pppoe: dhcpv6-pd: T421: initial support

The following configuration will assign a /64 prefix out of a /56 delegation
to eth0. The IPv6 address assigned to eth0 will be <prefix>::ffff/64.
If you do not know the prefix size delegated to you, start with sla-len 0.

  pppoe pppoe0 {
      authentication {
          password vyos
          user vyos
      }
      description sadfas
      dhcpv6-options {
          delegate eth0 {
              interface-id 65535
              sla-id 0
              sla-len 8
          }
      }
      ipv6 {
          address {
              autoconf
          }
          enable
      }
      source-interface eth1
  }

  vyos@vyos:~$ show interfaces
  Codes: S - State, L - Link, u - Up, D - Down, A - Admin Down
  Interface        IP Address                        S/L  Description
  ---------        ----------                        ---  -----------
  eth0             2001:db8:8003:400::ffff/64        u/u

1 files changed, 33 insertions, 2 deletions

diff --git a/data/templates/dhcp-client/ipv6.tmpl b/data/templates/dhcp-client/ipv6.tmpl
index 8485c9d78..6cfe24d3e 100644
--- a/data/templates/dhcp-client/ipv6.tmpl
+++ b/data/templates/dhcp-client/ipv6.tmpl
@@ -1,10 +1,41 @@
 # generated by dhcp.py
+# man https://www.unix.com/man-page/debian/5/dhcp6c.conf/
+
 interface {{ ifname }} {
-	request domain-name-servers,domain-name;
+    request domain-name-servers;
+    request domain-name;
 {%  if dhcpv6_prm_only %}
     information-only;
 {%  endif %}
 {%  if not dhcpv6_temporary %}
-    send ia-na 0;
+    send ia-na 1; # non-temporary address
 {%  endif %}
+{%  if dhcpv6_pd %}
+    send ia-pd 2; # prefix delegation
+{%  endif %}
+};
+
+{%  if not dhcpv6_temporary %}
+id-assoc na 1 {
+    # Identity association NA
 };
+{%  endif %}
+
+{%  if dhcpv6_pd %}
+id-assoc pd 2 {
+{%  for intf in dhcpv6_pd %}
+    prefix-interface {{ intf.ifname }} {
+{%    if intf.sla_id %}
+        sla-id {{ intf.sla_id }};
+{%    endif %}
+{%    if intf.sla_len %}
+        sla-len {{ intf.sla_len }};
+{%    endif %}
+{%    if intf.if_id %}
+        ifid {{ intf.if_id  }};
+{%    endif %}
+    };
+{%  endfor %}
+};
+{%  endif %}
+