Plesk: same UID, different user

The problem

When a new (s)FTP user is added to a subscription in Plesk, he is given the same UID as the original system user created for that subscription.
According to Plesk gurus, nothing is strange here. There is, however, a huge problem with this:

Upon login through ssh with the “main” user account, you won’t know which user you will be logged-in as.

Plesk asserts that the system will automatically know that the “real” user associated with a UID is the first matching entry in /etc/passwd. This is, in my experience, bullshit, specifically as far as sshd is concerned.

In consequence, upon ssh login as myrealuser@myserver, you don’t know ahead of time which user you will be logged-in as. You may end up as sftpuser1, myrealuser or any other user associated with the subscription, even if that user does not have shell access.

This is only the first problem. Consider:

• Each user can have a different crontab,
• Each user can have a different value for ~ BUT the same $HOME, which means
• Any script or executable that depends on something being in ~ (such as git or ssh) will behave unpredictably.

It is completely possible to ssh as a random user, edit a random crontab that will run scripts that don’t know which directory ~ will be.

My workarounds

Steps can be taken to ensure scripts will behave predictably in this situation.
As all users of a subscription share the same uid, ownership or ACL are not an issue.

$HOME and ~

If you end up not being logged-in as the real user, ~ can’t be trusted because of reasons. $HOME, however, can.

This has consequences, notably with ssh.

ssh

By default, ssh will look for a configuration file in ~/.ssh/config and for a known_hosts file in ~/.ssh/known_hosts. You have to provide paths with $HOME explicitly:

ssh -F $USER_HOME/.ssh/config -o UserKnownHostsFile=$USER_HOME/.ssh/known_hosts

git

If you use git over ssh, you need to provide the GIT_SSH_COMMAND environment variable that uses $HOME instead of ~:

GIT_SSH_COMMAND="ssh -F $USER_HOME/.ssh/config -o UserKnownHostsFile=$USER_HOME/.ssh/known_hosts"

Crontab

This command will actually give you the first entry in /etc/passwd matching your uid:

awk -F ':' -v uid="$(id -u)" '$3 == uid {print $1; exit}' /etc/passwd

This can then be used, for example, to edit the crontab:

USERID=`id -u`
USERNAME=`awk -F ':' -v uid="$USERID" '$3 == uid {print $1; exit}' /etc/passwd`

cat crontab_template | crontab -u $USERNAME -