Chapter 3. Providing DHCP services

• For DHCPv4: /var/lib/dhcpd/dhcpd.leases
• For DHCPv6: /var/lib/dhcpd/dhcpd6.leases

Prerequisites

• You are logged in as the root user.

• You have only a subnet definition for the 192.0.2.0/24 network in the /etc/dhcp/dhcpd.conf file.
• The enp0s1 network interface is connected to the 192.0.2.0/24 subnet.
• The enp7s0 interface is connected to a different subnet.

• IPv4 networks
• IPv6 networks

Prerequisites

• You are logged in as the root user.
• The dhcp-server package is installed.

Procedure

• For IPv4 networks:

  1. Copy the /usr/lib/systemd/system/dhcpd.service file to the /etc/systemd/system/ directory:

     # cp /usr/lib/systemd/system/dhcpd.service /etc/systemd/system/

     Do not edit the /usr/lib/systemd/system/dhcpd.service file. Future updates of the dhcp-server package can override the changes.

  2. Edit the /etc/systemd/system/dhcpd.service file, and append the names of the interface, that dhcpd should listen on to the command in the ExecStart parameter:

     ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid $DHCPDARGS enp0s1 enp7s0

     This example configures that dhcpd listens only on the enp0s1 and enp7s0 interfaces.

  3. Reload the systemd manager configuration:

     # systemctl daemon-reload

  4. Restart the dhcpd service:

     # systemctl restart dhcpd.service

• For IPv6 networks:

  1. Copy the /usr/lib/systemd/system/dhcpd6.service file to the /etc/systemd/system/ directory:

     # cp /usr/lib/systemd/system/dhcpd6.service /etc/systemd/system/

     Do not edit the /usr/lib/systemd/system/dhcpd6.service file. Future updates of the dhcp-server package can override the changes.

  2. Edit the /etc/systemd/system/dhcpd6.service file, and append the names of the interface, that dhcpd should listen on to the command in the ExecStart parameter:

     ExecStart=/usr/sbin/dhcpd -f -6 -cf /etc/dhcp/dhcpd6.conf -user dhcpd -group dhcpd --no-pid $DHCPDARGS enp0s1 enp7s0

     This example configures that dhcpd listens only on the enp0s1 and enp7s0 interfaces.

  3. Reload the systemd manager configuration:

     # systemctl daemon-reload

  4. Restart the dhcpd6 service:

     # systemctl restart dhcpd6.service

• IPv4 networks
• IPv6 networks

Prerequisites

• You are logged in as the root user.
• The dhcp-server package is installed.

Procedure

• For IPv4 networks:

  1. Edit the /etc/dhcp/dhcpd.conf file:

     1. Optionally, add global parameters that dhcpd uses as default if no other directives contain these settings:

        option domain-name "example.com";
        default-lease-time 86400;

        This example sets the default domain name for the connection to example.com, and the default lease time to 86400 seconds (1 day).

     2. Add the authoritative statement on a new line:

        authoritative;

        Important

        Without the authoritative statement, the dhcpd service does not answer DHCPREQUEST messages with DHCPNAK if a client asks for an address that is outside of the pool.

     3. For each IPv4 subnet directly connected to an interface of the server, add a subnet declaration:

        subnet 192.0.2.0 netmask 255.255.255.0 {
          range 192.0.2.20 192.0.2.100;
          option domain-name-servers 192.0.2.1;
          option routers 192.0.2.1;
          option broadcast-address 192.0.2.255;
          max-lease-time 172800;
        }

        This example adds a subnet declaration for the 192.0.2.0/24 network. With this configuration, the DHCP server assigns the following settings to a client that sends a DHCP request from this subnet:

        • A free IPv4 address from the range defined in the range parameter
        • IP of the DNS server for this subnet: 192.0.2.1
        • Default gateway for this subnet: 192.0.2.1
        • Broadcast address for this subnet: 192.0.2.255
        • The maximum lease time, after which clients in this subnet release the IP and send a new request to the server: 172800 seconds (2 days)

  2. Optionally, configure that dhcpd starts automatically when the system boots:

     # systemctl enable dhcpd

  3. Start the dhcpd service:

     # systemctl start dhcpd

• For IPv6 networks:

  1. Edit the /etc/dhcp/dhcpd6.conf file:

     1. Optionally, add global parameters that dhcpd uses as default if no other directives contain these settings:

        option dhcp6.domain-search "example.com";
        default-lease-time 86400;

        This example sets the default domain name for the connection to example.com, and the default lease time to 86400 seconds (1 day).

