On 27/12/2020 14:27, Chris Green wrote:
> The Natural Philosopher  wrote:
>> On 27/12/2020 12:42, Chris Green wrote:
>>> Are there any simple 3G/4G add-ons for a Pi that will allow it to
>>> power up into a mode where it has the ability to be connected to via
>>> the mobile data connection?
>>>
>>> I want a system that I can connect to from my home desktop/laptop on
>>> demand.
>>>
>>> Sort of 'thinking out loud' about this:-
>>>
>>>       I suppose a USB 3G/4G dongle could provide the hardware required
>>>       to make a mobile connection to the PI, recommendations?
>>>
>>>       It has to 'connect on demand' dialling *to* the Pi, it can't keep
>>>       the line open all the time, very expensive!
>>
>> Why? a smart phone sim is a charge per gigabyte, They are always on for
>> data!
>>
> So does just trying to connect *to* the device which has the data sim
> dongle wake up the connection?  It means one needs a dynamic DNS
> service but that's not a big problem.
>
>
>>>
>>>       So, is there software for Linux (and thus for the Pi) which will
>>>       handle incoming calls to allow ssh login?
>>>
>> Just use an IP capable dongle forget about 'incoming calls' and ssh
>> riught in. Biggest problem will be on what IP address you appear,
>>
> Yes, as I said it needs a dynamic DNS service but that's all.
>
>>>       How does one manage the other end?  Is there Linux desktop
>>>       software to allow one to dial up a remote system and then squirt
>>>       ssh down the connection?
>>>
>> 3G/4G is not 'dial up!
>>
>>> Any/all ideas would be very welcome.
>>>
>>
>> Look I think you have got the wrong end of the stick about how mobile IP
>> works...where is this pi going to be? Do you have a fixed IP address on
>> your ISP interface?
>>
> I probably have got it all wrong!  The Pi is actually a Beaglebone
> Black but that's irrelevant.
>
> All I want to do is be able to use ssh to connect *to* the BBB from
> home computers which have an internet connection.  The BBB doesn't
> have WiFi available.
>
>
>> Its almost impossible to set up a mobile connection to receive
>> unsolicited  IP. The mobile provider will absolutely do massive NAT. So
>> you need the Pi to be always online and connected to something that you
>> can contact, and the figure out a way to hijack the link. Just running
>> keepalive packets wont break the bank.
>>
> Currently it uses (rather flaky) marina WiFi and sets up ssh reverse
> tunnels by connecting to an intermediate system so that I can then
> connect *to* it via the intermediate system.
>
> I am looking for something more reliable.
>
>> sshing in wont be trivial but without knowing what you want to do with
>> the pi its hard to say whether you need to,. For example just polling a
>> webserver the pi could detect a request for data and uplaod it to the
>> webserver where you could download it., Even to the point of downloading
>> a command line off the server executing it on the pi and sending stdout
>> back to the server...
>>
>>
>>> I'm happy with fairly low-level stuff, I am a retired software
>>> engineer, grew up with Unix (solaris) command line and I'm also quite
>>> into home-build electronics projects.
>>>
>>>
>> Start with the assumption that you cant open a connection to a mobile
>> equpped pi, but 'always on' FROM the pi  wont be costly.
>>
> So, if one has 'always on' FROM the pi does that then just require
> some sort of dynamic dns service to be able to ssh *to* it?
>
No, not even that will work.

Because that will take you to the ISPS NAT router and there will be no
way to route onward to the Pi.

*Only if the Pi initiates the connection* will the NAT router set up a
mapping between public IP/port and PI IP/port.

Its analogous to your current wifi setup.  The Pi will have to be online
and permanently connected in some way to a publiclly accessible server
that you can use as a gateway.

Thats how stuff like whatsapp or wificalling or skype, work on a mobile,
  they are constantly polling a server registering what ip address and
socket they can be accessed on and when another phone contacts the
server it simply proxies the traffic or possibly tells them what ip to use.

You will need a server in public internet space I think. A virtual
private server can be VERY cheap if all you are running in it is a gateway.

I am not up in VPS so I would code up some custom daemon on one of my
VPSes. Then your clients would contact that daemon which would 'know'
where the Pi was, and perhaps start relaying packets down the pipe to te
pi.


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