1. Configurer un serveur DHCP et DNS
Prérequis : Ubuntu serveur 12.04 ;
La première chose à faire avec le serveur est de configurer les services DHCP et DNS pour le réseau. Ce qui permettra d’associer automatiquement le nom des machines avec leur adresse IP (et inversement). Ainsi toute les machines du réseau sauront que bidules.exemple.local et l’IP 192.168.1.2 sont une seule et même machine, ce qui est essentiel pour l’utilisation de Kerberos par la suite.
Attention, il ne peut pas y avoir plusieurs serveurs DHCP sur un réseau local, sans créer de conflits.
Deux possibilités :
- avoir une adresse ip static sur le serveur et désactiver le service DHCP du routeur
- utiliser une machine sur un réseau LAN isolée pour un test
Pour avoir une adresse IP statique, il faut modifier le fichier /etc/network/interfaces :
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.1.2
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
gateway 192.168.1.1
Installation de dnsmasq :
sudo aptitude install dnsmasq
Dnsmasq va s’occuper des services DNS et DHCP.
Nous allons le configurer de manière a ce qu’il ajoute les adresses IP alloué par DHCP dans le serveur DNS (requis par Kerberos).
Le fichier de configuration de Dnsmasq (/etc/dnsmasq.conf) est assez grand, mais bien commenté.
Les lignes importantes sont mises en rouge.
# Configuration file for dnsmasq.
#
# Format is one option per line, legal options are the same
# as the long options legal on the command line. See
# « /usr/sbin/dnsmasq –help » or « man 8 dnsmasq » for details.# The following two options make you a better netizen, since they
# tell dnsmasq to filter out queries which the public DNS cannot
# answer, and which load the servers (especially the root servers)
# uneccessarily. If you have a dial-on-demand link they also stop
# these requests from bringing up the link uneccessarily.# Never forward plain names (without a dot or domain part)
domain-needed
# Never forward addresses in the non-routed address spaces.
#bogus-priv# Uncomment this to filter useless windows-originated DNS requests
# which can trigger dial-on-demand links needlessly.
# Note that (amongst other things) this blocks all SRV requests,
# so don’t use it if you use eg Kerberos, SIP, XMMP or Google-talk.
# This option only affects forwarding, SRV records originating for
# dnsmasq (via srv-host= lines) are not suppressed by it.
#filterwin2k# Change this line if you want dns to get its upstream servers from
# somewhere other that /etc/resolv.conf
#resolv-file=# By default, dnsmasq will send queries to any of the upstream
# servers it knows about and tries to favour servers to are known
# to be up. Uncommenting this forces dnsmasq to try each query
# with each server strictly in the order they appear in
# /etc/resolv.conf
#strict-order# If you don’t want dnsmasq to read /etc/resolv.conf or any other
# file, getting its servers from this file instead (see below), then
# uncomment this.
#no-resolv# If you don’t want dnsmasq to poll /etc/resolv.conf or other resolv
# files for changes and re-read them then uncomment this.
#no-poll# Add other name servers here, with domain specs if they are for
# non-public domains.
server=/localnet/192.168.1.2
server=/#/8.8.8.8
server=/#/8.8.4.4# Example of routing PTR queries to nameservers: this will send all
# address->name queries for 192.168.3/24 to nameserver 10.1.2.3
#server=/3.168.192.in-addr.arpa/10.1.2.3# Add local-only domains here, queries in these domains are answered
# from /etc/hosts or DHCP only.
#local=/localnet/# Add domains which you want to force to an IP address here.
# The example below send any host in doubleclick.net to a local
# webserver.
#address=/doubleclick.net/127.0.0.1# –address (and –server) work with IPv6 addresses too.
