Re: gsmtap design/extensions?

On Apr 12, 2019, at 5:24 AM, Johannes Berg <[email protected]> =
wrote:

> On Thu, 2019-04-11 at 01:32 +0200, Harald Welte wrote:
>=20
>> GSMTAP was designed as a format to encapsulate protocols normally not =
spoken over IP
>> (such as classic GSM radio protocols, e.g. the Layer 2 LAPDm over GSM =
Um)
>> inside an IP transport.
>=20
> Sure, but wifi is also not spoken over IP, yet we don't encapsulate =
our
> 802.11 frames into IP to show them in wireshark :-)

That's just because the rpcap protocol hasn't been revved yet to handle =
the new create/set options/activate mechanism in libpcap to allow =
monitor-mode capturing, so you only get to see fake Ethernet frames. :-)

I.e., there's a split there between "capture" and "getting the packets =
from a capture delivered to you over an IP network".

Perhaps there should be a GSMTAP link-layer header type, so you can have =
a GSMTAP pcap file or GSMTAP interface in a pcapng file, combined with a =
more general "remote capture" mechanism in libpcap so that you could =
capture on gsmtap://host:port and capture from a host using the =
GSMTAP-over-UDP encapsulation - or capture using rpcap.

>> No userspace source would ever be able to generate such data and =
stream
>> it real-time into wireshark, would it?  Sure, I can write pcap file =
with
>> such ARPHDR_* values, but I could never do this in real-time.  For =
many
>> but not all use cases, that's really what it is: A vehicle to stream
>> real-time non-IP protocol traces into wireshark so it can visualize
>> the protocol traces.
>=20
> I think you can pipe a stream into wireshark?

1) You could pipe into libpcap or otherwise have a way for a libpcap =
module to connect to a user space source and get packets from it.

2) You could pipe a pcap file into tcpdump/Wireshark/etc..

3) You could have an extcap program:

	=
https://www.wireshark.org/docs/wsdg_html_chunked/ChCaptureExtcap.html

provide packets to Wireshark.

> To me it feels like the wrong thing to actually make wireshark listen =
on
> "lo" or "eth0" or something to get data from the cellular that's
> (locally) generated by another application, but I guess that's only
> about how you think about it - and if it's not generated locally then
> that's an easy transport. I'm not sure it makes *sense* because then =
you
> need permissions to capture on the wired network etc.

Depending on how your system is set up:

	$ ls -l /dev/bpf*
	crw-rw----  1 root  access_bpf   23,   0 Apr 10 22:57 /dev/bpf0
	crw-rw----  1 root  access_bpf   23,   1 Apr 10 22:56 /dev/bpf1

		...

and it could just be rw-rw-rw-.  Perhaps other systems make it harder to =
grant capture privileges.

> where you don't *really* need that for this stream

If there's a need for that, whatever provides the packets could impose =
that (by finding out the UID/GID of the other process if this is going =
over a UNIX-domain socket, at least on some UN*Xes, or by requiring a =
login if this is going over a network).=