Re: [gpsd-dev] Draft Stratum 1 Microserver HOWTO is up

[Paul Fertser]

Hi,

On Sat, May 21, 2016 at 10:53:51AM -0400, Eric S. Raymond wrote:
> > > ssh address@hidden
> > 
> > That assumes your Linux box supports the client side of mDNS, or something 
> > like that.  It may be common on desktop setups that are trying to do 
> > everything for everybody but I'm not sure it is universal.
> 
> The actual precondition is that your desktop has to be running
> avahi-daemon, or some other zeroconf implementation, so it will do
> mutual discovery with the avahi-daemon instance on the Pi.

Host name lookup on common GNU/Linux systems goes through libnss and
so the actual precondition for the above command to work is that the
"hosts" line in /etc/nsswitch.conf has mdns[_minimal] before "dns"
(mdns4[_minimal] might work too if the LAN is not IPv6-only). I do not
think it needs avahi-daemon running since the mDNS name resolution
doesn't need to be mutual. Some people have to disable mDNS discovery
due to misconfigured networks (e.g. windows AD domains use .local names
by default and that clashes with mDNS), I've seen that happen both on
ISP LANs and corporate networks.

I have an alternative proposal for reliable address discovery, but I'm
not sure you'll like it:

1. Connect your host's Ethernet port to the SBC's port directly with a
patchcord (cross- or not doesn't matter thanks to Auto-MDIX feature);

2. Discover your ethernet interface name, it might be something like
"eth0" or "enp6s2", I'll use eth0 as an example in the following
steps;

3. Do "ping6 -I eth0 ff02::1" and you should see only two addresses
answering:

$ ping6 -I eth0 ff02::1
PING ff02::1(ff02::1) 56 data bytes
64 bytes from fe80::250:70ff:fee7:41ec%eth0: icmp_seq=1 ttl=64 time=0.116 ms
64 bytes from fe80::227:19ff:fece:3bcd%eth0: icmp_seq=1 ttl=64 time=0.465 ms 
(DUP!)
64 bytes from fe80::250:70ff:fee7:41ec%eth0: icmp_seq=2 ttl=64 time=0.107 ms
64 bytes from fe80::227:19ff:fece:3bcd%eth0: icmp_seq=2 ttl=255 time=0.478 ms 
(DUP!)

The one that's marked (DUP!) is your SBC;

4. Connect to it by specifying address with the scope:

ssh address@hidden::227:19ff:fece:3bcd%eth0

That said, I'd probably just write something like "try
address@hidden, if that doesn't work, attach physical keyboard
and display to the SBC and figure out the assigned address with "ip a"
command". A reader familiar with computer networks will promptly
realise that if there's some other method of learning the address in
the specific conditions (like nmapping the LAN or looking through DHCP
server logs etc) he or she could employ that as well.

> Added: "Another possibility is that your host doesn't have a zeroconf
> implementation like Linux's 'avahi-daemon' installed.  Try to fix
> that.  If you can't, your host setup is outside the scope of what
> this HOWTO can handle; get expert help."

I think Debian ships the code in question in the libnss-mdns package.

HTH at least a little bit
-- 
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