@samitouri / QOSAMI-WSL / commits / 225f1f87

Mark routes via a link-local gateway as onlink to fix VPN /32 networking failures (#41003)

* Mark routes via a link-local gateway as onlink in the guest GnsEngine adds a default route via the link-local NAT gateway (169.254.254.1). When the guest interface has no on-link prefix covering that gateway (e.g. a mirrored host address plumbed as a /32), the kernel rejects the route add with ENETUNREACH. In wslc this aborts ConfigureNetworking, kills /gns, and fails session creation, surfacing to the user as "Catastrophic failure / E_UNEXPECTED". A link-local next hop is only ever reachable on-link, so set RTNH_F_ONLINK for any route whose gateway is link-local (169.254.0.0/16 or fe80::/10). This bypasses the kernel reachability check and has no effect on NAT gateways, which are not link-local. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> * IsLinkLocal: return false for non-IP families instead of throwing Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> --------- Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com> Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Ben Hillis committed Jul 9, 2026 at 11:26 UTC 225f1f87860f0ea76b7b477e26b0e4e54058ed37

3 files changed +21

src/linux/netlinkutil/Address.cpp

+16

@@ -141,6 +141,22 @@ bool Address::IsIpv4() const noexcept

141 return m_family == AF_INET;

142 }

143

144 +bool Address::IsLinkLocal() const

145 +{

146 + if (m_family == AF_INET)

147 + {

148 + return (ntohl(AsBytes<in_addr>().s_addr) & 0xFFFF0000) == 0xA9FE0000;

149 + }

150 +

151 + if (m_family == AF_INET6)

152 + {

153 + const auto bytes = AsBytes<in6_addr>();

154 + return bytes.s6_addr[0] == 0xFE && (bytes.s6_addr[1] & 0xC0) == 0x80;

155 + }

156 +

157 + return false;

158 +}

159 +

160 bool Address::IsPrefixRouteAutogenerationDisabled() const noexcept

161 {

162 return m_disableAutogeneratedPrefixRoute;

src/linux/netlinkutil/RoutingTable.cpp

@@ -139,6 +139,10 @@ void RoutingTable::SendMessage(const Route& route, int operation, int flags, con

139 message.route.rtm_type = route.IsMulticast() ? RTN_MULTICAST : RTN_UNICAST;

140 message.route.rtm_scope = route.IsOnlink() ? RT_SCOPE_LINK : RT_SCOPE_UNIVERSE;

141 message.route.rtm_flags = RTM_F_NOTIFY;

142 + if (route.via.has_value() && route.via.value().IsLinkLocal())

143 + {

144 + message.route.rtm_flags |= RTNH_F_ONLINK;

145 + }

146 message.route.rtm_dst_len = route.to.has_value() ? route.to.value().PrefixLength() : 0;

147

148 utils::InitializeIntegerAttribute(message.tableId, m_table, RTA_TABLE);

src/linux/netlinkutil/address.h

@@ -75,6 +75,7 @@ public:

75 int Scope() const noexcept;

76 int PreferredLifetime() const noexcept;

77 bool IsIpv4() const noexcept;

78 + bool IsLinkLocal() const;

79 bool IsPrefixRouteAutogenerationDisabled() const noexcept;

81 private: