[PATCH v5 1/5] Show debug message whenever we learn a new guest MAC address

[PATCH v5 1/5] Show debug message whenever we learn a new guest MAC address

[Volker Diels-Grabsch]

Signed-off-by: Volker Diels-Grabsch <v@njh.eu>
---
 tap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tap.c b/tap.c
index 7ba6399..3c08126 100644
--- a/tap.c
+++ b/tap.c
@@ -1096,7 +1096,11 @@ 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);
+		debug("Guest MAC address: %s",
+		      eth_ntop(c->guest_mac, bufmac, sizeof(bufmac)));
 		proto_update_l2_buf(c->guest_mac, NULL);
 	}
 
-- 
@@ -1096,7 +1096,11 @@ 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);
+		debug("Guest MAC address: %s",
+		      eth_ntop(c->guest_mac, bufmac, sizeof(bufmac)));
 		proto_update_l2_buf(c->guest_mac, NULL);
 	}
 
-- 
2.47.3

[PATCH v5 2/5] Improve clarity of comment

[Volker Diels-Grabsch]

Signed-off-by: Volker Diels-Grabsch <v@njh.eu>
---
 tap.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tap.c b/tap.c
index 3c08126..d4d19eb 100644
--- a/tap.c
+++ b/tap.c
@@ -1507,9 +1507,9 @@ 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));
 		break;
-- 
@@ -1507,9 +1507,9 @@ 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));
 		break;
-- 
2.47.3

Re: [PATCH v5 2/5] Improve clarity of comment

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1270 bytes --]

On Thu, Sep 11, 2025 at 10:18:59PM +0200, Volker Diels-Grabsch wrote:
> Signed-off-by: Volker Diels-Grabsch <v@njh.eu>

Commit message should probably say specifically what was inaccurate /
misleading about the original version.

Btw, for some reason I didn't receive 1/5 in this series.

> ---
>  tap.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 3c08126..d4d19eb 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1507,9 +1507,9 @@ 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));
>  		break;
> -- 
> 2.47.3
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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Re: [PATCH v5 2/5] Improve clarity of comment

[Stefano Brivio]

On Fri, 12 Sep 2025 11:17:16 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Sep 11, 2025 at 10:18:59PM +0200, Volker Diels-Grabsch wrote:
> > Signed-off-by: Volker Diels-Grabsch <v@njh.eu>  
> 
> Commit message should probably say specifically what was inaccurate /
> misleading about the original version.
> 
> Btw, for some reason I didn't receive 1/5 in this series.

Your email relay accepted it at 20:19:35.98982 UTC:

2025-09-11T22:19:35.989820+02:00 ... : *****: to=<david@gibson.dropbear.id.au>, relay=mx.ozlabs.org[********]:25, delay=8.2, delays=0.05/0.01/3.6/4.5, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as *****)

If needed:

  https://archives.passt.top/passt-dev/20250911201900.237677-2-v@njh.eu/raw

...maybe you confused it with a cover letter that's missing in this
case? By the way, I don't see an issue with that if it's mostly a bunch
of trivial changes.

-- 
Stefano

[PATCH v5 3/5] Introduce constant MAC_BROADCAST

[Volker Diels-Grabsch]

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

diff --git a/tap.c b/tap.c
index d4d19eb..ba6d7b4 100644
--- a/tap.c
+++ b/tap.c
@@ -1511,7 +1511,7 @@ void tap_backend_init(struct ctx *c)
 		 * 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

Re: [PATCH v5 3/5] Introduce constant MAC_BROADCAST

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1433 bytes --]

On Thu, Sep 11, 2025 at 10:19:01PM +0200, Volker Diels-Grabsch wrote:
> Signed-off-by: Volker Diels-Grabsch <v@njh.eu>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tap.c  | 2 +-
>  util.h | 2 ++
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/tap.c b/tap.c
> index d4d19eb..ba6d7b4 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1511,7 +1511,7 @@ void tap_backend_init(struct ctx *c)
>  		 * 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))
>  
> -- 
> 2.47.3
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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[PATCH v5 4/5] Fix --no-icmp description and make it imply --no-ndp

[Volker Diels-Grabsch]

Signed-off-by: Volker Diels-Grabsch <v@njh.eu>
---
 conf.c  | 3 +++
 passt.1 | 4 ++--
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/conf.c b/conf.c
index f47f48e..02e903b 100644
--- a/conf.c
+++ b/conf.c
@@ -2067,6 +2067,9 @@ void conf(struct ctx *c, int argc, char **argv)
 
 	isolate_user(uid, gid, !netns_only, userns, c->mode);
 
+	if (c->no_icmp)
+		c->no_ndp = 1;
+
 	if (c->pasta_conf_ns)
 		c->no_ra = 1;
 
diff --git a/passt.1 b/passt.1
index cef98b2..dd00b08 100644
--- a/passt.1
+++ b/passt.1
@@ -319,8 +319,8 @@ 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 protocol handler. ICMP and ICMPv6 requests coming from
+guest or target namespace will be silently dropped. Implies \fB--no-ndp\fR.
 
