The term resolver refers not to a special
application, but to the resolver library. This is a collection of
functions that can be found in the standard C library. The central
routines are gethostbyname(2) and
gethostbyaddr (2), which look up all IP addresses
associated with a host name, and vice versa. They may be configured to
simply look up the information in hosts, to query
a number of DNS name servers, or to use the hosts
database of Network Information Service (NIS).
The resolver functions read configuration files when they are
invoked. From these configuration files, they determine what databases
to query, in which order, and other details relevant to how you've
configured your environment. The older Linux standard library, libc,
used /etc/host.conf as its master configuration
file, but Version 2 of the GNU standard library, glibc, uses
/etc/nsswitch.conf. We'll describe each in turn,
since both are commonly used.
The /etc/host.conf tells the older Linux standard library
resolver functions which services to use, and in what order.
Options in host.conf must appear on separate lines.
Fields may be separated by white space (spaces or tabs). A hash sign
(#) introduces a comment that extends
to the next newline. The following options are available:
option determines the order in which the resolving services are
tried. Valid options are bind
for querying the name server, hosts for lookups
in /etc/hosts, and nis for NIS lookups. Any or all of them
may be specified. The order in which they appear on the line
determines the order in which the respective services are tried.
multi takes on or
off as options. This determines if a
host in /etc/hosts is allowed to have several IP
addresses, which is usually referred to as being “multi-homed.”
The default is off. This flag has no effect on DNS or NIS queries.
As we'll explain in the section Section 6.2.4,” DNS allows you to find the
hostname belonging to an IP address by using the in-addr.arpa domain. Attempts by name
servers to supply a false hostname are called
spoofing. To guard against this, the resolver can
be configured to check whether the original IP address is in fact
associated with the obtained hostname. If not, the name is rejected
and an error is returned. This behavior is turned on by setting
This option takes on or
off as arguments. If it is turned on,
any spoof attempts will cause the resolver to log a message to the
This option takes an argument specifying a domain name that will be
removed from hostnames before lookup. This is useful for
hosts entries, for which you might only want to
specify hostnames without a local domain. If you specify your local
domain name here, it will be removed from a lookup of a host with the
local domain name appended, thus allowing the lookup in
/etc/hosts to succeed. The domain name you add
must end with the (.) character (e.g.,
:linux.org.au.) if trim is to
trim options accumulate; you
can consider your host as being local to several domains.
A sample file for vlager is shown in
Example 6-1. Sample host.conf File
# We have named running, but no NIS (yet)
# Allow multiple addrs
# Guard against spoof attempts
# Trim local domain (not really necessary).
The settings from host.conf may be overridden using a
number of environment variables:
This variable specifies a file to be read instead of
This variable overrides the order option given in
host.conf. Services are given as hosts, bind, and nis, separated by a space, comma, colon,
This variable determines the measures taken against spoofing. It is
completely disabled by off.
The values warn and
warn off enable spoof
checking by turning logging on and off, respectively. A value of
* turns on spoof checks, but
leaves the logging facility as defined in
This variable uses a value of on or off to override the multi options from
This variable specifies a list of trim domains that override those
given in host.conf. Trim domains were explained
earlier when we discussed the trim keyword.
This variable specifies a list of trim domains that are added to those
given in host.conf.
Version 2 of the GNU standard library includes a more powerful and
flexible replacement for the older host.conf
mechanism. The concept of the name service has been extended to
include a variety of different types of information. Configuration
options for all of the different functions that query these databases
have been brought back into a single configuration file; the
The nsswitch.conf file allows the system
administrator to configure a wide variety of different
databases. We'll limit our discussion to options that relate to host
and network IP address resolution. You can easily find more
information about the other features by reading the GNU standard
Options in nsswitch.conf must appear on separate
lines. Fields may be separated by whitespace (spaces or tabs). A hash
sign (#) introduces a comment
that extends to the next newline. Each line describes a particular
service; hostname resolution is one of these. The first field in each
line is the name of the database, ending with a colon. The database
name associated with host address resolution is hosts. A related database is networks, which is used for resolution of
network names into network addresses. The remainder of each line
stores options that determine the way lookups for that database
The following options are available:
Use the Domain Name System (DNS) service to resolve the address. This makes
sense only for host address resolution, not network address
resolution. This mechanism uses the
/etc/resolv.conf file that we'll describe later
in the chapter.
Search a local file for the host or network name and its corresponding address.
This option uses the traditional /etc/hosts and
- nis or nisplus
Use the Network Information System (NIS) to resolve the host or
network address. NIS and NIS+ are discussed in detail in Chapter 13.
The order in which the services to be queried are listed determines the
order in which they are queried when attempting to resolve a name.
The query-order list is in the service description in the
/etc/nsswitch.conf file. The services are queried
from left to right and by default searching stops when a resolution is
A simple example of host and network database specification that would
mimic our configuration using the older libc standard library is shown
in Example 6-2.
Example 6-2. Sample nsswitch.conf File
# Example configuration of GNU Name Service Switch functionality.
# Information about this file is available in the `libc6-doc' package.
hosts: dns files
This example causes the system to look up hosts first in the Domain
Name System, and the /etc/hosts file, if that
can't find them. Network name lookups would be attempted using only
the /etc/networks file.
You are able to control the lookup
behavior more precisely using “action items” that describe
what action to take given the result of the previous lookup
attempt. Action items appear between service specifications, and are
enclosed within square brackets, [
]. The general syntax of the action statement is:
[ [!] status = action ... ]
There are two possible actions:
Controls returns to the program that attempted the name resolution. If a lookup attempt was successful, the resolver will return with the details,
otherwise it will return a zero result.
