Ubuntu – Reserved UIDs/GIDs and semi-reserverd UIDs/GIDs

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When installing a new system, depending on selected packages, some uids for some packages aren't always the same.

eg: 'sshd' may be UID 102, sometimes another value. On the other hand, 'www-data' is always '33'

How can I figure out all packages with reserved UIDs (ie www-data) and all the packages that generate a pseudo-random administration UID (<1000, ie: sshd)? This goes for GIDs too.

Is there a list of all those packages? Or is there an aptitude command that let's me find out all those packages, even the ones not installed?

Thanks

Best Answer

The UIDs and GIDs 0-99 are fixed and always the same. They are created by the package base-passwd.

The 100-999 are dynamically allocated, usually in the order the users/groups are created on your system.

See the section "UID and GID classes" in the Ubuntu Policy Manual for some more classes.

As far as I know there is no list of packages that create users or groups.

Packages create the additional users and groups in their *.preinst and *.postinst scripts. So to find the installed packages that have created users

grep 'adduser ' /var/lib/dpkg/info/*inst

may be a starting point (replace adduser with addgroup for groups).

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