Dale Hagglund is spot on. So I'm just going to say the same thing but in a different way, with some specifics and examples. ☺
The right thing to do in the Unix and Linux worlds is:
- to have a small, simple, easily auditable, program that runs as the superuser and binds the listening socket;
- to have another small, simple, easily auditable, program that drops privileges, spawned by the first program;
- to have the meat of the service, in a separate third program, run under a non-superuser account and chain loaded by the second program, expecting to simply inherit an open file descriptor for the socket.
You have the wrong idea of where the high risk is. The high risk is in reading from the network and acting upon what is read not in the simple acts of opening a socket, binding it to a port, and calling listen(). It's the part of a service that does the actual communication that is the high risk. The parts that open, bind(), and listen(), and even (to an extent) the part that accepts(), are not the high risk and can be run under the aegis of the superuser. They don't use and act upon (with the exception of source IP addresses in the accept() case) data that are under the control of untrusted strangers over the network.
There are many ways of doing this.
inetd
As Dale Hagglund says, the old "network superserver"inetd does this. The account under which the service process is run is one of the columns in inetd.conf. It doesn't separate the listening part and the dropping privileges part into two separate programs, small and easily auditable, but it does separate off the main service code into a separate program, exec()ed in a service process that it spawns with an open file descriptor for the socket.
The difficulty of auditing isn't that much of a problem, as one only has to audit the one program. inetd's major problem is not auditing so much but is rather that it doesn't provide simple fine-grained runtime service control, compared to more recent tools.
UCSPI-TCP and daemontools
Daniel J. Bernstein's UCSPI-TCP and daemontools packages were designed to do this in conjunction. One can alternatively use Bruce Guenter's largely equivalent daemontools-encore toolset.
The program to open the socket file descriptor and bind to the privileged local port is tcpserver, from UCSPI-TCP. It does both the listen() and the accept().
tcpserver then spawns either a service program that drops root privileges itself (because the protocol being served involves starting out as the superuser and then "logging on", as is the case with, for example, an FTP or an SSH daemon) or setuidgid which is a self-contained small and easily auditable program that solely drops privileges and then chain loads to the service program proper (no part of which thus ever runs with superuser privileges, as is the case with, say, qmail-smtpd).
A service run script would thus be for example (this one for dummyidentd for providing null IDENT service):
#!/bin/sh -eexec 2>&1exec \tcpserver 0 113 \setuidgid nobody \dummyidentd.plnosh
My nosh package is designed to do this. It has a small setuidgid utility, just like the others. One slight difference is that it's usable with systemd-style "LISTEN_FDS" services as well as with UCSPI-TCP services, so the traditional tcpserver program is replaced by two separate programs: tcp-socket-listen and tcp-socket-accept.
Again, single-purpose utilities spawn and chain load one another. One interesting quirk of the design is that one can drop superuser privileges after listen() but before even accept(). Here's a run script for qmail-smtpd that indeed does exactly that:
#!/bin/noshfdmove -c 2 1clearenv --keep-path --keep-localeenvdir env/softlimit -m 70000000tcp-socket-listen --combine4and6 --backlog 2 ::0 smtpsetuidgid qmaildsh -c 'exec \tcp-socket-accept -v -l "${LOCAL:-0}" -c "${MAXSMTPD:-1}" \ucspi-socket-rules-check \qmail-smtpd \'The programs that run under the aegis of the superuser are the small service-agnostic chain-loading tools fdmove, clearenv, envdir, softlimit, tcp-socket-listen, and setuidgid. By the point that sh is started, the socket is open and bound to the smtp port, and the process no longer has superuser privileges.
s6, s6-networking, and execline
Laurent Bercot's s6 and s6-networking packages were designed to do this in conjunction. The commands are structurally very similar to those of daemontools and UCSPI-TCP.
run scripts would be much the same, except for the substitution of s6-tcpserver for tcpserver and s6-setuidgid for setuidgid. However, one might also choose to make use of M. Bercot's execline toolset at the same time.
Here's an example of an FTP service, lightly modified from Wayne Marshall's original, that uses execline, s6, s6-networking, and the FTP server program from publicfile:
#!/command/execlineb -PWmultisubstitute { define CONLIMIT 41 define FTP_ARCHIVE "/var/public/ftp"}fdmove -c 2 1s6-envuidgid pubftp s6-softlimit -o25 -d250000 s6-tcpserver -vDRH -l0 -b50 -c ${CONLIMIT} -B '220 Features: a p .' 0 21 ftpd ${FTP_ARCHIVE}ipsvd
Gerrit Pape's ipsvd is another toolset that runs along the same lines as ucspi-tcp and s6-networking. The tools are chpst and tcpsvd this time, but they do the same thing, and the high risk code that does the reading, processing, and writing of things sent over the network by untrusted clients is still in a separate program.
Here's M. Pape's example of running fnord in a run script:
#!/bin/shexec 2>&1cd /public/10.0.5.4exec \chpst -m300000 -Uwwwuser \tcpsvd -v 10.0.5.4 443 sslio -v -unobody -//etc/fnord/jail -C./cert.pem \fnordsystemd
systemd, the new service supervision and init system that can be found in some Linux distributions, is intended to do what inetd can do. However, it doesn't use a suite of small self-contained programs. One has to audit systemd in its entirety, unfortunately.
With systemd one creates configuration files to define a socket that systemd listens on, and a service that systemd starts. The service "unit" file has settings that allow one a great deal of control over the service process, including what user it runs as.
With that user set to be a non-superuser, systemd does all of the work of opening the socket, binding it to a port, and calling listen() (and, if required, accept()) in process #1 as the superuser, and the service process that it spawns runs without superuser privileges.