YPSERV(8) BSD System Manager's Manual YPSERV(8)NAMEypserv — NIS database server
SYNOPSISypserv [-n] [-d] [-P port] [-p path]
DESCRIPTION
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 /etc/hosts, /etc/passwd and /etc/group, which tend to require frequent
changes in most environments, NIS allows groups of computers to share one
set of data which can be updated from a single location.
The ypserv utility is the server that distributes NIS databases to client
systems within an NIS domain. Each client in an NIS domain must have its
domainname set to one of the domains served by ypserv using the
domainname(1) command. The clients must also run ypbind(8) in order to
attach to a particular server, since it is possible to have several
servers within a single NIS domain.
The databases distributed by ypserv are stored in /var/yp/[domainname]
where domainname is the name of the domain being served. There can be
several such directories with different domainnames, and you need only
one ypserv daemon to handle them all.
The databases, or maps as they are often called, are created by
/var/yp/Makefile using several system files as source. The database
files are in db(3) format to help speed retrieval when there are many
records involved. In FreeBSD, 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 consid‐
ered good general practice.
The ypserv utility is started by /etc/rc.d/ypserv if it has been enabled
in /etc/rc.conf.
SPECIAL FEATURES
There are some problems associated with distributing a FreeBSD password
database via NIS: FreeBSD normally only stores encrypted passwords in
/etc/master.passwd, which is readable and writable only by root. By
turning this file into an NIS map, this security feature would be com‐
pletely defeated.
To make up for this, the FreeBSD version of ypserv handles the
master.passwd.byname and 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 shadow.byname or 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 privi‐
leged user. Any requests made by non-privileged users are therefore
rejected.
Furthermore, the getpwent(3) routines in the FreeBSD standard C library
will only attempt to retrieve data from the master.passwd.byname and
master.passwd.byuid maps for the superuser: if a normal user calls any of
these functions, the standard passwd.byname and passwd.byuid maps will be
accessed instead. The latter two maps are constructed by
/var/yp/Makefile by parsing the 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 master.passwd
database is maintained through NIS.
NOTES
Setting Up Master and Slave Servers
ypinit(8) is a convenient script that will help setup master and slave
NIS servers.
Limitations
There are two problems inherent with password shadowing in NIS that users
should be aware of:
1. The ‘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).
2. If you plan to use a FreeBSD system to serve non-FreeBSD
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 UNSECURE=True entry in
/var/yp/Makefile. This will cause the standard passwd.byname
and passwd.byuid maps to be generated with valid encrypted
password fields, which is necessary in order for non-FreeBSD
clients to perform user authentication through NIS.
Security
In general, any remote user can issue an RPC to ypserv and retrieve the
contents of your NIS maps, provided the remote user knows your domain
name. To prevent such unauthorized transactions, ypserv supports a fea‐
ture called securenets which can be used to restrict access to a given
set of hosts. At startup, ypserv will attempt to load the securenets
information from a file called /var/yp/securenets. (Note that this path
varies depending on the path specified with the -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 start‐
ing with “#” are considered to be comments. A sample securenets file
might look like this:
# 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
If ypserv 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 /var/yp/securenets file does not exist, ypserv will allow
connections from any host.
The ypserv utility also has support for Wietse Venema's tcpwrapper pack‐
age. This allows the administrator to use the tcpwrapper configuration
files (/etc/hosts.allow and /etc/hosts.deny) for access control instead
of /var/yp/securenets.
Note: while both of these access control mechanisms provide some secu‐
rity, they, like the privileged port test, are both vulnerable to “IP
spoofing” attacks.
NIS v1 compatibility
This version of ypserv has some support for serving NIS v1 clients. The
FreeBSD NIS implementation only uses the NIS v2 protocol, however other
implementations include support for the v1 protocol for backwards compat‐
ibility with older systems. The ypbind(8) daemons supplied with these
systems will try to establish a binding to an 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
ypserv does not handle v1 map transfer requests; consequently, it cannot
be used as a master or slave in conjunction with older NIS servers that
only support the v1 protocol. Fortunately, there probably are not any
such servers still in use today.
NIS servers that are also NIS clients
Care must be taken when running ypserv in a multi-server domain where the
server machines are also 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).
Refer to the ypbind(8) man page for details on how to force it to bind to
a particular server.
OPTIONS
The following options are supported by ypserv:
-n This option affects the way ypserv handles yp_match requests for
the hosts.byname and hosts.byaddress maps. By default, if ypserv
cannot find an entry for a given host in its hosts maps, it will
return an error and perform no further processing. With the -n
flag, ypserv will go one step further: rather than giving up imme‐
diately, it will try to resolve the hostname or address using a DNS
nameserver query. If the query is successful, ypserv will con‐
struct a fake database record and return it to the client, thereby
making it seem as though the client's yp_match request succeeded.
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 NIS for hostname and address resolution. The FreeBSD
resolver can be configured to do DNS queries directly, therefore it
is not necessary to enable this option when serving only FreeBSD
NIS clients.
-d Cause the server to run in debugging mode. Normally, ypserv
reports only unusual errors (access violations, file access fail‐
ures) using the 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, ypserv will not spawn any additional subprocesses as it nor‐
mally 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.
-P port
Force ypserv to bind to a specific TCP/UDP port, rather than
selecting its own.
-p path
Normally, ypserv assumes that all NIS maps are stored under
/var/yp. The -p flag may be used to specify an alternate NIS root
path, allowing the system administrator to move the map files to a
different place within the file system.
FILES
/var/yp/[domainname]/[maps] the NIS maps
/etc/nsswitch.conf name switch configuration file
/var/yp/securenets host access control file
SEE ALSOypcat(1), db(3), hosts_access(5), rpc.yppasswdd(8), yp(8), ypbind(8),
ypinit(8), yppush(8), ypxfr(8)HISTORY
This version of ypserv first appeared in FreeBSD 2.2.
AUTHORS
Bill Paul ⟨wpaul@ctr.columbia.edu⟩
BSD June 25, 2009 BSD
