- Setting up ping within one machine is trivial
- Setting up ping between two machines is relatively easy
- Setting up ping with 3 machines can be hard to get right and to get working.
Most documentation describes how to set up PING between two machines, and does not mention anything more complex.
Ancient history (well 1980’s)
To understand modern TCPIP networks, it helps to know about the origins of Ethernet.
Originally with Ethernet there was a bus; a very long cable. You plugged your computer into this bus. Each computer on the bus had a unique 48 bit MAC address. You could send a request to another computer on this bus using their Ethernet address. You could send a request to ALL computers on this bus. These days this might be, send to all computers:”does anyone have this IP address…?”
An Ethernet bus is connected to an Ethernet router which can route packets to other routers.
Ethernet has evolved and instead of a long bus shared by all users, you have a switch device, and you plug the Ethernet cable from your computer to the switch. Conceptually it is the same, but with the switch you get much better performance and usability. You still have routers to get between different Ethernet environments. The request send to all computers:”does anyone have this IP address…?” goes to the switch, and the switch sends it to all plugged in computers.
When you start TCPIP, it configures the Ethernet hardware adapter with the addresses is should listen for.
Other terminology and concepts
- A router is a networking device which forwards packets between different networks. Packets of data get sent from the originator through zero or more routers to the final destination.
- A gateway has several meanings
- It can connect different network types, for example act as a protocol translator, it can have a built-in firewall
- It can route IP traffic between different IP subnets
- It can loosely mean a router
Usually a router or gateway is a dedicated device or hardware.
You can have a computer act as a router or gateway; it takes data from one interface and routes it to a different interface. The computer can pass the data through a fire wall, or transform it, for example converting internal IP addresses to external IP addresses (NAT translation).
At a concept level, a computer’s network adapter is configured only to listen for packets which match the IP addresses of the interface, and ignore the rest.
If you want a computer to act as a router or gateway, you need to configure the network adapter to listen to all traffic, and to process the traffic. This is important when routing traffic from computer A through computer B to computer C.
A simple one hop ping, first time
I have set up my laptop to talk to a server over Ethernet.
I have configured my laptop using
sudo ip -6 addr add 2001:7::/64 dev enp0s31f6
This says for any IP V6 address starting with 2001:0007:0000:0000 then try sending it down the Ethernet cable with interface name enp0s31f6.
Ping a non existing address
If I try to ping an address say 2001:7::99 a packet is sent to all computers on the Ethernet bus
from myipAddress to everyone on the Ethernet bus, does anyone have IP address 2001:7::99?
There is no reply because no computer on the bus, has the address.
Ping the adjacent box
On the server, the IP address of the Ethernet cable is 2001:7::2.
If I ping this address, there are the following flows
From myipAddress (and myMAC) to everyone on the Ethernet bus, does anyone have IP address 2001:7::2?
From 2001:7::2 to myipAddress (MAC), yes I have that IP address. My Ethernet MAC address is …
My laptop puts the remote IP address and its MAC address into its neighbour cache, then issues the ping request.
The ping request looks in the neighbour cache find the IP address and MAC address and issues.
From myipAddress to 2001:7::2 at macAddress .. ping request.
The second and later times I issue a ping
The ping request looks in the neighbour cache find the IP address and MAC address and issues the ping
From myipAddress to 2001:7::2 at macAddress .. ping request.
So while the neighbour cache still has the IP of the target, and it’s MAC address, the ping can be routed to the next hop.
Setting up a multi hop ping
I have my laptop connected via a physical Ethernet cable to my server. The server is connected to z/OS via a virtual Ethernet connection. The IP address of the z/OS end of the virtual Ethernet cable is 2001:7::4.
The ping 2001:7::4 request on my laptop, does not work (as we saw above), because TCP asked everyone on the Ethernet bus if it has address 2001:7::4 – and no machine replies.
You need to define the link to the server machine as a gateway or router-like device which can handle IP addresses which are not on the Ethernet bus. You define it like
sudo ip -6 route add 2001:7::/64 via 2001:7::2
This says for any traffic with IP address 2001:0007:0000:0000:* send it via address 2001:7::2.
This requires 2001:7::2 to be known about, and so needs the following to be configured first
sudo ip -6 route add 2001:7::2 dev enp0s31f6
for TCPIP to know where to send the traffic to.
The route add 2001:7::2 dev enp0s31f6 command sends a broadcast on the Ethernet bus asking – does anyone have 2001:7::2. My server replies saying yes I have it. This is the same as for the ping above.