The resolver will move on to the next service in the list and attempt
resolution using it.
The optional (!) character specifies that the status value should
be inverted before testing; that is, it means “not.”
The available status values on which we can act are:
The requested entry was found without error. The default action for
this status is return.
There was no error in the lookup, but the target host or network could not be
found. The default action for this status is
The service queried was unavailable. This could mean that the
hosts or networks file was unreadable
for the files service or that a
name server or NIS server did not respond for the
The default action for this status is
This status means the service is temporarily unavailable. For the
files files service, this would
usually indicate that the relevant file was locked by some
process. For other services, it may mean the server was temporarily
unable to accept connections. The default action for this status is
A simple example of how you might use this mechanism is shown in
Example 6-3. Sample nsswitch.conf File Using an Action Statement
# Example configuration of GNU Name Service Switch functionality.
# Information about this file is available in the `libc6-doc' package.
hosts: dns [!UNAVAIL=return] files
This example attempts host resolution using the Domain Name Service
system. If the return status is anything other than unavailable, the
resolver returns whatever it has found. If, and only if, the DNS
lookup attempt returns an unavailable status, the resolver attempts to
use the local /etc/hosts. This means that we
should use the hosts file only if our name server
is unavailable for some reason.
When configuring the resolver library to use the BIND name service for host
lookups, you also have to tell it which name servers to use. There is a
separate file for this called resolv.conf. If this file
does not exist or is empty, the resolver assumes the name server is on your
To run a name server on your local host, you have to set it up separately,
as will be explained in the following section. If you are on a local network
and have the opportunity to use an existing name server, this should always
be preferred. If you use a dialup IP connection to the Internet, you
would normally specify the name server of your service provider in the
The most important option in resolv.conf is
name server, which gives the IP address
of a name server to use. If you specify several name servers by giving the
name server option several times, they
are tried in the order given. You should therefore put the most reliable
server first. The current implementation allows you to have up to three
name server statements in
resolv.conf. If no
name server option is given, the resolver
attempts to connect to the name server on the local host.
Two other options, domain and
search, let you use shortcut names
for hosts in your local domain. Usually, when just telnetting to another
host in your local domain, you don't want to type in the fully qualified
hostname, but use a name like gauss
on the command line and have the resolver tack on the
This is just the domain
statement's purpose. It lets you specify a default domain name to be
appended when DNS fails to look up a hostname. For instance, when
given the name gauss, the
resolver fails to find gauss. in DNS, because there is no such
top-level domain. When given mathematics.groucho.edu as a default
domain, the resolver repeats the query for gauss with the default domain appended,
this time succeeding.
That's just fine, you may think, but as soon you get out of the Math
department's domain, you're back to those fully qualified domain names.
Of course, you would also want to have shorthands like
quark.physics for hosts in the
Physics department's domain.
This is when the search list comes in. A search
list can be specified using the search option, which is a generalization
of the domain statement.
Where the latter gives a single default domain, the former specifies a
whole list of them, each to be tried in turn until a lookup
succeeds. This list must be separated by blanks or tabs.
The search and
domain statements are mutually exclusive
and may not appear more than once. If neither option is given, the resolver
will try to guess the default domain from the local hostname using the
getdomainname(2) system call. If the local hostname
doesn't have a domain part, the default domain will be assumed to be the
If you decide to put a search statement
into resolv.conf, you should be careful about what
domains you add to this list. Resolver libraries prior to BIND 4.9 used to
construct a default search list from the domain name when no search list was
given. This default list was made up of the default domain itself, plus all
of its parent domains up to the root. This caused some problems because DNS
requests wound up at name servers that were never meant to see them.
Assume you're at the Virtual Brewery and want to log in to foot.groucho.edu. By a slip
of your fingers, you mistype foot as foo, which doesn't exist. GMU's name
server will therefore tell you that it knows no such host. With the
old-style search list, the resolver would now go on trying the name
with vbrew.com and
com appended. The latter is
problematic because groucho.edu.com might actually be a valid
domain name. Their name server might then even find foo in their domain, pointing you to one
of their hosts—which clearly was not intended.
For some applications, these bogus host lookups can be a security problem.
Therefore, you should usually limit the domains on your search list to your
local organization, or something comparable. At the Mathematics department of
Groucho Marx University, the search list would commonly be set to
If default domains sound confusing to you, consider this sample
resolv.conf file for the Virtual Brewery:
# Our domain
# We use vlager as central name server:
name server 172.16.1.1
When resolving the name vale, the
resolver looks up vale and,
failing this, vale.vbrew.com.
When running a LAN inside a larger network, you definitely should use
central name servers if they are available. The name servers develop
rich caches that speed up repeat queries, since all queries are
forwarded to them. However, this scheme has a drawback: when a fire
destroyed the backbone cable at Olaf's university, no more work was
possible on his department's LAN because the resolver could no longer
reach any of the name servers. This situation caused difficulties with
most network services, such as X terminal logins and printing.
Although it is not very common for campus backbones to go down in flames,
one might want to take precautions against cases like this.
One option is to set up a local name server that resolves hostnames from your
local domain and forwards all queries for other hostnames to the main servers.
Of course, this is applicable only if you are running your own domain.
Alternatively, you can maintain a backup host table for your domain or LAN in
/etc/hosts. This is very simple to do. You simply ensure
that the resolver library queries DNS first, and the hosts file next. In an
/etc/host.conf file you'd use
and in an /etc/nsswitch.conf file you'd use
“hosts: dns files”,
to make the resolver fall back to the hosts file if the central name server is