Security wrappers and Bernstein chaining

One very common use of wrapper scripts is as security wrappers. A security script may call a gatekeeper program to check some sort of credential, then conditionally execute another based on the status value returned by the gatekeeper.

Bernstein chaining is a specialized security-wrapper technique invented by Daniel J. Bernstein, who has used it in a number of his packages. Conceptually, a Bernstein chain is like a pipeline, but each successive stage replaces the previous one rather than running concurrently with it.

The usual application is to confine security-privileged applications to some sort of gatekeeper program, which can then hand state to a less privileged one. The technique pastes several programs together using execs, or possibly a combination of forks and execs. The programs are all named on one command line. Each program performs some function and (if successful) runs exec(2) on the rest of its command line.

Bernstein's rblsmtpd package is a prototypical example. It serves to look up a host in the anti-spam DNS zone of the Mail Abuse Prevention System. It does this by doing a DNS query on the IP address passed into it in the TCPREMOTEIP environment variable. If the query is successful, then rblsmtpd runs its own SMTP that discards the mail. Otherwise the remaining command-line arguments are presumed to constitute a mail transport agent that knows the SMTP protocol, and handed to exec(2) to be run.

Another example may be found in Bernstein's qmail package. It contains a program called condredirect. The first parameter is an email address, and the remainder a program and arguments. Condredirect forks and execs those parameters, to run the program. If it exits successfully, the email pending on stdin is forwarded to the email address. In this case, opposite to that of rblsmtpd, the security decision is made by the child; this case is a bit more like a classical shellout.

A more elaborate example is the qmail POP3 server. It consists of three programs, qmail-popup, checkpassword and qmail-pop3d. Checkpassword comes from a separate package cleverly called checkpassword, and unsurprisingly it checks the password. The POP3 protocol has an authentication phase and mailbox phase. Once you enter the mailbox phase you cannot go back to the authentication phase. This is a perfect application for Bernstein chaining.

The first parameter of qmail-popup is the hostname to use in the POP3 prompts. The rest of its parameters are forked and execed, after the POP3 username and password have been fetched. If the program returns failure, the password must be wrong, so qmail-popup reports that and waits for a different password. Otherwise, the program is presumed to have finished the POP3 conversation, so qmail-popup exits.

The program named on qmail-popup's command line is expected to read three null-terminated strings from file descriptor 3 [31]. These are the username, password, and response to a cryptographic challenge, if any. This time it's checkpassword which accepts as parameters the name of qmail-pop3d and its parameters. The checkpassword program exits with failure if the password does not match; otherwise it changes to the user's uid, gid, and home directory, and executes the rest of its command line on behalf of that user.

Bernstein chaining is useful for situations in which the application needs setuid or setgid privileges to initialize a connection, or acquire some credential, and then drop those privileges so that following code does not have to be trusted. Following the exec, the child program cannot setreuid back to root. It's also more flexible than a single process, because you can modify the behavior of the system by inserting another program into the chain.

For example, rblsmtpd (mentioned above) can be inserted into a Bernstein chain, inbetween tcpserver (from the ucspi-tcp package) and the real SMTP server, typically qmail-smtpd. However, it works with inetd(8) and sendmail -bs as well.

As another example, Russ Nelson has written a qmail-popbull package. Without any modifications to qmail's POP3 server, qmail-popbull will insert a bulletin into the user's mailbox. It gets inserted into the Bernstein chain after checkpassword.

[31] qmail-popup's standard input and standard output are the socket, and standard error (which will be file descriptor 2) goes to a log file. File descriptor 3 is guaranteed to be the next to be allocated. As Ken Thompson once said: “You are not expected to understand this.”