I was testing out tracing TCP/IP on z/OS and wondered why I was not seeing packets with a bad destination address in the trace. I’ve now found out WHY I cannot see them, it’s just a little extra complexity of TCP/IP.
I had configured my z/OS to have an IP address of 7.1.168.74, and I could trace the packet. When I defined a route to z/OS for 7.168.1.>75< it did not appear in my trace even as an undeliverable packet.
My configuration was
INTERFACE TAP1
DEFINE IPAQENET
CHPIDTYPE OSD
IPADDR 7.168.1.74
; PRIROUTER
PORTNAME PORTB
START TAP1
If you add the PRIROUTER option, my packet appeared in the trace. The documentation says
The primary router stack is the only stack to which OSA forwards packets when the destination IP address has not been previously registered with OSA. If no active TCP/IP instance using this device is defined as the primary router (PRIROUTER) or secondary router (SECROUTER), the device discards the datagram.
I think of an OSA as a clever hardware socket where you plug your Ethernet cable into the side of the z/OS box. Amongst other things, it drops traffic not destined for the z/OS image(s).
The quoted text means the OSA is doing some of the network work, and dropping packets not destined for the z/OS image(s), and so less work for z/OS.
With PRIROUTER I had a trace record, and the drop reason code was 4167, IP_NO_FWD: the packet cannot be forwarded, as expected.
Can I see this in NETSTAT output?
When I use NETSTAT STATS, it now has
Received Address Errors = 5
When I use NETSTAT DEVLINKS, the interface now has
Inbound Packets In Error = 5
When PRIROUTE is not specified, these values are 0.
ColinPaice 