And this is the logs for a failure: default 11:21:42.260263+0200 kernel if_updown: utun15 is not attached default 11:21:42.260356+0200 kernel ifnet_attach: Waiting for all kernel threads created for interface utun15 to get scheduled at least once. default 11:21:42.260365+0200 kernel ifnet_attach: All kernel threads created for interface utun15 have been scheduled at least once. Proceeding. default 11:21:42.260591+0200 nesessionmanager getsockopt(UTUN_OPT_IFNAME) returned virtual interface name utun15 default 11:21:42.260653+0200 nesessionmanager Created a new NEVirtualInterface "utun15" (D8510BB8-5EA6-48DC-A597-CC69E3580118) default 11:21:42.261913+0200 airportd Driver Discovery: <airport[1189]> _interfaceAdded: utun15 added. default 11:21:42.262898+0200 kernel utun15: is now delegating en0 (type 0x6, family 2, sub-family 3) default 11:21:42.263102+0200 com.my.cool.network.extension Using interface name utun15 default 11:21:42.263186+0200 com.my.cool.network.extension Created a new NEVirtualInterface "utun15" from socket default 11:21:42.319041+0200 airportd Driver Discovery: <airport[1189]> _interfaceAdded: utun15 is not an AirPort interface. default 11:22:12.378941+0200 nesessionmanager utun15: removing DNS default 11:22:12.379537+0200 nesessionmanager utun15: removing IPv4 default 11:22:12.379745+0200 nesessionmanager utun15: removing IPv6 default 11:22:12.380048+0200 nesessionmanager utun15: removing proxies default 11:22:12.380099+0200 nesessionmanager utun15: removing service dictionary default 11:22:12.380584+0200 nesessionmanager Tearing down utun15 default 11:22:12.380740+0200 nesessionmanager Tearing down utun15 default 11:22:12.381736+0200 com.my.cool.network.extension Tearing down utun15 default 11:22:12.386054+0200 kernel ifnet_detach_final: Waiting for IO references on utun15 interface to be released default 11:22:12.389328+0200 com.my.cool.network.extension Tearing down utun15 NESessionManager creates the virtual interface from the utun but doesn't configure it.

Some more information on the topic. Here are the logs from the Console app for a successful sequence: default 12:17:49.259885+0200 kernel if_updown: utun12 is not attached default 12:17:49.259927+0200 kernel utun12: attached with 0 suspended link-layer multicast membership(s) default 12:17:49.259933+0200 kernel ifnet_attach: Waiting for all kernel threads created for interface utun12 to get scheduled at least once. default 12:17:49.259941+0200 kernel ifnet_attach: All kernel threads created for interface utun12 have been scheduled at least once. Proceeding. default 12:17:49.260128+0200 nesessionmanager getsockopt(UTUN_OPT_IFNAME) returned virtual interface name utun12 default 12:17:49.260163+0200 airportd Driver Discovery: <airport[1189]> _interfaceAdded: utun12 added. default 12:17:49.260186+0200 nesessionmanager Created a new NEVirtualInterface "utun12" (7A0B6CE9-D92F-49F7-B9B7-2B4713D78034) default 12:17:49.261063+0200 kernel utun12: is now delegating en0 (type 0x6, family 2, sub-family 3) default 12:17:49.261166+0200 com.my.cool.network.extension Using interface name utun12 default 12:17:49.261219+0200 com.my.cool.network.extension Created a new NEVirtualInterface "utun12" from socket default 12:17:49.306250+0200 airportd Driver Discovery: <airport[1189]> _interfaceAdded: utun12 is not an AirPort interface. default 12:17:49.936284+0200 nesessionmanager utun12: removing DNS default 12:17:49.936371+0200 nesessionmanager utun12: removing IPv4 default 12:17:49.936440+0200 nesessionmanager utun12: removing IPv6 default 12:17:49.936542+0200 nesessionmanager utun12: removing proxies default 12:17:49.936568+0200 nesessionmanager utun12: removing service dictionary default 12:17:49.936976+0200 nesessionmanager utun12: setting DNS to dictionary with keys: ( ServerAddresses, SupplementalMatchDomains, SearchDomains ) default 12:17:49.937052+0200 nesessionmanager utun12: removing IPv4 default 12:17:49.937133+0200 nesessionmanager utun12: removing IPv6 default 12:17:49.937211+0200 nesessionmanager utun12: setting proxies to dictionary with keys: ( FTPPassive ) default 12:17:49.937241+0200 nesessionmanager utun12: removing service dictionary default 12:17:50.003511+0200 kernel _dlil_attach_flowswitch_nexus: not attaching flowswitch to utun12 default 12:17:50.003520+0200 kernel _dlil_attach_flowswitch_nexus: not attaching flowswitch to utun12 default 12:17:50.007412+0200 kernel _dlil_attach_flowswitch_nexus: not attaching flowswitch to utun12 default 12:17:50.007422+0200 kernel _dlil_attach_flowswitch_nexus: not attaching flowswitch to utun12 default 12:17:50.012506+0200 nesessionmanager utun12: setting DNS to dictionary with keys: ( ConfirmedServiceID, ServerAddresses, SearchDomains, SupplementalMatchDomains ) default 12:17:50.013268+0200 nesessionmanager utun12: setting IPv4 to dictionary with keys: ( Router, AdditionalRoutes, ServerAddress, InterfaceName, Addresses ) default 12:17:50.025320+0200 mDNSResponder *** Network Configuration Change *** SC key: State:/Network/Interface/utun12/IPv4 default 12:17:50.033711+0200 mDNSResponder *** Network Configuration Change *** SC key: State:/Network/Interface/utun12/IPv6 default 12:17:50.049101+0200 nesessionmanager utun12: setting IPv6 to dictionary with keys: ( PrefixLength, AdditionalRoutes, Router, ServerAddress, InterfaceName, Addresses ) default 12:17:50.094795+0200 mDNSResponder Interface not represented in list; marking active and retriggering queries - ifid: 31, ifname: utun12, ifaddr: <mask.hash: '8a4YjRU426n4aoobHYIE+Q=='> default 12:17:50.094837+0200 mDNSResponder SetupActiveInterfaces: Registered utun12 (31) BSSID <mask.hash: 'kJtfT6erwO/YvBLN2AXs/Q=='> Struct addr 0x156024c00, primary 0x156024c00, <mask.hash: '8a4YjRU426n4aoobHYIE+Q=='>/64 (Primary) default 12:17:50.094852+0200 mDNSResponder Interface already represented in list - ifid: 31, ifname: utun12, ifaddr: <mask.hash: 'LXjHHzNKtevmjyvB+o2I2w=='> default 12:17:50.094878+0200 mDNSResponder SetupActiveInterfaces: Registered utun12 (31) BSSID <mask.hash: 'kJtfT6erwO/YvBLN2AXs/Q=='> Struct addr 0x15681aa00, primary 0x156024c00, <mask.hash: 'LXjHHzNKtevmjyvB+o2I2w=='>/120 default 12:17:50.095516+0200 nesessionmanager utun12: setting proxies to dictionary with keys: ( FTPPassive ) default 12:17:50.095675+0200 nesessionmanager utun12: setting primary service dictionary to { PrimaryRank = First; } default 12:17:50.129605+0200 configd effective_ifindex_get: utun12 (31): effective en0 (13) default 12:17:50.133021+0200 configd Host 10.64.5.7 Gate 10.64.5.7 Ifp utun12 Ifa 10.64.5.7 [first] [force] default 12:17:50.133399+0200 configd Host fc00:bbbb:bbbb:bb01::a40:507 Gate fc00:bbbb:bbbb:bb01::a40:507 Ifp utun12 Ifa fc00:bbbb:bbbb:bb01::a40:507 [first] [force] default 12:17:50.175750+0200 configd network changed: v4(utun12+:10.64.5.7, en0) v6(utun12+:fc00:bbbb:bbbb:bb01::a40:507, en0) DNS! Proxy+ SMB default 12:17:50.196686+0200 sharingd PrimaryIPv6 changed: fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.196775+0200 sharingd PrimaryIP changed: IPv4 10.64.5.7, IPv6 fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.196815+0200 rapportd PrimaryIPv6 changed: fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.196908+0200 rapportd PrimaryIP changed: IPv4 10.64.5.7, IPv6 fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.198332+0200 AirPlayXPCHelper PrimaryIPv6 changed: fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.198395+0200 AirPlayXPCHelper PrimaryIP changed: IPv4 10.64.5.7, IPv6 fe80::82a9:97ff:fe2d:3990%utun12 default 12:17:50.258420+0200 nesessionmanager Changing primary physical interface: en0 => utun12

Yes, this is macOS