а вот я даже не пробовал использовать wide-dhcpv6

слишком уж не стандартное оно (wide-dhcpv6).. тем более если справляется  DHCPCD -- то зачем ставить не стандартный софт (?)..

dhclient -- да -- вроде бы не умеет IPv6 PD как пишут всякие там Wiki :-) -- это о нём новость?

пример скрипта для владельцев DHCPCD:

------------------------------------------------------------
файл etc/ppp/ipv6-up.d/99-my-internet.sh :

#!/usr/bin/bash
 
if [ -z "$IFNAME" ] || [ "$LINKNAME" != "my_provider_linkname_blahblahblah" ]
then
  true
  exit
fi
 
lan_iface="br-lan"
modprobe ip6_tables
modprobe ip6table_filter
ip6tables -P FORWARD DROP && sysctl net.ipv6.conf.default.forwarding=1 && sysctl net.ipv6.conf.all.forwarding=1
if ! ip6tables -t mangle -C FORWARD -i "$lan_iface" -o "$IFNAME" -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
then
  ip6tables -t mangle -A FORWARD -i "$lan_iface" -o "$IFNAME" -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
fi
if ! ip6tables -C FORWARD -i "$lan_iface" -o "$IFNAME" -j ACCEPT
then
  ip6tables -A FORWARD -i "$lan_iface" -o "$IFNAME" -j ACCEPT
fi
if ! ip6tables -C FORWARD -i "$IFNAME" -o "$lan_iface" -j ACCEPT
then
  ip6tables -A FORWARD -i "$IFNAME" -o "$lan_iface" -j ACCEPT
fi
cat >/var/run/custom-internet-dhcpcd.conf <<__END__
  ipv6only
  noipv6rs
ia_pd 1
__END__
cat >/var/run/custom-internet-dhcpcd-hooks <<__END__
#!/usr/bin/bash
if [ "\$reason" == "REBIND6" ] && [ "\$new_dhcp6_prefix" == "\$old_dhcp6_prefix" ]
then
  true  
  exit
fi
if [ "\$reason" == "BOUND6" ] || [ "\$reason" == "REBIND6" ] || [ "\$reason" == "STOP6" ]
then
  if [ -f /var/run/custom-internet-radvd.conf ]
  then
    rm /var/run/custom-internet-radvd.conf
  fi
  if [ -f /var/run/custom-internet-radvd.pid ]
  then
    radvd_pid="\$(cat /var/run/custom-internet-radvd.pid)"
    rm /var/run/custom-internet-radvd.pid
    kill "\$radvd_pid"
  fi
  if [ -f /var/run/custom-internet-dhcpcd-addr.data ]
  then
    ip -6 addr del "\$(cat /var/run/custom-internet-dhcpcd-addr.data)" dev $lan_iface
    rm /var/run/custom-internet-dhcpcd-addr.data
  fi
fi
if [ "\$reason" == "BOUND6" ] || [ "\$reason" == "REBIND6" ]
then
  new_dhcp6_addr="\${new_dhcp6_prefix/%::\\/64/::1/64}"
  
  if [ "\$new_dhcp6_addr" != "\$new_dhcp6_prefix" ]
  then
    echo "\$new_dhcp6_addr" >/var/run/custom-internet-dhcpcd-addr.data
    ip -6 addr add "\$new_dhcp6_addr" dev $lan_iface
    
    cat >/var/run/custom-internet-radvd.conf <<__INNER_END__
interface $lan_iface {
  AdvSendAdvert on;
  
  MaxRtrAdvInterval 10;     # hack for broken mobile wifi-devices (normal default is 600)
  AdvDefaultLifetime 1800;  # hack for broken mobile wifi-devices (normal default is MaxRtrAdvInterval*3)
                            # see http://code.google.com/p/android/issues/detail?id=32662
  
  prefix \$new_dhcp6_prefix
  {
  };
  
  RDNSS 2001:4860:4860::8888 2001:4860:4860::8844
  {
  };
};
__INNER_END__
    
    radvd -C /var/run/custom-internet-radvd.conf -p /var/run/custom-internet-radvd.pid
  fi
fi
true
__END__
chmod +x /var/run/custom-internet-dhcpcd-hooks
dhcpcd -B -f /var/run/custom-internet-dhcpcd.conf -c /var/run/custom-internet-dhcpcd-hooks "$IFNAME" &
true

------------------------------------------------------------
файл /etc/ppp/ipv6-down.d/99-my-internet.sh :

#!/usr/bin/bash
if [ -z "$IFNAME" ] || [ "$LINKNAME" != "my_provider_linkname_blahblahblah" ]
then
  true
  exit
fi
lan_iface="br-lan"
modprobe ip6_tables
modprobe ip6table_filter
ip6tables -t mangle -D FORWARD -i "$lan_iface" -o "$IFNAME" -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
ip6tables -D FORWARD -i "$lan_iface" -o "$IFNAME" -j ACCEPT
ip6tables -D FORWARD -i "$IFNAME" -o "$lan_iface" -j ACCEPT
dhcpcd -x "$IFNAME"
rm /var/run/custom-internet-dhcpcd.conf
rm /var/run/custom-internet-dhcpcd-hooks
rm /var/lib/dhcpcd/dhcpcd-"$IFNAME".lease6
true