     2. Add the authoritative statement on a new line:

        authoritative;

        Important

        Without the authoritative statement, the dhcpd service does not answer DHCPREQUEST messages with DHCPNAK if a client asks for an address that is outside of the pool.

     3. For each IPv6 subnet directly connected to an interface of the server, add a subnet declaration:

        subnet6 2001:db8:0:1::/64 {
          range6 2001:db8:0:1::20 2001:db8:0:1::100;
          option dhcp6.name-servers 2001:db8:0:1::1;
          max-lease-time 172800;
        }

        This example adds a subnet declaration for the 2001:db8:0:1::/64 network. With this configuration, the DHCP server assigns the following settings to a client that sends a DHCP request from this subnet:

        • A free IPv6 address from the range defined in the range6 parameter.
        • The IP of the DNS server for this subnet is 2001:db8:0:1::1.

        • The maximum lease time, after which clients in this subnet release the IP and send a new request to the server is 172800 seconds (2 days).

          Note that IPv6 requires uses router advertisement messages to identify the default gateway.

  2. Optionally, configure that dhcpd6 starts automatically when the system boots:

     # systemctl enable dhcpd6

  3. Start the dhcpd6 service:

     # systemctl start dhcpd6

• IPv4 networks
• IPv6 networks

Prerequisites

• You are logged in as the root user.
• The dhcp-server package is installed.

Procedure

• For IPv4 networks:

  1. Edit the /etc/dhcp/dhcpd.conf file:

     1. Optionally, add global parameters that dhcpd uses as default if no other directives contain these settings:

        option domain-name "example.com";
        default-lease-time 86400;

        This example sets the default domain name for the connection to example.com, and the default lease time to 86400 seconds (1 day).

     2. Add the authoritative statement on a new line:

        authoritative;

        Important

        Without the authoritative statement, the dhcpd service does not answer DHCPREQUEST messages with DHCPNAK if a client asks for an address that is outside of the pool.

     3. Add a shared-network declaration, such as the following, for IPv4 subnets that are not directly connected to an interface of the server:

        shared-network example {
          option domain-name-servers 192.0.2.1;
          ...
        
          subnet 192.0.2.0 netmask 255.255.255.0 {
            range 192.0.2.20 192.0.2.100;
            option routers 192.0.2.1;
          }
        
          subnet 198.51.100.0 netmask 255.255.255.0 {
            range 198.51.100.20 198.51.100.100;
            option routers 198.51.100.1;
          }
          ...
        }

        This example adds a shared network declaration, that contains a subnet declaration for both the 192.0.2.0/24 and 198.51.100.0/24 networks. With this configuration, the DHCP server assigns the following settings to a client that sends a DHCP request from one of these subnets:

        • The IP of the DNS server for clients from both subnets is: 192.0.2.1.
        • A free IPv4 address from the range defined in the range parameter, depending on from which subnet the client sent the request.
        • The default gateway is either 192.0.2.1 or 198.51.100.1 depending on from which subnet the client sent the request.

     4. Add a subnet declaration for the subnet the server is directly connected to and that is used to reach the remote subnets specified in shared-network above:

        subnet 203.0.113.0 netmask 255.255.255.0 {
        }

        Note

        If the server does not provide DHCP service to this subnet, the subnet declaration must be empty as shown in the example. Without a declaration for the directly connected subnet, dhcpd does not start.

  2. Optionally, configure that dhcpd starts automatically when the system boots:

     # systemctl enable dhcpd

  3. Start the dhcpd service:

     # systemctl start dhcpd

• For IPv6 networks:

  1. Edit the /etc/dhcp/dhcpd6.conf file:

     1. Optionally, add global parameters that dhcpd uses as default if no other directives contain these settings:

        option dhcp6.domain-search "example.com";
        default-lease-time 86400;

        This example sets the default domain name for the connection to example.com, and the default lease time to 86400 seconds (1 day).

     2. Add the authoritative statement on a new line:

        authoritative;

        Important

        Without the authoritative statement, the dhcpd service does not answer DHCPREQUEST messages with DHCPNAK if a client asks for an address that is outside of the pool.

     3. Add a shared-network declaration, such as the following, for IPv6 subnets that are not directly connected to an interface of the server:

        shared-network example {
          option domain-name-servers 2001:db8:0:1::1:1
          ...
        
          subnet6 2001:db8:0:1::1:0/120 {
            range6 2001:db8:0:1::1:20 2001:db8:0:1::1:100
          }
        
          subnet6 2001:db8:0:1::2:0/120 {
            range6 2001:db8:0:1::2:20 2001:db8:0:1::2:100
          }
          ...
        }

        This example adds a shared network declaration that contains a subnet6 declaration for both the 2001:db8:0:1::1:0/120 and 2001:db8:0:1::2:0/120 networks. With this configuration, the DHCP server assigns the following settings to a client that sends a DHCP request from one of these subnets:

        • The IP of the DNS server for clients from both subnets is 2001:db8:0:1::1:1.

        • A free IPv6 address from the range defined in the range6 parameter, depending on from which subnet the client sent the request.

          Note that IPv6 requires uses router advertisement messages to identify the default gateway.

     4. Add a subnet6 declaration for the subnet the server is directly connected to and that is used to reach the remote subnets specified in shared-network above:

        subnet6 2001:db8:0:1::50:0/120 {
        }

        Note

        If the server does not provide DHCP service to this subnet, the subnet6 declaration must be empty as shown in the example. Without a declaration for the directly connected subnet, dhcpd does not start.

  2. Optionally, configure that dhcpd6 starts automatically when the system boots:

     # systemctl enable dhcpd6

  3. Start the dhcpd6 service:

     # systemctl start dhcpd6

• IPv4 networks
• IPv6 networks

Prerequisites

• The dhcpd service is configured and running.
• You are logged in as the root user.

Procedure

• For IPv4 networks:

  1. Edit the /etc/dhcp/dhcpd.conf file:

     1. Add a host declaration:

        host server.example.com {
        	hardware ethernet 52:54:00:72:2f:6e;
        	fixed-address 192.0.2.130;
        }

        This example configures the DHCP server to always assign the 192.0.2.130 IP address to the host with the 52:54:00:72:2f:6e MAC address.

        The dhcpd service identifies systems by the MAC address specified in the fixed-address parameter, and not by the name in the host declaration. As a consequence, you can set this name to any string that does not match other host declarations. To configure the same system for multiple networks, use a different name, otherwise, dhcpd fails to start.

     2. Optionally, add further settings to the host declaration that are specific for this host.

  2. Restart the dhcpd service:

     # systemctl start dhcpd

• For IPv6 networks:

  1. Edit the /etc/dhcp/dhcpd6.conf file:

     1. Add a host declaration:

        host server.example.com {
        	hardware ethernet 52:54:00:72:2f:6e;
        	fixed-address6 2001:db8:0:1::200;
        }

        This example configures the DHCP server to always assign the 2001:db8:0:1::20 IP address to the host with the 52:54:00:72:2f:6e MAC address.

        The dhcpd service identifies systems by the MAC address specified in the fixed-address6 parameter, and not by the name in the host declaration. As a consequence, you can set this name to any string, provided that it is unique to other host declarations. To configure the same system for multiple networks, use a different name because, otherwise, dhcpd fails to start.

     2. Optionally, add further settings to the host declaration that are specific for this host.

  2. Restart the dhcpd6 service:

     # systemctl start dhcpd6

• IPv4 networks
• IPv6 networks

Prerequisites

• The dhcpd service is configured and running.
• You are logged in as the root user.

Procedure

• For IPv4 networks:

  1. Edit the /etc/dhcp/dhcpd.conf file:

     1. Add a group declaration:

        group {
          option domain-name-servers 192.0.2.1;
        
          host server1.example.com {
            hardware ethernet 52:54:00:72:2f:6e;
            fixed-address 192.0.2.130;
          }
        
          host server2.example.com {
            hardware ethernet 52:54:00:1b:f3:cf;
            fixed-address 192.0.2.140;
          }
        }

        This group definition groups two host entries. The dhcpd service applies the value set in the option domain-name-servers parameter to both hosts in the group.

     2. Optionally, add further settings to the group declaration that are specific for these hosts.

  2. Restart the dhcpd service:

     # systemctl start dhcpd

• For IPv6 networks:

  1. Edit the /etc/dhcp/dhcpd6.conf file:

     1. Add a group declaration:

        group {
          option dhcp6.domain-search "example.com";
        
          host server1.example.com {
            hardware ethernet 52:54:00:72:2f:6e;
            fixed-address 2001:db8:0:1::200;
          }
        
          host server2.example.com {
            hardware ethernet 52:54:00:1b:f3:cf;
            fixed-address 2001:db8:0:1::ba3;
          }
        }

        This group definition groups two host entries. The dhcpd service applies the value set in the option dhcp6.domain-search parameter to both hosts in the group.