In summary
To send traffic on the same Ethernet bus you use
sudo ip -6 route add 2001:7::2 dev enp0s31f6
To route it via a router, switch, or computer acting as a router or switch you need.
sudo ip -6 route add 2001:7::/64 via 2001:7::2 or
sudo ip -6 route add 2001:7::/64 via 2001:7::2 <dev enp0s31f6>
The computer acting as a router or switch may need additional configuration. For example to allow it to route traffic from one Ethernet bus to another Ethernet bus you need to enable packet forwarding. On Linux to enable forwarding for all interfaces use
sysctl -w net.ipv6.conf.all.forwarding=1
On z/OS you use a static route such as
ROUTE 2001:7::/64 2001:7::3 JFPORTCP6
Where 2001:7::/64 is the range of addresses 2001:7::3 is (one of) the addresses of the interface at the remote end of the cabl, and JFPORTCP6 is the interface name. This is similar to the Linux route statement above.
You might need to set up the firewall. On my server I needed
sudo ufw route allow in on eno1 out on tap2
What IP Address does the sender have?
This is where is starts to get more complex.
Every network connection has at least one IP address.
With IP V6
- Each interface gets an “internal” IP address such as fe80::9b07:33a1:aa30:e272
- You can allocate an external address using the Linux command sudo ip -6 addr add 2001:7::1 dev enp0s31f6
- If the interface has one external IP address configured, then this will be used.
- If the interface has more than one external address configured then the first in the list may be used (not always).
- If the interface does not have an external IP address, then TCPIP will find one from another interface, or allocate one for the duration of the request, such as 2a00:23c5:978f:9999:210a:1e9b:94a4:c8e. This address comes from my wireless connection
When my laptop is started, only the wireless connection has an IPV6 address. A ping request to 2001:7::4 had the origin IP address of 2a00:23c5:978f:9999:210a:1e9b:94a4:c8e which is the first address in the list for the wireless connection.
You can tell ping which address to use, for example
ping -I 2a00:23c5:978f:9999:cff1:dc13:4fc6:f21b 2007:1::4 (this fails because the server does now know to send the response to the requester)
I defined a new address for the interface using
sudo ip -6 addr rep 2002:7::1 dev enp0s31f6
I could issue ping -I 2002:7::1 2001:7::4, but this failed to get a response, because the back-end and intermediate nodes, did not know how to get the response back to 2002:7::1
Does this sender address matter?
Yes, because the remote end needs to have a definition to be able to send the response back.
I had my laptop connected to a Linux server over Ethernet, which in turn was connected to z/OS over a virtual Ethernet.
On z/OS I could see the ping request arrive, but it could not send the response back because it did not know how to get to 2a00:23c5:978f:9999….
I configured the laptop end of the Ethernet to give it an IP address 2001:7::1
sudo ip -6 addr add 2001:7::1/64 dev enp0s31f6
I configured the server to laptop to have an IP address of 2001:7::2
sudo ip -6 route add 2001:7::1/128 dev eno1
I configured the server to z/OS with an IP address and a route
sudo ip -6 addr add 2001:7::3/128 dev tap2
sudo ip -6 route add 2001:7::4/128 dev tap2
Now when I did the ping, the originator was 2001:7::1.
I configured the z/OS interface to send stuff back
INTERFACE JFPORTCP6
DEFINE IPAQENET6
CHPIDTYPE OSD
PORTNAME PORTC
INTERFACE JFPORTCP6
ADDADDR 2001:DB8:8::9
INTERFACE JFPORTCP6
ADDADDR 2001:DB8::9
INTERFACE JFPORTCP6
ADDADDR 2001:7::4
START JFPORTCP6
and the routing
BEGINRoutes
; Destination Gateway LinkName Size
ROUTE 2001:7::/64 2001:7::3 JFPORTCP6 MTU 5000
...
ENDRoutes
This says that all traffic with destination address 2001:0007:0000:000…. send to interface JFPORTCP6. This interface is connected to a gateway with the address of the remote end of the Ethernet (so on the server) of 2001:7::3.
The server machine needs to act as a router between the different Ethernet buses. You can display and configure this using
sysctl -a |grep forwarding
sudo sysctl -w net.ipv6.conf.all.forwarding=1
Packet forwarding on z/OS
By default the z/OS interface only listens for packets with one of the IP addresses of the interface. For z/OS to be able to be a router; accept all packets, and route them to other interfaces you need:
- IPCONFIG DATAGRAMFWD in the TCP/IP Profile
- PRIROUTER on the Interface definition . This configures the Ethernet adapter (hardware) so If a datagram is received at this device for an unknown IP address, the datagram is routed to this TCP/IP instance.
But normally if you are running z/OS you use a cheaper, physical router, rather than use the z/OS to do your routing. It might only be people like me who run z/OS on their laptop who want to try routing through z/OS.
ColinPaice 