 .TP
 .BR \-\-no-dhcp
-- 
@@ -319,8 +319,8 @@ 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 protocol handler. ICMP and ICMPv6 requests coming from
+guest or target namespace will be silently dropped. Implies \fB--no-ndp\fR.
 
 .TP
 .BR \-\-no-dhcp
-- 
2.47.3

Re: [PATCH v5 4/5] Fix --no-icmp description and make it imply --no-ndp

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 1355 bytes --]

On Thu, Sep 11, 2025 at 10:19:03PM +0200, Volker Diels-Grabsch wrote:
> Signed-off-by: Volker Diels-Grabsch <v@njh.eu>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  conf.c  | 3 +++
>  passt.1 | 4 ++--
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index f47f48e..02e903b 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -2067,6 +2067,9 @@ void conf(struct ctx *c, int argc, char **argv)
>  
>  	isolate_user(uid, gid, !netns_only, userns, c->mode);
>  
> +	if (c->no_icmp)
> +		c->no_ndp = 1;
> +
>  	if (c->pasta_conf_ns)
>  		c->no_ra = 1;
>  
> diff --git a/passt.1 b/passt.1
> index cef98b2..dd00b08 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -319,8 +319,8 @@ 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 protocol handler. ICMP and ICMPv6 requests coming from
> +guest or target namespace will be silently dropped. Implies \fB--no-ndp\fR.
>  
>  .TP
>  .BR \-\-no-dhcp
> -- 
> 2.47.3
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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[PATCH v5 5/5] Send an initial ARP and NDP request to resolve the guest IP address

[Volker Diels-Grabsch]

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>
---
v4:
  - Shorten debug() messages
  - Split out unrelated improvements into separate patches
---
 arp.c   | 34 ++++++++++++++++++++++++++++++++++
 arp.h   |  1 +
 ndp.c   | 20 ++++++++++++++++++++
 ndp.h   |  1 +
 passt.1 |  4 ++--
 tap.c   |  5 +++++
 6 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/arp.c b/arp.c
index 44677ad..ad088b1 100644
--- a/arp.c
+++ b/arp.c
@@ -112,3 +112,37 @@ 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 for guest MAC address");
+	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..588b48f 100644
--- a/ndp.c
+++ b/ndp.c
@@ -438,3 +438,23 @@ 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 for guest MAC address");
+	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 dd00b08..af5726a 100644
--- a/passt.1
+++ b/passt.1
@@ -330,8 +330,8 @@ 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 ba6d7b4..2a8e11b 100644
--- a/tap.c
+++ b/tap.c
@@ -1359,6 +1359,11 @@ 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)
+		ndp_send_init_req(c);
 }
 
 /**
-- 
@@ -1359,6 +1359,11 @@ 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)
+		ndp_send_init_req(c);
 }
 
 /**
-- 
2.47.3

Re: [PATCH v5 5/5] Send an initial ARP and NDP request to resolve the guest IP address

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 5692 bytes --]

On Thu, Sep 11, 2025 at 10:19:05PM +0200, Volker Diels-Grabsch wrote:
11;rgb:ffff/ffff/ffff> 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>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
> v4:
>   - Shorten debug() messages
>   - Split out unrelated improvements into separate patches
> ---
>  arp.c   | 34 ++++++++++++++++++++++++++++++++++
>  arp.h   |  1 +
>  ndp.c   | 20 ++++++++++++++++++++
>  ndp.h   |  1 +
>  passt.1 |  4 ++--
>  tap.c   |  5 +++++
>  6 files changed, 63 insertions(+), 2 deletions(-)
> 
> diff --git a/arp.c b/arp.c
> index 44677ad..ad088b1 100644
> --- a/arp.c
> +++ b/arp.c
> @@ -112,3 +112,37 @@ 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 for guest MAC address");
> +	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..588b48f 100644
> --- a/ndp.c
> +++ b/ndp.c
> @@ -438,3 +438,23 @@ 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 for guest MAC address");
> +	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 dd00b08..af5726a 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -330,8 +330,8 @@ 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 ba6d7b4..2a8e11b 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1359,6 +1359,11 @@ 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)
> +		ndp_send_init_req(c);
>  }
>  
>  /**
> -- 
> 2.47.3
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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