Table of Contents
Use of the Domain Name System has been discussed in previous chapters, without going into detail on the setup of the server providing the service. This chapter describes setting up a simple, small domain with one Domain Name System (DNS) nameserver on a NetBSD system. It includes a brief explanation and overview of the DNS; further information can be obtained from the DNS Resources Directory (DNSRD) at http://www.dns.net/dnsrd/.
The DNS is a widely used naming service on the Internet and other TCP/IP networks. The network protocols, data and file formats, and other aspects of the DNS are Internet Standards, specified in a number of RFC documents, and described by a number of other reference and tutorial works. The DNS has a distributed, client-server architecture. There are reference implementations for the server and client, but these are not part of the standard. There are a number of additional implementations available for many platforms.
Naming services are used to provide a mapping between textual names and configuration data of some form. A nameserver maintains this mapping, and clients request the nameserver to resolve a name into its attached data.
The reader should have a good understanding of basic hosts to IP address mapping and IP address class specifications, see Section 22.6, “Name Service Concepts”.
In the case of the DNS, the configuration data bound to a name is in the form of standard Resource Records (RR's). These textual names conform to certain structural conventions.
The DNS presents a hierarchical name space, much like a UNIX filesystem, pictured as an inverted tree with the root at the top.
TOP-LEVEL .org
|
MID-LEVEL .diverge.org
______________________|________________________
| | |
BOTTOM-LEVEL strider.diverge.org samwise.diverge.org wormtongue.diverge.org
The system can also be logically divided even further if one wishes at different points. The example shown above shows three nodes on the diverge.org domain, but we could even divide diverge.org into subdomains such as "strider.net1.diverge.org", "samwise.net2.diverge.org" and "wormtongue.net2.diverge.org"; in this case, 2 nodes reside in "net2.diverge.org" and one in "net1.diverge.org".
There are directories of names, some of which may be sub-directories of further names. These directories are sometimes called zones. There is provision for symbolic links, redirecting requests for information on one name to the records bound to another name. Each name recognised by the DNS is called a Domain Name, whether it represents information about a specific host, or a directory of subordinate Domain Names (or both, or something else).
Unlike most filesystem naming schemes, however, Domain Names are written with the innermost name on the left, and progressively higher-level domains to the right, all the way up to the root directory if necessary. The separator used when writing Domain Names is a period, ".".
Like filesystem pathnames, Domain Names can be written in an
absolute or relative manner, though there are some differences
in detail. For instance, there is no way to indirectly refer
to the parent domain like with the UNIX
.. directory. Many (but not all)
resolvers offer a search path facility, so that
partially-specified names can be resolved relative to
additional listed sub-domains other than the client's own
domain. Names that are completely specified all the way to the
root are called Fully Qualified Domain
Names or FQDNs. A defining
characteristic of an FQDN is that it is written with a
terminating period. The same name, without the terminating
period, may be considered relative to some other
sub-domain. It is rare for this to occur without malicious
intent, but in part because of this possibility, FQDNs are
required as configuration parameters in some circumstances.
On the Internet, there are some established conventions for the names of the first few levels of the tree, at which point the hierarchy reaches the level of an individual organisation. This organisation is responsible for establishing and maintaining conventions further down the tree, within its own domain.
Resource Records for a domain are stored in a standardised format in an ASCII text file, often called a zone file. The following Resource Records are commonly used (a number of others are defined but not often used, or no longer used). In some cases, there may be multiple RR types associated with a name, and even multiple records of the same type.
Common DNS Resource Records
This record contains the numerical IP address associated with the name.
This record contains the Canonical Name (an FQDN with an associated A record) of the host name to which this record is bound. This record type is used to provide name aliasing, by providing a link to another name with which other appropriate RR's are associated. If a name has a CNAME record bound to it, it is an alias, and no other RR's are permitted to be bound to the same name.
It is common for these records to be used to point to hosts providing a particular service, such as an FTP or HTTP server. If the service must be moved to another host, the alias can be changed, and the same name will reach the new host.
This record contains a textual name. These records are bound to names built in a special way from numerical IP addresses, and are used to provide a reverse mapping from an IP address to a textual name. This is described in more detail in Section 25.1.8, “Reverse Resolution”.
This record type is used to delegate a sub-tree of the Domain Name space to another nameserver. The record contains the FQDN of a DNS nameserver with information on the sub-domain, and is bound to the name of the sub-domain. In this manner, the hierarchical structure of the DNS is established. Delegation is described in more detail in Section 25.1.4, “Delegation”.
This record contains the FQDN for a host that will accept SMTP electronic mail for the named domain, together with a priority value used to select an MX host when relaying mail. It is used to indicate other servers that are willing to receive and spool mail for the domain if the primary MX is unreachable for a time. It is also used to direct email to a central server, if desired, rather than to each and every individual workstation.
Contains two strings, intended for use to describe the host hardware and operating system platform. There are defined strings to use for some systems, but their use is not enforced. Some sites, because of security considerations, do not publicise this information.
A free-form text field, sometimes used as a comment field, sometimes overlaid with site-specific additional meaning to be interpreted by local conventions.
This record is required to appear for each zone file. It lists the primary nameserver and the email address of the person responsible for the domain, together with default values for a number of fields associated with maintaining consistency across multiple servers and caching of the results of DNS queries.
Using NS records, authority for portions of the DNS namespace below a certain point in the tree can be delegated, and further sub-parts below that delegated again. It is at this point that the distinction between a domain and a zone becomes important. Any name in the DNS is called a domain, and the term applies to that name and to any subordinate names below that one in the tree. The boundaries of a zone are narrower, and are defined by delegations. A zone starts with a delegation (or at the root), and encompasses all names in the domain below that point, excluding names below any subsequent delegations.
This distinction is important for implementation - a zone is a single administrative entity (with a single SOA record), and all data for the zone is referred to by a single file, called a zone file. A zone file may contain more than one period-separated level of the namespace tree, if desired, by including periods in the names in that zone file. In order to simplify administration and prevent overly-large zone files, it is quite legal for a DNS server to delegate to itself, splitting the domain into several zones kept on the same server.
For redundancy, it is common (and often administratively required) that there be more than one nameserver providing information on a zone. It is also common that at least one of these servers be located at some distance (in terms of network topology) from the others, so that knowledge of that zone does not become unavailable in case of connectivity failure. Each nameserver will be listed in an NS record bound to the name of the zone, stored in the parent zone on the server responsible for the parent domain. In this way, those searching the name hierarchy from the top down can contact any one of the servers to continue narrowing their search. This is occasionally called walking the tree.
There are a number of nameservers on the Internet which are called root nameservers. These servers provide information on the very top levels of the domain namespace tree. These servers are special in that their addresses must be pre-configured into nameservers as a place to start finding other servers. Isolated networks that cannot access these servers may need to provide their own root nameservers.
In order to maintain consistency between these servers, one is usually configured as the primary server, and all administrative changes are made on this server. The other servers are configured as secondaries, and transfer the contents of the zone from the primary. This operational model is not required, and if external considerations require it, multiple primaries can be used instead, but consistency must then be maintained by other means. DNS servers that store Resource Records for a zone, whether they be primary or secondary servers, are said to be authoritative for the zone. A DNS server can be authoritative for several zones.
When nameservers receive responses to queries, they can cache the results. This has a significant beneficial impact on the speed of queries, the query load on high-level nameservers, and network utilisation. It is also a major contributor to the memory usage of the nameserver process.
There are a number of parameters that are important to maintaining consistency amongst the secondaries and caches. The values for these parameters for a particular domain zone file are stored in the SOA record. These fields are:
Fields of the SOA Record
A serial number for the zone file. This should be incremented any time the data in the domain is changed. When a secondary wants to check if its data is up-to-date, it checks the serial number on the primary's SOA record.
A time, in seconds, specifying how often the secondary should check the serial number on the primary, and start a new transfer if the primary has newer data.
If a secondary fails to connect to the primary when the refresh time has elapsed (for example, if the host is down), this value specifies, in seconds, how often the connection should be retried.
If the retries fail to reach the primary within this number of seconds, the secondary destroys its copies of the zone data file(s), and stops answering requests for the domain. This stops very old and potentially inaccurate data from remaining in circulation.
This field specifies a time, in seconds, that the resource records in this zone should remain valid in the caches of other nameservers. If the data is volatile, this value should be short. TTL is a commonly-used acronym, that stands for "Time To Live".
DNS clients are configured with the addresses of DNS servers. Usually, these are servers which are authoritative for the domain of which they are a member. All requests for name resolution start with a request to one of these local servers. DNS queries can be of two forms:
A recursive query asks the nameserver to resolve a name completely, and return the result. If the request cannot be satisfied directly, the nameserver looks in its configuration and caches for a server higher up the domain tree which may have more information. In the worst case, this will be a list of pre-configured servers for the root domain. These addresses are returned in a response called a referral. The local nameserver must then send its request to one of these servers.
Normally, this will be an iterative query, which asks the second nameserver to either respond with an authoritative reply, or with the addresses of nameservers (NS records) listed in its tables or caches as authoritative for the relevant zone. The local nameserver then makes iterative queries, walking the tree downwards until an authoritative answer is found (either positive or negative) and returned to the client.
In some configurations, such as when firewalls prevent direct IP communications between DNS clients and external nameservers, or when a site is connected to the rest of the world via a slow link, a nameserver can be configured with information about a forwarder. This is an external nameserver to which the local nameserver should make requests as a client would, asking the external nameserver to perform the full recursive name lookup, and return the result in a single query (which can then be cached), rather than reply with referrals.
The DNS provides resolution from a textual name to a resource record, such as an A record with an IP address. It does not provide a means, other than exhaustive search, to match in the opposite direction; there is no mechanism to ask which name is bound to a particular RR.
For many RR types, this is of no real consequence, however it is often useful to identify by name the host which owns a particular IP address. Rather than complicate the design and implementation of the DNS database engine by providing matching functions in both directions, the DNS utilises the existing mechanisms and creates a special namespace, populated with PTR records, for IP address to name resolution. Resolving in this manner is often called reverse resolution, despite the inaccurate implications of the term.
The manner in which this is achieved is as follows:
A normal domain name is reserved and defined to be for the purpose of mapping IP addresses. The domain name used is "in-addr.arpa." which shows the historical origins of the Internet in the US Government's Defence Advanced Research Projects Agency's funding program.
This domain is then subdivided and delegated according to the structure of IP addresses. IP addresses are often written in decimal dotted quad notation, where each octet of the 4-octet long address is written in decimal, separated by dots. IP address ranges are usually delegated with more and more of the left-most parts of the address in common as the delegation gets smaller. Thus, to allow delegation of the reverse lookup domain to be done easily, this is turned around when used with the hierarchical DNS namespace, which places higher level domains on the right of the name.
Each byte of the IP address is written, as an ASCII text representation of the number expressed in decimal, with the octets in reverse order, separated by dots and appended with the in-addr.arpa. domain name. For example, to determine the hostname of a network device with IP address 11.22.33.44, this algorithm would produce the string "44.33.22.11.in-addr.arpa." which is a legal, structured Domain Name. A normal nameservice query would then be sent to the nameserver asking for a PTR record bound to the generated name.
The PTR record, if found, will contain the FQDN of a host.
One consequence of this is that it is possible for mismatch to occur. Resolving a name into an A record, and then resolving the name built from the address in that A record to a PTR record, may not result in a PTR record which contains the original name. There is no restriction within the DNS that the "reverse" mapping must coincide with the "forward" mapping. This is a useful feature in some circumstances, particularly when it is required that more than one name has an A record bound to it which contains the same IP address.
While there is no such restriction within the DNS, some application server programs or network libraries will reject connections from hosts that do not satisfy the following test:
the state information included with an incoming connection includes the IP address of the source of the request.
a PTR lookup is done to obtain an FQDN of the host making the connection
an A lookup is then done on the returned name, and the connection rejected if the source IP address is not listed amongst the A records that get returned.
This is done as a security precaution, to help detect and prevent malicious sites impersonating other sites by configuring their own PTR records to return the names of hosts belonging to another organisation.
Now let's look at actually setting up a small DNS enabled network. We will continue to use the examples mentioned in Chapter 23, Setting up TCP/IP on NetBSD in practice, i.e. we assume that:
Our IP networking is working correctly
We have IPNAT working correctly
Currently all hosts use the ISP for DNS
Our Name Server will be the “strider” host which also runs IPNAT, and our two clients use "strider" as a gateway. It is not really relevant as to what type of interface is on "strider", but for argument's sake we will say a 56k dial up connection.
So, before going any further, let's look at our
/etc/hosts file on
"strider" before we have made the alterations to use DNS.
Example 25.1. strider's /etc/hosts file
127.0.0.1 localhost 192.168.1.1 strider 192.168.1.2 samwise sam 192.168.1.3 wormtongue worm
This is not exactly a huge network, but it is worth noting that the same rules apply for larger networks as we discuss in the context of this section.
The other assumption we want to make is that the domain we want
to set up is diverge.org, and that the domain
is only known on our internal network, and not worldwide. Proper
registration of the nameserver's IP address as primary would be
needed in addition to a static IP. These are mostly
administrative issues which are left out here.
The NetBSD operating system provides a set of config files for you
to use for setting up DNS. They are stored in the
/etc/namedb directory, I strongly suggest
making a backup copy of this directory for reference
purposes.
The default directory contains the following files:
named.conf
localhost
127
loopback.v6
root.cache
You will see modified versions of these files in this section.
The examples in this chapter refer to BIND major version 8, however, it should be noted that format of the name database and other config files are almost 100% compatible between version. The only difference I noticed was that the “$TTL” information was not required.
The first file we want to look at is
/etc/namedb/named.conf. This file is the
config file for bind (hence the
catchy name). Setting up system like the one we are doing is
relatively simple. First, here is what mine looks like:
options {
directory "/etc/namedb";
allow-transfer { 192.168.1.0/24; };
allow-query { 192.168.1.0/24; };
listen-on port 53 { 192.168.1.1; };
};
zone "localhost" {
type master;
notify no;
file "localhost";
};
zone "127.IN-ADDR.ARPA" {
type master;
notify no;
file "127";
};
zone "0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.int" {
type master;
file "loopback.v6";
};
zone "diverge.org" {
type master;
notify no;
file "diverge.org";
};
zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "1.168.192";
};
zone "." in {
type hint;
file "root.cache";
};
Note that in my named.conf the root (".")
section is last, that is because there is another domain
called diverge.org on the internet (I happen to own it) so I
want the resolver to look out on the internet last. This is
not normally the case on most systems.
Another very important thing to remember here is that if you have an internal setup, in other words no live internet connection and/or no need to do root server lookups, comment out the root (".") zone. It may cause lookup problems if a particular client decides it wants to reference a domain on the internet, which our server couldn't resolve itself.
Looks like a pretty big mess, upon closer examination it is revealed that many of the lines in each section are somewhat redundant. So we should only have to explain them a few times.
Lets go through the sections of
named.conf:
This section defines some global parameters, most noticeable
is the location of the DNS tables, on this particular
system, they will be put in
/etc/namedb as indicated by the
"directory" option.
Following are the rest of the params:
This option lists which remote DNS servers acting as secondaries are allowed to do zone transfers, i.e. are allowed to read all DNS data at once. For privacy reasons, this should be restricted to secondary DNS servers only.
This option defines hosts from what network may query this name server at all. Restricting queries only to the local network (192.168.1.0/24) prevents queries arriving on the DNS server's external interface, and prevent possible privacy issues.
This option defined the port and associated IP addresses this server will run named(8) on. Again, the "external" interface is not listened here, to prevent queries getting received from "outside".
The rest of the named.conf file
consists of “zone”s. A zone is an area that can
have items to resolve attached, e.g. a domain can have
hostnames attached to resolve into IP addresses, and a
reverse-zone can have IP addresses attached that get
resolved back into hostnames. Each zone has a file
associated with it, and a table within that file for
resolving that particular zone. As is readily apparent,
their format in named.conf is
strikingly similar, so I will highlight just one of their
records:
The type of a zone is usually of type "master" in all cases except for the root zone “.” and for zones that a secondary (backup) service is provided - the type obviously is "secondary" in the latter case.
Do you want to send out notifications to secondaries when your zone changes? Obviously not in this setup, so this is set to "no".
This option sets the filename in our
/etc/namedb directory
where records about this particular zone may be
found. For the "diverge.org" zone, the file
/etc/namedb/diverge.org is used.
For the most part, the zone files look quite similar, however,
each one does have some unique properties. Here is what the
localhost file looks like:
Example 25.2. localhost
1|$TTL 3600 2|@ IN SOA strider.diverge.org. root.diverge.org. ( 3| 1 ; Serial 4| 8H ; Refresh 5| 2H ; Retry 6| 1W ; Expire 7| 1D) ; Minimum TTL 8| IN NS localhost. 9|localhost. IN A 127.0.0.1 10| IN AAAA ::1
Line by line:
This is the Time To Live for lookups, which defines how long other DNS servers will cache that value before discarding it. This value is generally the same in all the files.
This line is generally the same in all zone files except
root.cache. It defines a so-called
"Start Of Authority" (SOA) header, which contains some
basic information about a zone. Of specific interest
on this line are
"strider.diverge.org." and "root.diverge.org." (note the
trailing dots!). Obviously one
is the name of this server and the other is the contact
for this DNS server, in most cases root seems a little
ambiguous, it is preferred that a regular email account
be used for the contact information, with the "@"
replaced by a "." (for example, mine
would be "jrf.diverge.org.").
This line is the serial number identifying the "version" of the zone's data set (file). The serial number should be incremented each time there is a change to the file, the usual format is to either start with a value of "1" and increase it for every change, or use a value of "YYYYMMDDNN" to encode year (YYYY), month (MM), day (DD) and change within one day (NN) in the serial number.
This is the refresh rate of the server, in this file it is set to once every 8 hours.
The retry rate.
Lookup expiry.
The minimum Time To Live.
This is the Nameserver line, which uses a "NS"
resource record to show that "localhost" is the only
DNS server handing out data for this zone (which is "@", which
indicates the zone name used in the
named.conf file,
i.e. "diverge.org") is, well, "localhost".
This is the localhost entry, which uses an "A" resource record to indicate that the name "localhost" should be resolved into the IP-address 127.0.0.1 for IPv4 queries (which specifically ask for the "A" record).
This line is the IPv6 entry, which returns ::1 when someone asks for an IPv6-address (by specifically asking for the AAAA record) of "localhost.".
This is the reverse lookup file (or zone) to resolve the special IP address 127.0.0.1 back to "localhost":
1| $TTL 3600 2| @ IN SOA strider.diverge.org. root.diverge.org. ( 3| 1 ; Serial 4| 8H ; Refresh 5| 2H ; Retry 6| 1W ; Expire 7| 1D) ; Minimum TTL 8| IN NS localhost. 9| 1.0.0 IN PTR localhost.
In this file, all of the lines are the same as the
localhost zonefile with exception of line 9, this is the
reverse lookup (PTR) record. The zone used here is "@" again,
which got set to the value given in
named.conf, i.e. "127.in-addr.arpa". This
is a special "domain" which is used to do reverse-lookup of IP
addresses back into hostnames. For it to work, the four bytes
of the IPv4 address are reserved, and the domain
"in-addr.arpa" attached, so to resolve the IP address
"127.0.0.1", the PTR record of "1.0.0.127.in-addr.arpa" is
queried, which is what is defined in that line.
This zone file is populated by records for all of our hosts. Here is what it looks like:
1| $TTL 3600 2| @ IN SOA strider.diverge.org. root.diverge.org. ( 3| 1 ; serial 4| 8H ; refresh 5| 2H ; retry 6| 1W ; expire 7| 1D ) ; minimum seconds 8| IN NS strider.diverge.org. 9| IN MX 10 strider.diverge.org. ; primary mail server 10| IN MX 20 samwise.diverge.org. ; secondary mail server 11| strider IN A 192.168.1.1 12| samwise IN A 192.168.1.2 13| www IN CNAME samwise.diverge.org. 14| worm IN A 192.168.1.3
There is a lot of new stuff here, so lets just look over each line that is new here:
This line shows our mail exchanger (MX), in this case it is "strider". The number that precedes "strider.diverge.org." is the priority number, the lower the number their higher the priority. The way we are setup here is if "strider" cannot handle the mail, then "samwise" will.
CNAME stands for canonical name, or an alias for an existing hostname, which must have an A record. So we have aliased the following:
www.diverge.org to samwise.diverge.org
The rest of the records are simply mappings of IP address to a full name (A records).
This zone file is the reverse file for all of the host
records, to map their IP numbers we use on our private network
back into hostnames. The format is similar to that of the
"localhost" version with the obvious exception being the
addresses are different via the different zone given in the
named.conf file,
i.e. "0.168.192.in-addr.arpa" here:
1|$TTL 3600 2|@ IN SOA strider.diverge.org. root.diverge.org. ( 3| 1 ; serial 4| 8H ; refresh 5| 2H ; retry 6| 1W ; expire 7| 1D ) ; minimum seconds 8| IN NS strider.diverge.org. 9|1 IN PTR strider.diverge.org. 10|2 IN PTR samwise.diverge.org. 11|3 IN PTR worm.diverge.org.
This file contains a list of root name servers for your server to query when it gets requests outside of its own domain that it cannot answer itself. Here are first few lines of a root zone file:
; ; This file holds the information on root name servers needed to ; initialize cache of Internet domain name servers ; (e.g. reference this file in the "cache . <file>" ; configuration file of BIND domain name servers). ; ; This file is made available by InterNIC ; under anonymous FTP as ; file /domain/db.cache ; on server FTP.INTERNIC.NET ; -OR- RS.INTERNIC.NET ; ; last update: Jan 29, 2004 ; related version of root zone: 2004012900 ; ; ; formerly NS.INTERNIC.NET ; . 3600000 IN NS A.ROOT-SERVERS.NET. A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4 ; ; formerly NS1.ISI.EDU ; . 3600000 NS B.ROOT-SERVERS.NET. B.ROOT-SERVERS.NET. 3600000 A 192.228.79.201 ; ; formerly C.PSI.NET ; . 3600000 NS C.ROOT-SERVERS.NET. C.ROOT-SERVERS.NET. 3600000 A 192.33.4.12 ; ...
This file can be obtained from ISC at http://www.isc.org/ and
usually comes with a distribution of BIND. A
root.cache file is included in the NetBSD
operating system's "etc" set.
This section has described the most important files and settings
for a DNS server. Please see the BIND documentation in
/usr/src/dist/bind/doc/bog and
named.conf(5) for more information.
In this section we will look at how to get DNS going and setup "strider" to use its own DNS services.
Setting up named to start automatically is quite simple.
In /etc/rc.conf simply set
named=yes. Additional
options can be specified in named_flags, for
example, I like to use -g nogroup -u nobody,
so a non-root account runs the "named" process.
In addition to being able to startup "named" at boot time, it can also be controlled with the ndc command. In a nutshell the ndc command can stop, start or restart the named server process. It can also do a great many other things. Before use, it has to be setup to communicate with the "named" process, see the ndc(8) and named.conf(5) man pages for more details on setting up communication channels between "ndc" and the "named" process.
Next we want to point "strider" to
itself for lookups. We have two simple steps, first, decide on
our resolution order. On a network this small, it is likely that
each host has a copy of the hosts table, so we can get away with
using /etc/hosts first, and then
DNS. However, on larger networks it is much
easier to use DNS. Either way, the file where order of
name services used for resolution is determined
is /etc/nsswitch.conf
(see Example 23.2, “nsswitch.conf”). Here is part of a
typical nsswitch.conf:
. . . group_compat: nis hosts: files dns netgroup: files [notfound=return] nis . . .
The line we are interested in is the "hosts" line. "files"
means the system uses the /etc/hosts file
first to determine ip to name translation, and if it can't find
an entry, it will try DNS.
The next file to look at is
/etc/resolv.conf, which is used to
configure DNS lookups ("resolution") on the client side. The
format is pretty self explanatory but we will go over it
anyway:
domain diverge.org search diverge.org nameserver 192.168.1.1
In a nutshell this file is telling the resolver that this machine belongs to the "diverge.org" domain, which means that lookups that contain only a hostname without a "." gets this domain appended to build a FQDN. If that lookup doesn't succeed, the domains in the "search" line are tried next. Finally, the "nameserver" line gives the IP addresses of one or more DNS servers that should be used to resolve DNS queries.
To test our nameserver we can use several commands, for example:
#host samsam.diverge.org has address 192.168.1.2
As can be seen, the domain was appended automatically here,
using the value from /etc/resolv.conf. Here
is another example, the output of running host
www.yahoo.com:
$host www.yahoo.comwww.yahoo.com is an alias for www.yahoo.akadns.net. www.yahoo.akadns.net has address 68.142.226.38 www.yahoo.akadns.net has address 68.142.226.39 www.yahoo.akadns.net has address 68.142.226.46 www.yahoo.akadns.net has address 68.142.226.50 www.yahoo.akadns.net has address 68.142.226.51 www.yahoo.akadns.net has address 68.142.226.54 www.yahoo.akadns.net has address 68.142.226.55 www.yahoo.akadns.net has address 68.142.226.32
Other commands for debugging DNS besides host(1) are
nslookup(8) and dig(1). Note that ping(8) is
not useful for debugging DNS, as it will
use whatever is configured in
/etc/nsswitch.conf to do the name-lookup.
At this point the server is configured properly.
The procedure for setting up the client hosts are easier,
you only need to setup /etc/nsswitch.conf
and /etc/resolv.conf to the same values as
on the server.
A caching only name server has no local zones; all the
queries it receives are forwarded to the root servers and the
replies are accumulated in the local cache. The next time the
query is performed the answer will be faster because the data is
already in the server's cache. Since this type of server
doesn't handle local zones, to resolve the names of the local
hosts it will still be necessary to use the already known
/etc/hosts file.
Since NetBSD supplies defaults for all the files needed by a
caching only server, it only needs to be enabled and started and
is immediately ready for use! To enable named, put
named=yes into
/etc/rc.conf, and tell the system to use it
adding the following line to the
/etc/resolv.conf file:
#cat /etc/resolv.confnameserver 127.0.0.1
Now we can start named:
#sh /etc/rc.d/named restart
Now that the server is running we can test it using the nslookup(8) program:
$nslookupDefault server: localhost Address: 127.0.0.1 >
Let's try to resolve a host name, for example "www.NetBSD.org":
> www.NetBSD.org
Server: localhost
Address: 127.0.0.1
Name: www.NetBSD.org
Address: 204.152.190.12
If you repeat the query a second time, the result is slightly different:
> www.NetBSD.org
Server: localhost
Address: 127.0.0.1
Non-authoritative answer:
Name: www.NetBSD.org
Address: 204.152.190.12
As you've probably noticed, the address is the same, but the message “Non-authoritative answer” has appeared. This message indicates that the answer is not coming from an authoritative server for the domain NetBSD.org but from the cache of our own server.
The results of this first test confirm that the server is working correctly.
We can also try the host(1) and dig(1) commands, which give the following result.
$host www.NetBSD.orgwww.NetBSD.org has address 204.152.190.12$$dig www.NetBSD.org; <<>> DiG 8.3 <<>> www.NetBSD.org ;; res options: init recurs defnam dnsrch ;; got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19409 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 0 ;; QUERY SECTION: ;; www.NetBSD.org, type = A, class = IN ;; ANSWER SECTION: www.NetBSD.org. 23h32m54s IN A 204.152.190.12 ;; AUTHORITY SECTION: NetBSD.org. 23h32m54s IN NS uucp-gw-1.pa.dec.com. NetBSD.org. 23h32m54s IN NS uucp-gw-2.pa.dec.com. NetBSD.org. 23h32m54s IN NS ns.NetBSD.org. NetBSD.org. 23h32m54s IN NS adns1.berkeley.edu. NetBSD.org. 23h32m54s IN NS adns2.berkeley.edu. ;; Total query time: 14 msec ;; FROM: miyu to SERVER: 127.0.0.1 ;; WHEN: Thu Nov 25 22:59:36 2004 ;; MSG SIZE sent: 32 rcvd: 175
As you can see dig(1) gives quite a bit of output, the expected answer can be found in the "ANSWER SECTION". The other data given may be of interest when debugging DNS problems.