sendmail, 4th Edition

The Child

The child is the copy of sendmail that will transform into the delivery agent. Before the child can transform, it must perform a few more necessary steps.

If sendmail was compiled with HASSETUSERCONTEXT defined (HAS... on page 114), it calls setusercontext(3) like this:

setusercontext(NULL, pwd,  user-id,  LOGIN_SETRESOURCES|LOGIN_SETPRIORITY);

Here, pwd is a pointer to a structure of type passwd for the user whose user-id is user-id. The user-id is that of the controlling user (Delivery to Files on page 466) or the recipient (F=o on page 777).

The sendmail program next sets its group-id as appropriate. If the DontInitGroups option (DontInitGroups on page 1023) is false, sendmail calls initgroups(3). The group identity used is that described under the DefaultUser option (DefaultUser on page 1000).

If the /= delivery agent equate (/=(forward slash) on page 737) has a non-NULL value, sendmail calls chroot(8) to change its topmost directory into a private directory tree.

If the N= delivery agent equate (N= on page 748) has a nonzero value, sendmail calls nice(3) to “re-nice” the delivery agent to that value.

The sendmail program then sets its user-id. The user identity used is chosen by the mailer F=S and U= equates and the DefaultUser option, as detailed in DefaultUser on page 1000.

The sendmail program then attempts to chdir(2) into one of the directories listed in the D= delivery agent equate (D= on page 741).

Next, sendmail dup(2)s the pipes created in the previous section.

Finally, ...