File /etc/machine-id

Akkara · Posted: Fri Mar 15, 2019 11:12 pm Post subject:

I wonder why the need for an id in the first place. Someone had to make that decision at some point and put in it. What problem were they solving? I tried to do some quick searches but most of what I get is bug reports along the lines of "it is required can we have it get generated please". Didn't find any statement of motivation for it.

I don't know much about systemd, dbus, and all that. But I am a big fan of keeping things as simple as possible unless reason is shown for needing something more complicated. And it seems to me, if you can open a local socket and talk to a service, you implicitly know both must be running on the same machine, without needing to ID anything. Much like if we can shake hands, we must be in the same room, no need to check the GPS coordinates.

Does dbus work across the network? (And if so, what does it do that needs that?) I know pulseaudio can play across the network maybe that has something to do with it. If one doesn't use pulseaudio does that dependency go away? Sorry just talking out loud here, and while I could go on and investigate all this more deeply, it seems like a huge timesink to be keeping up with all the potential issues that the constant flux of software brings. Alas it is much easier for one guy to write bad code than for a team of ten to go in and find the bugs and fix the bad architectural decisions.
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eccerr0r · Posted: Sat Mar 16, 2019 12:45 am Post subject:

The ID is for having shared storage space.

Temp files in machine unique /tmp have notoriously been security issues. So it's best to store temp files in user accounts.

However if you have multiple machine sharing the same directory with multiple logins...collision city! So what do you do? Unique machine ID. Granted a hostname usually is sufficient, but I suspect the machine ID is used in case dynamic IP is used and changes from time to time.

Dbus can be used over network, it has always been for IPC and well, processes can be running on different computers too. Yes these features were designed for clustering in mind, us with single PCs it's kind of useless...
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pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

eccerr0r · Posted: Sat Mar 16, 2019 6:35 am Post subject:

Well, if you want to workaround this by randomly select a host id, that's probably the finest workaround; but really you need to trust your software. I'd think Pulseaudio is exempt because it's necessary to keep track of these sessions. Likewise Avahi, to keep machines from stepping on each other if IP address changes from DHCP so other machines can keep track of services available. LVM is reasonable too if you move disks around between machines, makes it easier for the subsystem to ensure similarly configured disks aren't accidentally swapped between machines.

Chrome however should have no right to this information, at least to the non-hash version. It's explicitly improper usage if the purpose is to upload it to the mothership. I don't see a reason why it would need the ID to detect collisions between multiple instances, this was a problem solved without machine id just fine for years.

IMHO I'm just going to leave it. Unfortunately I am a bit alarmed about the Chrome usage of this id. The other software above I have no problems with, even journald which also uses it. Fortunately I don't use Chrome much (it's in its own VM that I use for only one specific website), so hopefully this limits exposure.
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pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

figueroa · Posted: Sat Mar 16, 2019 5:36 pm Post subject:

eccerr0r · Posted: Sat Mar 16, 2019 8:18 pm Post subject:

mike155 · Posted: Sat Mar 16, 2019 10:01 pm Post subject:

figueroa · Posted: Sat Mar 16, 2019 10:07 pm Post subject:

figueroa · Posted: Sat Mar 16, 2019 10:10 pm Post subject:

eccerr0r · Posted: Sat Mar 16, 2019 10:43 pm Post subject:

So the question remains, why is it peeking at /etc/machine-id ... ?
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Naib · Posted: Sat Mar 16, 2019 11:37 pm Post subject:

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

figueroa · Posted: Sun Mar 17, 2019 3:36 am Post subject:

Akkara · Posted: Sun Mar 17, 2019 7:00 am Post subject:

Thank you, eccerr0r, for the detailed explanation.

krinn · Watchman Joined: 02 May 2003 Posts: 7470

a non solve to the problem.
If you move files to home directory to protect them from other users, the same could be achieve with proper rights and owernship on the file in /tmp ; i don't see where it could be problem as malicious program would need to have rights to alter it too.
If it was to create some sharing, i don't see how it help solving the problem to move the file into user home, as it should be restrict area from others too.
(without even speaking about other issue it could add, root have always a minimum space protect by fs to limit out of space issue, home have no such protection for users, a bad and kept open tmp file in home may put out of space home directory and user loosing valuable datas because of a crappy program filing his space with crap tmp datas)

And my first message about it was to point out, that there's no security at all in crypting a file that anyone could read ; it's even worst because from a privacy point of view, the crypted file is of lower value (depeding how you crypt it the result may change, while the original one, crypt or not, if you can source it and its content never change is golden value). Nobody cares how machine-id content is generated, as long as you can read it and it never change, it's gold.
So there's no security given by machine-id, only a 100% good fingerprint given to spys.

eccerr0r · Posted: Sun Mar 17, 2019 4:41 pm Post subject:

Well, the name in /tmp would need to be randomized. If another user started creating a /tmp/home-$otheruser-tmp/ directory, that would cause pain. In someone elses home directory, it's difficult to create such directory. Putting it in /home also enforces quota on temp space...

I suspect that the /run/user/$ID/* was created by systemd-logind/consolekit to mitigate this. I'm not sure why pulseaudio doesn't use this directory, that seems more logical.

I'm not 100% sure of avahi's capabilities are (namely the possibility of unprivileged services) and don't know what this may entail but advertising a service on a host that changed their IP address, it would be nice to know which machine you were running on, and the machine id would be helpful. Again this would hopefully be a trusted LAN environment so these don't go out on the open network.

LVM is a different problem.
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