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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45ef358c07bsm5968003f8f.36.2026.05.29.13.04.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 29 May 2026 13:04:16 -0700 (PDT) From: Stefano Brivio To: Tj Cc: =?UTF-8?B?w43DsWlnbw==?= Huguet , Thorsten Leemhuis , Fernando Fernandez Mancera , Jakub Kicinski , netdev@vger.kernel.org, Yumei Huang , Ido Schimmel , Justin Iurman , David Ahern , David Gibson , Linux kernel regressions list , Beniamino Galvani Subject: Re: Problem with IPv6 privacy addresses in 7.0 Message-ID: <20260529220415.22d0be8d@elisabeth> In-Reply-To: References: Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Date: Fri, 29 May 2026 22:04:15 +0200 (CEST) On Fri, 29 May 2026 18:28:58 +0000 Tj wrote: > I believe I hit this on a router using Debian 13 with v7.0.* kernel this week that > uses systemd-networkd to configure IPV6 RA and prefix delegation after > moving from v6.19.*. > > Symptom was the router could no longer reach public IPv6 addresses > itself but forwarding was unaffected. > > The ISP (Starlink) provides a /64 prefix via RA and a /56 via DHCPv6. networkd > allocates a static suffix address from both to the WAN-side interface. > > I discovered after much experimentation that instead of the usual /56 > address being the source it was choosing the /64 and failing. Do you really mean an address configured as /56, or a /64 address that systemd-networkd derives from a /56 delegated prefix? Because more specific addresses / longest matching prefixes (RFC 6724 Section 5., Rule 8, implemented by ipv6_get_saddr_eval()) should anyway be preferred as source addresses, regardless of the order of insertion of addresses with the same scope. I'm looking into possible assumptions made by systemd-networkd in this case. *If* this is confirmed, I also start thinking that a revert and exporting the correct implementation as non-default using a netlink flag would be preferable. at this point. > Router uses policy routing so my work-around was to add a rule so the > /64 address is added to the WAN interface's route table. > > [RoutingPolicyRule] > To=::/0 > From=2a0d:3344:aaaa:bbbb::/64 > Priority=30100 > Table=starlink > > The WAN interface config for RA and PD is: > > [IPv6AcceptRA] > UseGateway=yes > UseDNS=no > UseDomains=no > Token=static:::ff > # when RouteTable is set a table name is explicitly required in any [Route] section without a Table= of its own > # names defined in /etc/systemd/networkd.conf.d/51-RouteTable.conf as: [Network] RouteTable= > RouteTable=starlink > > [DHCPv6] > UseAddress=no > UseDNS=no > UseNTP=no > UseHostname=no > UseDomains=no > UseDelegatedPrefix=yes > PrefixDelegationHint=::/56 > ## asked for in RFE https://github.com/systemd/systemd/issues/31566 > ##RouteTable=starlink > > [DHCPPrefixDelegation] > Announce=false > UplinkInterface=:self > Assign=yes > Token=static:::1 > SubnetId=0xff -- Stefano