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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



rcmd, rcmd_af, rresvport, rresvport_af, ruserok - routines for returning a stream to a remote command


#include <netdb.h>
#include <unistd.h>

int rcmd(char **ahost, unsigned short inport, const char *luser,
     const char *ruser, const char *cmd, int *fd2p);
int rcmd_af(char **ahost, unsigned short inport, const char *luser,
     const char *ruser, const char *cmd, int *fd2p, int af);
int rresvport(int *port);
int rresvport_af(int *port, int af);
int ruserok(const char *rhost, int suser, const char *ruser,
     const char *luser);


The rcmd() and rcmd_af() functions execute a command on a remote machine with an authentication scheme based on privileged port numbers. The {PRIV_NET_PRIVADDR} privilege is required to use these functions so they can bind a socket to a privileged port number. An AF_INET socket is returned with rcmd(). The rcmd_af() function supports AF_INET, AF_INET6, or AF_UNSPEC for the address family. An application can choose which type of socket is returned by passing AF_INET or AF_INET6 as the address family. The use of AF_UNSPEC means that the caller will accept any address family. Choosing AF_UNSPEC provides a socket that best suits the connectivity to the remote host.

The rcmd() and rcmd_af() functions look up the host *ahost using getaddrinfo(3C) and return −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 Internet port inport.

If the connection succeeds, a socket in the Internet domain of type SOCK_STREAM is returned to the caller. The socket is given to the remote command as standard input (file descriptor 0) and standard output (file descriptor 1). If fd2p is non-zero, an auxiliary channel to a control process is set up and a descriptor for it is placed in *fd2p. The control process returns diagnostic output file (descriptor 2) from the command on the auxiliary channel. The control process also accepts bytes on this channel as signal numbers to be forwarded to the process group of the command. If fd2p is 0, the standard error (file descriptor 2) of the remote command is made the same as its standard output. No provision is made for sending arbitrary signals to the remote process, other than possibly sending out-of-band data.

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

The rresvport() and rresvport_af() functions are used to obtain a socket bound to a privileged port number. Privileged Internet ports are those in the range 1 to 1023. The {PRIV_NET_PRIVADDR} privilege is required to bind a socket to a privileged port number. The application must pass in port, which must be in the range 512 to 1023. The system first tries to bind to that port number. If it fails, the system then tries to bind to another unused privileged port, if one is available.

The rresvport() function returns a descriptor to a socket in the Internet domain of type SOCK_STREAM with an address in the AF_INET address family. The rresvport_af() function is the equivalent to rresvport(), except that you can choose AF_INET or AF_INET6 as the socket address family to be returned by rresvport_af(). AF_UNSPEC does not apply to the rresvport_af() function.

The ruserok() function is a routine used by servers to authenticate clients that request a service with rcmd(). The ruserok() function takes a remote host name returned by the gethostbyaddr() function with two user names and a flag to indicate whether the local user's name is that of the superuser. See gethostbyname(3C). The ruserok() function then checks the files /etc/hosts.equiv and possibly .rhosts in the local user's home directory to see if the request for service is allowed. A 0 value is returned if the machine name is listed in the /etc/hosts.equiv file, or if the host and remote user name are found in the .rhosts file. Otherwise, the ruserok() function returns −1. If the superuser flag is 1, the /etc/hosts.equiv is not checked.

Return Values

The rcmd() and rcmd_af() functions return a valid socket descriptor upon success. The functions return −1 upon error and print a diagnostic message to standard error.

The rresvport() and rresvport_af() functions return a valid, bound socket descriptor upon success. The functions return −1 upon error with the global value errno set according to the reason for failure.

The error code EAGAIN is overloaded to mean “All privileged network ports in use.”



system trusted hosts and users


user's trusted hosts and users


The protocols underlying these functions use weak authentication and offer no protection against spoofing or snooping of traffic. The in.rshd(8) server is disabled by default on Oracle Solaris and most other modern operating systems. Use of the sshd(8) server is strongly recommended instead, and a programming interface to it is available via the libssh2 library.

The range of privileged ports used by these functions does not take into account changes to the smallest-nonpriv-port and extra-priv-ports properties by ipadm(8).


See attributes(7) for descriptions of the following attributes:

Interface Stability

This interface is Unsafe in multithreaded applications. Unsafe interfaces should be called only from the main thread.

See Also

rlogin(1), rsh(1), Intro(2), getaddrinfo(3C), gethostbyname(3C), rexec(3C), attributes(7), privileges(7), in.rexecd(8), in.rshd(8)


The rcmd_af() and rresvport_af() functions were added to Oracle Solaris in the Solaris 8 release.

The rcmd(), rresvport(), and ruserok() functions have been present since the initial release of Solaris.