Chapter 3. Providing DHCP services The dynamic host configuration protocol (DHCP) is a network protocol that automatically assigns IP information to clients. You can set up the dhcpd service to provide a DHCP server and DHCP relay in your network. 3.1. The difference between static and dynamic IP addressing Static IP addressing When you assign a static IP address to a device, the address does not change over time unless you change it manually. Use static IP addressing if you want: To ensure network address consistency for servers such as DNS, and authentication servers. To use out-of-band management devices that work independently of other network infrastructure. Dynamic IP addressing When you configure a device to use a dynamic IP address, the address can change over time. For this reason, dynamic addresses are typically used for devices that connect to the network occasionally because the IP address can be different after rebooting the host. Dynamic IP addresses are more flexible, easier to set up, and administer. The Dynamic Host Control Protocol (DHCP) is a traditional method of dynamically assigning network configurations to hosts. Note There is no strict rule defining when to use static or dynamic IP addresses. It depends on user's needs, preferences, and the network environment. 3.2. DHCP transaction phases The DHCP works in four phases: Discovery, Offer, Request, Acknowledgement, also called the DORA process. DHCP uses this process to provide IP addresses to clients. Discovery The DHCP client sends a message to discover the DHCP server in the network. This message is broadcasted at the network and data link layer. Offer The DHCP server receives messages from the client and offers an IP address to the DHCP client. This message is unicast at the data link layer but broadcast at the network layer. Request The DHCP client requests the DHCP server for the offered IP address. This message is unicast at the data link layer but broadcast at the network layer. Acknowledgment The DHCP server sends an acknowledgment to the DHCP client. This message is unicast at the data link layer but broadcast at the network layer. It is the final message of the DHCP DORA process. 3.3. The differences when using dhcpd for DHCPv4 and DHCPv6 The dhcpd service supports providing both DHCPv4 and DHCPv6 on one server. However, you need a separate instance of dhcpd with separate configuration files to provide DHCP for each protocol. DHCPv4 Configuration file: /etc/dhcp/dhcpd.conf Systemd service name: dhcpd DHCPv6 Configuration file: /etc/dhcp/dhcpd6.conf Systemd service name: dhcpd6 3.4. The lease database of the dhcpd service A DHCP lease is the period for which the dhcpd service allocates a network address to a client. The dhcpd service stores the DHCP leases in the following databases: For DHCPv4: /var/lib/dhcpd/dhcpd.leases For DHCPv6: /var/lib/dhcpd/dhcpd6.leases Warning Manually updating the database files can corrupt the databases. The lease databases contain information about the allocated leases, such as the IP address assigned to a media access control (MAC) address or the time stamp when the lease expires. Note that all time stamps in the lease databases are in Coordinated Universal Time (UTC). The dhcpd service recreates the databases periodically: The service renames the existing files: /var/lib/dhcpd/dhcpd.leases to /var/lib/dhcpd/dhcpd.leases~ /var/lib/dhcpd/dhcpd6.leases to /var/lib/dhcpd/dhcpd6.leases~ The service writes all known leases to the newly created /var/lib/dhcpd/dhcpd.leases and /var/lib/dhcpd/dhcpd6.leases files. Additional resources dhcpd.leases(5) man page on your system Restoring a corrupt lease database 3.5. Comparison of DHCPv6 to radvd In an IPv6 network, only router advertisement messages provide information about an IPv6 default gateway. As a consequence, if you want to use DHCPv6 in subnets that require a default gateway setting, you must additionally configure a router advertisement service, such as Router Advertisement Daemon ( radvd ). The radvd service uses flags in router advertisement packets to announce the availability of a DHCPv6 server. The following table compares features of DHCPv6 and radvd : DHCPv6 radvd Provides information about the default gateway no yes Guarantees random addresses to protect privacy yes no Sends further network configuration options yes no Maps media access control (MAC) addresses to IPv6 addresses yes no 3.6. Configuring the radvd service for IPv6 routers The router advertisement daemon ( radvd ) sends router advertisement messages that are required for IPv6 stateless autoconfiguration. This enables users to automatically configure their addresses, settings, routes, and to choose a default router based on these advertisements. Note You can only set /64 prefixes in the radvd service. To use other prefixes, use DHCPv6. Prerequisites You are logged in as the root user. Procedure Install the radvd package: Edit the /etc/radvd.conf file, and add the following configuration: These settings configures radvd to send router advertisement messages on the enp1s0 device for the 2001:db8:0:1::/64 subnet. The AdvManagedFlag on setting defines that the client should receive the IP address from a DHCP server, and the AdvOtherConfigFlag parameter set to on defines that clients should receive non-address information from the DHCP server as well. Optional: Configure that radvd automatically starts when the system boots: Start the radvd service: Verficiation Display the content of router advertisement packages and the configured values radvd sends: Additional resources radvd.conf(5) man page on your system /usr/share/doc/radvd/radvd.conf.example file Can I use a prefix length other than 64 bits in IPv6 Router Advertisements? 