Enabling IPv6 SLAAC removes all IPv4 VM connectivity

[SimonVa]

Hi,

I've been trying to configure IPv6 on my Proxmox V8 host. Everytime I enable my IPv6 config, all of my virtual machines lose connectivity, despite having ipv6 disabled (with net.ipv6.conf.ens18.disable_ipv6 = 1).

My server provider provides an IPv6 SLAAC configuration which i've tried to enable with this interfaces file :

auto lo
iface lo inet loopback

iface enp1s0f0 inet manual

iface enp1s0f1 inet manual
        mtu 9000

auto vmbr0

# Public IPv4 network
iface vmbr0 inet static
        address X.X.X.X/24
        gateway X.X.X.X
        bridge-ports enp1s0f0
        bridge-stp off
        bridge-fd 0

# Public IPv6 network
iface vmbr0 inet6 auto
    autoconf 1
    accept_ra 2

# Private Network
auto vmbr1
iface vmbr1 inet dhcp
        bridge-ports enp1s0f1
        bridge-stp off
        bridge-fd 0
        mtu 9000

Here is my /etc/sysctl.conf file :

net.ipv4.conf.all.rp_filter=1
net.ipv4.icmp_echo_ignore_broadcasts=1
net.ipv4.conf.default.forwarding=1
net.ipv4.conf.default.proxy_arp = 0
net.ipv4.ip_forward=1
kernel.sysrq = 1
net.ipv4.conf.default.send_redirects = 1
net.ipv4.conf.all.send_redirects = 0

net.ipv6.conf.all.forwarding=1
net.ipv6.conf.vmbr0.accept_ra=2

When all of this is enabled, ping and ping6 work on Proxmox, and nothing works on the virtual machines. When I disable iface vmbr0 inet6 auto, ping works again on my virtual machines (after a reboot, systemctl restart networking is not enough in this case...)

Do you have an idea ?

Thanks,
Simon