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 # The Domain Name System  This page was moved to:
   [The NetBSD Guide - The Domain Name System](//www.NetBSD.org/docs/guide/en/chap-dns.html)
 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/](http://www.dns.net/dnsrd/).  
   
 ## DNS Background and Concepts  
   
 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  
   
 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  
 [[Name Service Concepts|guide/net-intro#nsconcepts]].  
   
 In the case of the DNS, the configuration data bound to a name is in the form of  
 standard *Resource Records* (RRs). These textual names conform to certain  
 structural conventions.  
   
 ### The DNS namespace  
   
 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  
 *FQDN*s. 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  
   
 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  
   
  * *A: Address* -- This record contains the numerical IP address associated with  
    the name.  
   
  * *CNAME: Canonical 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.  
   
  * *PTR: Pointer* -- 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 [[Reverse Resolution|guide/dns#bg-reverse]].  
   
  * *NS: Name Server* -- 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  
    [[Delegation|guide/dns#bg-delegation]].  
   
  * *MX: Mail eXchange* -- 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.  
   
  * *HINFO: Host Information* -- 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.  
   
  * *TXT: Text* -- A free-form text field, sometimes used as a comment field,  
    sometimes overlaid with site-specific additional meaning to be interpreted by  
    local conventions.  
   
  * *SOA: Start of Authority* -- 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.  
   
 ### Delegation  
   
 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.  
   
 ### Delegation to multiple servers  
   
 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.  
   
 ### Secondaries, Caching, and the SOA record  
   
 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  
   
  * *Serial* -- 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.  
   
  * *Refresh* -- 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.  
   
  * *Retry* -- 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.  
   
  * *Expire* -- 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.  
   
  * *TTL* -- 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".  
   
 ### Name Resolution  
   
 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.  
   
 ### Reverse Resolution  
   
 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.  
   
 ## The DNS Files  
   
 Now let's look at actually setting up a small DNS enabled network. We will  
 continue to use the examples mentioned in [Chapter 24, *Setting up TCP/IP on  
 NetBSD in practice*](chap-net-practice.html "Chapter 24. 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 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. Along with a default `/etc/named.conf`, the following files are  
 stored in the `/etc/namedb` directory:  
   
  * `localhost`  
  * `127`  
  * `loopback.v6`  
  * `root.cache`  
   
 You will see modified versions of these files in this section, and I strongly  
 suggest making a backup copy of the original files for reference purposes.  
   
 *Note*: 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.  
   
 ### /etc/named.conf  
   
 The first file we want to look at is `/etc/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`:  
   
 #### options  
   
 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:  
   
  * `allow-transfer` -- 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.  
   
  * `allow-query` -- 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.  
   
  * `listen-on port` -- This option defined the port and associated IP addresses  
    this server will run  
    [named(8)](http://netbsd.gw.com/cgi-bin/man-cgi?named+8+NetBSD-5.0.1+i386)  
    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:  
   
 #### zone diverge.org  
   
  * `type` -- 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.  
   
  * `notify` -- Do you want to send out notifications to secondaries when your  
    zone changes? Obviously not in this setup, so this is set to "no".  
   
  * `file` -- 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.  
   
 ### /etc/namedb/localhost  
   
 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:  
   
      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:  
   
  * *Line 1*: 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.  
   
  * *Line 2*: 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.").  
   
  * *Line 3*: 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.  
   
  * *Line 4*: This is the refresh rate of the server, in this file it is set to  
    once every 8 hours.  
   
  * *Line 5*: The retry rate.  
   
  * *Line 6*: Lookup expiry.  
   
  * *Line 7*: The minimum Time To Live.  
   
  * *Line 8*: 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".  
   
  * *Line 9*: 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).  
   
  * *Line 10*: 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.".  
   
 ### /etc/namedb/zone.127.0.0  
   
 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.  
   
 ### /etc/namedb/diverge.org  
   
 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:  
   
  * *Line 9*: 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.  
   
  * *Line 11*: CNAME stands for canonical name, or an alias for an existing  
    hostname, which must have an A record. So we have aliased `www.diverge.org`  
    to `samwise.diverge.org`.  
   
 The rest of the records are simply mappings of IP address to a full name (A  
 records).  
   
 ### /etc/namedb/1.168.192  
   
 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.  
   
 ### /etc/namedb/root.cache  
   
 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)](http://netbsd.gw.com/cgi-bin/man-cgi?named.conf+5+NetBSD-5.0.1+i386)  
 for more information.  
   
 ## Using DNS  
   
 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)](http://netbsd.gw.com/cgi-bin/man-cgi?ndc+8+NetBSD-5.0.1+i386)  
 and  
 [named.conf(5)](http://netbsd.gw.com/cgi-bin/man-cgi?named.conf+5+NetBSD-5.0.1+i386)  
 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  
 [[`nsswitch.conf`|guide/net-practice#ex-nsswitch]]. 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 sam  
     sam.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.com  
     www.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)](http://netbsd.gw.com/cgi-bin/man-cgi?host+1+NetBSD-5.0.1+i386) are  
 [nslookup(8)](http://netbsd.gw.com/cgi-bin/man-cgi?nslookup+8+NetBSD-5.0.1+i386)  
 and  
 [dig(1)](http://netbsd.gw.com/cgi-bin/man-cgi?dig+1+NetBSD-5.0.1+i386). Note  
 that  
 [ping(8)](http://netbsd.gw.com/cgi-bin/man-cgi?ping+8+NetBSD-5.0.1+i386)  
 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.  
   
 ## Setting up a caching only name 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.conf  
     nameserver 127.0.0.1  
   
 Now we can start named:  
   
     # sh /etc/rc.d/named restart  
   
 ### Testing the server  
   
 Now that the server is running we can test it using the  
 [nslookup(8)](http://netbsd.gw.com/cgi-bin/man-cgi?nslookup+8+NetBSD-5.0.1+i386)  
 program:  
   
     $ nslookup  
     Default 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)](http://netbsd.gw.com/cgi-bin/man-cgi?host+1+NetBSD-5.0.1+i386) and  
 [dig(1)](http://netbsd.gw.com/cgi-bin/man-cgi?dig+1+NetBSD-5.0.1+i386) commands,  
 which give the following result.  
   
     $ host www.NetBSD.org  
     www.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)](http://netbsd.gw.com/cgi-bin/man-cgi?dig+1+NetBSD-5.0.1+i386) 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.  
   

Removed from v.1.2  
changed lines
  Added in v.1.6


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