From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202508 header.b=LPFSyBsD; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id A7AE45A0271 for ; Thu, 18 Sep 2025 04:43:03 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202508; t=1758163380; bh=B2PyN1dqw8wMr0Hm/8nNzH39CPiFiIZd2d/Y8y5sRZo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LPFSyBsDkL7lTb2M5pphbBiaRudVGdRGgEgTMKrjh72TsA47T09a2i1R8Y6IoRYzE hn9v/jWOJq3Jx0TNXamUGSDNYfIfqiXrbPp0Q1iVQTMZK40o4Qy27cOnpn/9LFKMGR zTH7c4UhjMC9Kz0aBLb8lLcA98K9B+urDUPaB6SNDeR3hlGDS2yjwob2zhCRLHMP0g yv2EnPWKdsdSudLN8stM3NbEKm7d0mTvhhaijYkxhAh6YyTsE4U5bu2slyjfIS7rBS aAtpuCaCcQ97J0sEMtU7A0ovGLzAL1TM+80sgBHEH4dtSNLJWnH3C2oq0XGOBDimp/ RG1ixVh73dY4w== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4cS0LD5l6Nz4wC3; Thu, 18 Sep 2025 12:43:00 +1000 (AEST) Date: Thu, 18 Sep 2025 12:32:20 +1000 From: David Gibson To: Volker Diels-Grabsch Subject: Re: [PATCH 4/5] Send an initial ARP and NDP request to resolve the guest IP address Message-ID: References: <20250916192209.3986375-1-v@njh.eu> <20250916192209.3986375-5-v@njh.eu> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="lnBtfhKI5keUmAvC" Content-Disposition: inline In-Reply-To: <20250916192209.3986375-5-v@njh.eu> Message-ID-Hash: 5GFSXZWQ5EDNDRNAVKVKTTNMGJHUDDWZ X-Message-ID-Hash: 5GFSXZWQ5EDNDRNAVKVKTTNMGJHUDDWZ X-MailFrom: dgibson@gandalf.ozlabs.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: --lnBtfhKI5keUmAvC Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Sep 16, 2025 at 09:21:15PM +0200, Volker Diels-Grabsch wrote: > 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > Signed-off-by: Volker Diels-Grabsch Reviewed-by: David Gibson > --- > 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(-) >=20 > 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) > =20 > 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 =3D 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 =3D htons(ARPOP_REQUEST); > + req.ah.ar_hrd =3D htons(ARPHRD_ETHER); > + req.ah.ar_pro =3D htons(ETH_P_IP); > + req.ah.ar_hln =3D ETH_ALEN; > + req.ah.ar_pln =3D 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__)); > =20 > int arp(const struct ctx *c, struct iov_tail *data); > +void arp_send_init_req(const struct ctx *c); > =20 > #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 tim= espec *now) > first: > next_ra =3D now->tv_sec + interval; > } > + > +/** > + * ndp_send_init_req() - Send initial NDP NS to retrieve guest MAC addre= ss > + * @c: Execution context > + */ > +void ndp_send_init_req(const struct ctx *c) > +{ > + struct ndp_ns ns =3D { > + .ih =3D { > + .icmp6_type =3D NS, > + .icmp6_code =3D 0, > + .icmp6_router =3D 0, /* Reserved */ > + .icmp6_solicited =3D 0, /* Reserved */ > + .icmp6_override =3D 0, /* Reserved */ > + }, > + .target_addr =3D 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); > =20 > #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. > =20 > .TP > .BR \-\-no-ndp > -Disable NDP responses. NDP messages coming from guest or target namespac= e will > -be ignored. > +Disable Neighbor Discovery. NDP messages coming from guest or target > +namespace will be ignored. No initial NDP message will be sent. > =20 > .TP > .BR \-\-no-dhcpv6 > diff --git a/tap.c b/tap.c > index 50e1a88..0f8ee25 100644 > --- a/tap.c > +++ b/tap.c > @@ -1359,6 +1359,11 @@ static void tap_start_connection(const struct ctx = *c) > ev.events =3D EPOLLIN | EPOLLRDHUP; > ev.data.u64 =3D 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); > } > =20 > /** > --=20 > 2.47.3 >=20 --=20 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 --lnBtfhKI5keUmAvC Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmjLbx0ACgkQzQJF27ox 2GcVgg/+MsPo/z+/zV3JKfr0cmKX7SFbklhYeoycngseOOGe3K1p7f2uRo3bc20F +ugmKV/Yw6kk2JqW5EeTpse9W8CT4udUi8p4bJqz0e5vgohHQrNd2xBWQZ/UgL8X hmV83O2vifg4nLc3eko+DFrwS30GV3aguTJFvBpx9cCPhWwrWmPDz2DPOmW8C0Xh Oh79dKxf/06X6hRvjI5CjuUYLkm5ZG4bX3V3IXUDkdEUuklIn2KL9qWw1/+52PoR 3cVoZKR+UceVVfnGM5E++6K/Hr62EQq0r6/UBx5OzYov2rYIJB7fKOXIfM2vW2YJ kM7ef4EqZnQSdZB/a3JGMrzHfIwyDivAd9LCvrlxDZWr0sSCVJsg04qz0SWQSZeB 46nbSXvyNvtxKHswAzxdvWCRjcjCnibZ9VoGcCdSzNGwaIVHs3dxELMkllO1bviq tfvh6v7usJ38Fh4NB4MEce7YHtFKVbe/QQDOkoZE820hx7qImPgzZG3F/QPtm/uR XZxd4BtwvLXT4o7IvVDCRRgWm1RIIIXaNFJlJPL/zLM7tEVYPFOrKFcD4oySauBU ljSqBXW6yUySig54B71PdWA5elui0C7eyYmCW5DakZWo1plkhCQEZF0cQqP2VNt4 XNnGxNuKecmozf/VaEW7966fI65aeT0OJl8urM+1MZuPfYbvE8s= =MVbk -----END PGP SIGNATURE----- --lnBtfhKI5keUmAvC--