[HECnet] Some SIMH weirdness on Raspbian

[Johnny Billquist]

That looks just like the IPv4 over IPv6 thingy...

   Johnny

On 2019-12-07 13:37, Keith Halewood wrote:
> Hi,
> 
> Perhaps this isn’t strictly HECnet related but as HECnet traffic is 
> traversing some part of this weird arrangement via pydecnet, I’m taking 
> a chance:
> 
> I run SIMH on Raspberry PIs under Raspbian Buster.
> 
> I have both IPv4 and IPv6 networking switched on and a 
> router/DHCP(v6)/DNS infrastructure to cope successfully with it.
> 
> (Nothing is wireless for what I’m about to describe, not that it would 
> make much difference)
> 
> SIMH’s simulated Ethernet devices on the PIs are TAP connections to a 
> bridge device connection to a real eth0 – no problem here.
> 
> SIMH instances’ consoles and terminal MUX devices are listening on 
> individual ports and I telnet into these usually from my PC via Putty.
> 
> The DNS servers do not have AAAA for the PIs, just A, so the PC connects 
> to the PIs via IPv4 – no problem here.
> 
> The PIs show the SIMH instances listening on the right TCP ports but 
> when I filter with -4, ie:
>                  netstat –a -4
> 
> I don’t see SIMH listening. When I filter with -6, ie:
> 
>                  netstat –a -6
> 
> I do see a listen on those ports.
> 
> I notice that, for example, ssh listens on 0.0.0.0:ssh AND [::]:ssh but 
> SIMH listens only on *:8601 (for example)*
> *The * seems to show up only when I restrict the search to the ipv6 family.
> 
> The * seems to indicate a listen with no ‘family’ preference.
> 
> An established connection to *:8601 seems even stranger.
> 
> It only shows up when netstat is run with -6 but it shows the correct 
> IPv4 addresses for each endpoint. It is an IPv4 connection anyway.
> 
> The ‘ss -6’ command shows up something even weirder for the established 
> (IPv4) connections:
> 
> The local address port is: [::ffff:192.168.2.42]:8601 and the remote 
> address port is: [::ffff:192.168.2.12]:61152
> 
> The IPv4 part of these ports is correct. Why are they ‘encapsulated’ in 
> some IPv6 syntax and listed as IPv6 connections?
> 
> Can anybody point me in the right direction for some explanation please? 
> My google keyword searching skills seem a little off today.
> 
> Regards,
> 
> Keith
> 

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol