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Revision 23 as of 2014-07-02 11:39:59
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Editor: ?SteveCrook
Comment: Add instruction for using localtime within chroot environment
Revision 24 as of 2014-11-20 19:51:00
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Editor: ?JeremyMorton
Comment: Fixed misleading wording; machine can be associated with multiple domains
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This file contains the local DNS server configuration, and this is where you declare the zones associated with the domain. This file contains the local DNS server configuration, and this is where you declare the zones associated with this server's domain(s).

Translation(s): French


Putting a DNS server on a network allows for the replacement of IP addresses of individual machines by a name. As a result, it's even possible to associate multiple names to the same machine to update the different available services. For example, www.example.com and pop.example.com, could both point to the primary server where the mail server and the business intranet reside, and the domain could be example.com. It's easy to remember that these two services are running on the same machine whose IP address is

Now imagine that our network administrator decides for some reason or another to move the mail server to the machine The only thing that has to be changed is the DNS server configuration file. You could always go and modify the host configuration for all the users, but that would be time consuming and inconvenient.


  • DNS : Domain Name System or Domain Name Server

  • Primary Server :

  • Secondary server :

  • Server cache :

Network Layout

We get internet access through an xxxbox (, two DNS servers provided by our ISP (, In fact, these two latter servers will ever be referred to in the configuration because the xxxbox will be in charge of resolving names if the packet destination isn't known. Consequently, I consider the xxxbox like a primary server outside of our domain. The “sid” server ( is connected to the xxxbox via its primary network card. It's also connected to the LAN ( by its secondary network interface( It's on this that we are going to install the primary DNS server for our domain example.com (RFC 2606) All the computers on the LAN are automatically assigned a single address by the DHCP service. The DHCP also provides the primary DNS server's address for our domain, and updatees the host names for the zone example.com so they can be associated with an ip address.

Server Management


The package bind9 will be used for installation.

# apt-get install bind9 

and then if you want to also install the documentation (very useful):

# apt-get install bind9-doc


After installation, you might want to get familiar with some of the configuration files. They are in the directory /etc/bind/

TSIG Signature

The purpose of this signature is to authenticate transactions with BIND. Thus, the DHCP server cannot update the example.com domain if it loses this key. Copy and paste an existing key

# cd /etc/bind/
# cat rndc.key
key "rndc-key" {
        algorithm hmac-md5;
        secret "QJc08cnP1xkoF4a/eSZZbw==";

# cp rndc.key ns-example-com_rndc-key

You can generate a new key with the following options:

  • algorithm HMAC-MD5 - identifies 157 (required for a TSIG signature and only algorithm supported by BIND)

  • length of 512 octets (multiple of 64 with a maximum length of 512 for the above algorithm)

  • name : ns-example-com_rndc-key

dnssec-keygen -a HMAC-MD5 -b 512 -n USER ns-example-com_rndc-key

The footprint associated with the key is 53334. We get two files, one with an extension key and the other with a private extension. This substitutes the key in the file ns-example-com_rndc-key with the one in one of these two files.

# cat Kns-example-com_rndc-key.+157+53334.private
Private-key-format: v1.2
Algorithm: 157 (HMAC_MD5)
Key: LZ5m+L/HAmtc9rs9OU2RGstsg+Ud0TMXOT+C4rK7+YNUo3vNxKx/197o2Z80t6gA34AEaAf3F+hEodV4K+SWvA==
Bits: AAA=

# cat ns-example-com_rndc-key
key "ns-example-com_rndc-key" {
        algorithm hmac-md5;
        secret "LZ5m+L/HAmtc9rs9OU2RGstsg+Ud0TMXOT+C4rK7+YNUo3vNxKx/197o2Z80t6gA34AEaAf3F+hEodV4K+SWvA==";

The file ns-example-com_rndc-key should not be made world readable for security reasons. This should be inserted into the bind configuration by an include because the bind configuration itself is world-readable. Also, it's a good idea to delete the key and private files generated before.

named.conf File

This file is the main configuration file for the DNS file.

// Managing acls
acl internals {;; };

// Load options
include "/etc/bind/named.conf.options";

// TSIG key used for the dynamic update
include "/etc/bind/ns-example-com_rndc-key";

// Configure the communication channel for Administrative BIND9 with rndc
// By default, they key is in the rndc.key file and is used by rndc and bind9 
// on the localhost
controls {
        inet port 953 allow {; };

// prime the server with knowledge of the root servers
zone "." {
        type hint;
        file "/etc/bind/db.root";

// be authoritative for the localhost forward and reverse zones, and for
// broadcast zones as per RFC 1912
zone "localhost" {
        type master;
        file "/etc/bind/db.local";
zone "127.in-addr.arpa" {
        type master;
        file "/etc/bind/db.127";
zone "0.in-addr.arpa" {
        type master;
        file "/etc/bind/db.0";
zone "255.in-addr.arpa" {
        type master;
        file "/etc/bind/db.255";

include "/etc/bind/named.conf.local";

File named.conf.options

This file contains all the configuration options for the DNS server

options {
        directory "/var/cache/bind";

        // Exchange port between DNS servers
        query-source address * port *;

        // Transmit requests to if
        // this server doesn't know how to resolve them
        forward only;
        forwarders {; };

        auth-nxdomain no;    # conform to RFC1035

        // Listen on local interfaces only(IPV4)
        listen-on-v6 { none; };
        listen-on {;; };

        // Do not transfer the zone information to the secondary DNS
        allow-transfer { none; };

        // Accept requests for internal network only
        allow-query { internals; };

        // Allow recursive queries to the local hosts
        allow-recursion { internals; };

        // Do not make public version of BIND
        version none;

The port associated with the query-source option must not in any case be frozen because it jeopardizes the DNS transactions in the case of a resolver.

M. Rash wrote an interesting article about this and how to force the source port randomly via the iptables: Mitigating DNS Cache Poisoning Attacks with iptables

To reduce the delay timeout for UDP connections, and thus highlight the randomization, which by default is 30s by tuple, simply update the parameter net.netfilter.nf_conntrack_udp_timeout

# sysctl -w net.netfilter.nf_conntrack_udp_timeout=10

to get timeout of 10s.

named.conf.local File

This file contains the local DNS server configuration, and this is where you declare the zones associated with this server's domain(s).

// Manage the file logs
include "/etc/bind/named.conf.log";

// Domain Management example.com
// ------------------------------
//  - The server is defined as the master on the domain.
//  - There are no forwarders for this domain.
//  - Entries in the domain can be added dynamically 
//    with the key ns-example-com_rndc-key
zone "example.com" {
        type master;
        file "/var/lib/bind/db.example.com";
        //forwarders {};
        // If we do not comment the ''forwarders'' "empty" clients of the local subnet in my case don't have access to the upstream DNS ?
        //allow-update { key ns-example-com_rndc-key; };
        allow-update { key rndc-key; };
        //confusion between the file name to import (ns-example-com_rndc-key) and the key label (rndc-key) ?
zone "0.168.192.in-addr.arpa" {
        type master;
        file "/var/lib/bind/db.example.com.inv";
        //see comment below (zone "example.com")
        //forwarders {};
        //allow-update { key ns-example-com_rndc-key; };
        allow-update { key rndc-key; };

// Consider adding the 1918 zones here, if they are not used in your
// organization
include "/etc/bind/zones.rfc1918";

named.conf.log File

logging {
        channel update_debug {
                file "/var/log/update_debug.log" versions 3 size 100k;
                severity debug;
                print-severity  yes;
                print-time      yes;
        channel security_info {
                file "/var/log/security_info.log" versions 1 size 100k;
                severity info;
                print-severity  yes;
                print-time      yes;
        channel bind_log {
                file "/var/log/bind.log" versions 3 size 1m;
                severity info;
                print-category  yes;
                print-severity  yes;
                print-time      yes;

        category default { bind_log; };
        category lame-servers { null; };
        category update { update_debug; };
        category update-security { update_debug; };
        category security { security_info; };

Here we define different log methods for the different categories. The first category is, as its name indicates the default category that is usually assigned to syslog. All categories not mentioned, are similar to the default category. For a list of the different categories, see the bind9 administrator reference manual. In terms of blade-servers, it ignores all the logs associated with them.

Resource Records (RR)

DNS is made up of several registrations, RR or Resource Records, defining the various domain information. The first is dedicated to name resolution, in our case, it is the file db.example.com. The second will be used for reverse name resolution, it is the file db.example.com.inv.


  • RR for name reso (db.example.com file)

$TTL    3600
@       IN      SOA     sid.example.com. root.example.com. (
                   2007010401           ; Serial
                         3600           ; Refresh [1h]
                          600           ; Retry   [10m]
                        86400           ; Expire  [1d]
                          600 )         ; Negative Cache TTL [1h]
@       IN      NS      sid.example.com.
@       IN      MX      10 sid.example.com.

sid     IN      A
etch    IN      A

pop     IN      CNAME   sid
www     IN      CNAME   sid
mail    IN      CNAME   sid
  • RR for inverse name resol ( db.example.com.inv file)

@ IN SOA        sid.example.com. root.example.com. (
                   2007010401           ; Serial
                         3600           ; Refresh [1h]
                          600           ; Retry   [10m]
                        86400           ; Expire  [1d]
                          600 )         ; Negative Cache TTL [1h]
@       IN      NS      sid.example.com.

1       IN      PTR     sid.example.com.
2       IN      PTR     etch.example.com.

Some Explanations :

$TTL : (Time To Live) expresses the duration (in seconds) validity, by default, of the information contained in the RRs. Once this time expires, it is necessary to recheck the data. Types :

  • SOA : Show romanization

to define information about the area. In this case the name of the primary DNS server "sid.example.com." and the email address of technical contact (root.example.com.; the @ is replaced by a dot). It is composed of several fields:

  • 1. Serial : is the whole non-signed 32 bits. This is the serial number to increment with each change of file. It allows the secondary server to reload the information they have. The general purpose is to format it this way YYYYMMDDXX, either for the first amendment 01/04/2007 -> 2007040101, for the second 2007040102.

  • 2. Refresh : defines the data refresh period.

  • 3. Retry : if an error occurs during the last refresh, it will be repeated at the end of time Retry.

  • 4. Expires': the server is considered unavailable after the time expires.

  • 5. Negative cache TTL': set the lifetime of a NXDOMAIN response from us.

  • 'NS: information on behalf of nameservers for the domain.

  • 'X.: information on the mail server. Many can be defined. Thus, it is possible to give them a priority, assigning a number. The lower the number, the higher the priority.

  • 'A: associates a host name to an IPv4 address (32 bits)

  • 'YYYY: associates a host name to an IPv6 address (128 bits)

  • 'CNAME: identifies the canonical name of an alias (a name that points to another name)

  • 'PTR: This is simply the inverse resolution (the opposite of type A).

The classes in the association determines the Internet class. Other classes are available (CH and HS). For more information please consult the RFC 1035

/etc/resolv.conf File

search example.com

It's no more complicated than that !

Bind Chroot

The named daemon is started using the bind user by default. This option is found in the bind service config file /etc/default/bind9:

OPTIONS="-u bind"

The bind start script /etc/init.d/bind9 reads this config file when the service is started.

Starting bind as a non root user is good practice but to run the daemon in a chroot environment we also need specify the chroot directory. This is done using the same OPTIONS variable in /etc/default/bind9.

To begin, start by stopping the bind service:

/etc/init.d/bind9 stop

Then edit /etc/default/bind9:

OPTIONS="-u bind -t /var/bind9/chroot"

Now create the chroot directory structure:

mkdir -p /var/bind9/chroot/{etc,dev,var/cache/bind,var/run/named}

Create the required device special files and set the correct permissions:

mknod /var/bind9/chroot/dev/null c 1 3
mknod /var/bind9/chroot/dev/random c 1 8
chmod 660 /var/bind9/chroot/dev/{null,random}

Move the current config directory into the new chroot directory:

mv /etc/bind /var/bind9/chroot/etc

Now create a symbolic link in /etc for compatibility:

ln -s /var/bind9/chroot/etc/bind /etc/bind 

If you want to use the local timezone in the chroot (e.g. for syslog):

cp /etc/localtime /var/bind9/chroot/etc/

Change the ownership on the files you've just moved over and the rest of the newly created chroot directory structure:

chown -R bind:bind /etc/bind/*
chmod 775 /var/bind9/chroot/var/{cache/bind,run/named}
chgrp bind /var/bind9/chroot/var/{cache/bind,run/named}

Edit the PIDFILE variable in /etc/init.d/bind9 to the correct path:


Finally tell rsyslog to listen to the bind logs in the correct place:

echo "\$AddUnixListenSocket /var/bind9/chroot/dev/log" > /etc/rsyslog.d/bind-chroot.conf

Restart rsyslog and start bind:

/etc/init.d/rsyslog restart; /etc/init.d/bind9 start

Client Manage

As I mentioned at the beginning, the assignment of IP addresses on the LAN is performed by the DHCP server. Thus, to set our DNS server to different clients, it is necessary to add the DHCP configuration file the following two lines:

option domain-name "example.com"

option domain-name-server sid.example.com

It must be added to the file (I think) the areas for which DHCP should automatically perform updates.

Syntax (everything after "=>" is my comments) :

zone [name.of.the.zone.] {

  • primary; => the primary DNS server is on the same machine as the DHCP

    key rndc-key; => it's necessary to provide the security key (via an include) in the beginning of the DHCP server configuration file,

    • this must be the same key that secures the allow-update for the zone in the named.conf.local of Bind9.


Examples de [name.of.the.zone.] (with the "." at the end) :

- example.com. : for the direct zone of this article,

- 0.168.192.in-addr.arpa. : for the inverse zone of this article.

For more information on the implementation of dynamic update of DNS records through DHCP is here

Testing tools

  • Dig Command : this can directly search the DNS server of your choice and get a lot of information in addition to name resolution and contrast resolution.

    $ dig nomade-frjo.stones.lan
    ; <<>> DiG 9.4.2 <<>> nomade-frjo.stones.lan
    ;; global options:  printcmd
    ;; Got answer:
    ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15760
    ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2
    ;nomade-frjo.stones.lan.                IN      A
    nomade-frjo.stones.lan. 900     IN      A
    stones.lan.             604800  IN      NS      emerald.stones.lan.
    stones.lan.             604800  IN      NS      diamond.stones.lan.
    diamond.stones.lan.     604800  IN      A
    emerald.stones.lan.     604800  IN      A
    ;; Query time: 20 msec
    ;; SERVER:
    ;; WHEN: Fri Mar 28 20:53:09 2008
    ;; MSG SIZE  rcvd: 131
    $ dig -x
    ; <<>> DiG 9.4.2 <<>> -x
    ;; global options:  printcmd
    ;; Got answer:
    ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 37702
    ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2
    ;    IN      PTR
    ;; ANSWER SECTION: 900 IN      PTR     nomade-frjo.stones.lan.
    0.168.192.in-addr.arpa. 604800  IN      NS      diamond.stones.lan.
    0.168.192.in-addr.arpa. 604800  IN      NS      emerald.stones.lan.
    diamond.stones.lan.     604800  IN      A
    emerald.stones.lan.     604800  IN      A
    ;; Query time: 19 msec
    ;; SERVER:
    ;; WHEN: Fri Mar 28 20:53:31 2008
    ;; MSG SIZE  rcvd: 155
  • nslookup : Kind of slow but still useful.

    $ nslookup etch
    Name:   etch.example.com
    $ nslookup
    Address:        name = etch.example.com.
  • named-checkconf : Verifies the syntax of the configuration files for Bind9.

    # named-checkconf -z
    zone localhost/IN: loaded serial 1
    zone 127.in-addr.arpa/IN: loaded serial 1
    zone 0.in-addr.arpa/IN: loaded serial 1
    zone 255.in-addr.arpa/IN: loaded serial 1
    zone estar.lan/IN: loaded serial 20080315
    zone 0.168.192.in-addr.arpa/IN: loaded serial 20080315
    zone 10.in-addr.arpa/IN: loaded serial 1
    zone 16.172.in-addr.arpa/IN: loaded serial 1
    zone 17.172.in-addr.arpa/IN: loaded serial 1
    zone 18.172.in-addr.arpa/IN: loaded serial 1
    zone 19.172.in-addr.arpa/IN: loaded serial 1
    zone 20.172.in-addr.arpa/IN: loaded serial 1
    zone 21.172.in-addr.arpa/IN: loaded serial 1
    zone 22.172.in-addr.arpa/IN: loaded serial 1
    zone 23.172.in-addr.arpa/IN: loaded serial 1
    zone 24.172.in-addr.arpa/IN: loaded serial 1
    zone 25.172.in-addr.arpa/IN: loaded serial 1
    zone 26.172.in-addr.arpa/IN: loaded serial 1
    zone 27.172.in-addr.arpa/IN: loaded serial 1
    zone 28.172.in-addr.arpa/IN: loaded serial 1
    zone 29.172.in-addr.arpa/IN: loaded serial 1
    zone 30.172.in-addr.arpa/IN: loaded serial 1
    zone 31.172.in-addr.arpa/IN: loaded serial 1
    zone 168.192.in-addr.arpa/IN: loaded serial 1
  • named-checkzone : Verifies the validity of zone files before resetting the configuration.

    # named-checkzone example.com /var/lib/bind/db.example.com
    zone example.com/IN: loaded serial 20080315
    # named-checkzone 0.168.192.in-addr.arpa /var/lib/bind/db.example.com.inv
    zone 0.168.192.in-addr.arpa/IN: loaded serial 20080315

Links and Resources

/!\ ?ToDos

  • End of
  • To add DNSSEC.