     2. Optionally, add further settings to the group declaration that are specific for these hosts.

  2. Restart the dhcpd6 service:

     # systemctl start dhcpd6

• Restoring the DHCPv4 lease database
• Restoring the DHCPv6 lease database

Prerequisites

• You are logged in as the root user.
• The lease database is corrupt.

Procedure

• Restoring the DHCPv4 lease database:

  1. Stop the dhcpd service:

     # systemctl stop dhcpd

  2. Rename the corrupt lease database:

     # mv /var/lib/dhcpd/dhcpd.leases /var/lib/dhcpd/dhcpd.leases.corrupt

  3. Restore the copy of the lease database that the dhcp service created when it refreshed the lease database:

     # cp -p /var/lib/dhcpd/dhcpd.leases~ /var/lib/dhcpd/dhcpd.leases

     Important

     If you have a more recent backup of the lease database, restore this backup instead.

  4. Start the dhcpd service:

     # systemctl start dhcpd

• Restoring the DHCPv6 lease database:

  1. Stop the dhcpd6 service:

     # systemctl stop dhcpd6

  2. Rename the corrupt lease database:

     # mv /var/lib/dhcpd/dhcpd6.leases /var/lib/dhcpd/dhcpd6.leases.corrupt

  3. Restore the copy of the lease database that the dhcp service created when it refreshed the lease database:

     # cp -p /var/lib/dhcpd/dhcpd6.leases~ /var/lib/dhcpd/dhcpd6.leases

     Important

     If you have a more recent backup of the lease database, restore this backup instead.

  4. Start the dhcpd6 service:

     # systemctl start dhcpd6

• IPv4 networks
• IPv6 networks

Prerequisites

• You are logged in as the root user.

Procedure

• For IPv4 networks:

  1. Install the dhcp-relay package:

     # yum install dhcp-relay

  2. Copy the /lib/systemd/system/dhcrelay.service file to the /etc/systemd/system/ directory:

     # cp /lib/systemd/system/dhcrelay.service /etc/systemd/system/

     Do not edit the /usr/lib/systemd/system/dhcrelay.service file. Future updates of the dhcp-relay package can override the changes.

  3. Edit the /etc/systemd/system/dhcrelay.service file, and append the -i interface parameter, together with a list of IP addresses of DHCPv4 servers that are responsible for the subnet:

     ExecStart=/usr/sbin/dhcrelay -d --no-pid -i enp1s0 192.0.2.1

     With these additional parameters, dhcrelay listens for DHCPv4 requests on the enp1s0 interface and forwards them to the DHCP server with the IP 192.0.2.1.

  4. Reload the systemd manager configuration:

     # systemctl daemon-reload

  5. Optionally, configure that the dhcrelay service starts when the system boots:

     # systemctl enable dhcrelay.service

  6. Start the dhcrelay service:

     # systemctl start dhcrelay.service

• For IPv6 networks:

  1. Install the dhcp-relay package:

     # yum install dhcp-relay

  2. Copy the /lib/systemd/system/dhcrelay.service file to the /etc/systemd/system/ directory and name the file dhcrelay6.service:

     # cp /lib/systemd/system/dhcrelay.service /etc/systemd/system/dhcrelay6.service

     Do not edit the /usr/lib/systemd/system/dhcrelay.service file. Future updates of the dhcp-relay package can override the changes.

  3. Edit the /etc/systemd/system/dhcrelay6.service file, and append the -l receiving_interface and -u outgoing_interface parameters:

     ExecStart=/usr/sbin/dhcrelay -d --no-pid -l enp1s0 -u enp7s0

     With these additional parameters, dhcrelay listens for DHCPv6 requests on the enp1s0 interface and forwards them to the network connected to the enp7s0 interface.

  4. Reload the systemd manager configuration:

     # systemctl daemon-reload

  5. Optionally, configure that the dhcrelay6 service starts when the system boots:

     # systemctl enable dhcrelay6.service

  6. Start the dhcrelay6 service:

     # systemctl start dhcrelay6.service