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Chapter 1. Setting up and configuring a BIND DNS server

BIND is a feature-rich DNS server that is fully compliant with the Internet Engineering Task Force (IETF) DNS standards and draft standards. For example, administrators frequently use BIND as:

  • Caching DNS server in the local network
  • Authoritative DNS server for zones
  • Secondary server to provide high availability for zones

1.1. Considerations about protecting BIND with SELinux or running it in a change-root environment

To secure a BIND installation, you can:

  • Run the named service without a change-root environment. In this case, SELinux in enforcing mode prevents exploitation of known BIND security vulnerabilities. By default, Red Hat Enterprise Linux uses SELinux in enforcing mode.

    Important

    Running BIND on RHEL with SELinux in enforcing mode is more secure than running BIND in a change-root environment.

  • Run the named-chroot service in a change-root environment.

    Using the change-root feature, administrators can define that the root directory of a process and its sub-processes is different to the / directory. When you start the named-chroot service, BIND switches its root directory to /var/named/chroot/. As a consequence, the service uses mount --bind commands to make the files and directories listed in /etc/named-chroot.files available in /var/named/chroot/, and the process has no access to files outside of /var/named/chroot/.

If you decide to use BIND:

  • In normal mode, use the named service.
  • In a change-root environment, use the named-chroot service. This requires that you install, additionally, the named-chroot package.

1.2. Configuring BIND as a caching DNS server

By default, the BIND DNS server resolves and caches successful and failed lookups. The service then answers requests to the same records from its cache. This significantly improves the speed of DNS lookups.

Prerequisites

  • The IP address of the server is static.

Procedure

  1. Install the bind and bind-utils packages:

    # dnf install bind bind-utils
  2. If you want to run BIND in a change-root environment install the bind-chroot package:

    # dnf install bind-chroot

    Note that running BIND on a host with SELinux in enforcing mode, which is default, is more secure.

  3. Edit the /etc/named.conf file, and make the following changes in the options statement:

    1. Update the listen-on and listen-on-v6 statements to specify on which IPv4 and IPv6 interfaces BIND should listen:

      listen-on port 53 { 127.0.0.1; 192.0.2.1; };
      listen-on-v6 port 53 { ::1; 2001:db8:1::1; };
    2. Update the allow-query statement to configure from which IP addresses and ranges clients can query this DNS server:

      allow-query { localhost; 192.0.2.0/24; 2001:db8:1::/64; };
    3. Add an allow-recursion statement to define from which IP addresses and ranges BIND accepts recursive queries:

      allow-recursion { localhost; 192.0.2.0/24; 2001:db8:1::/64; };
      Warning

      Do not allow recursion on public IP addresses of the server. Otherwise, the server can become part of large-scale DNS amplification attacks.

    4. By default, BIND resolves queries by recursively querying from the root servers to an authoritative DNS server. Alternatively, you can configure BIND to forward queries to other DNS servers, such as the ones of your provider. In this case, add a forwarders statement with the list of IP addresses of the DNS servers that BIND should forward queries to:

      forwarders { 198.51.100.1; 203.0.113.5; };

      As a fall-back behavior, BIND resolves queries recursively if the forwarder servers do not respond. To disable this behavior, add a forward only; statement.

  4. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  5. Update the firewalld rules to allow incoming DNS traffic:

    # firewall-cmd --permanent --add-service=dns
    # firewall-cmd --reload
  6. Start and enable BIND:

    # systemctl enable --now named

    If you want to run BIND in a change-root environment, use the systemctl enable --now named-chroot command to enable and start the service.

Verification

  1. Use the newly set up DNS server to resolve a domain:

    # dig @localhost www.example.org
    ...
    www.example.org.    86400    IN    A    198.51.100.34
    
    ;; Query time: 917 msec
    ...

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

    After querying a record for the first time, BIND adds the entry to its cache.

  2. Repeat the previous query:

    # dig @localhost www.example.org
    ...
    www.example.org.    85332    IN    A    198.51.100.34
    
    ;; Query time: 1 msec
    ...

    Because of the cached entry, further requests for the same record are significantly faster until the entry expires.

Next steps

  • Configure the clients in your network to use this DNS server. If a DHCP server provides the DNS server setting to the clients, update the DHCP server’s configuration accordingly.

Additional resources

1.3. Configuring logging on a BIND DNS server

The configuration in the default /etc/named.conf file, as provided by the bind package, uses the default_debug channel and logs messages to the /var/named/data/named.run file. The default_debug channel only logs entries when the server’s debug level is non-zero.

Using different channels and categories, you can configure BIND to write different events with a defined severity to separate files.

Prerequisites

  • BIND is already configured, for example, as a caching name server.
  • The named or named-chroot service is running.

Procedure

  1. Edit the /etc/named.conf file, and add category and channel phrases to the logging statement, for example:

    logging {
        ...
    
        category notify { zone_transfer_log; };
        category xfer-in { zone_transfer_log; };
        category xfer-out { zone_transfer_log; };
        channel zone_transfer_log {
            file "/var/named/log/transfer.log" versions 10 size 50m;
            print-time yes;
            print-category yes;
            print-severity yes;
            severity info;
         };
    
         ...
    };

    With this example configuration, BIND logs messages related to zone transfers to /var/named/log/transfer.log. BIND creates up to 10 versions of the log file and rotates them if they reach a maximum size of 50 MB.

    The category phrase defines to which channels BIND sends messages of a category.

    The channel phrase defines the destination of log messages including the number of versions, the maximum file size, and the severity level BIND should log to a channel. Additional settings, such as enabling logging the time stamp, category, and severity of an event are optional, but useful for debugging purposes.

  2. Create the log directory if it does not exist, and grant write permissions to the named user on this directory:

    # mkdir /var/named/log/
    # chown named:named /var/named/log/
    # chmod 700 /var/named/log/
  3. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  4. Restart BIND:

    # systemctl restart named

    If you run BIND in a change-root environment, use the systemctl restart named-chroot command to restart the service.

Verification

  • Display the content of the log file:

    # cat /var/named/log/transfer.log
    ...
    06-Jul-2022 15:08:51.261 xfer-out: info: client @0x7fecbc0b0700 192.0.2.2#36121/key example-transfer-key (example.com): transfer of 'example.com/IN': AXFR started: TSIG example-transfer-key (serial 2022070603)
    06-Jul-2022 15:08:51.261 xfer-out: info: client @0x7fecbc0b0700 192.0.2.2#36121/key example-transfer-key (example.com): transfer of 'example.com/IN': AXFR ended

Additional resources

  • named.conf(5) man page

1.4. Writing BIND ACLs

Controlling access to certain features of BIND can prevent unauthorized access and attacks, such as denial of service (DoS). BIND access control list (acl) statements are lists of IP addresses and ranges. Each ACL has a nickname that you can use in several statements, such as allow-query, to refer to the specified IP addresses and ranges.

Warning

BIND uses only the first matching entry in an ACL. For example, if you define an ACL { 192.0.2/24; !192.0.2.1; } and the host with IP address 192.0.2.1 connects, access is granted even if the second entry excludes this address.

BIND has the following built-in ACLs:

  • none: Matches no hosts.
  • any: Matches all hosts.
  • localhost: Matches the loopback addresses 127.0.0.1 and ::1, as well as the IP addresses of all interfaces on the server that runs BIND.
  • localnets: Matches the loopback addresses 127.0.0.1 and ::1, as well as all subnets the server that runs BIND is directly connected to.

Prerequisites

  • BIND is already configured, for example, as a caching name server.
  • The named or named-chroot service is running.

Procedure

  1. Edit the /etc/named.conf file and make the following changes:

    1. Add acl statements to the file. For example, to create an ACL named internal-networks for 127.0.0.1, 192.0.2.0/24, and 2001:db8:1::/64, enter:

      acl internal-networks { 127.0.0.1; 192.0.2.0/24; 2001:db8:1::/64; };
      acl dmz-networks { 198.51.100.0/24; 2001:db8:2::/64; };
    2. Use the ACL’s nickname in statements that support them, for example:

      allow-query { internal-networks; dmz-networks; };
      allow-recursion { internal-networks; };
  2. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  3. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

Verification

  • Execute an action that triggers a feature which uses the configured ACL. For example, the ACL in this procedure allows only recursive queries from the defined IP addresses. In this case, enter the following command on a host that is not within the ACL’s definition to attempt resolving an external domain:

    # dig +short @192.0.2.1 www.example.com

    If the command returns no output, BIND denied access, and the ACL works. For a verbose output on the client, use the command without +short option:

    # dig @192.0.2.1 www.example.com
    ...
    ;; WARNING: recursion requested but not available
    ...

1.5. Configuring zones on a BIND DNS server

A DNS zone is a database with resource records for a specific sub-tree in the domain space. For example, if you are responsible for the example.com domain, you can set up a zone for it in BIND. As a result, clients can, resolve www.example.com to the IP address configured in this zone.

1.5.1. The SOA record in zone files

The start of authority (SOA) record is a required record in a DNS zone. This record is important, for example, if multiple DNS servers are authoritative for a zone but also to DNS resolvers.

A SOA record in BIND has the following syntax:

name class type mname rname serial refresh retry expire minimum

For better readability, administrators typically split the record in zone files into multiple lines with comments that start with a semicolon (;). Note that, if you split a SOA record, parentheses keep the record together:

@ IN SOA ns1.example.com. hostmaster.example.com. (
                          2022070601 ; serial number
                          1d         ; refresh period
                          3h         ; retry period
                          3d         ; expire time
                          3h )       ; minimum TTL
Important

Note the trailing dot at the end of the fully-qualified domain names (FQDNs). FQDNs consist of multiple domain labels, separated by dots. Because the DNS root has an empty label, FQDNs end with a dot. Therefore, BIND appends the zone name to names without a trailing dot. A hostname without a trailing dot, for example, ns1.example.com would be expanded to ns1.example.com.example.com., which is not the correct address of the primary name server.

These are the fields in a SOA record:

  • name: The name of the zone, the so-called origin. If you set this field to @, BIND expands it to the zone name defined in /etc/named.conf.
  • class: In SOA records, you must set this field always to Internet (IN).
  • type: In SOA records, you must set this field always to SOA.
  • mname (master name): The hostname of the primary name server of this zone.
  • rname (responsible name): The email address of who is responsible for this zone. Note that the format is different. You must replace the at sign (@) with a dot (.).
  • serial: The version number of this zone file. Secondary name servers only update their copies of the zone if the serial number on the primary server is higher.

    The format can be any numeric value. A commonly-used format is <year><month><day><two-digit-number>. With this format, you can, theoretically, change the zone file up to a hundred times per day.

  • refresh: The amount of time secondary servers should wait before checking the primary server if the zone was updated.
  • retry: The amount of time after that a secondary server retries to query the primary server after a failed attempt.
  • expire: The amount of time after that a secondary server stops querying the primary server, if all previous attempts failed.
  • minimum: RFC 2308 changed the meaning of this field to the negative caching time. Compliant resolvers use it to determine how long to cache NXDOMAIN name errors.
Note

A numeric value in the refresh, retry, expire, and minimum fields define a time in seconds. However, for better readability, use time suffixes, such as m for minute, h for hours, and d for days. For example, 3h stands for 3 hours.

Additional resources

  • RFC 1035: Domain names - implementation and specification
  • RFC 1034: Domain names - concepts and facilities
  • RFC 2308: Negative caching of DNS queries (DNS cache)

1.5.2. Setting up a forward zone on a BIND primary server

Forward zones map names to IP addresses and other information. For example, if you are responsible for the domain example.com, you can set up a forward zone in BIND to resolve names, such as www.example.com.

Prerequisites

  • BIND is already configured, for example, as a caching name server.
  • The named or named-chroot service is running.

Procedure

  1. Add a zone definition to the /etc/named.conf file:

    zone "example.com" {
        type master;
        file "example.com.zone";
        allow-query { any; };
        allow-transfer { none; };
    };

    These settings define:

    • This server as the primary server (type master) for the example.com zone.
    • The /var/named/example.com.zone file is the zone file. If you set a relative path, as in this example, this path is relative to the directory you set in directory in the options statement.
    • Any host can query this zone. Alternatively, specify IP ranges or BIND access control list (ACL) nicknames to limit the access.
    • No host can transfer the zone. Allow zone transfers only when you set up secondary servers and only for the IP addresses of the secondary servers.
  2. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  3. Create the /var/named/example.com.zone file, for example, with the following content:

    $TTL 8h
    @ IN SOA ns1.example.com. hostmaster.example.com. (
                              2022070601 ; serial number
                              1d         ; refresh period
                              3h         ; retry period
                              3d         ; expire time
                              3h )       ; minimum TTL
    
                      IN NS   ns1.example.com.
                      IN MX   10 mail.example.com.
    
    www               IN A    192.0.2.30
    www               IN AAAA 2001:db8:1::30
    ns1               IN A    192.0.2.1
    ns1               IN AAAA 2001:db8:1::1
    mail              IN A    192.0.2.20
    mail              IN AAAA 2001:db8:1::20

    This zone file:

    • Sets the default time-to-live (TTL) value for resource records to 8 hours. Without a time suffix, such as h for hour, BIND interprets the value as seconds.
    • Contains the required SOA resource record with details about the zone.
    • Sets ns1.example.com as an authoritative DNS server for this zone. To be functional, a zone requires at least one name server (NS) record. However, to be compliant with RFC 1912, you require at least two name servers.
    • Sets mail.example.com as the mail exchanger (MX) of the example.com domain. The numeric value in front of the host name is the priority of the record. Entries with a lower value have a higher priority.
    • Sets the IPv4 and IPv6 addresses of www.example.com, mail.example.com, and ns1.example.com.
  4. Set secure permissions on the zone file that allow only the named group to read it:

    # chown root:named /var/named/example.com.zone
    # chmod 640 /var/named/example.com.zone
  5. Verify the syntax of the /var/named/example.com.zone file:

    # named-checkzone example.com /var/named/example.com.zone
    zone example.com/IN: loaded serial 2022070601
    OK
  6. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

Verification

  • Query different records from the example.com zone, and verify that the output matches the records you have configured in the zone file:

    # dig +short @localhost AAAA www.example.com
    2001:db8:1::30
    
    # dig +short @localhost NS example.com
    ns1.example.com.
    
    # dig +short @localhost A ns1.example.com
    192.0.2.1

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

1.5.3. Setting up a reverse zone on a BIND primary server

Reverse zones map IP addresses to names. For example, if you are responsible for IP range 192.0.2.0/24, you can set up a reverse zone in BIND to resolve IP addresses from this range to hostnames.

Note

If you create a reverse zone for whole classful networks, name the zone accordingly. For example, for the class C network 192.0.2.0/24, the name of the zone is 2.0.192.in-addr.arpa. If you want to create a reverse zone for a different network size, for example 190.0.2.0/28, the name of the zone is 28-2.0.192.in-addr.arpa.

Prerequisites

  • BIND is already configured, for example, as a caching name server.
  • The named or named-chroot service is running.

Procedure

  1. Add a zone definition to the /etc/named.conf file:

    zone "2.0.192.in-addr.arpa" {
        type master;
        file "2.0.192.in-addr.arpa.zone";
        allow-query { any; };
        allow-transfer { none; };
    };

    These settings define:

    • This server as the primary server (type master) for the 2.0.192.in-addr.arpa reverse zone.
    • The /var/named/2.0.192.in-addr.arpa.zone file is the zone file. If you set a relative path, as in this example, this path is relative to the directory you set in directory in the options statement.
    • Any host can query this zone. Alternatively, specify IP ranges or BIND access control list (ACL) nicknames to limit the access.
    • No host can transfer the zone. Allow zone transfers only when you set up secondary servers and only for the IP addresses of the secondary servers.
  2. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  3. Create the /var/named/2.0.192.in-addr.arpa.zone file, for example, with the following content:

    $TTL 8h
    @ IN SOA ns1.example.com. hostmaster.example.com. (
                              2022070601 ; serial number
                              1d         ; refresh period
                              3h         ; retry period
                              3d         ; expire time
                              3h )       ; minimum TTL
    
                      IN NS   ns1.example.com.
    
    1                 IN PTR  ns1.example.com.
    30                IN PTR  www.example.com.

    This zone file:

    • Sets the default time-to-live (TTL) value for resource records to 8 hours. Without a time suffix, such as h for hour, BIND interprets the value as seconds.
    • Contains the required SOA resource record with details about the zone.
    • Sets ns1.example.com as an authoritative DNS server for this reverse zone. To be functional, a zone requires at least one name server (NS) record. However, to be compliant with RFC 1912, you require at least two name servers.
    • Sets the pointer (PTR) record for the 192.0.2.1 and 192.0.2.30 addresses.
  4. Set secure permissions on the zone file that only allow the named group to read it:

    # chown root:named /var/named/2.0.192.in-addr.arpa.zone
    # chmod 640 /var/named/2.0.192.in-addr.arpa.zone
  5. Verify the syntax of the /var/named/2.0.192.in-addr.arpa.zone file:

    # named-checkzone 2.0.192.in-addr.arpa /var/named/2.0.192.in-addr.arpa.zone
    zone 2.0.192.in-addr.arpa/IN: loaded serial 2022070601
    OK
  6. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

Verification

  • Query different records from the reverse zone, and verify that the output matches the records you have configured in the zone file:

    # dig +short @localhost -x 192.0.2.1
    ns1.example.com.
    
    # dig +short @localhost -x 192.0.2.30
    www.example.com.

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

1.5.4. Updating a BIND zone file

In certain situations, for example if an IP address of a server changes, you must update a zone file. If multiple DNS servers are responsible for a zone, perform this procedure only on the primary server. Other DNS servers that store a copy of the zone will receive the update through a zone transfer.

Prerequisites

  • The zone is configured.
  • The named or named-chroot service is running.

Procedure

  1. Optional: Identify the path to the zone file in the /etc/named.conf file:

    options {
        ...
        directory       "/var/named";
    }
    
    zone "example.com" {
        ...
        file "example.com.zone";
    };

    You find the path to the zone file in the file statement in the zone’s definition. A relative path is relative to the directory set in directory in the options statement.

  2. Edit the zone file:

    1. Make the required changes.
    2. Increment the serial number in the start of authority (SOA) record.

      Important

      If the serial number is equal to or lower than the previous value, secondary servers will not update their copy of the zone.

  3. Verify the syntax of the zone file:

    # named-checkzone example.com /var/named/example.com.zone
    zone example.com/IN: loaded serial 2022062802
    OK
  4. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

Verification

  • Query the record you have added, modified, or removed, for example:

    # dig +short @localhost A ns2.example.com
    192.0.2.2

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

1.5.5. DNSSEC zone signing using the automated key generation and zone maintenance features

You can sign zones with domain name system security extensions (DNSSEC) to ensure authentication and data integrity. Such zones contain additional resource records. Clients can use them to verify the authenticity of the zone information.

If you enable the DNSSEC policy feature for a zone, BIND performs the following actions automatically:

  • Creates the keys
  • Signs the zone
  • Maintains the zone, including re-signing and periodically replacing the keys.
Important

To enable external DNS servers to verify the authenticity of a zone, you must add the public key of the zone to the parent zone. Contact your domain provider or registry for further details on how to accomplish this.

This procedure uses the built-in default DNSSEC policy in BIND. This policy uses single ECDSAP256SHA key signatures. Alternatively, create your own policy to use custom keys, algorithms, and timings.

Prerequisites

  • The zone for which you want to enable DNSSEC is configured.
  • The named or named-chroot service is running.
  • The server synchronizes the time with a time server. An accurate system time is important for DNSSEC validation.

Procedure

  1. Edit the /etc/named.conf file, and add dnssec-policy default; to the zone for which you want to enable DNSSEC:

    zone "example.com" {
        ...
        dnssec-policy default;
    };
  2. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

  3. BIND stores the public key in the /var/named/K<zone_name>.+<algorithm>+<key_ID>.key file. Use this file to display the public key of the zone in the format that the parent zone requires:

    • DS record format:

      # dnssec-dsfromkey /var/named/Kexample.com.+013+61141.key
      example.com. IN DS 61141 13 2 3E184188CF6D2521EDFDC3F07CFEE8D0195AACBD85E68BAE0620F638B4B1B027
    • DNSKEY format:

      # grep DNSKEY /var/named/Kexample.com.+013+61141.key
      example.com. 3600 IN DNSKEY 257 3 13 sjzT3jNEp120aSO4mPEHHSkReHUf7AABNnT8hNRTzD5cKMQSjDJin2I3 5CaKVcWO1pm+HltxUEt+X9dfp8OZkg==
  4. Request to add the public key of the zone to the parent zone. Contact your domain provider or registry for further details on how to accomplish this.

Verification

  1. Query your own DNS server for a record from the zone for which you enabled DNSSEC signing:

    # dig +dnssec +short @localhost A www.example.com
    192.0.2.30
    A 13 3 28800 20220718081258 20220705120353 61141 example.com. e7Cfh6GuOBMAWsgsHSVTPh+JJSOI/Y6zctzIuqIU1JqEgOOAfL/Qz474 M0sgi54m1Kmnr2ANBKJN9uvOs5eXYw==

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

  2. After the public key has been added to the parent zone and propagated to other servers, verify that the server sets the authenticated data (ad) flag on queries to the signed zone:

    #  dig @localhost example.com +dnssec
    ...
    ;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
    ...

1.6. Configuring zone transfers among BIND DNS servers

Zone transfers ensure that all DNS servers that have a copy of the zone use up-to-date data.

Prerequisites

  • On the future primary server, the zone for which you want to set up zone transfers is already configured.
  • On the future secondary server, BIND is already configured, for example, as a caching name server.
  • On both servers, the named or named-chroot service is running.

Procedure

  1. On the existing primary server:

    1. Create a shared key, and append it to the /etc/named.conf file:

      # tsig-keygen example-transfer-key | tee -a /etc/named.conf
      key "example-transfer-key" {
              algorithm hmac-sha256;
              secret "q7ANbnyliDMuvWgnKOxMLi313JGcTZB5ydMW5CyUGXQ=";
      };

      This command displays the output of the tsig-keygen command and automatically appends it to /etc/named.conf.

      You will require the output of the command later on the secondary server as well.

    2. Edit the zone definition in the /etc/named.conf file:

      1. In the allow-transfer statement, define that servers must provide the key specified in the example-transfer-key statement to transfer a zone:

        zone "example.com" {
            ...
            allow-transfer { key example-transfer-key; };
        };

        Alternatively, use BIND access control list (ACL) nicknames in the allow-transfer statement.

      2. By default, after a zone has been updated, BIND notifies all name servers which have a name server (NS) record in this zone. If you do not plan to add an NS record for the secondary server to the zone, you can, configure that BIND notifies this server anyway. For that, add the also-notify statement with the IP addresses of this secondary server to the zone:

        zone "example.com" {
            ...
            also-notify { 192.0.2.2; 2001:db8:1::2; };
        };
    3. Verify the syntax of the /etc/named.conf file:

      # named-checkconf

      If the command displays no output, the syntax is correct.

    4. Reload BIND:

      # systemctl reload named

      If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

  2. On the future secondary server:

    1. Edit the /etc/named.conf file as follows:

      1. Add the same key definition as on the primary server:

        key "example-transfer-key" {
                algorithm hmac-sha256;
                secret "q7ANbnyliDMuvWgnKOxMLi313JGcTZB5ydMW5CyUGXQ=";
        };
      2. Add the zone definition to the /etc/named.conf file:

        zone "example.com" {
            type slave;
            file "slaves/example.com.zone";
            allow-query { any; };
            allow-transfer { none; };
            masters {
              192.0.2.1 key example-transfer-key;
              2001:db8:1::1 key example-transfer-key;
            };
        };

        These settings state:

        • This server is a secondary server (type slave) for the example.com zone.
        • The /var/named/slaves/example.com.zone file is the zone file. If you set a relative path, as in this example, this path is relative to the directory you set in directory in the options statement. To separate zone files for which this server is secondary from primary ones, you can store them, for example, in the /var/named/slaves/ directory.
        • Any host can query this zone. Alternatively, specify IP ranges or ACL nicknames to limit the access.
        • No host can transfer the zone from this server.
        • The IP addresses of the primary server of this zone are 192.0.2.1 and 2001:db8:1::2. Alternatively, you can specify ACL nicknames. This secondary server will use the key named example-transfer-key to authenticate to the primary server.
    2. Verify the syntax of the /etc/named.conf file:

      # named-checkconf
    3. Reload BIND:

      # systemctl reload named

      If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

  3. Optional: Modify the zone file on the primary server and add an NS record for the new secondary server.

Verification

On the secondary server:

  1. Display the systemd journal entries of the named service:

    # journalctl -u named
    ...
    Jul 06 15:08:51 ns2.example.com named[2024]: zone example.com/IN: Transfer started.
    Jul 06 15:08:51 ns2.example.com named[2024]: transfer of 'example.com/IN' from 192.0.2.1#53: connected using 192.0.2.2#45803
    Jul 06 15:08:51 ns2.example.com named[2024]: zone example.com/IN: transferred serial 2022070101
    Jul 06 15:08:51 ns2.example.com named[2024]: transfer of 'example.com/IN' from 192.0.2.1#53: Transfer status: success
    Jul 06 15:08:51 ns2.example.com named[2024]: transfer of 'example.com/IN' from 192.0.2.1#53: Transfer completed: 1 messages, 29 records, 2002 bytes, 0.003 secs (667333 bytes/sec)

    If you run BIND in a change-root environment, use the journalctl -u named-chroot command to display the journal entries.

