rcmd(3)

💖 NAME 💖

rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af, ruserok_af — routines for returning a stream to a remote command

okay so bestie, welcome to the r-word corner of libc 💅 this whole page lives in the Library Functions Manual, it's rcmd(3), and it is lowkey giving vintage internet energy. let's go 🦋

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📚 LIBRARY 📚

Standard C library (libc, -lc)

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✨ SYNOPSIS ✨

#include <netdb.h>    /* Or <unistd.h> on some systems */

int rcmd(char **restrict ahost, unsigned short inport,
            const char *restrict locuser,
            const char *restrict remuser,
            const char *restrict cmd, int *restrict fd2p);

int rresvport(int *port);

int iruserok(uint32_t raddr, int superuser,
            const char *ruser, const char *luser);
int ruserok(const char *rhost, int superuser,
            const char *ruser, const char *luser);

int rcmd_af(char **restrict ahost, unsigned short inport,
            const char *restrict locuser,
            const char *restrict remuser,
            const char *restrict cmd, int *restrict fd2p,
            sa_family_t af);

int rresvport_af(int *port, sa_family_t af);

int iruserok_af(const void *restrict raddr, int superuser,
            const char *restrict ruser, const char *restrict luser,
            sa_family_t af);
int ruserok_af(const char *rhost, int superuser,
            const char *ruser, const char *luser,
            sa_family_t af);

🧚 Feature Test Macro Requirements for glibc (see feature_test_macros(7)): 🧚

rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(),
iruserok_af(), ruserok(), ruserok_af():
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.19 and earlier:
        _BSD_SOURCE

yes bestie, you literally have to define a feature test macro to even see these. it's giving ✨gated content✨. _DEFAULT_SOURCE on new glibc, _BSD_SOURCE on glibc 2.19 and earlier. no macro, no function. periodt.

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💅 DESCRIPTION 💅

so here's the vibe check. 📡

The rcmd() function is used by the superuser to execute a command on a remote machine using an authentication scheme based on privileged port numbers. The rresvport() function returns a file descriptor to a socket with an address in the privileged port space. The iruserok() and ruserok() functions are used by servers to authenticate clients requesting service with rcmd(). All four functions are used by the rshd(8) server (among others). 🫶

🚀 rcmd() — the main character

The rcmd() function looks up the host *ahost using gethostbyname(3), returning -1 if the host does not exist. Otherwise, *ahost is set to the standard name of the host and a connection is established to a server residing at the well-known Internet port inport.

If the connection succeeds, a socket in the Internet domain of type SOCK_STREAM is returned to the caller, and given to the remote command as stdin and stdout. If fd2p is nonzero, then an auxiliary channel to a control process will be set up, and a file descriptor for it will be placed in *fd2p. The control process will return diagnostic output from the command (unit 2) on this channel, and will also accept bytes on this channel as being UNIX signal numbers, to be forwarded to the process group of the command. If fd2p is 0, then the stderr (unit 2 of the remote command) will be made the same as the stdout and no provision is made for sending arbitrary signals to the remote process, although you may be able to get its attention by using out-of-band data. 😭

The protocol is described in detail in rshd(8).

🔒 rresvport() — getting you a bougie port

The rresvport() function is used to obtain a socket with a privileged port bound to it. This socket is suitable for use by rcmd() and several other functions. Privileged ports are those in the range 0 to 1023. Only a privileged process (on Linux, a process that has the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace) is allowed to bind to a privileged port. In the glibc implementation, this function restricts its search to the ports from 512 to 1023. The port argument is value-result: the value it supplies to the call is used as the starting point for a circular search of the port range; on (successful) return, it contains the port number that was bound to. 🎀

👯 iruserok() and ruserok() — the bouncers at the door

The iruserok() and ruserok() functions take a remote host's IP address or name, respectively, two usernames and a flag indicating whether the local user's name is that of the superuser. Then, if the user is not the superuser, it checks the /etc/hosts.equiv file. If that lookup is not done, or is unsuccessful, the .rhosts in the local user's home directory is checked to see if the request for service is allowed. 🕵️

If this file does not exist, is not a regular file, is owned by anyone other than the user or the superuser, is writable by anyone other than the owner, or is hardlinked anywhere, the check automatically fails. Zero is returned if the machine name is listed in the hosts.equiv file, or the host and remote username are found in the .rhosts file; otherwise, iruserok() and ruserok() return -1. If the local domain (as obtained from gethostname(2)) is the same as the remote domain, only the machine name need be specified.

If the IP address of the remote host is known, iruserok() should be used in preference to ruserok(), as it does not require trusting the DNS server for the remote host's domain. trust issues, honestly. 🙈

🌐 *_af() variants — the multiverse arc

All of the functions described above work with IPv4 (AF_INET) sockets. The "_af" variants take an extra argument that allows the socket address family to be specified. For these functions, the af argument can be specified as AF_INET or AF_INET6. In addition, rcmd_af() supports the use of AF_UNSPEC. slay. ✨

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🎁 RETURN VALUE 🎁

The rcmd() function returns a valid socket descriptor on success. It returns -1 on error and prints a diagnostic message on the standard error. 😭

The rresvport() function returns a valid, bound socket descriptor on success. On failure, it returns -1 and sets errno to indicate the error. The error code EAGAIN is overloaded to mean: "All network ports in use". (dramatic but accurate 💀)

For information on the return from ruserok() and iruserok(), see above. 👆

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🌈 ATTRIBUTES 🌈

For an explanation of the terms used in this section, see attributes(7).

• rcmd(), rcmd_af() — Thread safety: MT-Unsafe
• rresvport(), rresvport_af() — Thread safety: MT-Safe
• iruserok(), ruserok(), iruserok_af(), ruserok_af() — Thread safety: MT-Safe locale

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⭐ STANDARDS ⭐

BSD.

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🦋 HISTORY 🦋

• iruserok_af(), rcmd_af(), rresvport_af(), ruserok_af() — glibc 2.2.
• Solaris, 4.2BSD. The "_af" variants are more recent additions, and are not present on as wide a range of systems.

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💀 BUGS 💀

iruserok() and iruserok_af() are declared in glibc headers only since glibc 2.12. yeah. iconic behavior honestly. 🙈

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🔥 SEE ALSO 🔥

rlogin(1), rsh(1), rexec(3), rexecd(8), rlogind(8), rshd(8)

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and that's the whole page bestie!! 💖✨

this was rcmd(3) from the Linux man-pages 6.20, dated 2025-09-21. go forth and bind responsibly, mwah 💅🎀🦋