So is there just a configuration I am missing that would allow the Juniper to forward/relay the DHCPv6 from the link-local address assigned to reth0.1 or is it just an absolute must for reth0.1 to have a GUA address assigned in order for DHCPv6 Relay to work?
DHCP relays should be able to use just about any src address for the given relayed packet towards the DHCP server, just as long as...
- the DHCPv6 server can unicast its RELAY-REPLY back to the src address the relay had used for the RELAY-FORW message.
- (as a corollary to the above) the DHCPv6 relay, for the RELAY-FORW messages, uses an ipv6 src address that is routeable/reachable from the given DHCPv6 server. A link local address not on a common segment with the DHCPv6 server (relay destination) can therefore not be used as src address for relay messages.
- the DHCPv6 relay adds enough information to the RELAY-FORW message to allow the DHCPv6 server to match the client's messages (typically a SOLICIT or REQUEST) to a configured DHCPv6 scope.
It seems that for the latter, Junos has some options you can set explicitely:
See section "Inserting DHCPv6 Interface-ID Option (Option 18) In DHCPv6 Packets" in https://www.juniper.net/documentation/us/en/software/junos/dhcp/topics/topic-map/dhcpv6-relay-agent.html, from where:
You can configure DHCPv6 relay agent to insert the DHCPv6 Interface-ID
(option 18) in the packets that the relay sends to a DHCPv6 server.
You can configure the option 18 support at either the DHCPv6 global or
group level.
When you configure option 18 support, you can optionally include the
following additional information:
Prefix - Specify the prefix option to add a prefix to the interface
identifier. The prefix can be any combination of hostname, logical
system name, and routing instance name.
Interface description - Specify the use-interface-description option to
include the textual interface description instead of the interface
identifier. You can include either the device interface description or
the logical interface description.
Option 82 Agent - Circuit ID suboption (suboption 1)—Specify the
use-option-82 option to include the DHCPv4 Option 82 Agent Circuit ID
suboption (suboption 1). This configuration is useful in a dual-stack
environment, which has both DHCPv4 and DHCPv6 subscribers that reside
over the same underlying logical interface. The router checks for the
option 82 suboption 1 value and inserts it into the outgoing packets.
If no DHCPv4 binding exists or if the binding does not have an option
82 suboption 1 value, the router sends the packets without adding an
option 18.
So, on the SRX acting as DHCPv6 relay, you might want to check...
- if you can force one of these options to be added to the RELAY-FORW message, and if setting them can be made independent of the given client facing interface's configuration (which is what you don't want to have in your case: an IPv6 GUA address on the client facing interface).
- if you can force-set the src address for RELAY-FORW messages from the DHCPv6 relay to the DHCPv6 server, to avoid having to use the client facing interface's ipv6 address for that purpose.
A bit of speculation: As long as the relay has a GUA on its client facing interface, it propably adds that address to the relayed request automatically, and sends the RELAY-FORW messages with that address as src. The DHCPv6 server then knows which address scope to match and where to send the RELAY-REPLY to.
If the relay does not have a GUA on the client facing interface, there must be another way to give a hint to the DHCPv6 server for proper address scope selection.
