I've got a /64 IPv6 subnet and I'd like to use additional v6 addresses now. => /etc/network/interfaces:

iface ens18 inet6 static
    address asdf:asdf:asdf:asdf::1
    netmask 64
    gateway fe80::1
    accept_ra 0
    autoconf 0
    privext 0
    dns-nameservers ::1 xxxx:xxxx::1:53 xxxx:xxxx:2:53

iface ens18 inet6 static
    address asdf:asdf:asdf:asdf::2
    netmask 64

iface ens18 inet6 static
    address asdf:asdf:asdf:asdf::3
    netmask 64

However, as soon as I add asdf:asdf:asdf:asdf::2 and/or asdf:asdf:asdf:asdf::3, I cannot ping any other IPv6 address.
When I remove these blocks and reboot, I can use IPv6 again.

# ip addr show ens18
2: ens18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether [...] brd ff:ff:ff:ff:ff:ff
    inet [...]/24 brd scope global ens18
    valid_lft forever preferred_lft forever
    inet6 asdf:asdf:asdf:asdf::1/64 scope global 
    valid_lft forever preferred_lft forever
    inet6 fe80::[...]/64 scope link 
    valid_lft forever preferred_lft forever
# ip -6 route
asdf:asdf:asdf:asdf::/64 dev ens18 proto kernel metric 256  pref medium
fe80::/64 dev ens18 proto kernel metric 256  pref medium
default via fe80::1 dev ens18 metric 1024  pref medium

What is the problem here?

  • Please show the output of ip a and ip -6 r while the system is having the problem. It's not very useful to see them while the system is working properly. – Michael Hampton Oct 26 '18 at 2:39
  • The problem were ICMPv6 messages. I did not know that these are very different to normal ICMP packages. – Hoeze Oct 26 '18 at 11:16

I found the problem: ICMPv6 messages! -.-

  1. You have to accept -p ipv6-icmp instead of -p icmp
  2. Do not use stateful allow for ICMP, e.g.:
    ip6tables -A OUTPUT -m state --state NEW,RELATED,ESTABLISHED -j ACCEPT
    This will keep ICMPv6 packages from being caught by that rule

I had a pretty hard time to debug this, since it was more or less working randomly...

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