Understanding BT Smart hub 2 and my IPv6 addresses

As part of some work trying to get IPV6 to connect, I spent too much time understanding the various IP addresses in my configuration. By accident I found the magic incantation that allowed my z/OS on zD&T to talk to the outside internet.

Under the covers the BT Smart Hub 2 has a router called Arcadyan_ae. You will see this in a Wireshark trace. My IP addresses start with 2A00:23C5:… This belongs to BT. I’ve replace the address with BT:a:b:c .

This Blog post is referred to in Connecting a zPDT z/OS to the internet to using IPV6 and explains the various IPv6 addresses

My BT Smart Hub 2

You can display configuration information from the Smart Hub by using a web browser and address 192.168.1.254.

The Smart Hub IPv6 information.

This is password protected in the Advanced Settings.

There is a section IPv6 WAN details, this was not of interest.
The section IPv6 LAN details has

• Link local address: fe80::6a2:22ff:feae:2871/64

For my Linux Server

the information in the BT Hub was

• Device Name: Colins Server
• Device Icon: None
• Connection Status: Connected
• IP address: 192.168.1.25 I think this came from my z/OS system running on the server!
• MAC address: CC:64…:C5
• Connection Type: Wireless
• Address assignment: DHCP
• Always using this address: YES
• IPv6 Addressing:
  • GUA(Temporary): BT:a:b:c:2f3d:acdb Assigned by device
  • GUA (Permanent) :BT:a:b:c:7cd3:8993 Assigned by device
  • Link local address:fe80::…:8c80:37a4 Assigned by device
    

The IPv4 address (192.168.1.25) varied from day to day. During the day, it tended to be the same value. I could not find what caused it to change.

On my Linux server

The command ip addr gave

wlxcc641aee92c5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 ...
    link/ether cc:64:1a...:c5 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.25/24 brd 192.168.1.255 scope global dynamic noprefixroute wlxcc641aee92c5
       ...
    inet6 BT:a:b:c:7cd3:8993/64 scope global temporary dynamic 
       ...
    inet6 BT:a:b:c:2f3d:acdb/64 scope global dynamic mngtmpaddr noprefixroute 
       ...
    inet6 fe80::...:8c80:37a4 /64 scope link noprefixroute 
       ...

Where

• the GUA (Permanent) is the same as the address with scope global temporary dynamic
• the GUA (Temporary) is the same as the address with scope global dynamic mngtmpaddr noprefixroute
• the Link local address is the same as the address with scope link noprefixroute. The word link gives it away.

Configure z/OS TCPIP Interface

I configured the interface like

 INTERFACE WIRE6 
     DEFINE IPAQENET6 
     CHPIDTYPE OSD 
     IPADDR  2001:DB7:8::1 
     PORTNAME PORTB 

 INTERFACE WIRE6 
     ADDADDR BT:a:b:c::1
 START WIRE6 

This interface has addresses 2001:DB7:8::1 and BT:a:b:c::1. The value BT:a:b:c is the same as the high part of the Smart Hub’s Global unicast address: BT:a:b:c:6a2:22ff:feae:2871. The value BT:a:b:cuniquely identifies my Smart Hub2 router. Any device in the subarea below this router, must have the same top part. I picked address 1(::1).

The TSO NETSTAT HOME command gave

IntfName:   WIRE6 
  Address:  2001:db7:8::1 
    Type:   Global 
    Flags: 
  Address:  BT:a:b:c::1
    Type:   Global 
    Flags: 
  Address:  fe80::cc64:1a02:ee:92c5 
    Type:   Link_Local 
    Flags:  Autoconfigured 

With the two addresses I configured, and an internally generated link local address.

The TSO NETSTAT ND gave

Query Neighbor cache for fe80::6a2:22ff:feae:2871 
  IntfName: WIRE6              IntfType: IPAQENET6 
  LinkLayerAddr: 04A222AE2871  State: Reachable 
  Type: Router                 AdvDfltRtr: Yes 

Where fe80::6a2:22ff:feae:2871 is the IPv6 address of the router – see the top of this blog post.

Routing

My routing table did not need an IPv6 default entry because TCPIP can deduce it.

BEGINRoutes 
;     Destination   SubnetMask    FirstHop       LinkName  Size 
ROUTE 10.0.0.0    255.0.0.0           =          TAP0 MTU 1492 

ROUTE DEFAULT                    192.168.1.254   WIRE  MTU 1492
 
ENDRoutes 

TSO NETSTAT ROUTE gave me

IPv6 Destinations 
DestIP:   Default 
  Gw:     fe80::6a2:22ff:feae:2871 
  Intf:   WIRE6             Refcnt:  0000000000 
  Flgs:   UGD               MTU:     1492 
               MTU:     65535 
DestIP:   2001:db7:8::1/128 
  Gw:     :: 
  Intf:   WIRE6             Refcnt:  0000000000 
  Flgs:   UH                MTU:     1492 
DestIP:   BT:a:b:c::/64 
  Gw:     :: 
  Intf:   WIRE6             Refcnt:  0000000000 
  Flgs:   UD                MTU:     1492 
DestIP:  aBT:a:b:c::1/128 
  Gw:     :: 
  Intf:   WIRE6             Refcnt:  0000000000 
  Flgs:   UH                MTU:     1492 
DestIP:   fe80::cc64:1a02:ee:92c5/128 
  Gw:     :: 
  Intf:   WIRE6             Refcnt:  0000000000 
  Flgs:   UH                MTU:     1492 

DestIP:   ::1/128 
  Gw:     :: 
  Intf:   LOOPBACK6         Refcnt:  0000000000 
  Flgs:   UH 

Where the Default is the value in TSO NETSTAT ND, and is the IP address of the Smart Hub2 – see above.

PING

When I did a TCPIP PING to an IPv6 address,

• the source was BT:a:b:c::1
• the destination was 2a04:abcd::81

The first ping

The first ping had the following flows

1. Ping the address
2. The router does not know where the originating IP address came from, so it asks all devices connected to it – Does anyone have this IP address?
3. Z/OS replies to the router saying “I’ve got that address”
4. The router says “Here is some data for you

In more technically

1. From BT:a:b:c::1 ping destination 2a04:abcd::81
2. (From the router) Source Address: fe80::6a2:22ff:feae:2871 to all devices on the subnet (ff02::1:ff00:1) Neighbour Solicitation for BT:a:b:c::1
3. (From z/OS) Source Address: fe80::cc64:1a03:ee:92c5 to the router (fe80::6a2:22ff:feae:2871) Neighbour Advertisement BT:a:b:c::1
4. From 2a04:abcd::81 to BT:a:b:c :1 ping response

The second ping request went directly to z/OS because the BT Hub had learned where the IP address was.