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Mychaela Falconia mychaela.falconia at gmail.comKeith wrote:
> No mention of 192.168.100.101.
>
> Maybe this is just what the ppp code in the pppd or in the kernel uses
> for the dstaddr of the p-t-p link if it is not getting this info elsewhere?
Harald Welte replied:
> No, I think it's what some component in your phone is inventing and returning
> to the ppp daemon on your Linux machine :)
My first thought in reaction to Keith's analysis was what Harald said,
so I decided to test it experimentally. I don't have my own Osmocom
network or any BTS of my own to test with, but I do have active service
on several SIMs with the local GSM&UMTS commercial network operator in
my neck of the woods (T-Mobile USA), and the available services include
mobile Internet in addition to calls and SMS. My thought was to make
a pppd connection from the same host (my Slackware laptop) to the same
T-Mobile network through two different GSM+GPRS(+UMTS) modem
implementations, and see if the P-t-P address reported in ifconfig
changes or not.
The first tested implementation is Huawei E303, a 3G data modem in the
USB stick form factor; it is Qualcomm-based to the best of my knowledge.
Here is the ifconfig output:
ppp0 Link encap:Point-to-Point Protocol
inet addr:21.227.117.124 P-t-P:10.64.64.64 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:33967 errors:0 dropped:0 overruns:0 frame:0
TX packets:29454 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:28077904 (26.7 Mb) TX bytes:2831375 (2.7 Mb)
The other tested implementation is my own FreeCalypso, or more
specifically an FCDEV3B modem board running FreeCalypso Magnetite
hybrid firmware, build date 2018-07-30T20:24:34Z. It is also using a
different SIM, although on the same billing account with T-Mobile and
with the same service configuration - I am too lazy to move the same
SIM back and forth between the USB stick modem and my own board. Here
is the ifconfig output:
ppp0 Link encap:Point-to-Point Protocol
inet addr:22.255.27.2 P-t-P:10.64.64.64 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:285 errors:0 dropped:0 overruns:0 frame:0
TX packets:341 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:79139 (77.2 Kb) TX bytes:35368 (34.5 Kb)
The dynamically assigned IP address is different because I had to shut
down pppd with one modem and start a new session with the other modem,
but notice that the P-t-P address is the same fixed 10.64.64.64.
One very special feature of my FreeCalypso GSM+GPRS modem implementation
is that it is compiled from source code that is freely published for
anyone to study or tweak as they like, so if anyone is interested, you
are more than welcome to study the PPP-to-GPRS translation code in
this modem implementation and see what it does. Unfortunately, though,
I don't have any spare cycles at the present time to do such a code
study myself - I am not the original author of this code, I inherited
it from TI, and while I strive to maintain it, I will only be able to
devote significant time to this task if someone buys my modem hardware
(the firmware is useless without the hardware) in a commercially
significant quantity, which hasn't happened yet. In the absence of
commercial customers for FreeCalypso modems, I can only spend very
little time on firmware maintenance, just enough to verify that it
works on a basic level, which it does - I am sending this post through
the mobile Internet GPRS connection going through my FreeCalypso modem.
But I consider it rather unlikely that two independent and very
different implementation of PPP-to-GPRS translation (or PPP-to-UMTS in
the case of the 3G modem) have the same 10.64.64.64 IP address hard-
coded in them, so in this case the P-t-P address is probably coming
from T-Mobile's GGSN and not from the modem.
> Many [modern] modems are doing some internal NAT so that the IP addresses on
> the host PC (pppd) side don't correspond to those on the modem-sgsn side.
While I don't have the time to study it and see for myself (see above),
I highly doubt that the implementation which we (FreeCalypso) inherited
from TI does such NAT.
M~