| Deployment GuideDNS (Domain Name System), also known as a nameserver, is a network system that associates hostnames with their respective IP addresses. For users, this has the advantage that they can refer to machines on the network by names that are usually easier to remember than the numerical network addresses. For system administrators, using the nameserver allows them to change the IP address for a host without ever affecting the name-based queries, or to decide which machines handle these queries.
14.1. Introduction to DNSDNS is usually implemented using one or more centralized servers that are authoritative for certain domains. When a client host requests information from a nameserver, it usually connects to port 53. The nameserver then attempts to resolve the name requested. If it does not have an authoritative answer, or does not already have the answer cached from an earlier query, it queries other nameservers, called root nameservers, to determine which nameservers are authoritative for the name in question, and then queries them to get the requested name. In a DNS server such as BIND (Berkeley Internet Name Domain), all information is stored in basic data elements called resource records (RR). The resource record is usually a fully qualified domain name (FQDN) of a host, and is broken down into multiple sections organized into a tree-like hierarchy. This hierarchy consists of a main trunk, primary branches, secondary branches, and so on. Example 14.1. A simple resource record Each level of the hierarchy is divided by a period (that is, .). In Example 14.1, "A simple resource record", com defines the top-level domain, example its subdomain, and sales the subdomain of example. In this case, bob identifies a resource record that is part of the sales.example.com domain. With the exception of the part furthest to the left (that is, bob), each of these sections is called a zone and defines a specific namespace. Zones are defined on authoritative nameservers through the use of zone files, which contain definitions of the resource records in each zone. Zone files are stored on primary nameservers (also called master nameservers), where changes are made to the files, and secondary nameservers (also called slave nameservers), which receive zone definitions from the primary nameservers. Both primary and secondary nameservers are authoritative for the zone and look the same to clients. Depending on the configuration, any nameserver can also serve as a primary or secondary server for multiple zones at the same time. There are two nameserver configuration types: - authoritative
Authoritative nameservers answer to resource records that are part of their zones only. This category includes both primary (master) and secondary (slave) nameservers. - recursive
Recursive nameservers offer resolution services, but they are not authoritative for any zone. Answers for all resolutions are cached in a memory for a fixed period of time, which is specified by the retrieved resource record.
Although a nameserver can be both authoritative and recursive at the same time, it is recommended not to combine the configuration types. To be able to perform their work, authoritative servers should be available to all clients all the time. On the other hand, since the recursive lookup takes far more time than authoritative responses, recursive servers should be available to a restricted number of clients only, otherwise they are prone to distributed denial of service (DDoS) attacks. 14.1.3. BIND as a NameserverBIND consists of a set of DNS-related programs. It contains a nameserver called named, an administration utility called rndc, and a debugging tool called dig. Refer to Chapter 10, Services and Daemons for more information on how to run a service in Red Hat Enterprise Linux. This chapter covers BIND (Berkeley Internet Name Domain), the DNS server included in Red Hat Enterprise Linux. It focuses on the structure of its configuration files, and describes how to administer it both locally and remotely. 14.2.1. Configuring the named ServiceTable 14.1. The named service configuration files | Path | Description |
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/etc/named.conf | The main configuration file. | /etc/named/ | An auxiliary directory for configuration files that are included in the main configuration file. |
The configuration file consists of a collection of statements with nested options surrounded by opening and closing curly brackets. Note that when editing the file, you have to be careful not to make any syntax error, otherwise the named service will not start. A typical /etc/named.conf file is organized as follows: statement-1 ["statement-1-name"] [statement-1-class] { option-1; option-2; option-N;};statement-2 ["statement-2-name"] [statement-2-class] { option-1; option-2; option-N;};statement-N ["statement-N-name"] [statement-N-class] { option-1; option-2; option-N;};
If you have installed the bind-chroot package, the BIND service will run in the /var/named/chroot environment. In that case, the initialization script will mount the above configuration files using the mount --bind command, so that you can manage the configuration outside this environment. 14.2.1.1. Common Statement TypesThe following types of statements are commonly used in /etc/named.conf: -
acl The acl (Access Control List) statement allows you to define groups of hosts, so that they can be permitted or denied access to the nameserver. It takes the following form: acl acl-name { match-element; ...}; The acl-name statement name is the name of the access control list, and the match-element option is usually an individual IP address (such as 10.0.1.1) or a CIDR (Classless Inter-Domain Routing) network notation (for example, 10.0.1.0/24). For a list of already defined keywords, see Table 14.2, "Predefined access control lists". Table 14.2. Predefined access control lists | Keyword | Description |
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any | Matches every IP address. | localhost | Matches any IP address that is in use by the local system. | localnets | Matches any IP address on any network to which the local system is connected. | none | Does not match any IP address. |
The acl statement can be especially useful in conjunction with other statements such as options. Example 14.2, "Using acl in conjunction with options" defines two access control lists, black-hats and red-hats, and adds black-hats on the blacklist while granting red-hats a normal access. Example 14.2. Using acl in conjunction with options acl black-hats { 10.0.2.0/24; 192.168.0.0/24; 1234:5678::9abc/24;};acl red-hats { 10.0.1.0/24;};options { blackhole { black-hats; }; allow-query { red-hats; }; allow-query-cache { red-hats; };};
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include The include statement allows you to include files in the /etc/named.conf, so that potentially sensitive data can be placed in a separate file with restricted permissions. It takes the following form: include "file-name" The file-name statement name is an absolute path to a file. Example 14.3. Including a file to /etc/named.conf include "/etc/named.rfc1912.zones";
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options The options statement allows you to define global server configuration options as well as to set defaults for other statements. It can be used to specify the location of the named working directory, the types of queries allowed, and much more. It takes the following form: options { option; ...};Table 14.3. Commonly used options | Option | Description |
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allow-query | Specifies which hosts are allowed to query the nameserver for authoritative resource records. It accepts an access control list, a collection of IP addresses, or networks in the CIDR notation. All hosts are allowed by default. | allow-query-cache | Specifies which hosts are allowed to query the nameserver for non-authoritative data such as recursive queries. Only localhost and localnets are allowed by default. | blackhole | Specifies which hosts are not allowed to query the nameserver. This option should be used when particular host or network floods the server with requests. The default option is none. | directory | Specifies a working directory for the named service. The default option is /var/named/. | dnssec-enable | Specifies whether to return DNSSEC related resource records. The default option is yes. | dnssec-validation | Specifies whether to prove that resource records are authentic via DNSSEC. The default option is yes. | forwarders | Specifies a list of valid IP addresses for nameservers to which the requests should be forwarded for resolution. | forward | Specifies the behavior of the forwarders directive. It accepts the following options: first - The server will query the nameservers listed in the forwarders directive before attempting to resolve the name on its own.
only - When unable to query the nameservers listed in the forwarders directive, the server will not attempt to resolve the name on its own.
| listen-on | Specifies the IPv4 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv4 interfaces are used by default. | listen-on-v6 | Specifies the IPv6 network interface on which to listen for queries. On a DNS server that also acts as a gateway, you can use this option to answer queries originating from a single network only. All IPv6 interfaces are used by default. | max-cache-size | Specifies the maximum amount of memory to be used for server caches. When the limit is reached, the server causes records to expire prematurely so that the limit is not exceeded. In a server with multiple views, the limit applies separately to the cache of each view. The default option is 32M. | notify | Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options: yes - The server will notify all secondary nameservers.
no - The server will not notify any secondary nameserver.
master-only - The server will notify primary server for the zone only.
explicit - The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
| pid-file | Specifies the location of the process ID file created by the named service. | recursion | Specifies whether to act as a recursive server. The default option is yes. | statistics-file | Specifies an alternate location for statistics files. The /var/named/named.stats file is used by default. |
To prevent distributed denial of service (DDoS) attacks, it is recommended that you use the allow-query-cache option to restrict recursive DNS services for a particular subset of clients only. Example 14.4. Using the options statement options { allow-query { localhost; }; listen-on port 53 { 127.0.0.1; }; listen-on-v6 port 53 { ::1; }; max-cache-size 256M; directory "/var/named"; statistics-file "/var/named/data/named_stats.txt"; recursion yes; dnssec-enable yes; dnssec-validation yes;};
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zone The zone statement allows you to define the characteristics of a zone, such as the location of its configuration file and zone-specific options, and can be used to override the global options statements. It takes the following form: zone zone-name [zone-class] { option; ...}; The zone-name attribute is the name of the zone, zone-class is the optional class of the zone, and option is a zone statement option as described in Table 14.4, "Commonly used options". The zone-name attribute is particularly important, as it is the default value assigned for the $ORIGIN directive used within the corresponding zone file located in the /var/named/ directory. The named daemon appends the name of the zone to any non-fully qualified domain name listed in the zone file. For example, if a zone statement defines the namespace for example.com, use example.com as the zone-name so that it is placed at the end of hostnames within the example.com zone file. Table 14.4. Commonly used options | Option | Description |
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allow-query | Specifies which clients are allowed to request information about this zone. This option overrides global allow-query option. All query requests are allowed by default. | allow-transfer | Specifies which secondary servers are allowed to request a transfer of the zone's information. All transfer requests are allowed by default. | allow-update | Specifies which hosts are allowed to dynamically update information in their zone. The default option is to deny all dynamic update requests. Note that you should be careful when allowing hosts to update information about their zone. Do not set IP addresses in this option unless the server is in the trusted network. Instead, use TSIG key as described in Section 14.2.5.3, "Transaction SIGnatures (TSIG)". | file | Specifies the name of the file in the named working directory that contains the zone's configuration data. | masters | Specifies from which IP addresses to request authoritative zone information. This option is used only if the zone is defined as type slave. | notify | Specifies whether to notify the secondary nameservers when a zone is updated. It accepts the following options: yes - The server will notify all secondary nameservers.
no - The server will not notify any secondary nameserver.
master-only - The server will notify primary server for the zone only.
explicit - The server will notify only the secondary servers that are specified in the also-notify list within a zone statement.
| type | Specifies the zone type. It accepts the following options: delegation-only - Enforces the delegation status of infrastructure zones such as COM, NET, or ORG. Any answer that is received without an explicit or implicit delegation is treated as NXDOMAIN. This option is only applicable in TLDs (Top-Level Domain) or root zone files used in recursive or caching implementations.
forward - Forwards all requests for information about this zone to other nameservers.
hint - A special type of zone used to point to the root nameservers which resolve queries when a zone is not otherwise known. No configuration beyond the default is necessary with a hint zone.
master - Designates the nameserver as authoritative for this zone. A zone should be set as the master if the zone's configuration files reside on the system.
slave - Designates the nameserver as a slave server for this zone. Master server is specified in masters directive.
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Most changes to the /etc/named.conf file of a primary or secondary nameserver involve adding, modifying, or deleting zone statements, and only a small subset of zone statement options is usually needed for a nameserver to work efficiently. In Example 14.5, "A zone statement for a primary nameserver", the zone is identified as example.com, the type is set to master, and the named service is instructed to read the /var/named/example.com.zone file. It also allows only a secondary nameserver ( 192.168.0.2) to transfer the zone. Example 14.5. A zone statement for a primary nameserver zone "example.com" IN { type master; file "example.com.zone"; allow-transfer { 192.168.0.2; };};
A secondary server's zone statement is slightly different. The type is set to slave, and the masters directive is telling named the IP address of the master server. In Example 14.6, "A zone statement for a secondary nameserver", the named service is configured to query the primary server at the 192.168.0.1 IP address for information about the example.com zone. The received information is then saved to the /var/named/slaves/example.com.zone file. Note that you have to put all slave zones to /var/named/slaves directory, otherwise the service will fail to transfer the zone. Example 14.6. A zone statement for a secondary nameserver zone "example.com" { type slave; file "slaves/example.com.zone"; masters { 192.168.0.1; };};
14.2.1.2. Other Statement TypesThe following types of statements are less commonly used in /etc/named.conf: -
controls The controls statement allows you to configure various security requirements necessary to use the rndc command to administer the named service. -
key The key statement allows you to define a particular key by name. Keys are used to authenticate various actions, such as secure updates or the use of the rndc command. Two options are used with key: algorithm algorithm-name - The type of algorithm to be used (for example, hmac-md5).
secret "key-value" - The encrypted key.
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logging The logging statement allows you to use multiple types of logs, so called channels. By using the channel option within the statement, you can construct a customized type of log with its own file name (file), size limit (size), versioning (version), and level of importance (severity). Once a customized channel is defined, a category option is used to categorize the channel and begin logging when the named service is restarted. By default, named sends standard messages to the rsyslog daemon, which places them in /var/log/messages. Several standard channels are built into BIND with various severity levels, such as default_syslog (which handles informational logging messages) and default_debug (which specifically handles debugging messages). A default category, called default, uses the built-in channels to do normal logging without any special configuration. Customizing the logging process can be a very detailed process and is beyond the scope of this chapter. For information on creating custom BIND logs, refer to the BIND 9 Administrator Reference Manual referenced in Section 14.2.7.1, "Installed Documentation". -
server The server statement allows you to specify options that affect how the named service should respond to remote nameservers, especially with regard to notifications and zone transfers. The transfer-format option controls the number of resource records that are sent with each message. It can be either one-answer (only one resource record), or many-answers (multiple resource records). Note that while the many-answers option is more efficient, it is not supported by older versions of BIND. -
trusted-keys -
view The view statement allows you to create special views depending upon which network the host querying the nameserver is on. This allows some hosts to receive one answer regarding a zone while other hosts receive totally different information. Alternatively, certain zones may only be made available to particular trusted hosts while non-trusted hosts can only make queries for other zones. Multiple views can be used as long as their names are unique. The match-clients option allows you to specify the IP addresses that apply to a particular view. If the options statement is used within a view, it overrides the already configured global options. Finally, most view statements contain multiple zone statements that apply to the match-clients list. Note that the order in which the view statements are listed is important, as the first statement that matches a particular client's IP address is used. For more information on this topic, refer to Section 14.2.5.1, "Multiple Views".
Additionally to statements, the /etc/named.conf file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to a user. The following are valid comment tags: -
// Any text after the // characters to the end of the line is considered a comment. For example: notify yes; // notify all secondary nameservers -
# Any text after the # character to the end of the line is considered a comment. For example: notify yes; # notify all secondary nameservers /* and */Any block of text enclosed in /* and */ is considered a comment. For example: notify yes; /* notify all secondary nameservers */
14.2.2. Editing Zone FilesAs outlined in Section 14.1.1, "Nameserver Zones", zone files contain information about a namespace. They are stored in the named working directory located in /var/named/ by default, and each zone file is named according to the file option in the zone statement, usually in a way that relates to the domain in question and identifies the file as containing zone data, such as example.com.zone. Table 14.5. The named service zone files | Path | Description |
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/var/named/ | The working directory for the named service. The nameserver is not allowed to write to this directory. | /var/named/slaves/ | The directory for secondary zones. This directory is writable by the named service. | /var/named/dynamic/ | The directory for other files, such as dynamic DNS (DDNS) zones or managed DNSSEC keys. This directory is writable by the named service. | /var/named/data/ | The directory for various statistics and debugging files. This directory is writable by the named service. |
A zone file consists of directives and resource records. Directives tell the nameserver to perform tasks or apply special settings to the zone, resource records define the parameters of the zone and assign identities to individual hosts. While the directives are optional, the resource records are required in order to provide name service to a zone. All directives and resource records should be entered on individual lines. 14.2.2.1. Common DirectivesDirectives begin with the dollar sign character followed by the name of the directive, and usually appear at the top of the file. The following directives are commonly used in zone files: -
$INCLUDE The $INCLUDE directive allows you to include another file at the place where it appears, so that other zone settings can be stored in a separate zone file. Example 14.7. Using the $INCLUDE directive $INCLUDE /var/named/penguin.example.com
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$ORIGIN The $ORIGIN directive allows you to append the domain name to unqualified records, such as those with the hostname only. Note that the use of this directive is not necessary if the zone is specified in /etc/named.conf, since the zone name is used by default. Example 14.8. Using the $ORIGIN directive
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$TTL The $TTL directive allows you to set the default Time to Live (TTL) value for the zone, that is, how long is a zone record valid. Each resource record can contain its own TTL value, which overrides this directive. Increasing this value allows remote nameservers to cache the zone information for a longer period of time, reducing the number of queries for the zone and lengthening the amount of time required to propagate resource record changes. Example 14.9. Using the $TTL directive
14.2.2.2. Common Resource RecordsThe following resource records are commonly used in zone files: -
A The Address record specifies an IP address to be assigned to a name. It takes the following form: hostname IN A IP-address
If the hostname value is omitted, the record will point to the last specified hostname. Example 14.10. Using the A resource record server1 IN A 10.0.1.3 IN A 10.0.1.5
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CNAME The Canonical Name record maps one name to another. Because of this, this type of record is sometimes referred to as an alias record. It takes the following form: alias-name IN CNAME real-name
CNAME records are most commonly used to point to services that use a common naming scheme, such as www for Web servers. However, there are multiple restrictions for their usage:
CNAME records should not point to other CNAME records. This is mainly to avoid possible infinite loops. CNAME records should not contain other resource record types (such as A, NS, MX, etc.). The only exception are DNSSEC related records (that is, RRSIG, NSEC, etc.) when the zone is signed. Other resource record that point to the fully qualified domain name (FQDN) of a host (that is, NS, MX, PTR) should not point to a CNAME record.
Example 14.11. Using the CNAME resource record server1 IN A 10.0.1.5www IN CNAME server1
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MX The Mail Exchange record specifies where the mail sent to a particular namespace controlled by this zone should go. It takes the following form: IN MX preference-value email-server-name The email-server-name is a fully qualified domain name (FQDN). The preference-value allows numerical ranking of the email servers for a namespace, giving preference to some email systems over others. The MX resource record with the lowest preference-value is preferred over the others. However, multiple email servers can possess the same value to distribute email traffic evenly among them. Example 14.12. Using the MX resource record example.com. IN MX 10 mail.example.com. IN MX 20 mail2.example.com.
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NS The Nameserver record announces authoritative nameservers for a particular zone. It takes the following form: IN NS nameserver-name The nameserver-name should be a fully qualified domain name (FQDN). Note that when two nameservers are listed as authoritative for the domain, it is not important whether these nameservers are secondary nameservers, or if one of them is a primary server. They are both still considered authoritative. Example 14.13. Using the NS resource record IN NS dns1.example.com.IN NS dns2.example.com.
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PTR The Pointer record points to another part of the namespace. It takes the following form: last-IP-digit IN PTR FQDN-of-system
The last-IP-digit directive is the last number in an IP address, and the FQDN-of-system is a fully qualified domain name (FQDN). -
SOA The Start of Authority record announces important authoritative information about a namespace to the nameserver. Located after the directives, it is the first resource record in a zone file. It takes the following form: @ IN SOA primary-name-server hostmaster-email ( serial-number time-to-refresh time-to-retry time-to-expire minimum-TTL ) The directives are as follows: The @ symbol places the $ORIGIN directive (or the zone's name if the $ORIGIN directive is not set) as the namespace being defined by this SOA resource record. The primary-name-server directive is the hostname of the primary nameserver that is authoritative for this domain. The hostmaster-email directive is the email of the person to contact about the namespace. The serial-number directive is a numerical value incremented every time the zone file is altered to indicate it is time for the named service to reload the zone. The time-to-refresh directive is the numerical value secondary nameservers use to determine how long to wait before asking the primary nameserver if any changes have been made to the zone. The time-to-retry directive is a numerical value used by secondary nameservers to determine the length of time to wait before issuing a refresh request in the event that the primary nameserver is not answering. If the primary server has not replied to a refresh request before the amount of time specified in the time-to-expire directive elapses, the secondary servers stop responding as an authority for requests concerning that namespace. In BIND 4 and 8, the minimum-TTL directive is the amount of time other nameservers cache the zone's information. In BIND 9, it defines how long negative answers are cached for. Caching of negative answers can be set to a maximum of 3 hours (that is, 3H).
When configuring BIND, all times are specified in seconds. However, it is possible to use abbreviations when specifying units of time other than seconds, such as minutes ( M), hours ( H), days ( D), and weeks ( W). Table 14.6, "Seconds compared to other time units" shows an amount of time in seconds and the equivalent time in another format. Table 14.6. Seconds compared to other time units | Seconds | Other Time Units |
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| 60 | 1M | | 1800 | 30M | | 3600 | 1H | | 10800 | 3H | | 21600 | 6H | | 43200 | 12H | | 86400 | 1D | | 259200 | 3D | | 604800 | 1W | | 31536000 | 365D |
Example 14.14. Using the SOA resource record @ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21600 ; refresh after 6 hours 3600 ; retry after 1 hour 604800 ; expire after 1 week 86400 ) ; minimum TTL of 1 day
Additionally to resource records and directives, a zone file can also contain comments. Comments are ignored by the named service, but can prove useful when providing additional information to the user. Any text after the semicolon character to the end of the line is considered a comment. For example: 604800 ; expire after 1 week The following examples show the basic usage of zone files. 14.2.2.4.1. A Simple Zone FileExample 14.15. A simple zone file $ORIGIN example.com.$TTL 86400@ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21600 ; refresh after 6 hours 3600 ; retry after 1 hour 604800 ; expire after 1 week 86400 ) ; minimum TTL of 1 day; IN NS dns1.example.com. IN NS dns2.example.com.dns1 IN A 10.0.1.1 IN AAAA aaaa:bbbb::1dns2 IN A 10.0.1.2 IN AAAA aaaa:bbbb::2;@ IN MX 10 mail.example.com. IN MX 20 mail2.example.com.mail IN A 10.0.1.5 IN AAAA aaaa:bbbb::5mail2 IN A 10.0.1.6 IN AAAA aaaa:bbbb::6; This sample zone file illustrates sharing the same IP addresses; for multiple services:;services IN A 10.0.1.10 IN AAAA aaaa:bbbb::10 IN A 10.0.1.11 IN AAAA aaaa:bbbb::11ftp IN CNAME services.example.com.www IN CNAME services.example.com.; In this example, the authoritative nameservers are set as dns1.example.com and dns2.example.com, and are tied to the 10.0.1.1 and 10.0.1.2 IP addresses respectively using the A record. The email servers configured with the MX records point to mail and mail2 via A records. Since these names do not end in a trailing period, the $ORIGIN domain is placed after them, expanding them to mail.example.com and mail2.example.com. Services available at the standard names, such as www.example.com ( WWW), are pointed at the appropriate servers using the CNAME record. This zone file would be called into service with a zone statement in the /etc/named.conf similar to the following: zone "example.com" IN { type master; file "example.com.zone"; allow-update { none; };};14.2.2.4.2. A Reverse Name Resolution Zone FileA reverse name resolution zone file is used to translate an IP address in a particular namespace into an fully qualified domain name (FQDN). It looks very similar to a standard zone file, except that the PTR resource records are used to link the IP addresses to a fully qualified domain name as shown in Example 14.16, "A reverse name resolution zone file". Example 14.16. A reverse name resolution zone file $ORIGIN 1.0.10.in-addr.arpa.$TTL 86400@ IN SOA dns1.example.com. hostmaster.example.com. ( 2001062501 ; serial 21600 ; refresh after 6 hours 3600 ; retry after 1 hour 604800 ; expire after 1 week 86400 ) ; minimum TTL of 1 day;@ IN NS dns1.example.com.;1 IN PTR dns1.example.com.2 IN PTR dns2.example.com.;5 IN PTR server1.example.com.6 IN PTR server2.example.com.;3 IN PTR ftp.example.com.4 IN PTR ftp.example.com. In this example, IP addresses 10.0.1.1 through 10.0.1.6 are pointed to the corresponding fully qualified domain name. This zone file would be called into service with a zone statement in the /etc/named.conf file similar to the following: zone "1.0.10.in-addr.arpa" IN { type master; file "example.com.rr.zone"; allow-update { none; };};There is very little difference between this example and a standard zone statement, except for the zone name. Note that a reverse name resolution zone requires the first three blocks of the IP address reversed followed by .in-addr.arpa. This allows the single block of IP numbers used in the reverse name resolution zone file to be associated with the zone. 14.2.3. Using the rndc UtilityThe rndc utility is a command line tool that allows you to administer the named service, both locally and from a remote machine. Its usage is as follows: rndc [option...] command [command-option]
14.2.3.1. Configuring the UtilityTo prevent unauthorized access to the service, named must be configured to listen on the selected port (that is, 953 by default), and an identical key must be used by both the service and the rndc utility. Table 14.7. Relevant files | Path | Description |
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/etc/named.conf | The default configuration file for the named service. | /etc/rndc.conf | The default configuration file for the rndc utility. | /etc/rndc.key | The default key location. |
The rndc configuration is located in /etc/rndc.conf. If the file does not exist, the utility will use the key located in /etc/rndc.key, which was generated automatically during the installation process using the rndc-confgen -a command. The named service is configured using the controls statement in the /etc/named.conf configuration file as described in Section 14.2.1.2, "Other Statement Types". Unless this statement is present, only the connections from the loopback address (that is, 127.0.0.1) will be allowed, and the key located in /etc/rndc.key will be used. To prevent unprivileged users from sending control commands to the service, make sure only root is allowed to read the /etc/rndc.key file: ~]# chmod o-rwx /etc/rndc.key 14.2.3.2. Checking the Service StatusTo check the current status of the named service, use the following command: ~]# rndc statusversion: 9.7.0-P2-RedHat-9.7.0-5.P2.el6CPUs found: 1worker threads: 1number of zones: 16debug level: 0xfers running: 0xfers deferred: 0soa queries in progress: 0query logging is OFFrecursive clients: 0/0/1000tcp clients: 0/100server is up and running 14.2.3.3. Reloading the Configuration and ZonesTo reload both the configuration file and zones, type the following at a shell prompt: ~]# rndc reloadserver reload successful This will reload the zones while keeping all previously cached responses, so that you can make changes to the zone files without losing all stored name resolutions. To reload a single zone, specify its name after the reload command, for example: ~]# rndc reload localhostzone reload up-to-date Finally, to reload the configuration file and newly added zones only, type: ~]# rndc reconfig If you intend to manually modify a zone that uses Dynamic DNS (DDNS), make sure you run the freeze command first: ~]# rndc freeze localhost Once you are finished, run the thaw command to allow the DDNS again and reload the zone: ~]# rndc thaw localhostThe zone reload and thaw was successful. 14.2.3.4. Updating Zone KeysTo update the DNSSEC keys and sign the zone, use the sign command. For example: ~]# rndc sign localhost Note that to sign a zone with the above command, the auto-dnssec option has to be set to maintain in the zone statement. For instance: zone "localhost" IN { type master; file "named.localhost"; allow-update { none; }; auto-dnssec maintain;};14.2.3.5. Enabling the DNSSEC ValidationTo enable the DNSSEC validation, type the following at a shell prompt: ~]# rndc validation on Similarly, to disable this option, type: ~]# rndc validation off 14.2.3.6. Enabling the Query LoggingTo enable (or disable in case it is currently enabled) the query logging, run the following command: ~]# rndc querylog 14.2.4. Using the dig UtilityThe dig utility is a command line tool that allows you to perform DNS lookups and debug a nameserver configuration. Its typical usage is as follows: dig [@server] [option...] name type
14.2.4.1. Looking Up a NameserverTo look up a nameserver for a particular domain, use the command in the following form: digname NS
Example 14.17. A sample nameserver lookup ~]$ dig example.com NS; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> example.com NS; global options: +cmd; Got answer:; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 57883; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0; QUESTION SECTION:;example.com. IN NS; ANSWER SECTION:example.com. 99374 IN NS a.iana-servers.net.example.com. 99374 IN NS b.iana-servers.net.; Query time: 1 msec; SERVER: 10.34.255.7#53(10.34.255.7); WHEN: Wed Aug 18 18:04:06 2010; MSG SIZE rcvd: 77 14.2.4.2. Looking Up an IP AddressTo look up an IP address assigned to a particular domain, use the command in the following form: digname A
Example 14.18. A sample IP address lookup ~]$ dig example.com A; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> example.com A; global options: +cmd; Got answer:; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4849; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0; QUESTION SECTION:;example.com. IN A; ANSWER SECTION:example.com. 155606 IN A 192.0.32.10; AUTHORITY SECTION:example.com. 99175 IN NS a.iana-servers.net.example.com. 99175 IN NS b.iana-servers.net.; Query time: 1 msec; SERVER: 10.34.255.7#53(10.34.255.7); WHEN: Wed Aug 18 18:07:25 2010; MSG SIZE rcvd: 93 14.2.4.3. Looking Up a HostnameTo look up a hostname for a particular IP address, use the command in the following form: dig -x address Example 14.19. A sample hostname lookup ~]$ dig -x 192.0.32.10; <<>> DiG 9.7.1-P2-RedHat-9.7.1-2.P2.fc13 <<>> -x 192.0.32.10; global options: +cmd; Got answer:; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29683; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 5, ADDITIONAL: 6; QUESTION SECTION:;10.32.0.192.in-addr.arpa. IN PTR; ANSWER SECTION:10.32.0.192.in-addr.arpa. 21600 IN PTR www.example.com.; AUTHORITY SECTION:32.0.192.in-addr.arpa. 21600 IN NS b.iana-servers.org.32.0.192.in-addr.arpa. 21600 IN NS c.iana-servers.net.32.0.192.in-addr.arpa. 21600 IN NS d.iana-servers.net.32.0.192.in-addr.arpa. 21600 IN NS ns.icann.org.32.0.192.in-addr.arpa. 21600 IN NS a.iana-servers.net.; ADDITIONAL SECTION:a.iana-servers.net. 13688 IN A 192.0.34.43b.iana-servers.org. 5844 IN A 193.0.0.236b.iana-servers.org. 5844 IN AAAA 2001:610:240:2::c100:ecc.iana-servers.net. 12173 IN A 139.91.1.10c.iana-servers.net. 12173 IN AAAA 2001:648:2c30::1:10ns.icann.org. 12884 IN A 192.0.34.126; Query time: 156 msec; SERVER: 10.34.255.7#53(10.34.255.7); WHEN: Wed Aug 18 18:25:15 2010; MSG SIZE rcvd: 310 14.2.5. Advanced Features of BINDMost BIND implementations only use the named service to provide name resolution services or to act as an authority for a particular domain. However, BIND version 9 has a number of advanced features that allow for a more secure and efficient DNS service. Before attempting to use advanced features like DNSSEC, TSIG, or IXFR (Incremental Zone Transfer), make sure that the particular feature is supported by all nameservers in the network environment, especially when you use older versions of BIND or non-BIND servers. Optionally, different information can be presented to a client depending on the network a request originates from. This is primarily used to deny sensitive DNS entries from clients outside of the local network, while allowing queries from clients inside the local network. To configure multiple views, add the view statement to the /etc/named.conf configuration file. Use the match-clients option to match IP addresses or entire networks and give them special options and zone data. 14.2.5.2. Incremental Zone Transfers (IXFR)Incremental Zone Transfers (IXFR) allow a secondary nameserver to only download the updated portions of a zone modified on a primary nameserver. Compared to the standard transfer process, this makes the notification and update process much more efficient. Note that IXFR is only available when using dynamic updating to make changes to master zone records. If manually editing zone files to make changes, Automatic Zone Transfer (AXFR) is used. 14.2.5.3. Transaction SIGnatures (TSIG)Transaction SIGnatures (TSIG) ensure that a shared secret key exists on both primary and secondary nameserver before allowing a transfer. This strengthens the standard IP address-based method of transfer authorization, since attackers would not only need to have access to the IP address to transfer the zone, but they would also need to know the secret key. Since version 9, BIND also supports TKEY, which is another shared secret key method of authorizing zone transfers. When communicating over an insecure network, do not rely on IP address-based authentication only. 14.2.5.4. DNS Security Extensions (DNSSEC)Domain Name System Security Extensions (DNSSEC) provide origin authentication of DNS data, authenticated denial of existence, and data integrity. When a particular domain is marked as secure, the SERFVAIL response is returned for each resource record that fails the validation. Note that to debug a DNSSEC-signed domain or a DNSSEC-aware resolver, you can use the dig utility as described in Section 14.2.4, "Using the dig Utility". Useful options are +dnssec (requests DNSSEC-related resource records by setting the DNSSEC OK bit), +cd (tells recursive nameserver not to validate the response), and +bufsize=512 (changes the packet size to 512B to get through some firewalls). 14.2.5.5. Internet Protocol version 6 (IPv6)14.2.6. Common Mistakes to AvoidThe following is a list of recommendations on how to avoid common mistakes users make when configuring a nameserver: - Use semicolons and curly brackets correctly
An omitted semicolon or unmatched curly bracket in the /etc/named.conf file can prevent the named service from starting. - Use period correctly
In zone files, a period at the end of a domain name denotes a fully qualified domain name. If omitted, the named service will append the name of the zone or the value of $ORIGIN to complete it. - Increment the serial number when editing a zone file
If the serial number is not incremented, the primary nameserver will have the correct, new information, but the secondary nameservers will never be notified of the change, and will not attempt to refresh their data of that zone. - Configure the firewall
If a firewall is blocking connections from the named service to other nameservers, the recommended practice is to change the firewall settings. According to the recent research in DNS security, using a fixed UDP source port for DNS queries is a potential security vulnerability that could allow an attacker to conduct cache-poisoning attacks more easily. To prevent this, configure your firewall to allow queries from a random UDP source port.
14.2.7. Additional ResourcesThe following sources of information provide additional resources regarding BIND. 14.2.7.1. Installed DocumentationBIND features a full range of installed documentation covering many different topics, each placed in its own subject directory. For each item below, replace version with the version of the bind package installed on the system: -
/usr/share/doc/bind-version/ The main directory containing the most recent documentation. -
/usr/share/doc/bind-version/arm/ The directory containing the BIND 9 Administrator Reference Manual in HTML and SGML formats, which details BIND resource requirements, how to configure different types of nameservers, how to perform load balancing, and other advanced topics. For most new users of BIND, this is the best place to start. -
/usr/share/doc/bind-version/draft/ The directory containing assorted technical documents that review issues related to the DNS service, and propose some methods to address them. -
/usr/share/doc/bind-version/misc/ The directory designed to address specific advanced issues. Users of BIND version 8 should consult the migration document for specific changes they must make when moving to BIND 9. The options file lists all of the options implemented in BIND 9 that are used in /etc/named.conf. -
/usr/share/doc/bind-version/rfc/ The directory providing every RFC document related to BIND.
There is also a number of man pages for the various applications and configuration files involved with BIND: -
man rndc The manual page for rndc containing the full documentation on its usage. -
man named The manual page for named containing the documentation on assorted arguments that can be used to control the BIND nameserver daemon. -
man lwresd The manual page for lwresd containing the full documentation on the lightweight resolver daemon and its usage. -
man named.conf The manual page with a comprehensive list of options available within the named configuration file. -
man rndc.conf The manual page with a comprehensive list of options available within the rndc configuration file.
14.2.7.2. Useful Websites- http://www.isc.org/software/bind
The home page of the BIND project containing information about current releases as well as a PDF version of the BIND 9 Administrator Reference Manual.
- DNS and BIND by Paul Albitz and Cricket Liu; O'Reilly & Associates
A popular reference that explains both common and esoteric BIND configuration options, and provides strategies for securing a DNS server. - The Concise Guide to DNS and BIND by Nicolai Langfeldt; Que
Looks at the connection between multiple network services and BIND, with an emphasis on task-oriented, technical topics.
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