#address=/www.thekelleys.org.uk/fe80::20d:60ff:fe36:f83# You can control how dnsmasq talks to a server: this forces
# queries to 10.1.2.3 to be routed via eth1
# –server=10.1.2.3@eth1# and this sets the source (ie local) address used to talk to
# 10.1.2.3 to 192.168.1.1 port 55 (there must be a interface with that
# IP on the machine, obviously).
# –server=10.1.2.3@192.168.1.1#55# If you want dnsmasq to change uid and gid to something other
# than the default, edit the following lines.
#user=
#group=# If you want dnsmasq to listen for DHCP and DNS requests only on
# specified interfaces (and the loopback) give the name of the
# interface (eg eth0) here.
# Repeat the line for more than one interface.
#interface=
# Or you can specify which interface _not_ to listen on
#except-interface=
# Or which to listen on by address (remember to include 127.0.0.1 if
# you use this.)
#listen-address=
# If you want dnsmasq to provide only DNS service on an interface,
# configure it as shown above, and then use the following line to
# disable DHCP on it.
#no-dhcp-interface=# On systems which support it, dnsmasq binds the wildcard address,
# even when it is listening on only some interfaces. It then discards
# requests that it shouldn’t reply to. This has the advantage of
# working even when interfaces come and go and change address. If you
# want dnsmasq to really bind only the interfaces it is listening on,
# uncomment this option. About the only time you may need this is when
# running another nameserver on the same machine.
#bind-interfaces# If you don’t want dnsmasq to read /etc/hosts, uncomment the
# following line.
no-hosts
# or if you want it to read another file, as well as /etc/hosts, use
# this.
addn-hosts=/etc/hosts.dnsmasq# Set this (and domain: see below) if you want to have a domain
# automatically added to simple names in a hosts-file.
#expand-hosts# Set the domain for dnsmasq. this is optional, but if it is set, it
# does the following things.
# 1) Allows DHCP hosts to have fully qualified domain names, as long
# as the domain part matches this setting.
# 2) Sets the « domain » DHCP option thereby potentially setting the
# domain of all systems configured by DHCP
# 3) Provides the domain part for « expand-hosts »
domain=exemple.fr# Set a different domain for a particular subnet
#domain=wireless.thekelleys.org.uk,192.168.2.0/24# Same idea, but range rather then subnet
#domain=reserved.thekelleys.org.uk,192.68.3.100,192.168.3.200# Uncomment this to enable the integrated DHCP server, you need
# to supply the range of addresses available for lease and optionally
# a lease time. If you have more than one network, you will need to
# repeat this for each network on which you want to supply DHCP
# service.
dhcp-range=192.168.1.50,192.168.1.150,12h# This is an example of a DHCP range where the netmask is given. This
# is needed for networks we reach the dnsmasq DHCP server via a relay
# agent. If you don’t know what a DHCP relay agent is, you probably
# don’t need to worry about this.
#dhcp-range=192.168.0.50,192.168.0.150,255.255.255.0,12h# This is an example of a DHCP range with a network-id, so that
# some DHCP options may be set only for this network.
#dhcp-range=red,192.168.0.50,192.168.0.150# Supply parameters for specified hosts using DHCP. There are lots
# of valid alternatives, so we will give examples of each. Note that
# IP addresses DO NOT have to be in the range given above, they just
# need to be on the same network. The order of the parameters in these
# do not matter, it’s permissble to give name,adddress and MAC in any order# Always allocate the host with ethernet address 11:22:33:44:55:66
# The IP address 192.168.0.60
#dhcp-host=11:22:33:44:55:66,192.168.0.60# Always set the name of the host with hardware address
# 11:22:33:44:55:66 to be « fred »
#dhcp-host=11:22:33:44:55:66,fred# Always give the host with ethernet address 11:22:33:44:55:66
# the name fred and IP address 192.168.0.60 and lease time 45 minutes
#dhcp-host=11:22:33:44:55:66,fred,192.168.0.60,45m# Give a host with ethernet address 11:22:33:44:55:66 or
# 12:34:56:78:90:12 the IP address 192.168.0.60. Dnsmasq will assume
# that these two ethernet interfaces will never be in use at the same
# time, and give the IP address to the second, even if it is already
# in use by the first. Useful for laptops with wired and wireless
# addresses.
#dhcp-host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.60# Give the machine which says its name is « bert » IP address
# 192.168.0.70 and an infinite lease
#dhcp-host=bert,192.168.0.70,infinite# Always give the host with client identifier 01:02:02:04
# the IP address 192.168.0.60
#dhcp-host=id:01:02:02:04,192.168.0.60# Always give the host with client identifier « marjorie »
# the IP address 192.168.0.60
#dhcp-host=id:marjorie,192.168.0.60# Enable the address given for « judge » in /etc/hosts
# to be given to a machine presenting the name « judge » when
# it asks for a DHCP lease.
#dhcp-host=judge# Never offer DHCP service to a machine whose ethernet
# address is 11:22:33:44:55:66
#dhcp-host=11:22:33:44:55:66,ignore# Ignore any client-id presented by the machine with ethernet
# address 11:22:33:44:55:66. This is useful to prevent a machine
# being treated differently when running under different OS’s or
# between PXE boot and OS boot.
#dhcp-host=11:22:33:44:55:66,id:*# Send extra options which are tagged as « red » to
# the machine with ethernet address 11:22:33:44:55:66
#dhcp-host=11:22:33:44:55:66,net:red# Send extra options which are tagged as « red » to
# any machine with ethernet address starting 11:22:33:
#dhcp-host=11:22:33:*:*:*,net:red# Ignore any clients which are specified in dhcp-host lines
# or /etc/ethers. Equivalent to ISC « deny unkown-clients ».
# This relies on the special « known » tag which is set when
# a host is matched.
#dhcp-ignore=#known# Send extra options which are tagged as « red » to any machine whose
# DHCP vendorclass string includes the substring « Linux »
#dhcp-vendorclass=red,Linux# Send extra options which are tagged as « red » to any machine one
# of whose DHCP userclass strings includes the substring « accounts »
#dhcp-userclass=red,accounts# Send extra options which are tagged as « red » to any machine whose
# MAC address matches the pattern.
#dhcp-mac=red,00:60:8C:*:*:*# If this line is uncommented, dnsmasq will read /etc/ethers and act
# on the ethernet-address/IP pairs found there just as if they had
# been given as –dhcp-host options. Useful if you keep
# MAC-address/host mappings there for other purposes.
#read-ethers# Send options to hosts which ask for a DHCP lease.
# See RFC 2132 for details of available options.
# Common options can be given to dnsmasq by name:
# run « dnsmasq –help dhcp » to get a list.
# Note that all the common settings, such as netmask and
# broadcast address, DNS server and default route, are given
# sane defaults by dnsmasq. You very likely will not need
# any dhcp-options. If you use Windows clients and Samba, there
# are some options which are recommended, they are detailed at the
# end of this section.# Override the default route supplied by dnsmasq, which assumes the
# router is the same machine as the one running dnsmasq.
#dhcp-option=3,192.168.0.1# Do the same thing, but using the option name
dhcp-option=option:router,192.168.1.1
# Override the default route supplied by dnsmasq and send no default
# route at all. Note that this only works for the options sent by
# default (1, 3, 6, 12, 28) the same line will send a zero-length option
# for all other option numbers.
#dhcp-option=3# Set the NTP time server addresses to 192.168.0.4 and 10.10.0.5
#dhcp-option=option:ntp-server,192.168.0.4,10.10.0.5
dhcp-option=option:ntp-server,192.168.1.2
# Set the NTP time server address to be the same machine as
# is running dnsmasq
#dhcp-option=42,0.0.0.0# Set the NIS domain name to « welly »
#dhcp-option=40,welly# Set the default time-to-live to 50
#dhcp-option=23,50# Set the « all subnets are local » flag
#dhcp-option=27,1# Send the etherboot magic flag and then etherboot options (a string).
#dhcp-option=128,e4:45:74:68:00:00
#dhcp-option=129,NIC=eepro100# Specify an option which will only be sent to the « red » network
# (see dhcp-range for the declaration of the « red » network)
# Note that the net: part must precede the option: part.
#dhcp-option = net:red, option:ntp-server, 192.168.1.1# The following DHCP options set up dnsmasq in the same way as is specified
# for the ISC dhcpcd in
# http://www.samba.org/samba/ftp/docs/textdocs/DHCP-Server-Configuration.txt
# adapted for a typical dnsmasq installation where the host running
# dnsmasq is also the host running samba.
# you may want to uncomment some or all of them if you use
# Windows clients and Samba.
dhcp-option=19,0 # option ip-forwarding off
dhcp-option=44,0.0.0.0 # set netbios-over-TCP/IP nameserver(s) aka WINS server(s)
dhcp-option=45,0.0.0.0 # netbios datagram distribution server
dhcp-option=46,8 # netbios node type# Send RFC-3397 DNS domain search DHCP option. WARNING: Your DHCP client
# probably doesn’t support this……
#dhcp-option=option:domain-search,eng.apple.com,marketing.apple.com# Send RFC-3442 classless static routes (note the netmask encoding)
#dhcp-option=121,192.168.1.0/24,1.2.3.4,10.0.0.0/8,5.6.7.8# Send vendor-class specific options encapsulated in DHCP option 43.