  2. Verify that BIND created the zone file:

    # ls -l /var/named/slaves/
    total 4
    -rw-r--r--. 1 named named 2736 Jul  6 15:08 example.com.zone

    Note that, by default, secondary servers store zone files in a binary raw format.

  3. Query a record of the transferred zone from the secondary server:

    # dig +short @192.0.2.2 AAAA www.example.com
    2001:db8:1::30

    This example assumes that the secondary server you set up in this procedure listens on IP address 192.0.2.2.

1.7. Configuring response policy zones in BIND to override DNS records

Using DNS blocking and filtering, administrators can rewrite a DNS response to block access to certain domains or hosts. In BIND, response policy zones (RPZs) provide this feature. You can configure different actions for blocked entries, such as returning an NXDOMAIN error or not responding to the query.

If you have multiple DNS servers in your environment, use this procedure to configure the RPZ on the primary server, and later configure zone transfers to make the RPZ available on your secondary servers.

Prerequisites

  • BIND is already configured, for example, as a caching name server.
  • The named or named-chroot service is running.

Procedure

  1. Edit the /etc/named.conf file, and make the following changes:

    1. Add a response-policy definition to the options statement:

      options {
          ...
      
          response-policy {
              zone "rpz.local";
          };
      
          ...
      }

      You can set a custom name for the RPZ in the zone statement in response-policy. However, you must use the same name in the zone definition in the next step.

    2. Add a zone definition for the RPZ you set in the previous step:

      zone "rpz.local" {
          type master;
          file "rpz.local";
          allow-query { localhost; 192.0.2.0/24; 2001:db8:1::/64; };
          allow-transfer { none; };
      };

      These settings state:

      • This server is the primary server (type master) for the RPZ named rpz.local.
      • The /var/named/rpz.local file is the zone file. If you set a relative path, as in this example, this path is relative to the directory you set in directory in the options statement.
      • Any hosts defined in allow-query can query this RPZ. Alternatively, specify IP ranges or BIND access control list (ACL) nicknames to limit the access.
      • No host can transfer the zone. Allow zone transfers only when you set up secondary servers and only for the IP addresses of the secondary servers.
  2. Verify the syntax of the /etc/named.conf file:

    # named-checkconf

    If the command displays no output, the syntax is correct.

  3. Create the /var/named/rpz.local file, for example, with the following content:

    $TTL 10m
    @ IN SOA ns1.example.com. hostmaster.example.com. (
                              2022070601 ; serial number
                              1h         ; refresh period
                              1m         ; retry period
                              3d         ; expire time
                              1m )       ; minimum TTL
    
                     IN NS    ns1.example.com.
    
    example.org      IN CNAME .
    *.example.org    IN CNAME .
    example.net      IN CNAME rpz-drop.
    *.example.net    IN CNAME rpz-drop.

    This zone file:

    • Sets the default time-to-live (TTL) value for resource records to 10 minutes. Without a time suffix, such as h for hour, BIND interprets the value as seconds.
    • Contains the required start of authority (SOA) resource record with details about the zone.
    • Sets ns1.example.com as an authoritative DNS server for this zone. To be functional, a zone requires at least one name server (NS) record. However, to be compliant with RFC 1912, you require at least two name servers.
    • Return an NXDOMAIN error for queries to example.org and hosts in this domain.
    • Drop queries to example.net and hosts in this domain.

    For a full list of actions and examples, see IETF draft: DNS Response Policy Zones (RPZ).

  4. Verify the syntax of the /var/named/rpz.local file:

    # named-checkzone rpz.local /var/named/rpz.local
    zone rpz.local/IN: loaded serial 2022070601
    OK
  5. Reload BIND:

    # systemctl reload named

    If you run BIND in a change-root environment, use the systemctl reload named-chroot command to reload the service.

Verification

  1. Attempt to resolve a host in example.org, that is configured in the RPZ to return an NXDOMAIN error:

    # dig @localhost www.example.org
    ...
    ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 30286
    ...

    This example assumes that BIND runs on the same host and responds to queries on the localhost interface.

  2. Attempt to resolve a host in the example.net domain, that is configured in the RPZ to drop queries:

    # dig @localhost www.example.net
    ...
    ;; connection timed out; no servers could be reached
    ...