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Old 02-29-2008, 12:40 PM   #1
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System usernames versus user numbers -- incompatibility?

I'm running Debian etch and I'm perplexed about the difference between usernames and user numbers. When I look at ps -aux, most of the processes are identified by their username:

www-data 16793  0.0  2.9  43052 26584 ?        S    Feb24   4:57 /usr/sbin/apache
mysql     1663  0.0  0.6 111588  5596 pts/2    Sl   Feb23   0:15 /usr/sbin/mysqld
but a few of them, notably Exim, are identified by their user number:
109       7580  0.0  0.1  11624  1604 ?        Ss   13:01   0:00 /usr/sbin/exim4 -bd -q30m
In /etc/passwd, there is a user called Debian-exim, and its user number is 109. What decides whether the name or the number will be used to identify the process?

This is not idle curiosity, because when Exim is running as user 109, it doesn't have permission to access files belonging to Debian-exim! That is a problem. How can I fix it?

Incidentally, chown doesn't seem to care about the difference between names and numbers:
root /etc/exim4 #ls -l passwd.client

-rw-r--r-- 1 root Debian-exim 205 Feb 28 12:49 passwd.client

root /etc/exim4 #chgrp 109 passwd.client

root /etc/exim4 #ls -l passwd.client

-rw-r--r-- 1 root Debian-exim 205 Feb 28 12:49 passwd.client
Old 02-29-2008, 03:09 PM   #2
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I seem to remember that ps displays the number when the name is longer than a certain length.

Old 02-29-2008, 05:18 PM   #3
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Dunno when ps display numbers or names, but it is true that it doesn't matter whether you use names or numbers. There are fully equal. (Why the two are different than???)

Shit happens when you use YP, or you are mounting from another computer and the name/number combinations are different from one machine to the other. I thought numbers had preference over names in such cases.

Old 02-29-2008, 05:39 PM   #4
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Give a specific example of failed access. it may not be permissions - does Debian use SELinux?
Old 03-01-2008, 10:53 AM   #5
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A specific example is that suddenly, starting the day before yesterday, exim can't read this file:

-rw-r----- 1 root Debian-exim 205 Dec 20 17:40 passwd.client

But if I make it world-readable (0644), suddenly exim can read it again. Exim is running as user 109, which is Debian-exim.

I can reproduce the problem easily with a chmod 0640 on that file; exim tells me "failed in cram_md5 authenticator: failed to open /etc/exim4/passwd.client for linear search: Permission denied (euid=109 egid=42)"

And yes, user 109 is a member of the Debian-exim group.

Is this a symptom of some other problem that I'm misunderstanding, perhaps?


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