Interface-ID missing in A10-NSP DHCPv6 Relay Reply - RFC6221

[SoerenBusse]

Describe the bug
According to RFC6221, the DHCPv6 Relay Reply packet must contain the interface-id received by in relay-forw. https://datatracker.ietf.org/doc/html/rfc6221#section-5.3.2

However, when BNG Blaster acts as an A10-NSP with DHCPv6 NA/PD enabled, the relay-reply doesn't contain the interface-id. This triggers undefined behavior in access-nodes, like dropping the response or not decapsulating the relay-reply.

I think it's related to this code here, where the interface-id is not copied to the outer dhcpv6.
https://github.com/rtbrick/bngblaster/blob/main/code/bngblaster/src/bbl_a10nsp.c#L485

To Reproduce
Version (bngblaster -v):

Version: DEV
Compiler: GNU (11.4.0)
GIT:
  REF: dev
  SHA: b8e26058bb6a41af4c5716883be5701aede17646

Add any other context about the problem here.ON configuration:
Demo config from here, where BNGBlaster also acts an LDRA: #164

[GIC-de]

Please verify the following fix:
https://github.com/rtbrick/bngblaster/pull/301/files

[SoerenBusse]

Hi @GIC-de,

sadly this doesn't fix the issue:

Version: DEV
Compiler: GNU (14.2.1)
GIT:
  REF: HEAD
  SHA: fd3569650c300bb49228ea06abbf090e3581c1ef
IO Modes: packet_mmap_raw (default), packet_mmap, raw

I see the Interface-ID inside the relay-forw, but it's still missing in the relay-reply:

I've used the following config for testing:

{
    "interfaces": {
        "a10nsp": [
            {
                "__comment__": "DHCP Server",
                "interface": "veth1.1"
            }
        ],
        "access": [
            {
                "__comment__": "DHCP Client",
                "interface": "veth1.2",
                "type": "ipoe",
                "outer-vlan-min": 1,
                "outer-vlan-max": 4000,
                "inner-vlan": 7,
                "stream-group-id": 1
            }
        ]
    },
    "access-line": {
        "agent-remote-id": "DEU.RTBRICK.{session-global}",
        "agent-circuit-id": "0.0.0.0/0.0.0.0 eth 0:{session-global}",
        "rate-up": 1024,
        "rate-down": 50000
    },
    "dhcp": {
        "enable": true,
        "broadcast": false
    },
    "dhcpv6": {
        "enable": true,
        "ldra": true
    },
    "session-traffic": {
        "ipv4-pps": 1,
        "ipv6-pps": 1,
        "ipv6pd-pps": 1
    }
}

[SoerenBusse]

When stepping through with the debugger, the interface-id is correctly set on dhcpv6_outer, however it isn't sent on the wire.

Edit 1:
I think the root cause is here:
https://github.com/rtbrick/bngblaster/blob/main/code/bngblaster/src/bbl_protocols.c#L273
interface-id is not part of access-lines but rather an option in the DHCP Relay Root, therefore it needs to be encoded here.

Edit 2:
Adding the following to encode_dhcpv6 fixed the issue:

diff --git a/code/bngblaster/src/bbl_protocols.c b/code/bngblaster/src/bbl_protocols.c
index d8a5c01..d57c677 100644
--- a/code/bngblaster/src/bbl_protocols.c
+++ b/code/bngblaster/src/bbl_protocols.c
@@ -289,6 +289,15 @@ encode_dhcpv6(uint8_t *buf, uint16_t *len,
             memset(buf, 0x0, sizeof(ipv6addr_t));
         }
         BUMP_WRITE_BUFFER(buf, len, sizeof(ipv6addr_t));
+        /* Interface-ID */
+        if (dhcpv6->interface_id) {
+            *(uint16_t*)buf = htobe16(DHCPV6_OPTION_INTERFACE_ID);
+            BUMP_WRITE_BUFFER(buf, len, sizeof(uint16_t));
+            *(uint16_t*)buf = htobe16(dhcpv6->interface_id_len);
+            BUMP_WRITE_BUFFER(buf, len, sizeof(uint16_t));
+            memcpy(buf, dhcpv6->interface_id, dhcpv6->interface_id_len);
+            BUMP_WRITE_BUFFER(buf, len, dhcpv6->interface_id_len);
+        }
     } else {
         /* Type/Transaction ID */
         *(uint32_t*)buf = htobe32(dhcpv6->xid);

[GIC-de]

Please verify the solution here:
#302