# The meaning of the options is defined by the vendor-class so
# options are sent only when the client supplied vendor class
# matches the class given here. (A substring match is OK, so « MSFT »
# matches « MSFT » and « MSFT 5.0 »). This example sets the
# mtftp address to 0.0.0.0 for PXEClients.
#dhcp-option=vendor:PXEClient,1,0.0.0.0# Send microsoft-specific option to tell windows to release the DHCP lease
# when it shuts down. Note the « i » flag, to tell dnsmasq to send the
# value as a four-byte integer – that’s what microsoft wants. See
# http://technet2.microsoft.com/WindowsServer/en/library/a70f1bb7-d2d4
# -49f0-96d6-4b7414ecfaae1033.mspx?mfr=true
#dhcp-option=vendor:MSFT,2,1i# Send the Encapsulated-vendor-class ID needed by some configurations of
# Etherboot to allow is to recognise the DHCP server.
#dhcp-option=vendor:Etherboot,60, »Etherboot »# Send options to PXELinux. Note that we need to send the options even
# though they don’t appear in the parameter request list, so we need
# to use dhcp-option-force here.
# See http://syslinux.zytor.com/pxe.php#special for details.
# Magic number – needed before anything else is recognised
#dhcp-option-force=208,f1:00:74:7e
# Configuration file name
#dhcp-option-force=209,configs/common
# Path prefix
#dhcp-option-force=210,/tftpboot/pxelinux/files/
# Reboot time. (Note ‘i’ to send 32-bit value)
#dhcp-option-force=211,30i# Set the boot filename for netboot/PXE. You will only need
# this is you want to boot machines over the network and you will need
# a TFTP server; either dnsmasq’s built in TFTP server or an
# external one. (See below for how to enable the TFTP server.)
#dhcp-boot=pxelinux.0# Boot for Etherboot gPXE. The idea is to send two different
# filenames, the first loads gPXE, and the second tells gPXE what to
# load. The dhcp-match sets the gpxe tag for requests from gPXE.
#dhcp-match=gpxe,175 # gPXE sends a 175 option.
#dhcp-boot=net:#gpxe,undionly.kpxe
#dhcp-boot=mybootimage# Encapsulated options for Etherboot gPXE. All the options are
# encapsulated within option 175
#dhcp-option=encap:175, 1, 5b # priority code
#dhcp-option=encap:175, 176, 1b # no-proxydhcp
#dhcp-option=encap:175, 177, string # bus-id
#dhcp-option=encap:175, 189, 1b # BIOS drive code
#dhcp-option=encap:175, 190, user # iSCSI username
#dhcp-option=encap:175, 191, pass # iSCSI password# Test for the architecture of a netboot client. PXE clients are
# supposed to send their architecture as option 93. (See RFC 4578)
#dhcp-match=peecees, option:client-arch, 0 #x86-32
#dhcp-match=itanics, option:client-arch, 2 #IA64
#dhcp-match=hammers, option:client-arch, 6 #x86-64
#dhcp-match=mactels, option:client-arch, 7 #EFI x86-64# Do real PXE, rather than just booting a single file, this is an
# alternative to dhcp-boot.
#pxe-prompt= »What system shall I netboot? »
# or with timeout before first available action is taken:
#pxe-prompt= »Press F8 for menu. », 60# Available boot services. for PXE.
#pxe-service=x86PC, « Boot from local disk »# Loads <tftp-root>/pxelinux.0 from dnsmasq TFTP server.
#pxe-service=x86PC, « Install Linux », pxelinux# Loads <tftp-root>/pxelinux.0 from TFTP server at 1.2.3.4.
# Beware this fails on old PXE ROMS.
#pxe-service=x86PC, « Install Linux », pxelinux, 1.2.3.4# Use bootserver on network, found my multicast or broadcast.
#pxe-service=x86PC, « Install windows from RIS server », 1# Use bootserver at a known IP address.
#pxe-service=x86PC, « Install windows from RIS server », 1, 1.2.3.4# If you have multicast-FTP available,
# information for that can be passed in a similar way using options 1
# to 5. See page 19 of
# http://download.intel.com/design/archives/wfm/downloads/pxespec.pdf# Enable dnsmasq’s built-in TFTP server
#enable-tftp# Set the root directory for files availble via FTP.
#tftp-root=/var/ftpd# Make the TFTP server more secure: with this set, only files owned by
# the user dnsmasq is running as will be send over the net.
#tftp-secure# This option stops dnsmasq from negotiating a larger blocksize for TFTP
# transfers. It will slow things down, but may rescue some broken TFTP
# clients.
#tftp-no-blocksize# Set the boot file name only when the « red » tag is set.
#dhcp-boot=net:red,pxelinux.red-net# An example of dhcp-boot with an external TFTP server: the name and IP
# address of the server are given after the filename.
# Can fail with old PXE ROMS. Overridden by –pxe-service.
#dhcp-boot=/var/ftpd/pxelinux.0,boothost,192.168.0.3# Set the limit on DHCP leases, the default is 150
#dhcp-lease-max=150# The DHCP server needs somewhere on disk to keep its lease database.
# This defaults to a sane location, but if you want to change it, use
# the line below.
#dhcp-leasefile=/var/lib/misc/dnsmasq.leases# Set the DHCP server to authoritative mode. In this mode it will barge in
# and take over the lease for any client which broadcasts on the network,
# whether it has a record of the lease or not. This avoids long timeouts
# when a machine wakes up on a new network. DO NOT enable this if there’s
# the slighest chance that you might end up accidentally configuring a DHCP
# server for your campus/company accidentally. The ISC server uses
# the same option, and this URL provides more information:
# http://www.isc.org/index.pl?/sw/dhcp/authoritative.php
dhcp-authoritative# Run an executable when a DHCP lease is created or destroyed.
# The arguments sent to the script are « add » or « del »,
# then the MAC address, the IP address and finally the hostname
# if there is one.
#dhcp-script=/bin/echo# Set the cachesize here.
#cache-size=150# If you want to disable negative caching, uncomment this.
#no-negcache# Normally responses which come form /etc/hosts and the DHCP lease
# file have Time-To-Live set as zero, which conventionally means
# do not cache further. If you are happy to trade lower load on the
# server for potentially stale date, you can set a time-to-live (in
# seconds) here.
#local-ttl=# If you want dnsmasq to detect attempts by Verisign to send queries
# to unregistered .com and .net hosts to its sitefinder service and
# have dnsmasq instead return the correct NXDOMAIN response, uncomment
# this line. You can add similar lines to do the same for other
# registries which have implemented wildcard A records.
#bogus-nxdomain=64.94.110.11# If you want to fix up DNS results from upstream servers, use the
# alias option. This only works for IPv4.
# This alias makes a result of 1.2.3.4 appear as 5.6.7.8
#alias=1.2.3.4,5.6.7.8
# and this maps 1.2.3.x to 5.6.7.x
#alias=1.2.3.0,5.6.7.0,255.255.255.0
# and this maps 192.168.0.10->192.168.0.40 to 10.0.0.10->10.0.0.40
#alias=192.168.0.10-192.168.0.40,10.0.0.0,255.255.255.0# Change these lines if you want dnsmasq to serve MX records.
# Return an MX record named « maildomain.com » with target
# servermachine.com and preference 50
#mx-host=maildomain.com,servermachine.com,50# Set the default target for MX records created using the localmx option.
#mx-target=servermachine.com# Return an MX record pointing to the mx-target for all local
# machines.
#localmx# Return an MX record pointing to itself for all local machines.
