The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on Internet Protocol (IP) networks for automatically assigning IP addresses and other communication parameters to devices connected to the network using a client–server architecture.
Installation
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| apt install isc-dhcp-server
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Usage
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| systemctl enable --now isc-dhcp-server.service
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| systemctl ( start | stop | restart ) isc-dhcp-server.service
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Troubleshooting
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| systemctl status isc-dhcp-server.service
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| journalctl -xeu isc-dhcp-server.service
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Network interface configuration
Edit /etc/default/isc-dhcp-server.
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| DHCPDv4_CONF=/etc/dhcp/dhcpd.conf
INTERFACESv4="enp0s1 enp0s2 ..."
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IP-Pool configuration
Edit /etc/dhcp/dhcpd.conf.
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| option domain-name "fritz.box";
option domain-name-servers "9.9.9.9, 192.168.0.1";
default-lease-time 600;
max-lease-time 7200;
authoritative;
subnet 192.168.0.0 netmask 255.255.255.0 {
range 192.168.0.50 192.168.0.100;
option routers 192.168.0.1;
}
host dhcp-server {
hardware ethernet fe:fe:fe:fe:fe:fe;
fixed-address 192.168.0.2;
option routers 192.168.0.1;
}
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| option | description |
|---|
| option domain-name | Specifies the locally used domain name, e.g. like local.lan or something |
| option domain-servers | Specifies used DNS-servers |
| default-lease-time | Like minimum handout of an IP-address from the pool. In seconds. |
| max-lease-time | Like maximum handout of an IP-address from the pool. In seconds. |
| authoritative | This DHCP-server is the master. |
| subnet 192.168.0.0 netmask 255.255.255.0 { } | Subnets or IP-Pools that the server is allowed to serve |
| host dhcp-server { } | Reservation for a specific host, this matches the MAC-address |
Logging / Get info about requests and offers
Up to Debian 11 you can use:
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| tail -f -n 100 /var/log/syslog | grep dhcp
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| option | description |
|---|
| -f | show all following messages |
| -nxx | show previous xx messages |
From Debian 12 and above use:
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| journalctl -fn100 -u isc-dhcp-server
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| option | description |
|---|
| -f | show all following messages |
| -nxx | show previous xx messages |
Possible output:
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| Apr 20 09:20:42 fw dhcpd[3217]: DHCPREQUEST for 10.0.100.51 (10.0.100.1) from be:98:60:d1:af:88 via ens19: wrong network.
Apr 20 09:20:42 fw dhcpd[3217]: DHCPNAK on 10.0.100.51 to be:98:60:d1:af:88 via ens19
Apr 20 09:20:43 fw dhcpd[3217]: reuse_lease: lease age 1 (secs) under 25% threshold, reply with unaltered, existing lease for 10.0.10.50
Apr 20 09:20:43 fw dhcpd[3217]: DHCPDISCOVER from be:98:60:d1:af:88 (archiso) via ens19
Apr 20 09:20:43 fw dhcpd[3217]: DHCPOFFER on 10.0.10.50 to be:98:60:d1:af:88 (archiso) via ens19
Apr 20 09:20:43 fw dhcpd[3217]: reuse_lease: lease age 1 (secs) under 25% threshold, reply with unaltered, existing lease for 10.0.10.50
Apr 20 09:20:43 fw dhcpd[3217]: DHCPREQUEST for 10.0.10.50 (10.0.10.1) from be:98:60:d1:af:88 (archiso) via ens19
Apr 20 09:20:43 fw dhcpd[3217]: DHCPACK on 10.0.10.50 to be:98:60:d1:af:88 (archiso) via ens19
Apr 20 09:22:27 fw dhcpd[3217]: DHCPDISCOVER from fe:fe:fe:fe:fe:fe via ens20
Apr 20 09:22:27 fw dhcpd[3217]: DHCPOFFER on 192.168.0.2 to fe:fe:fe:fe:fe:fe via ens20
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DHCP-Relay-Agent
A Dynamic Host Configuration Protocol (DHCP) relay agent is a network service that forwards DHCP requests from clients on one network to a DHCP server on another network. This is particularly useful in larger network environments where multiple subnets exist and maintaining a DHCP server on each subnet is not practical. Configuring a DHCP relay agent ensures that all clients, regardless of their network location, can obtain IP configuration from a centralized DHCP server.
Installation
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| apt install isc-dhcp-relay
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Configuration
Edit /etc/default/isc-dhcp-relay.
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| SERVERS="192.168.19.11"
INTERFACES="enp0s9 enp0s10"
OPTIONS=""
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| option | description |
|---|
| SERVERS=”” | Here you have to specifiy the IP of your actual DHCP-Server. |
| INTERFACES=”” | You must specifiy all interfaces which transferring dhcp packets. So it have to be 2 at least. |
| OPTIONS=”” | Additional command-line options for the relay agent (optional) |
Usage
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| systemctl enable isc-dhcp-relay
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| systemctl ( start | stop | restart ) isc-dhcp-relay
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Troubleshooting
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| systemctl status isc-dhcp-relay
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| grep isc-dhcp-relay /var/log/syslog
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You can pipe the second command through tail -n*xx* to filter the output
DHCP-Client
To troubleshoot clients in your network and test if the configured DHCP-server is working properly here some basic commands to force the client to obtain a new IP or renew the lease of the actual offered IP.
Installation
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| apt install isc-dhcp-common
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Usage
Do a new IP-request:
Do an IP-Renew:
