1rcmd(3)                    Library Functions Manual                    rcmd(3)
2
3
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

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <netdb.h>    /* Or <unistd.h> on some systems */
14
15       int rcmd(char **restrict ahost, unsigned short inport,
16                   const char *restrict locuser,
17                   const char *restrict remuser,
18                   const char *restrict cmd, int *restrict fd2p);
19
20       int rresvport(int *port);
21
22       int iruserok(uint32_t raddr, int superuser,
23                   const char *ruser, const char *luser);
24       int ruserok(const char *rhost, int superuser,
25                   const char *ruser, const char *luser);
26
27       int rcmd_af(char **restrict ahost, unsigned short inport,
28                   const char *restrict locuser,
29                   const char *restrict remuser,
30                   const char *restrict cmd, int *restrict fd2p,
31                   sa_family_t af);
32
33       int rresvport_af(int *port, sa_family_t af);
34
35       int iruserok_af(const void *restrict raddr, int superuser,
36                   const char *restrict ruser, const char *restrict luser,
37                   sa_family_t af);
38       int ruserok_af(const char *rhost, int superuser,
39                   const char *ruser, const char *luser,
40                   sa_family_t af);
41
42   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
43
44       rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(),
45       iruserok_af(), ruserok(), ruserok_af():
46           Since glibc 2.19:
47               _DEFAULT_SOURCE
48           glibc 2.19 and earlier:
49               _BSD_SOURCE
50

DESCRIPTION

52       The  rcmd() function is used by the superuser to execute a command on a
53       remote machine using an authentication scheme based on privileged  port
54       numbers.   The  rresvport()  function  returns  a  file descriptor to a
55       socket with an address in the privileged port  space.   The  iruserok()
56       and ruserok() functions are used by servers to authenticate clients re‐
57       questing service with rcmd().  All  four  functions  are  used  by  the
58       rshd(8) server (among others).
59
60   rcmd()
61       The  rcmd()  function  looks up the host *ahost using gethostbyname(3),
62       returning -1 if the host does not exist.  Otherwise, *ahost is  set  to
63       the  standard  name  of  the  host and a connection is established to a
64       server residing at the well-known Internet port inport.
65
66       If the connection succeeds, a socket in the  Internet  domain  of  type
67       SOCK_STREAM  is returned to the caller, and given to the remote command
68       as stdin and stdout.  If fd2p is nonzero, then an auxiliary channel  to
69       a  control process will be set up, and a file descriptor for it will be
70       placed in *fd2p.  The control process  will  return  diagnostic  output
71       from  the  command (unit 2) on this channel, and will also accept bytes
72       on this channel as being UNIX signal numbers, to be  forwarded  to  the
73       process group of the command.  If fd2p is 0, then the stderr (unit 2 of
74       the remote command) will be made the same as the stdout and  no  provi‐
75       sion  is  made for sending arbitrary signals to the remote process, al‐
76       though you may be able to get its attention by using out-of-band data.
77
78       The protocol is described in detail in rshd(8).
79
80   rresvport()
81       The rresvport() function is used to obtain a socket with  a  privileged
82       port  bound  to it.  This socket is suitable for use by rcmd() and sev‐
83       eral other functions.  Privileged ports are those in  the  range  0  to
84       1023.   Only  a  privileged  process  (on Linux, a process that has the
85       CAP_NET_BIND_SERVICE capability in the  user  namespace  governing  its
86       network  namespace)  is  allowed  to bind to a privileged port.  In the
87       glibc implementation, this function restricts its search to  the  ports
88       from 512 to 1023.  The port argument is value-result: the value it sup‐
89       plies to the call is used as the starting point for a  circular  search
90       of  the port range; on (successful) return, it contains the port number
91       that was bound to.
92
93   iruserok() and ruserok()
94       The iruserok() and ruserok() functions take a remote host's IP  address
95       or  name, respectively, two usernames and a flag indicating whether the
96       local user's name is that of the superuser.  Then, if the user  is  not
97       the  superuser, it checks the /etc/hosts.equiv file.  If that lookup is
98       not done, or is unsuccessful, the .rhosts in the local user's home  di‐
99       rectory is checked to see if the request for service is allowed.
100
101       If  this file does not exist, is not a regular file, is owned by anyone
102       other than the user or the superuser, is writable by anyone other  than
103       the  owner,  or  is hardlinked anywhere, the check automatically fails.
104       Zero is returned if the machine name is listed in the hosts.equiv file,
105       or  the  host and remote username are found in the .rhosts file; other‐
106       wise iruserok() and ruserok() return -1.  If the local domain  (as  ob‐
107       tained  from gethostname(2)) is the same as the remote domain, only the
108       machine name need be specified.
109
110       If the IP address of the remote host is  known,  iruserok()  should  be
111       used  in  preference  to ruserok(), as it does not require trusting the
112       DNS server for the remote host's domain.
113
114   *_af() variants
115       All of the functions described above work with IPv4 (AF_INET)  sockets.
116       The  "_af"  variants  take an extra argument that allows the socket ad‐
117       dress family to be specified.  For these functions, the af argument can
118       be  specified  as AF_INET or AF_INET6.  In addition, rcmd_af() supports
119       the use of AF_UNSPEC.
120

RETURN VALUE

122       The rcmd() function returns a valid socket descriptor on  success.   It
123       returns -1 on error and prints a diagnostic message on the standard er‐
124       ror.
125
126       The rresvport() function returns a valid, bound  socket  descriptor  on
127       success.   On failure, it returns -1 and sets errno to indicate the er‐
128       ror.  The error code EAGAIN is overloaded to mean: "All  network  ports
129       in use".
130
131       For information on the return from ruserok() and iruserok(), see above.
132

ATTRIBUTES

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

STANDARDS

149       BSD.
150

HISTORY

152       iruserok_af()
153       rcmd_af()
154       rresvport_af()
155       ruserok_af()
156              glibc 2.2.
157
158       Solaris, 4.2BSD.  The "_af" variants are more recent additions, and are
159       not present on as wide a range of systems.
160

BUGS

162       iruserok() and iruserok_af() are declared in glibc headers  only  since
163       glibc 2.12.
164

SEE ALSO

166       rlogin(1), rsh(1), rexec(3), rexecd(8), rlogind(8), rshd(8)
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169
170Linux man-pages 6.04              2023-03-30                           rcmd(3)
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