Why is the guest OS not pinging from the host over IPv6?

Question

Ping

Good afternoon!

KVM virtualization environment (OS Centos 6.5) was raised on the Hetzner server.
Several VMs are running on a virtual machine on Centos 6.5.
The host machine has white IPv4.

Virtual machines use "gray" IPv4 behind NAT.
IPv6 must be used next to IPv4.

IPv6 is configured on both the host and guest systems. Inside the virtual network, guest machines ping each other, the host machine, and external network resources over IPv6. From the host machine or from external resources, ping6 goes inside the virtual network to a specific guest machine only after you ping the host machine from this guest or if there is other network activity from this guest system to the host or to external resources.

# virsh net-edit default

<network>
  <name>default</name>
  <uuid>e5e50b4a-8f28-4736-b7e1-daa3b5dbe22b</uuid>
  <forward mode='nat'/>
  <bridge name='virbr0' stp='on' delay='0' />
  <mac address='mac-адрес'/>
  <ip address='192.168.122.1' netmask='255.255.255.0'>
    <dhcp>
      <range start='192.168.122.3' end='192.168.122.254' />
    </dhcp>
  </ip>
  <ip family='ipv6' address='2a01:4f8:xxx:xxxx::2' prefix='64'>
  </ip>
</network>

sysctl.conf

### Hetzner Online AG installimage
# sysctl config
net.ipv4.ip_forward=1
net.ipv4.conf.all.rp_filter=1
net.ipv4.icmp_echo_ignore_broadcasts=1
# ipv6 settings (no autoconfiguration)
net.ipv6.conf.all.autoconf=0
net.ipv6.conf.all.accept_dad=0
net.ipv6.conf.all.accept_ra=0
net.ipv6.conf.all.accept_ra_defrtr=0
net.ipv6.conf.all.accept_ra_rtr_pref=0
net.ipv6.conf.all.accept_ra_pinfo=0
net.ipv6.conf.all.accept_source_route=0
net.ipv6.conf.all.accept_redirects=0
net.ipv6.conf.all.forwarding=1

# route -A inet6 -n

Kernel IPv6 routing table
Destination                                 Next Hop                                Flags Metric Ref    Use Iface
2a01:4f8:xxx:xxxx::1/128                    ::                                      U     1024   1        0 eth0    
2a01:4f8:xxx:xxxx::5/128                    2a01:4f8:xxx:xxxx::5                    UC    0      1        0 eth0    
2a01:4f8:xxx:xxxx::/64                      ::                                      U     256    0        0 eth0    
2a01:4f8:xxx:xxxx::/64                      ::                                      U     256    0        0 virbr0  
fe80::/64                                   ::                                      U     256    0        0 eth0    
fe80::/64                                   ::                                      U     256    0        0 virbr0  
fe80::/64                                   ::                                      U     256    0        0 vnet0   
fe80::/64                                   ::                                      U     256    0        0 vnet1   
fe80::/64                                   ::                                      U     256    0        0 vnet2   
fe80::/64                                   ::                                      U     256    0        0 vnet3   
::/0                                        fe80::1                                 UG    1      18590       0 eth0    
::/0                                        2a01:4f8:xxx:xxxx::1                    UG    1024   0        0 eth0    
::1/128                                     ::                                      U     0      13       1 lo      
2a01:4f8:xxx:xxxx::/128                     ::                                      U     0      0        1 lo      
2a01:4f8:xxx:xxxx::/128                     ::                                      U     0      0        1 lo      
2a01:4f8:xxx:xxxx::2/128                    ::                                      U     0      802       1 lo      
2a01:4f8:xxx:xxxx::2/128                    ::                                      U     0      774       1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::/128                                  ::                                      U     0      0        1 lo      
fe80::5054:ff:fed6:7ec1/128                 ::                                      U     0      1457       1 lo      
fe80::6e62:6dff:feb2:a454/128               ::                                      U     0      914       1 lo      
fe80::fc54:ff:feb1:3998/128                 ::                                      U     0      0        1 lo      
fe80::fc54:ff:fec7:87a7/128                 ::                                      U     0      0        1 lo      
fe80::fc54:ff:fed3:6720/128                 ::                                      U     0      0        1 lo      
fe80::fc54:ff:fedb:904a/128                 ::                                      U     0      0        1 lo      
ff00::/8                                    ::                                      U     256    0        0 eth0    
ff00::/8                                    ::                                      U     256    0        0 virbr0  
ff00::/8                                    ::                                      U     256    0        0 vnet0   
ff00::/8                                    ::                                      U     256    0        0 vnet1   
ff00::/8                                    ::                                      U     256    0        0 vnet2   
ff00::/8                                    ::                                      U     256    0        0 vnet3

# brctl show

bridge name	bridge id		STP enabled	interfaces
virbr0		8000.525400d67ec1	yes		virbr0-nic
              vnet0
              vnet1
              vnet2
              vnet3

# ip6tables -L -n -v

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 ACCEPT     udp      virbr0 *       ::/0                 ::/0                udp dpt:53 
    0     0 ACCEPT     tcp      virbr0 *       ::/0                 ::/0                tcp dpt:53 
  104 12504 ACCEPT     all      *      *       2a01:4f8:xxx:xxxx::/64  ::/0                
    0     0 ACCEPT     all      lo     *       ::/0                 ::/0                
  564 2701K ACCEPT     all      *      *       ::/0                 ::/0                state RELATED,ESTABLISHED 
 3560  252K ACCEPT     icmpv6    *      *       ::/0                 ::/0                
    0     0 REJECT     all      *      *       ::/0                 ::/0                reject-with icmp6-adm-prohibited 

Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
  497  141K ACCEPT     all      *      virbr0  ::/0                 2a01:4f8:xxx:xxxx::/64 
 2308  195K ACCEPT     all      virbr0 *       2a01:4f8:xxx:xxxx::/64  ::/0                
    0     0 ACCEPT     all      virbr0 virbr0  ::/0                 ::/0                
    0     0 REJECT     all      *      virbr0  ::/0                 ::/0                reject-with icmp6-port-unreachable 
    0     0 REJECT     all      virbr0 *       ::/0                 ::/0                reject-with icmp6-port-unreachable 
    0     0 ACCEPT     all      virbr0 *       2a01:4f8:xxx:xxxx::/64  ::/0                
 4096  335K REJECT     all      *      *       ::/0                 ::/0                reject-with icmp6-adm-prohibited 

Chain OUTPUT (policy ACCEPT 8765 packets, 875K bytes)
 pkts bytes target     prot opt in     out     source               destination

ifconfig doesn't fit...

Answer 1

The solution, as always, turned out to be ingeniously simple!
Must be added to the /etc/sysconfig/network file
IPV6FORWARDING=yes

Answer 2

https://debian.pro/1345
What is there about v4 - you can ignore