Static Home Directory Rant

Date: 01 January 2007

Updated: 01 March 2011

Ok, I have to get this off my chest because, 1) it drives me fracking bonkers everytime and see it and 2) I want a place I can point people at when they ask why this is a bad idea. What I’m talking about is the excessive number of HOWTOs that think it’s a good idea to use the same home directory for all virtual users. Most of these HOWTOs discuss using postfix with courier, but I have seen courier-mta HOWTOs that do this too.

Let me give a little background first. The courier mail server supports the concept of virtual users. These are users that do not have system accounts. They only have accounts that exist for courier. (Note: Other mail servers support virtual users so, when I say courier, keep in mind that his applies to other servers as well.) When you setup a virtual user for courier, you create an entry in some sort of database, like userdb, MySQL or LDAP, that has the username (or email address), password, home directory, maildir, user and group IDs for directory ownership, etc., and a few other things that are not relevent to this discussion. The systems are configured through various auth*rc files, e.g. authmysqlrc is the config file for MySQL. In the case of MySQL and PostgreSQL, you specify column names (or SQL snippets) that will tell courier where everything is. (I talk about this a little in How to Allow Users to Login With user%domain.)

Let me give you an example from a MySQL setup to show you what I’m complaining about. Note: In the example I used a static value instead of a column name for the home directory. You get the same effect if you just set everyone’s homedirs to be the same.

MYSQL_HOME_FIELD "/home/vmail"
MYSQL_MAILDIR_FIELD concat(domain, '/', email, '/')

Every user on a system configured like this will have the same home directory, /home/vmail, and a maildir that’s looks like inside that directory. At this point, those of you that haven’t wandered off are asking, “But PerlStalker, what’s wrong with that? It seems to make sense. The user’s mail is kept separate and it all works.” The problem is that courier (and a few other tools) use the user’s home directory to store user-specific settings. For example, maildrop (a common mail delivery agent (MDA) used with courier) looks for user-specific filters in $HOME/.mailfilter (where $HOME is the user’s home directory). courier-mta will look for $HOME/.courier* files to see if there are user-specific delivery instructions. (postfix calls these files .forward while qmail uses .qmail, but the idea is the same.) But that’s not all, some tools such as SpamAssassin can be configured to keep a user’s settings in $HOME/.spamassassin or similar files. This gets to be a problem when you have more than one user trying to use the same $HOME.

User configs are not the only problem. You may want to configure your system so that each domain is owned by a different uid/gid pair to allow, for example, a domain admin to make changes to their domain but no others or to tie the domain’s mail store into a global quota for that user. The catch is that courier will try to suid to the uid/gid pair and change to the user’s $HOME. You get a couple of things happening now: 1) courier may not be able to cd to $HOME because it’s owned by someone else and 2) courier will complain that $HOME is owned by someone else.

What should be becoming apparent here is that what is lost when every user has the same home directory is the flexability to do user-specific things. As an admin, you might not need any of that flexability right now, but you might at some point in the future. If you take the small step of forking out the users' homes into separate directories in the beginning, it can save you a lot of pain and suffering later. So, here is my recommendation. I suggest that you use a different home dir for each user and use the default maildir. For example, a user might have a home directory of /home/vmail/ with a maildir of $HOME/Maildir. (Some systems will change the default maildir to .maildir.)

He’s our authmysqlrc file again. Notice that I commented out MYSQL_MAILDIR_FIELD. That’s because I’ll be using the default which is set in imapd. You could, of course, uncomment here instead.

MYSQL_HOME_FIELD concat('/home/vmail/', domain, '/', email)

Some people recommend setting the maildir to ./ so that it is the same as the home directory. I don’t like to for the simple reason that most of the confiuration files that get written in $HOME start with a leading dot, e.g. .mailfilter and .spamassassin. Courier uses directories with a leading dot to signify sub-folders so having files with leading dots could cause problems and /or confusion.

Ok, so that’s the end of my rant. Most of what I said can be applied to any mail server with virtual users. The idea is to plan ahead and give your self the flexibility to adapt to new things in the future.

Update 2011-03-01: I recovered this post from the old site database but I don’t have the original creation date. It was sometime back in 2007, I think.