Updating baseband on Samsung devices when you are running aftermarket ROMs.

Some time ago I wrote about heimdall and how you could flash a Samsung official ROM using it. This time we'll be talking quite about the opposite.

When you are running an aftermarket ROM (one that is not the one from the maker of your phone) like an official LineageOS or crDroid ROM you get regular updates from them, but what you typically don't get is updates for the baseband.

The baseband is the software that runs the communication processor or modem, it is an important part of your phone as it is in charge of all the communications, so having it updated is important, not only because of the functionalities, but also because of the security issues.

So... the problem here is... how do you update your baseband now that you are no longer running your phone's official ROM? You could backup and flash your updated Samsung official ROM and then reinstall your favourite ROM and restore from backup, but that's a nonsense, so...

How about flashing just the modem software and leave your favourite ROM untouched?

Well, for that we can rely on heimdall, just make sure that you run the latest version of heimdall (2.0.2 as I write this). First you must download the Samsung updated ROM from your favourite site, we need the CP archive, something like...

CP_A705FNXXU5DWB1_CP23709798_CL24363203_QB62257661_REV00_user_low_ship_MULTI_CERT.tar.md5

for the A70, note that while this ends on md5 it is just a normal tar file, then extract the files from it, in this case modem.bin.lz4 and uncompress it with unlz4 and you'll get the modem.bin file that you must flash, now we just need to find where we need to flash it, we look for the name of this file on the heimdall print-pit output and we'll get the Partition Name, in this case MODEM, so we run...

heimdall flash --MODEM modem.bin

That's it, enjoy your new baseband ;-)