host(1M) host(1M)
NAME
host - look up host names using domain server
SYNOPSIS
host [-v] [-w] [-r] [-d] [-t querytype] [-a] [-l] host [server]
DESCRIPTION
host looks for information about Internet hosts. It gets this informa-
tion from a set of interconnected servers that are spread across the
country. By default, it simply converts between host names and Inter-
net addresses. However with the -t or -a options, it can be used to
find all of the information about this host that is maintained by the
domain server.
The arguments can be either host names or host numbers. The program
first attempts to interpret them as host numbers. If this fails, it
will treat them as host names. A host number consists of first decimal
numbers separated by dots, e.g. 128.6.4.194.
A host name consists of names separated by dots, e.g.
topaz.rutgers.edu. Unless the name ends in a dot, the local domain is
automatically tacked on the end. Thus a Rutgers user can say "host
topaz", and it will actually look up "topaz.rutgers.edu". If this
fails, the name is tried unchanged (in this case, "topaz"). This same
convention is used for mail and other network utilities. The actual
suffix to tack on the end is obtained by looking at the results of a
"hostname" call, and using everything starting at the first dot. (See
below for a description of how to customize the host name lookup.)
The first argument is the host name you want to look up. If this is a
number, an "inverse query" is done, i.e. the domain system looks in a
separate set of databases used to convert numbers to names.
The second argument is optional. It allows you to specify a particular
server to query. If you don't specify this argument, the default
server is used.
If a name is specified, you may see output of three different kinds.
Here is an example that shows all of them:
% host sni4
sni4.rutgers.edu is a nickname for ATHOS.RUTGERS.EDU
ATHOS.RUTGERS.EDU has address 128.6.5.46
ATHOS.RUTGERS.EDU has address 128.6.4.4
ATHOS.RUTGERS.EDU mail is handled by ARAMIS.RUTGERS.EDU
The user has typed the command "host sni4". The first line indicates
that the name "sni4.rutgers.edu" is actually a nickname. The official
host name is "ATHOS.RUTGERS.EDU". The next two lines show the address.
If a system has more than one network interface, there will be a
separate address for each. The last line indicates that
"ATHOS.RUTGERS.EDU" does not receive its own mail. Mail for it is
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taken by "ARAMIS.RUTGERS.EDU". There may be more than one such line,
since some systems have more than one other system that will handle
mail for them. Technically, every system that can receive mail is sup-
posed to have an entry of this kind. If the system receives its own
mail, there should be an entry the mentions the system itself, for
example "XXX mail is handled by XXX". However many systems that
receive their own mail do not bother to mention that fact. If a system
has a "mail is handled by" entry, but no address, this indicates that
it is not really part of the Internet, but a system that is on the
network will forward mail to it. Systems on Usenet, Bitnet, and a
number of other networks have entries of this kind.
OPTIONS
There are a number of options that can be used before the host name.
Most of these options are meaningful only to the staff who have to
maintain the domain database.
-v Causes printout to be in a "verbose" format. This is the official
domain master file format, which is documented in named(1M).
Without this option, output still follows this format in general
terms, but some attempt is made to make it more intelligible to
normal users. Without -v, a, mx, and cname records are written
out as "has address", "mail is handled by", and "is a nickname
for", and TTL and class fields are not shown.
-w Causes host to wait forever for a response. Normally it will time
out after around a minute.
-r Causes recursion to be turned off in the request. This means that
the name server will return only data it has in its own database.
It will not ask other servers for more information.
-d Turns on debugging. Network transactions are shown in detail.
-t Allows you to specify a particular type of information to be
looked up. The arguments are defined in named(1M). Currently sup-
ported types are a, ns, md, mf, cname, soa, mb, mg, mr, null,
wks, ptr, hinfo, minfo, mx, uinfo, uid, gid, unspec, and the
wildcard, which may be written as either "any" or "*". Types must
be given in lower case. Note that the default is to look first
for a, and then mx, except that if the verbose option is turned
on, the default is only a.
-a The option -a (for "all") is equivalent to -v -t any.
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-l The option -l causes a listing of a complete domain. E.g.
host -l rutgers.edu
will give a listing of all hosts in the rutgers.edu domain. The
-t option is used to filter what information is presented, as you
would expect. The default is address information, which also
include PTR and NS records. The command
host -l -v -t any rutgers.edu
will give a complete download of the zone data for rutgers.edu,
in the official master file format. (However the SOA record is
listed twice.) Note: -l is implemented by doing a complete zone
transfer and then filtering out the information the you have
asked for. This command should be used only if it is absolutely
necessary.
CUSTOMIZING HOST NAME LOOKUP
In general, if the name supplied by the user does not have any dots in
it, a default domain is appended to the end. This domain can be
defined in /etc/resolv.conf, but is normally derived by taking the
local hostname after its first dot. The user can override this, and
specify a different default domain, using the environment variable
LOCALDOMAIN. In addition, the user can supply his own abbreviations
for host names. They should be in a file consisting of one line per
abbreviation. Each line contains an abbreviation, a space, and then
the full host name. This file must be pointed to by an environment
variable HOSTALIASES, which is the name of the file.
NOTES
You may get unexpected results if you type in a name that is not part
of the local domain. Always bear in mind that the local domain name is
appended to every name unless it ends in a dot. The original name is
used only if it was not possible to append the local domain name.
The -l option only tries the first name server listed for the domain
that you have requested. If this server is dead, you may need to
specify a server manually. For example, to get a listing of foo.edu,
you could try "host -t ns foo.edu" to get a list of all the name
servers for foo.edu, and then try "host -l foo.edu xxx" for all xxx on
the list of name servers, until you find one that works.
FILES
/usr/lib/named/bog.ascii
/usr/lib/named/bog.ps
Name Server Operations Guide for BIND ("BOG"); ASCII and
PostScript format
SEE ALSO
named(1M), resolv.conf(4).
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