#selfmx# Change the following lines if you want dnsmasq to serve SRV
# records. These are useful if you want to serve ldap requests for
# Active Directory and other windows-originated DNS requests.
# See RFC 2782.
# You may add multiple srv-host lines.
# The fields are <name>,<target>,<port>,<priority>,<weight>
# If the domain part if missing from the name (so that is just has the
# service and protocol sections) then the domain given by the domain=
# config option is used. (Note that expand-hosts does not need to be
# set for this to work.)# A SRV record sending LDAP for the example.com domain to
# ldapserver.example.com port 289
#srv-host=_ldap._tcp.example.com,ldapserver.example.com,389# A SRV record sending LDAP for the example.com domain to
# ldapserver.example.com port 289 (using domain=)
#domain=example.com
#srv-host=_ldap._tcp,ldapserver.example.com,389# Two SRV records for LDAP, each with different priorities
#srv-host=_ldap._tcp.example.com,ldapserver.example.com,389,1
#srv-host=_ldap._tcp.example.com,ldapserver.example.com,389,2# A SRV record indicating that there is no LDAP server for the domain
# example.com
#srv-host=_ldap._tcp.example.com# The following line shows how to make dnsmasq serve an arbitrary PTR
# record. This is useful for DNS-SD. (Note that the
# domain-name expansion done for SRV records _does_not
# occur for PTR records.)
#ptr-record=_http._tcp.dns-sd-services, »New Employee Page._http._tcp.dns-sd-services »# Change the following lines to enable dnsmasq to serve TXT records.
# These are used for things like SPF and zeroconf. (Note that the
# domain-name expansion done for SRV records _does_not
# occur for TXT records.)#Example SPF.
#txt-record=example.com, »v=spf1 a -all »#Example zeroconf
#txt-record=_http._tcp.example.com,name=value,paper=A4# Provide an alias for a « local » DNS name. Note that this _only_ works
# for targets which are names from DHCP or /etc/hosts. Give host
# « bert » another name, bertrand
#cname=bertand,bert# For debugging purposes, log each DNS query as it passes through
# dnsmasq.
#log-queries# Log lots of extra information about DHCP transactions.
#log-dhcp# Include a another lot of configuration options.
#conf-file=/etc/dnsmasq.more.conf
#conf-dir=/etc/dnsmasq.d# Server DNS settings… this is required as the server itself will
# not be obtaining it’s IP address via DHCP and therefore would
# not be automatically added to the DNS records for forward/reverse
# DNS queries as required by Kerberos
ptr-record=2.1.168.192.in-addr.arpa., »bidule.exemple.fr »
address=/bidule.exemple.fr/192.168.1.2# Kerberos automatic stuff…
# This maps kerberos.example.fr and
# ldap.example.fr to the server and also makes all
# dhcp clients aware of the kerberos realm… magic 😀
address=/kerberos.exemple.fr/192.168.1.2
address=/ldap.exemple.fr/192.168.1.2
txt-record=_kerberos.exemple.fr, »EXEMPLE.FR »
srv-host=_kerberos._udp.exemple.fr, »kerberos.exemple.fr »,88
srv-host=_kerberos._tcp.exemple.fr, »kerberos.exemple.fr »,88
srv-host=_kerberos-master._udp.exemple.fr, »kerberos.exemple.fr »,88
srv-host=_kerberos-adm._tcp.exemple.fr, »kerberos.exemple.fr »,749
srv-host=_kpasswd._udp.exemple.fr, »kerberos.exemple.fr »,464
srv-host=_ldap._tcp.exemple.fr, »ldap.exemple.fr »,389
Par défaut, dnsmasq va lire les informations des hôtes à partir du fichier /etc/hosts mais Ubuntu (et les autres dérivées de Debian) a un enregistrement pour associer l’adresse IP 127.0.1.1 au nom d’hôte local.
Cet enregistrement va poser problème lors de la configuration d’OpenLDAP et peut empêcher la bonne résolution de nom de « bidule » sur le réseau.
Nous allons donc dire à dnsmasq d’utiliser son propre fichier /etc/hosts.dnsmasq pour toutes demande d’informations sur les hôtes à adresses IP statique.
Nous allons créer ce fichier :
sudo touch /etc/hosts.dnsmasq
Dnsmasq est maintenant configuré pour être serveur DHCP et les clients utiliserons
le DNS local pour les requêtes de nom de domaine.
il suffit de redémarrer le service pour que la configuration soit prise en compte :
sudo service dnsmasq restart
Webographie :
Résolution des noms avec resolvconf sous Linux Debian
http://www.drazzib.com/docs:admin:dnsmasq
http://www.jopa.fr/index.php/2008/10/30/dnsmasq-dns-cache-et-dhcp/