		break;
+	case MODE_PASTA:
+		break;
+	case MODE_VU:
+		break;
+	}
 }
 
 /**
@@ -1504,8 +1514,9 @@ void tap_backend_init(struct ctx *c)
 		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.
+		 * sends us packets (e.g. responds to our initial ARP request).
+		 * Until then, use the broadcast address so that our first
+		 * packets will reach it.
 		 */
 		memset(&c->guest_mac, 0xff, sizeof(c->guest_mac));
 		break;
-- 
@@ -1355,6 +1355,16 @@ 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);
+
+	switch (c->mode) {
+	case MODE_PASST:
+		send_initial_arp_req(c);
+		break;
+	case MODE_PASTA:
+		break;
+	case MODE_VU:
+		break;
+	}
 }
 
 /**
@@ -1504,8 +1514,9 @@ void tap_backend_init(struct ctx *c)
 		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.
+		 * sends us packets (e.g. responds to our initial ARP request).
+		 * Until then, use the broadcast address so that our first
+		 * packets will reach it.
 		 */
 		memset(&c->guest_mac, 0xff, sizeof(c->guest_mac));
 		break;
-- 
2.47.2