host-only network: IPv6 Routing does not work as expected

[3lding]

Hey 👋

I've already viewed all current issues and did not find any which applies to my problem.

I'm currently trying to reach my VMs from my host-only network from my host with the correct route information which was automatically applied by UTM itself. I've also checked the interface configuration made by UTM and that looked fine as well. The neighbor discovery table is working as expected and is displaying my neighbor machines. But no matter what I've tried no ping or SSH connection to my machines is working.

Here are my outputs:

• IP addresses of one of my machines (the bold one was my target address of my host-only network):

$ utmctl ip-address 5DFE8D01-71FF-4AE0-9842-E06217331F23
192.168.64.4
172.16.0.3
10.42.2.0
10.42.2.1
fd94:15bf:6918:7a8b:342e:1aff:fe2e:d0d2
fe80::342e:1aff:fe2e:d0d2
**fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9**
fe80::f031:9dff:fedf:5cb9
fe80::4a:41ff:fe04:9544
fe80::1c81:faff:fe92:3950
fe80::d02e:4cff:fea8:6569

• ifconfig of the UTM bridge:

bridge101: flags=8a63<UP,BROADCAST,SMART,RUNNING,ALLMULTI,SIMPLEX,MULTICAST> mtu 1500
	options=3<RXCSUM,TXCSUM>
	ether ae:07:75:c3:78:65
	inet 172.16.0.1 netmask 0xffffff00 broadcast 172.16.0.255
	inet6 fe80::ac07:75ff:fec3:7865%bridge101 prefixlen 64 scopeid 0x1a
	**inet6 fd4b:8268:b2d1:bdba:cdd:7c88:d7db:f905 prefixlen 64 autoconf secured**
	Configuration:
		id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
		maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
		root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
		ipfilter disabled flags 0x0
	member: vmenet1 flags=3<LEARNING,DISCOVER>
	        ifmaxaddr 0 port 25 priority 0 path cost 0
	member: vmenet5 flags=3<LEARNING,DISCOVER>
	        ifmaxaddr 0 port 30 priority 0 path cost 0
	member: vmenet3 flags=3<LEARNING,DISCOVER>
	        ifmaxaddr 0 port 28 priority 0 path cost 0
	Address cache:
		f2:31:9d:df:5c:b9 Vlan1 vmenet3 1199 flags=0<>
		92:1e:e7:88:61:2e Vlan1 vmenet5 1200 flags=0<>
		5a:f6:9b:ed:ff:3c Vlan1 vmenet1 1200 flags=0<>
	nd6 options=201<PERFORMNUD,DAD>
	media: autoselect
	status: active

• IPv6 route table

fd4b:8268:b2d1:bdba::/64                link#26                                 UC              bridge101
fd4b:8268:b2d1:bdba::                   link#26                                 UHLWI           bridge101
fd4b:8268:b2d1:bdba::1                  link#26                                 UHLWI           bridge101
fd4b:8268:b2d1:bdba::4                  link#26                                 UHLWI           bridge101
fd4b:8268:b2d1:bdba:cdd:7c88:d7db:f905  ae.7.75.c3.78.65                        UHL                   lo0
fd4b:8268:b2d1:bdba:901e:e7ff:fe88:612e link#26                                 UHLWI           bridge101
fd4b:8268:b2d1:bdba:c4bc:22ff:fe4d:6581 c6.bc.22.4d.65.81                       UHLWI           bridge101
fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9 link#26                                 UHLWI           bridge101

• Neighbor Discovery Protocol:

fd4b:8268:b2d1:bdba:cdd:7c88:d7db:f905  ae:7:75:c3:78:65  bridge101 permanent R
fd4b:8268:b2d1:bdba:c4bc:22ff:fe4d:6581 c6:bc:22:4d:65:81 bridge101 11h59m13s S
fe80::58f6:9bff:feed:ff3c%bridge101     5a:f6:9b:ed:ff:3c bridge101 7h42m10s  S
fe80::901e:e7ff:fe88:612e%bridge101     92:1e:e7:88:61:2e bridge101 17h20m29s S
fe80::ac07:75ff:fec3:7865%bridge101     ae:7:75:c3:78:65  bridge101 permanent R  R
fe80::c4bc:22ff:fe4d:6581%bridge101     c6:bc:22:4d:65:81 bridge101 11h59m18s S
fe80::f031:9dff:fedf:5cb9%bridge101     f2:31:9d:df:5c:b9 bridge101 8h25m24s  S

• Ping link-local:

PING6(56=40+8+8 bytes) fe80::ac07:75ff:fec3:7865%bridge101 --> fe80::f031:9dff:fedf:5cb9%bridge101
ping6: sendmsg: No route to host
ping6: wrote fe80::f031:9dff:fedf:5cb9%bridge101 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fe80::f031:9dff:fedf:5cb9%bridge101 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fe80::f031:9dff:fedf:5cb9%bridge101 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fe80::f031:9dff:fedf:5cb9%bridge101 16 chars, ret=-1
--- fe80::f031:9dff:fedf:5cb9%bridge101 ping6 statistics ---
4 packets transmitted, 0 packets received, 100.0% packet loss

• Ping ULA:

PING6(56=40+8+8 bytes) fd4b:8268:b2d1:bdba:cdd:7c88:d7db:f905 --> fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9
ping6: sendmsg: No route to host
ping6: wrote fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9 16 chars, ret=-1
ping6: sendmsg: No route to host
ping6: wrote fd4b:8268:b2d1:bdba:f031:9dff:fedf:5cb9 16 chars, ret=-1

The funny thing - my vm is able to reach the host without any issues or special config:

Why is that so? Am I misinterpreting the design of a host-only network in UTM or overlooking something? Is this issue maybe not related to UTM but to macOS instead?

Configuration

• UTM Version: 4.7.5 (116)
• macOS Version: 26.2 (25C56)
• Mac Chip: M4