HardenedBSD/usr.sbin/ypserv/ypserv.8

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.\"	Bill Paul <wpaul@ctr.columbia.edu>.  All rights reserved.
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.Dd December 13, 2009
.Dt YPSERV 8
.Os
.Sh NAME
.Nm ypserv
.Nd NIS database server
.Sh SYNOPSIS
.Nm
.Op Fl n
.Op Fl d
.Op Fl P Ar port
.Op Fl p Ar path
.Sh DESCRIPTION
.Tn NIS
is an RPC-based service designed to allow a number of UNIX-based
machines to share a common set of configuration files.
Rather than
requiring a system administrator to update several copies of files
such as
.Pa /etc/hosts ,
.Pa /etc/passwd
and
.Pa /etc/group ,
which tend to require frequent changes in most environments,
.Tn NIS
allows groups of computers to share one set of data which can be
updated from a single location.
.Pp
The
.Nm
utility is the server that distributes
.Tn NIS
databases to client systems within an
.Tn NIS
.Em domain .
Each client in an
.Tn NIS
domain must have its domainname set to
one of the domains served by
.Nm
using the
.Xr domainname 1
command.
The clients must also run
.Xr ypbind 8
in order to attach to a particular server, since it is possible to
have several servers within a single
.Tn NIS
domain.
.Pp
The databases distributed by
.Nm
are stored in
.Pa /var/yp/[domainname]
where
.Pa domainname
is the name of the domain being served.
There can be several
such directories with different domainnames, and you need only one
.Nm
daemon to handle them all.
.Pp
The databases, or
.Pa maps
as they are often called,
are created by
.Pa /var/yp/Makefile
using several system files as source.
The database files are in
.Xr db 3
format to help speed retrieval when there are many records involved.
In
.Fx ,
the maps are always readable and writable only by root for security
reasons.
Technically this is only necessary for the password
maps, but since the data in the other maps can be found in
other world-readable files anyway, it does not hurt and it is considered
good general practice.
.Pp
The
.Nm
utility is started by
.Pa /etc/rc.d/ypserv
if it has been enabled in
.Pa /etc/rc.conf .
.Sh SPECIAL FEATURES
There are some problems associated with distributing a
.Fx
password
database via
.Tn NIS :
.Fx
normally only stores encrypted passwords
in
.Pa /etc/master.passwd ,
which is readable and writable only by root.
By turning this file
into an
.Tn NIS
map, this security feature would be completely defeated.
.Pp
To make up for this, the
.Fx
version of
.Nm
handles the
.Pa master.passwd.byname
and
.Pa master.passwd.byuid
maps in a special way.
When the server receives a request to access
either of these two maps (or in fact either of the
.Pa shadow.byname
or
.Pa shadow.byuid
maps), it will check the TCP port from which the
request originated and return an error if the port number is greater
than 1023.
Since only the superuser is allowed to bind to TCP ports
with values less than 1024, the server can use this test to determine
whether or not the access request came from a privileged user.
Any requests made by non-privileged users are therefore rejected.
.Pp
Furthermore, the
.Xr getpwent 3
routines in the
.Fx
standard C library will only attempt to retrieve
data from the
.Pa master.passwd.byname
and
.Pa master.passwd.byuid
maps for the superuser: if a normal user calls any of these functions,
the standard
.Pa passwd.byname
and
.Pa passwd.byuid
maps will be accessed instead.
The latter two maps are constructed by
.Pa /var/yp/Makefile
by parsing the
.Pa master.passwd
file and stripping out the password fields, and are therefore
safe to pass on to unprivileged users.
In this way, the shadow password
aspect of the protected
.Pa master.passwd
database is maintained through
.Tn NIS .
.Sh NOTES
.Ss Setting Up Master and Slave Servers
.Xr ypinit 8
is a convenient script that will help setup master and slave
.Tn NIS
servers.
.Ss Limitations
There are two problems inherent with password shadowing in
.Tn NIS
that users should
be aware of:
.Bl -enum -offset indent
.It
The
.Sq TCP port less than 1024
test is trivial to defeat for users with
unrestricted access to machines on your network (even those machines
which do not run UNIX-based operating systems).
.It
If you plan to use a
.Fx
system to serve
.No non- Ns Fx
clients that
have no support for password shadowing (which is most of them), you
will have to disable the password shadowing entirely by uncommenting the
.Em UNSECURE=True
entry in
.Pa /var/yp/Makefile .
This will cause the standard
.Pa passwd.byname
and
.Pa passwd.byuid
maps to be generated with valid encrypted password fields, which is
necessary in order for
.No non- Ns Fx
clients to perform user
authentication through
.Tn NIS .
.El
.Ss Security
In general, any remote user can issue an RPC to
.Nm
and retrieve the contents of your
.Tn NIS
maps, provided the remote user
knows your domain name.
To prevent such unauthorized transactions,
.Nm
supports a feature called
.Pa securenets
which can be used to restrict access to a given set of hosts.
At startup,
.Nm
will attempt to load the securenets information from a file
called
.Pa /var/yp/securenets .
(Note that this path varies depending on the path specified with
the
.Fl p
option, which is explained below.)
This file contains entries
that consist of a network specification and a network mask separated
by white space.
Lines starting with
.Dq \&#
are considered to be comments.
A
sample securenets file might look like this:
.Bd -unfilled -offset indent
# allow connections from local host -- mandatory
127.0.0.1     255.255.255.255
# allow connections from any host
# on the 192.168.128.0 network
192.168.128.0 255.255.255.0
# allow connections from any host
# between 10.0.0.0 to 10.0.15.255
10.0.0.0      255.255.240.0
.Ed
.Pp
If
.Nm
receives a request from an address that matches one of these rules,
it will process the request normally.
If the address fails to match
a rule, the request will be ignored and a warning message will be
logged.
If the
.Pa /var/yp/securenets
file does not exist,
.Nm
will allow connections from any host.
.Pp
The
.Nm
utility also has support for Wietse Venema's
.Em tcpwrapper
package.
This allows the administrator to use the tcpwrapper
configuration files
.Pa ( /etc/hosts.allow
and
.Pa /etc/hosts.deny )
for access control instead of
.Pa /var/yp/securenets .
.Pp
Note: while both of these access control mechanisms provide some
security, they, like the privileged port test, are both vulnerable
to
.Dq IP spoofing
attacks.
.Ss NIS v1 compatibility
This version of
.Nm
has some support for serving
.Tn NIS
v1 clients.
The
.Fx
.Tn NIS
implementation only uses the
.Tn NIS
v2 protocol, however other implementations
include support for the v1 protocol for backwards compatibility
with older systems.
The
.Xr ypbind 8
daemons supplied with these systems will try to establish a binding
to an
.Tn NIS
v1 server even though they may never actually need it (and they may
persist in broadcasting in search of one even after they receive a
response from a v2 server).
Note that while
support for normal client calls is provided, this version of
.Nm
does not handle v1 map transfer requests; consequently, it cannot
be used as a master or slave in conjunction with older
.Tn NIS
servers that
only support the v1 protocol.
Fortunately, there probably are not any
such servers still in use today.
.Ss NIS servers that are also NIS clients
Care must be taken when running
.Nm
in a multi-server domain where the server machines are also
.Tn NIS
clients.
It is generally a good idea to force the servers to
bind to themselves rather than allowing them to broadcast bind
requests and possibly become bound to each other: strange failure
modes can result if one server goes down and
others are dependent upon on it.
(Eventually all the clients will
time out and attempt to bind to other servers, but the delay
involved can be considerable and the failure mode is still present
since the servers might bind to each other all over again).
.Pp
Refer to the
.Xr ypbind 8
man page for details on how to force it to bind to a particular
server.
.Sh OPTIONS
The following options are supported by
.Nm :
.Bl -tag -width flag
.It Fl n
This option affects the way
.Nm
handles yp_match requests for the
.Pa hosts.byname
and
.Pa hosts.byaddress
maps.
By default, if
.Nm
cannot find an entry for a given host in its hosts maps, it will
return an error and perform no further processing.
With the
.Fl n
flag,
.Nm
will go one step further: rather than giving up immediately, it
will try to resolve the hostname or address using a DNS nameserver
query.
If the query is successful,
.Nm
will construct a fake database record and return it to the client,
thereby making it seem as though the client's yp_match request
succeeded.
.Pp
This feature is provided for compatibility with SunOS 4.1.x,
which has brain-damaged resolver functions in its standard C
library that depend on
.Tn NIS
for hostname and address resolution.
The
.Fx
resolver can be configured to do DNS
queries directly, therefore it is not necessary to enable this
option when serving only
.Fx
.Tn NIS
clients.
.It Fl d
Cause the server to run in debugging mode.
Normally,
.Nm
reports only unusual errors (access violations, file access failures)
using the
.Xr syslog 3
facility.
In debug mode, the server does not background
itself and prints extra status messages to stderr for each
request that it receives.
Also, while running in debug mode,
.Nm
will not spawn any additional subprocesses as it normally does
when handling yp_all requests or doing DNS lookups.
(These actions
often take a fair amount of time to complete and are therefore handled
in subprocesses, allowing the parent server process to go on handling
other requests.)
This makes it easier to trace the server with
a debugging tool.
.It Fl h Ar addr
Specify a specific address to bind to for requests.  This option may be
specified multiple times.  If no
.Fl h
option is specified,
.Nm
will bind to default passive address
.Pq e.g. INADDR_ANY for IPv4
for each transport.
.It Fl P Ar port
Force ypserv to bind to a specific TCP/UDP port, rather than selecting
its own.
.It Fl p Ar path
Normally,
.Nm
assumes that all
.Tn NIS
maps are stored under
.Pa /var/yp .
The
.Fl p
flag may be used to specify an alternate
.Tn NIS
root path, allowing
the system administrator to move the map files to a different place
within the file system.
.El
.Sh FILES
.Bl -tag -width Pa -compact
.It Pa /var/yp/[domainname]/[maps]
the
.Tn NIS
maps
.It Pa /etc/nsswitch.conf
name switch configuration file
.It Pa /var/yp/securenets
host access control file
.El
.Sh SEE ALSO
.Xr ypcat 1 ,
.Xr db 3 ,
.Xr hosts_access 5 ,
.Xr rpc.yppasswdd 8 ,
.Xr yp 8 ,
.Xr ypbind 8 ,
.Xr ypinit 8 ,
.Xr yppush 8 ,
.Xr ypxfr 8
.Sh HISTORY
This version of
.Nm
first appeared in
.Fx 2.2 .
.Sh AUTHORS
.An Bill Paul Aq Mt wpaul@ctr.columbia.edu