Getting a unique id from a unix-like system

I want to get from any Unix-like system (if this is possible) a unique id that will be persistent every time my application runs in the same machine. If it is possible, I want to get the same id from Linux or FreeBSD or Solaris, etc... I don't want to generate a new id for each machine, but get an already existent id, and I prefer this id to come from the operating system and I don't prefer to use something like the MAC address.

If there is no other option available, I can use MAC in combination with something else, for example the id can be the md5 hash of the combination of the MAC address and something else.

I would like to listen to your suggestions.

If it is useful, my application is written in C/C++.

The aim of all this is to prevent a user to run my application for two or more times. I want to run just once.

How about the UUID of the root filesystem? You can get the root filesystem device from /etc/fstab, either by manually parsing the file or by using getfsent (3) or getfsfile (3). Once you have the device, you can get the UUID by either checking the links in /dev/disk/by-uuid or from the blkid command.

Both Solaris and Linux provide the hostid(1) utility

The best way is, as usual, to see how other people already solved the same problem.

FLEXlm also uses a host identifier for its node-locked licenses. The most common host identifier it uses is the ethernet MAC address for one of your network interfaces, smashed together without any separator.

It can also use (on Windows) the volume serial number of the C: drive (again smashed together without any separators), and on Solaris the output of the hostid command (IIRC, on Sun computers, this number is actually unique, and located on a small removable EEPROM on the system board).

While the MAC address is extremly easy to fake, it is a nearly universal identifer nowadays (almost all new computers have at least one ethernet port, and it is very common for them to be onboard), and actually intended to be globally unique (in fact, the Ethernet protocols depend on this uniqueness). The main problems you would have with this approach:

• Some computers have several ethernet addresses; some of them are on the main board, some are on separate removable cards.
• They are extremly easy to fake (and some protocols depend on being able to change them).
• Some virtualized environment generate random ethernet addresses on each boot (but they usually have a way to force a fixed value).

Another option is to use information derived from dmidecode, a command present on linux. This information is decoded from /dev/mem, therefore requiring root access.

The information dmidecode reads is known to be flawed, as some motherboard manufacturers lie or fake some of the fields.

There is no general and reliable way to get what you want.