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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e7521bfdc6sm6933044f8f.8.2025.09.10.07.01.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Sep 2025 07:01:57 -0700 (PDT) Date: Wed, 10 Sep 2025 16:01:55 +0200 From: Stefano Brivio To: Volker Diels-Grabsch Subject: Re: [PATCH] Send an initial ARP and NDP request to resolve the guest IP address Message-ID: <20250910160155.0a11f81e@elisabeth> In-Reply-To: References: <20250909145516.762957-1-v@njh.eu> <20250909145516.762957-2-v@njh.eu> <20250910112906.5e1b7e5a@elisabeth> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: KLEEVUg-On5_xTlH_oUvvr0yzz-PLW5xtPKfP1PJlwk_1757512920 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: F3OI4ZITI3RUEOPGTOMNZYSOOIHC426X X-Message-ID-Hash: F3OI4ZITI3RUEOPGTOMNZYSOOIHC426X X-MailFrom: sbrivio@redhat.com 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, David Gibson 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: On Wed, 10 Sep 2025 12:33:44 +0200 Volker Diels-Grabsch wrote: > Dear Stefano, > > Thanks for your timely review. I agree with almost everything of it. > > Just a small clarification on this one: > > Stefano Brivio wrote: > > > @@ -1503,11 +1510,12 @@ 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 (e.g. responds to our initial ARP or > > > > I don't think the response is an example, so I wouldn't use "e.g." > > here, rather "i.e." / "that is", if that's the expected behaviour. > > The reason for using "e.g." is the following: There is still the > "usual" case where the passt client (QEMU) was freshly started > together with passt. > > In that case, it *will not* respond to neither our ARP nor NDP request, > simply because it won't recognize the IPv4/6 addresses, because it > doesn't yet have any. In that situation, we'll learn about the guest's > MAC address only after it sends a DCHP request, or NDP, or similar to > us, on its own initiative - not as a response to anything we might > have sent. > > So I'd propose to either keep the "e.g." wording, or to extend the > comment by enumerating all possible cases. Ah, sorry, I see what you meant now. Of course, it makes sense. > (Regarding the latter, I don't feel confident enough to be able to > really enumerate them all. In addition to DCHP, NDP NS and DHCPv6, a > sufficient strange client network stack might even broadcast nonsense > packets to us, which we might not process further, but still learn the > guest's MAC address from.) I guess the version in v4 is more terse without any real loss of generality or clarity. -- Stefano