xref: /openbsd-src/lib/libc/net/rcmd.3 (revision 36bf49e74679c453fed2dad43d3abb15e64e4b1c)

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.Dd $Mdocdate: May 28 2016 $
.Dt RCMD 3
.Os
.Sh NAME
.Nm rcmd ,
.Nm rcmd_af ,
.Nm rresvport ,
.Nm rresvport_af ,
.Nm ruserok
.Nd routines for returning a stream to a remote command
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn rcmd "char **ahost" "int inport" "const char *locuser" "const char *remuser" "const char *cmd" "int *fd2p"
.Ft int
.Fn rcmd_af "char **ahost" "int inport" "const char *locuser" "const char *remuser" "const char *cmd" "int *fd2p" "int af"
.Ft int
.Fn rresvport "int *port"
.Ft int
.Fn rresvport_af "int *port" "int af"
.Ft int
.Fn ruserok "const char *rhost" "int superuser" "const char *ruser" "const char *luser"
.Sh DESCRIPTION
The
.Fn rcmd
function is used by the superuser to execute a command on a remote
machine using an authentication scheme based on reserved
port numbers.
If the calling process is not setuid, the
.Ev RSH
environment variable is set, and
.Fa inport
is
.Dq shell/tcp ,
.Xr rcmdsh 3
is called instead with the value of
.Ev RSH .
Alternately, if the user is not the superuser,
.Fn rcmd
will invoke
.Xr rcmdsh 3
to run the command via
.Xr ssh 1 .
While
.Fn rcmd
can handle IPv4 cases only,
the
.Fn rcmd_af
function can handle other cases as well.
.Pp
The
.Fn rresvport
and
.Fn rresvport_af
functions return a descriptor to a socket
with an address in the privileged port space.
The
.Fn ruserok
function is used by servers
to authenticate clients requesting service with
.Fn rcmd .
.Pp
The
.Fn rcmd
function looks up the host
.Fa *ahost
using
.Xr getaddrinfo 3
and, if the host exists,
.Fa *ahost
is set to the canonical name of the host.
A connection is then established to a server
residing at the well-known Internet port
.Fa inport .
If the user is not the superuser, the only valid port is
.Dq shell/tcp
(usually port 514).
.Pp
If the connection succeeds,
a socket in the Internet domain of type
.Dv SOCK_STREAM
is returned to the caller, and given to the remote
command as stdin and stdout.
If
.Fa fd2p
is non-zero, then an auxiliary channel to a control
process will be set up, and a descriptor for it will be placed
in
.Fa *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
.Tn UNIX
signal numbers, to be
forwarded to the process group of the command.
If
.Fa fd2p
is
.Va NULL ,
then the standard error (unit 2 of the remote command) will be made
the same as the standard output 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.
Note that if the user is not the superuser,
.Fa fd2p
must be
.Va NULL .
.Pp
.Fn rcmd_af
takes address family in the last argument.
If the last argument is
.Dv AF_UNSPEC ,
interpretation of
.Fa *ahost
will obey the underlying address resolution like DNS.
.Pp
The
.Fn rresvport
and
.Fn rresvport_af
functions are used to obtain a socket with a privileged
address bound to it.
This socket is suitable for use by
.Fn rcmd
and several other functions.
Privileged Internet ports are those in the range 0 to
.Va IPPORT_RESERVED - 1 ,
which happens to be 1023.
Only the superuser is allowed to bind an address of this sort to a socket.
.Fn rresvport
and
.Fn rresvport_af
need to be seeded with a port number; if that port
is not available these functions will find another.
.Pp
The
.Fn ruserok
function takes a remote host's name, two user names,
and a flag indicating whether the local user's
name is that of the superuser.
Then, if the user is
.Em not
the superuser, it checks the
.Pa /etc/hosts.equiv
file.
If that lookup is not done, or is unsuccessful, the
.Pa .rhosts
in the local user's home directory is checked to see if the request for
service is allowed.
.Pp
If this file does not exist, is not a regular file, is owned by anyone
other than the user or the superuser, or is writeable by anyone other
than the owner, the check automatically fails.
Zero is returned if the machine name is listed in the
.Pa hosts.equiv
file, or the host and remote user name are found in the
.Pa .rhosts
file; otherwise
.Fn ruserok
returns \-1.
If the local domain (as obtained from
.Xr getaddrinfo 3 )
is the same as the remote domain, only the machine name need be specified.
.Pp
.Fn ruserok
implicitly requires trusting the DNS server for the remote host's domain.
.Sh DIAGNOSTICS
The
.Fn rcmd
function returns a valid socket descriptor on success.
It returns \-1 on error and prints a diagnostic message on the standard error.
.Pp
The
.Fn rresvport
and
.Fn rresvport_af
functions return a valid, bound socket descriptor on success.
It returns \-1 on error with the global value
.Va errno
set according to the reason for failure.
The error code
.Er EAGAIN
is overloaded to mean
.Dq all network ports in use .
.Sh SEE ALSO
.Xr ssh 1 ,
.Xr intro 2 ,
.Xr bindresvport 3 ,
.Xr bindresvport_sa 3 ,
.Xr rcmdsh 3
.Sh HISTORY
These
functions appeared in
.Bx 4.2 .
.Pp
The
.Fn iruserok
and
.Fn iruserok_sa
functions, IP address based versions of
.Fn ruserok ,
were removed in
.Ox 6.0 .