IPv4/DHCP/Server
DHCP (Dynamic Host Configuration Protocol)
Allgemein
- Erweiterung des Bootstrap-Protokolls (BOOTP)
- Hintergrundprozess (Dienst/Daemon)
- RFC 2131
- Transport durch UDP
- Standard-Ports:
- IPv4: 67 (Server) 68 (Client)
- IPv6: 547 (Server) 546 (Client)
Aufgabe
- Clients automatisch in ein bestehendes Netz einbinden,
- Übermittlung der Konfigurationsparameter...
- IP-Adresse,
- Netzmaske,
- Gateway, Router ins nächst höhere 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,
Betriebsmodi
- dynamische Zuordnung:
- automatische Zuordnung mit Lease-Time
- Lease-Time: Festlegung wie lange eine IP-Adresse an Client „verliehen“ wird, bevor Client eine „Verlängerung“ beantragen muss
- bei „Nicht-Verlängerung“ wird IP-Adresse frei und neu vergeben
- 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 (siehe /var/lib/dhcpd.leases)
- Neue Clients erhalten keine IP-Adresse, wenn Adressbereich vergeben, auch wenn IP-Adressen nicht aktiv genutzt werden
- statische Zuordnung:
- IP-Adressen werden MAC-Adressen fest zugeordnet; wichtig bei Port-Weiterleitungen oder wenn DHCP-Client Server-Dienste zur Verfügung stellt
- kein „einfaches“ Einbinden von neuen Clients
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
- DHCP-NAK: Ablehnung einer DHCPREQUEST-Anforderung durch den DHCP-Server.
- DHCP-DECLINE: Ablehnung durch den Client, da die IP-Adresse schon verwendet wird.
- DHCP-RELEASE: Der Client gibt die eigene Konfiguration frei, damit die Parameter wieder für andere Clients zur Verfügung stehen.
- DHCP-INFORM: Anfrage eines Clients nach weiteren Konfigurationsparametern, z. B. weil der Client eine statische IP-Adresse besitzt.
DHCP-Refresh (nur bei dynamischer Zuordnung)
DHCPACK-Nachricht IP-Adresse,"lease time" und zwei Fristen: Die "Renewal-Time" T1 und die "Rebinding-Time" T2.
- T1 1/2 von T2
- T2 7/8 der Lease-Time
Zum Beispiel:
Lease-Time = 100s => "Renewal-Time" T1 = 50s und "Rebinding-Time" T2 = 87,5s
Nach Ablauf T1 versucht der Client, seine Lease-Time zu verlängern.
- DHCPREQUESTs per Unicast an bisherigen Server
- Erfolgt DHCP-ACK: Client erhält bisherige Konfiguration mit frischer Lease-Time
- Erfolgt kein DHCP-ACK: Client nutzt Konfiguration ohne Einschränkungen weiter
Nach Ablauf von T2 sendet Client DHCPREQUESTs per Broadcast an irgendeinen anderen DHCP-Server
Läuft die Lease-Time ab so verfällt die Konfiguration und Client beginnt erneut mit eunem DHCP-DISCOVER.
Sind alle IP-Adressen bereits vergeben, sendet Server ein DHCP-NAK (DHCP-Not Acknowledged).
Einrichtung unter Linux/Debian am Beispiel "isc-dhcp-server"
Installation
root@router0230:~# apt install isc-dhcp-server
Konfiguration
/etc/dhcp/dhcpd.conf
- Lease-Time in Sekunden
default-lease-time 600; max-lease-time 7200;
- Subnet-Blöcke
subnet 10.30.30.0 netmask 255.255.255.0
- Range
range 10.30.30.10 10.30.30.99;
- Options
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;
Nach der Änderung der Konfiguration, muss die dhcpd.conf erneut eingelesen werden.
root@router0230:~# systemctl restart isc-dhcp-server.service
Damit Änderungen bei bereits aktivem Client wirksam werden, muss dieser sie neu anfordern.
Bisherige Konfiguration verwerfen.
root@router0230:~# dhclient -r
Neue Konfiguration anfordern.
root@router0230:~# dhclient
Fehlerauslesung
/var/log/syslog
root@router0230:~# cat /var/log/syslog | grep "dhcp"
Erst Ordner /var/lib/dhcpd erzeugen.
root@router0230:~# touch /var/lib/dhcpd
Dann Datei dhcpd.leases.
root@router0230:~# touch /var/lib/dhcpd/dhcpd.leases
Danach wird diese Datei erst beschrieben und kann ausgelesen werden.
root@router0230:~# cat /var/lib/dhcpd/dhcpd.leases
Konfigurationsdatei auf Server
/etc/dhcp/dhcpd.conf
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; host user { hardware ethernet 00:1d:7d:c8:de:bd; fixed-address 10.30.30.5; option host-name "user"; } host robert { hardware ethernet 74:27:ea:e1:b2:b4; fixed-address 10.30.30.6; option host-name "robert"; } host meik { hardware ethernet 74:27:ea:cc:fe:a0; fixed-address 10.30.30.7; option host-name "meik"; } host ufuk { hardware ethernet 74:27:ea:e1:ba:b1; fixed-address 10.30.30.8; option host-name "ufuk"; } host tanja { hardware ethernet 74:27:ea:e1:b1:e4; fixed-address 10.30.30.9; option host-name "tanja"; } } # 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; # } #}
Konfigurationsdatei auf Client
/etc/dhcp/dhclient.conf
# Configuration file for /sbin/dhclient. # # This is a sample configuration file for dhclient. See dhclient.conf's # man page for more information about the syntax of this file # and a more comprehensive list of the parameters understood by # dhclient. # # Normally, if the DHCP server provides reasonable information and does # not leave anything out (like the domain name, for example), then # few changes must be made to this file, if any. # option rfc3442-classless-static-routes code 121 = array of unsigned integer 8; send host-name = gethostname(); request subnet-mask, broadcast-address, time-offset, routers, domain-name, domain-name-servers, domain-search, host-name, dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers, netbios-name-servers, netbios-scope, interface-mtu, rfc3442-classless-static-routes, ntp-servers; #send dhcp-client-identifier 1:0:a0:24:ab:fb:9c; #send dhcp-lease-time 3600; #supersede domain-name "fugue.com home.vix.com"; #prepend domain-name-servers 127.0.0.1; #require subnet-mask, domain-name-servers; #timeout 60; #retry 60; #reboot 10; #select-timeout 5; #initial-interval 2; #script "/sbin/dhclient-script"; #media "-link0 -link1 -link2", "link0 link1"; #reject 192.33.137.209; #alias { # interface "eth0"; # fixed-address 192.5.5.213; # option subnet-mask 255.255.255.255; #} #lease { # interface "eth0"; # fixed-address 192.33.137.200; # medium "link0 link1"; # option host-name "andare.swiftmedia.com"; # option subnet-mask 255.255.255.0; # option broadcast-address 192.33.137.255; # option routers 192.33.137.250; # option domain-name-servers 127.0.0.1; # renew 2 2000/1/12 00:00:01; # rebind 2 2000/1/12 00:00:01; # expire 2 2000/1/12 00:00:01; #}