DHCP (Dynamic Host Configuration Protocol)

Allgemein

ist Erweiterung des Bootstrap-Protokolls (BOOTP)
ist ein Hintergrundprozess (Dienst/Daemon)
definiert in RFC 2131
Transport durch UDP
Standard-Ports:
- IPv4: 67 (Server od. Relay-Agent) 68 (Client)
- IPv6: 547 (Server od. Relay-Agent) 546 (Client)

Aufgabe

angeschlossene Clients ohne manuelle Konfiguration der Schnittstelle in ein bestehendes Netz einbinden,
das heißt die Konfigurationsparameter...
- IP-Adresse, die Adresse eines Gerätes im Netzwerk,
- Netzmaske, Adressierung des Netz- und Hostteils einer IP-Adresse,
- Gateway, die Brücke von einem ins andere Netz,
- Rechnername,
- Broadcast-Adresse,
- Boot-Image für plattenlose Workstations,
- NetBIOS Nameserver und Name Server (DNS)
- Time- und NTP-Server, die für die Synchronisierung der Uhrzeit zuständig sind,
...können automatisch vergeben werden

Betriebsmodi

manuelle Zuordnung (statisches DHCP):

IP-Adressen werden bestimmte MAC-Adressen fest zugeordnet; z.B. wichtig bei Port-Weiterleitungen oder wenn DHCP-Client Server-Dienste zur Verfügung stellt

Nachteil: kein „einfaches“ Einbinden von neuen Clients

automatische Zuordnung:

am DHCP-Server wird ein Bereich (range) von IP-Adressen definiert IP-Adressen werden automatisch an die MAC-Adressen von neuen DHCP-Clients zugewiesen und keinem anderen Host mehr zugewiesen
Zuweisungen sind permanent, werden nicht entfernt und in einer Tabelle eingetragen (/var/lib/dhcpd.leases)

Nachteil: neue Clients erhalten keine IP-Adresse, wenn der gesamte Adressbereich vergeben ist, auch wenn die bereits vergebenen IP-Adressen nicht aktiv genutzt werden

dynamische Zuordnung:

automatische Zuordnung mit Lease-Time
in Konfigurationdatei (/etc/dhcp/dhcpd.conf) wird festgelegt, wie lange eine IP-Adresse an Client „verliehen“ wird, bevor Client beim Server eine „Verlängerung“ beantragen muss

Vorteil: bei „Nicht-Verlängerung“ durch Client wird IP-Adresse frei und neu vergeben

Ablauf

DHCP-DISCOVER: Client sucht per Broadcast nach DHCP-Server
DHCP-OFFER: DHCP-Server bietet Client per Unicast Konfigurationsparameter der Schnittstelle an
DHCP-REQUEST: Client fordert angebotene Konfigurationsparameter bei DHCP-Server an (z.B. Mietanfrage)
DHCP-ACK: DHCP-Server sendet Konfigurationsparameter an Client

Einrichtung unter Linux/Debian am Beispiel "isc-dhcp-server"

Installation

root@router0230:~# apt install isc-dhcp-server

Konfiguration

wichtige Dateien:

"/etc/default/isc-dhcp-server"
"/etc/dhcp/dhcpd.conf"

/etc/default/isc-dhcp-server

Hier muss festgelegt werden, an welches Netzwerkgerät der DHCP-Server gebunden werden soll. Hier soll das Netzwerkgerät "enp5s1" an den DHCP-Server gebunden werden.

# On what interfaces should the DHCP server (dhcpd) serve DHCP requests?
# Separate multiple interfaces with spaces, e.g. "eth0 eth1".
INTERFACESv4="enp5s1"
INTERFACESv6=""

/etc/dhcp/dhcpd.conf

Festlegung von ...

Authoritative-Statement

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
authoritative;

Lease-Time

default-lease-time 600;
max-lease-time 7200;

Subnet-Blöcke
Range
Options - z.B. Router, Domain-Name-Server

# No service will be given on this subnet, but declaring it helps the 
# DHCP server to understand the network topology.

#subnet 10.152.187.0 netmask 255.255.255.0 {
#}

# This is a very basic subnet declaration.

#subnet 10.254.239.0 netmask 255.255.255.224 {
#  range 10.254.239.10 10.254.239.20;
#  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
#}

# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.

#subnet 10.254.239.32 netmask 255.255.255.224 {
#  range dynamic-bootp 10.254.239.40 10.254.239.60;
#  option broadcast-address 10.254.239.31;
#  option routers rtr-239-32-1.example.org;
#}

# A slightly different configuration for an internal subnet.
subnet 10.30.30.0 netmask 255.255.255.0
       range 10.30.30.10 10.30.30.99;
       option subnet-mask 255.255.255.0;
       option domain-name-servers 10.0.0.3, 10.0.0.4;
       option domain-name "raum102.itw";
       option domain-search "raum102.itw";
       option routers 10.30.30.1;
       option broadcast-address 10.30.30.255;
       default-lease-time 600;
       max-lease-time 7200;
}

# dhcpd.conf
#
# Sample configuration file for ISC dhcpd
#

# option definitions common to all supported networks...
#option domain-name "example.org";
#option domain-name-servers ns1.example.org, ns2.example.org;

#default-lease-time 600;
#max-lease-time 7200;

# The ddns-updates-style parameter controls whether or not the server will
# attempt to do a DNS update when a lease is confirmed. We default to the
# behavior of the version 2 packages ('none', since DHCP v2 didn't
# have support for DDNS.)
#ddns-update-style none;

# If this DHCP server is the official DHCP server for the local
# network, the authoritative directive should be uncommented.
authoritative;

# Use this to send dhcp log messages to a different log file (you also
# have to hack syslog.conf to complete the redirection).
log-facility local7;

# No service will be given on this subnet, but declaring it helps the 
# DHCP server to understand the network topology.

#subnet 10.152.187.0 netmask 255.255.255.0 {
#}

# This is a very basic subnet declaration.

#subnet 10.254.239.0 netmask 255.255.255.224 {
#  range 10.254.239.10 10.254.239.20;
#  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org;
#}

# This declaration allows BOOTP clients to get dynamic addresses,
# which we don't really recommend.

#subnet 10.254.239.32 netmask 255.255.255.224 {
#  range dynamic-bootp 10.254.239.40 10.254.239.60;
#  option broadcast-address 10.254.239.31;
#  option routers rtr-239-32-1.example.org;
#}

# A slightly different configuration for an internal subnet.
subnet 10.30.30.0 netmask 255.255.255.0
       range 10.30.30.10 10.30.30.99;
       option subnet-mask 255.255.255.0;
       option domain-name-servers 10.0.0.3, 10.0.0.4;
       option domain-name "raum102.itw";
       option domain-search "raum102.itw";
       option routers 10.30.30.1;
       option broadcast-address 10.30.30.255;
       default-lease-time 600;
       max-lease-time 7200;
}

# Hosts which require special configuration options can be listed in
# host statements.   If no address is specified, the address will be
# allocated dynamically (if possible), but the host-specific information
# will still come from the host declaration.
#host passacaglia {
#  hardware ethernet 0:0:c0:5d:bd:95;
#  filename "vmunix.passacaglia";
#  server-name "toccata.example.com";
#}

# Fixed IP addresses can also be specified for hosts.   These addresses
# should not also be listed as being available for dynamic assignment.
# Hosts for which fixed IP addresses have been specified can boot using
# BOOTP or DHCP.   Hosts for which no fixed address is specified can only
# be booted with DHCP, unless there is an address range on the subnet
# to which a BOOTP client is connected which has the dynamic-bootp flag
# set.
#host fantasia {
#  hardware ethernet 08:00:07:26:c0:a5;
#  fixed-address fantasia.example.com;
#}

# You can declare a class of clients and then do address allocation
# based on that.   The example below shows a case where all clients
# in a certain class get addresses on the 10.17.224/24 subnet, and all
# other clients get addresses on the 10.0.29/24 subnet.

#class "foo" {
#  match if substring (option vendor-class-identifier, 0, 4) = "SUNW";
#}

#shared-network 224-29 {
#  subnet 10.17.224.0 netmask 255.255.255.0 {
#    option routers rtr-224.example.org;
#  }
#  subnet 10.0.29.0 netmask 255.255.255.0 {
#    option routers rtr-29.example.org;
#  }
#  pool {
#    allow members of "foo";
#    range 10.17.224.10 10.17.224.250;
#  }
#  pool {
#    deny members of "foo";
#    range 10.0.29.10 10.0.29.230;
#  }
#}