Local transports handle deliveries to files and pipes. (The `autoreply' transport can be thought of as similar to a pipe.) Whenever a local transport is run, Exim forks a subprocess for it. Before running the transport code, it sets a specific uid and gid by calling `setuid()' and `setgid()'. It also sets a current file directory; for some transports a home directory setting is also relevant. The `pipe' transport is the only one which sets up environment variables; see section "Environment variables" in chapter "The pipe transport" for details.
The values used for the uid, gid, and the directories may come from several different places. In many cases the director that handles the address associates settings with that address. However, values may also be given in the transport's own configuration, and these override anything that comes with the address. The sections below contain a summary of the possible sources of the values, and how they interact with each other.
All local transports have the options `group' and `user'. If `group' is set, it overrides any group that may be set in the address, even if `user' is not set. This makes it possible, for example, to run local mail delivery under the uid of the recipient, but in a special group. For example:
group_delivery:
driver = appendfile
file = /var/spool/mail/${local_part}
group = mail
If `user' is set for a transport, its value overrides what is set in the address. If `user' is non-numeric and `group' is not set, the gid associated with the user is used. If `user' is numeric, then `group' must be set.
The `pipe' transport contains the special option `pipe_as_creator'. If this is set and `user' is not set, the uid of the process that called Exim to receive the message is used, and if `group' is not set, the corresponding original gid is also used.
When the uid is taken from the transport's configuration, the `initgroups()' function is called for the groups associated with that uid if the `initgroups' option is set for the transport; `pipe' is the only transport that has such an option.
When the uid is not specified by the transport, but is associated with the address by a director or router, the option for calling `initgroups()' is taken from the director or router configuration. All directors and routers have `group', `user', and `initgroups' options, which are used as follows:
For the `aliasfile' director they specify the uid and gid for local deliveries generated directly -- that is, deliveries to pipes or files. They have no effect on generated addresses that are processed independently.
The `forwardfile' director's `check_local_user' option causes a password file lookup for the local part of an address. The uid and gid obtained from this lookup are used for any directly generated local deliveries, but they can be overridden by the `group' and `user' options of the director. As for `aliasfile', these values are not used for generated addresses that are processed independently.
The `localuser' director looks up local parts in the password file, and sets the uid and gid from that file for local deliveries, but these values can be overridden by the director's options.
For the `smartuser' director and all the routers, the `group', `user', and `initgroups' options are used only if the driver sets up a delivery to a local transport.
The `pipe' transport has a `home_directory' option. If this is set, it overrides any home directory set by the director for the address. The value of the home directory is set in the environment variable HOME while running the pipe. It need not be set, in which case HOME is not defined.
The `appendfile' transport does not have a `home_directory' option. The only use for a home directory in this transport is if the expansion variable `$home' is used in one of its options, in which case the value set by the director is used.
The `appendfile' and `pipe' transports have a `current_directory' option. If this is set, it overrides any current directory set by the director for the address. If neither the director nor the transport sets a current directory, then Exim uses the value of the home directory, if set. Otherwise it sets the current directory to `/' before running a local transport.
The `aliasfile', `forwardfile', and `localuser' directors all have `current_directory' and `home_directory' options, which are associated with any addresses they explicitly direct to a local transport.
For `forwardfile', if `home_directory' is not set and there is a `file_directory' value, that is used instead. If it too is not set, but `check_local_user' is set, the user's home directory is used. For `localuser', if `home_directory' is not set, the home directory is taken from the password file entry that this director looks up. There are no defaults for `current_directory' in the directors, because it defaults to the value of `home_directory' if it is not set at transport time.
The `smartuser' director and all the routers have no means of setting up home and current directory strings; consequently any local transport that they use must specify them for itself if they are required.
Normally a local delivery is handling a single address, and in that case the variables such as `$domain' and `$local_part' are set during local deliveries. However, in some circumstances more than one address may be handled at once (for example, while writing batch SMTP for onward transmission by some other means). In this case, the variables associated with the local part are never set, `$domain' is set only if all the addresses have the same domain, and `$original_domain' is never set.
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