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Delivery Agents
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30.6 How Executed

For safety and efficiency, sendmail undertakes a complicated series of steps to run (execute) a delivery agent. [14] Some (such as setting the environment) are intended to improve security. Others (such as forking) are intended to improve efficiency by creating parallel actions. Here, we discuss those steps in the order in which they are taken by sendmail.

[14] For the purpose of this discussion we will exclude the internal agents (such as IPC) and focus on actual programs (such as /bin/mail).

30.6.1 Fork

When sendmail performs delivery, it cannot simply replace itself with the delivery agent program. Instead, it must fork(2), and the child will replace itself.

If sendmail is running in verbose mode (see Section 34.8.76, Verbose), it shows that it is about to start this process:

Connecting to delivery agent

If a traffic-logging file was specified with -X command-line switch (see Section 26.4, "Log Transactions with -X"), sendmail appends the following line to that file:

pid === EXEC the expanded A= here

Here, the A= equate (see Section 30.4.1) from the delivery agent's declaration is printed with all its macros expanded and with the recipients listed.

Next sendmail creates a pipe so that it will be able to print the email message to the delivery agent and so that it can read errors emitted by the delivery agent. See the -d11 debugging switch (see Section 37.5.44, -d11.1) for a description of what can go wrong.

If all has gone well, sendmail fork(2)s a copy of itself. The parent then pipes the email message to the child.

30.6.2 The Child

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

If sendmail was compiled with HASSETUSERCONTEXT defined (see Section 18.8.9, HAS...), it calls setusercontext(3) like this:

setusercontext(NULL, pwd, uid, LOGIN_SETRESOURCES|LOGIN_SETPRIORITY);

Here, pwd is a pointer to a structure of type passwd for the user whose uid is uid. The uid is that of the controlling user (see Section 24.2.2, "Delivery to Files") or the recipient (see Section 30.8.33, F=o).

If the N= equate (see Section 30.4.8) has a nonzero value, sendmail calls nice(3) to "re-nice" the delivery agent to that value.

The sendmail program next sets its uid and gid as appropriate. If the DontInitGroups option (see Section 34.8.19, DontInitGroups) is false, sendmail calls initgroups(3). The identity used is that described under the DefaultUser option (see Section 34.8.15).

Next sendmail attempts to chdir(2) into one of the directories listed in the D= equate (see Section 30.4.3). This process can be watched with the -d11.20 debugging switch (see Section 37.5.46).

Finally, sendmail calls setsid(2) to become a process-group leader and execve(2) to become the delivery agent. That latter call looks like this:

execve(agent, argv, envp);

Here, agent is the full path of the delivery agent as specified in the P= equate (see Section 30.4.9). The argument vector (contents of the A= equate with all the macros expanded and all the recipients added) is passed as argv. The environment is that originally given to sendmail, massaged for security and augmented by the E= configuration command (see Section 22.2.1).


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