1RCMD(3)                    Linux Programmer's Manual                   RCMD(3)
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4

NAME

6       rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af,
7       ruserok_af - routines for returning a stream to a remote command
8

SYNOPSIS

10       #include <netdb.h>    /* Or <unistd.h> on some systems */
11
12       int rcmd(char **restrict ahost, unsigned short inport,
13                   const char *restrict locuser,
14                   const char *restrict remuser,
15                   const char *restrict cmd, int *restrict fd2p);
16
17       int rresvport(int *port);
18
19       int iruserok(uint32_t raddr, int superuser,
20                   const char *ruser, const char *luser);
21       int ruserok(const char *rhost, int superuser,
22                   const char *ruser, const char *luser);
23
24       int rcmd_af(char **restrict ahost, unsigned short inport,
25                   const char *restrict locuser,
26                   const char *restrict remuser,
27                   const char *restrict cmd, int *restrict fd2p,
28                   sa_family_t af);
29
30       int rresvport_af(int *port, sa_family_t af);
31
32       int iruserok_af(const void *restrict raddr, int superuser,
33                   const char *restrict ruser, const char *restrict luser,
34                   sa_family_t af);
35       int ruserok_af(const char *rhost, int superuser,
36                   const char *ruser, const char *luser,
37                   sa_family_t af);
38
39   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
40
41       rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(),
42       iruserok_af(), ruserok(), ruserok_af():
43           Since glibc 2.19:
44               _DEFAULT_SOURCE
45           Glibc 2.19 and earlier:
46               _BSD_SOURCE
47

DESCRIPTION

49       The  rcmd() function is used by the superuser to execute a command on a
50       remote machine using an authentication scheme based on privileged  port
51       numbers.   The  rresvport()  function  returns  a  file descriptor to a
52       socket with an address in the privileged port  space.   The  iruserok()
53       and ruserok() functions are used by servers to authenticate clients re‐
54       questing service with rcmd().  All  four  functions  are  used  by  the
55       rshd(8) server (among others).
56
57   rcmd()
58       The  rcmd()  function  looks up the host *ahost using gethostbyname(3),
59       returning -1 if the host does not exist.  Otherwise, *ahost is  set  to
60       the  standard  name  of  the  host and a connection is established to a
61       server residing at the well-known Internet port inport.
62
63       If the connection succeeds, a socket in the  Internet  domain  of  type
64       SOCK_STREAM  is returned to the caller, and given to the remote command
65       as stdin and stdout.  If fd2p is nonzero, then an auxiliary channel  to
66       a  control process will be set up, and a file descriptor for it will be
67       placed in *fd2p.  The control process  will  return  diagnostic  output
68       from  the  command (unit 2) on this channel, and will also accept bytes
69       on this channel as being UNIX signal numbers, to be  forwarded  to  the
70       process group of the command.  If fd2p is 0, then the stderr (unit 2 of
71       the remote command) will be made the same as the stdout and  no  provi‐
72       sion  is  made for sending arbitrary signals to the remote process, al‐
73       though you may be able to get its attention by using out-of-band data.
74
75       The protocol is described in detail in rshd(8).
76
77   rresvport()
78       The rresvport() function is used to obtain a socket with  a  privileged
79       port  bound  to it.  This socket is suitable for use by rcmd() and sev‐
80       eral other functions.  Privileged ports are those in  the  range  0  to
81       1023.   Only  a  privileged  process  (on Linux, a process that has the
82       CAP_NET_BIND_SERVICE capability in the  user  namespace  governing  its
83       network  namespace)  is  allowed  to bind to a privileged port.  In the
84       glibc implementation, this function restricts its search to  the  ports
85       from 512 to 1023.  The port argument is value-result: the value it sup‐
86       plies to the call is used as the starting point for a  circular  search
87       of  the port range; on (successful) return, it contains the port number
88       that was bound to.
89
90   iruserok() and ruserok()
91       The iruserok() and ruserok() functions take a remote host's IP  address
92       or  name, respectively, two usernames and a flag indicating whether the
93       local user's name is that of the superuser.  Then, if the user  is  not
94       the  superuser, it checks the /etc/hosts.equiv file.  If that lookup is
95       not done, or is unsuccessful, the .rhosts in the local user's home  di‐
96       rectory is checked to see if the request for service is allowed.
97
98       If  this file does not exist, is not a regular file, is owned by anyone
99       other than the user or the superuser, is writable by anyone other  than
100       the  owner,  or  is hardlinked anywhere, the check automatically fails.
101       Zero is returned if the machine name is listed in the hosts.equiv file,
102       or  the  host and remote username are found in the .rhosts file; other‐
103       wise iruserok() and ruserok() return -1.  If the local domain  (as  ob‐
104       tained  from gethostname(2)) is the same as the remote domain, only the
105       machine name need be specified.
106
107       If the IP address of the remote host is  known,  iruserok()  should  be
108       used  in  preference  to ruserok(), as it does not require trusting the
109       DNS server for the remote host's domain.
110
111   *_af() variants
112       All of the functions described above work with IPv4 (AF_INET)  sockets.
113       The  "_af"  variants  take an extra argument that allows the socket ad‐
114       dress family to be specified.  For these functions, the af argument can
115       be  specified  as AF_INET or AF_INET6.  In addition, rcmd_af() supports
116       the use of AF_UNSPEC.
117

RETURN VALUE

119       The rcmd() function returns a valid socket descriptor on  success.   It
120       returns -1 on error and prints a diagnostic message on the standard er‐
121       ror.
122
123       The rresvport() function returns a valid, bound  socket  descriptor  on
124       success.   On failure, it returns -1 and sets errno to indicate the er‐
125       ror.  The error code EAGAIN is overloaded to mean: "All  network  ports
126       in use".
127
128       For information on the return from ruserok() and iruserok(), see above.
129

VERSIONS

131       The    functions    iruserok_af(),   rcmd_af(),   rresvport_af(),   and
132       ruserok_af() functions are provide in glibc since version 2.2.
133

ATTRIBUTES

135       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
136       tributes(7).
137
138       ┌─────────────────────────────────────┬───────────────┬────────────────┐
139Interface                            Attribute     Value          
140       ├─────────────────────────────────────┼───────────────┼────────────────┤
141rcmd(), rcmd_af()                    │ Thread safety │ MT-Unsafe      │
142       ├─────────────────────────────────────┼───────────────┼────────────────┤
143rresvport(), rresvport_af()          │ Thread safety │ MT-Safe        │
144       ├─────────────────────────────────────┼───────────────┼────────────────┤
145iruserok(), ruserok(),               │ Thread safety │ MT-Safe locale │
146iruserok_af(), ruserok_af()          │               │                │
147       └─────────────────────────────────────┴───────────────┴────────────────┘
148

CONFORMING TO

150       Not in POSIX.1.  Present on the BSDs, Solaris, and many other  systems.
151       These functions appeared in 4.2BSD.  The "_af" variants are more recent
152       additions, and are not present on as wide a range of systems.
153

BUGS

155       iruserok() and iruserok_af() are declared in glibc headers  only  since
156       version 2.12.
157

SEE ALSO

159       rlogin(1), rsh(1), rexec(3), rexecd(8), rlogind(8), rshd(8)
160

COLOPHON

162       This  page  is  part of release 5.12 of the Linux man-pages project.  A
163       description of the project, information about reporting bugs,  and  the
164       latest     version     of     this    page,    can    be    found    at
165       https://www.kernel.org/doc/man-pages/.
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169Linux                             2021-03-22                           RCMD(3)
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