3.7. Setting network interfaces for the DHCP servers By default, the dhcpd service processes requests only on network interfaces that have an IP address in the subnet defined in the configuration file of the service. For example, in the following scenario, dhcpd listens only on the enp0s1 network interface: 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. Only follow this procedure if the DHCP server contains multiple network interfaces connected to the same network but the service should listen only on specific interfaces. Depending on whether you want to provide DHCP for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites You are logged in as the root user. The dhcp-server package is installed. Procedure For IPv4 networks: Copy the /usr/lib/systemd/system/dhcpd.service file to the /etc/systemd/system/ directory: Do not edit the /usr/lib/systemd/system/dhcpd.service file. Future updates of the dhcp-server package can override the changes. 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: This example configures that dhcpd listens only on the enp0s1 and enp7s0 interfaces. Reload the systemd manager configuration: Restart the dhcpd service: For IPv6 networks: Copy the /usr/lib/systemd/system/dhcpd6.service file to the /etc/systemd/system/ directory: Do not edit the /usr/lib/systemd/system/dhcpd6.service file. Future updates of the dhcp-server package can override the changes. 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: This example configures that dhcpd listens only on the enp0s1 and enp7s0 interfaces. Reload the systemd manager configuration: Restart the dhcpd6 service: 3.8. Setting up the DHCP service for subnets directly connected to the DHCP server Use the following procedure if the DHCP server is directly connected to the subnet for which the server should answer DHCP requests. This is the case if a network interface of the server has an IP address of this subnet assigned. Depending on whether you want to provide DHCP for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites You are logged in as the root user. The dhcp-server package is installed. Procedure For IPv4 networks: Edit the /etc/dhcp/dhcpd.conf file: Optional: Add global parameters that dhcpd uses as default if no other directives contain these settings: This example sets the default domain name for the connection to example.com , and the default lease time to 86400 seconds (1 day). Add the authoritative statement on a new line: 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. For each IPv4 subnet directly connected to an interface of the server, add a subnet declaration: 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) Optional: Configure that dhcpd starts automatically when the system boots: Start the dhcpd service: For IPv6 networks: Edit the /etc/dhcp/dhcpd6.conf file: Optional: Add global parameters that dhcpd uses as default if no other directives contain these settings: This example sets the default domain name for the connection to example.com , and the default lease time to 86400 seconds (1 day). Add the authoritative statement on a new line: 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. For each IPv6 subnet directly connected to an interface of the server, add a subnet declaration: 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. Optional: Configure that dhcpd6 starts automatically when the system boots: Start the dhcpd6 service: Additional resources The dhcp-options(5) and dhcpd.conf(5) man pages on your system /usr/share/doc/dhcp-server/dhcpd.conf.example file /usr/share/doc/dhcp-server/dhcpd6.conf.example file 3.9. Setting up the DHCP service for subnets that are not directly connected to the DHCP server Use the following procedure if the DHCP server is not directly connected to the subnet for which the server should answer DHCP requests. This is the case if a DHCP relay agent forwards requests to the DHCP server, because none of the DHCP server's interfaces is directly connected to the subnet the server should serve. Depending on whether you want to provide DHCP for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites You are logged in as the root user. The dhcp-server package is installed. Procedure For IPv4 networks: Edit the /etc/dhcp/dhcpd.conf file: Optional: Add global parameters that dhcpd uses as default if no other directives contain these settings: This example sets the default domain name for the connection to example.com , and the default lease time to 86400 seconds (1 day). Add the authoritative statement on a new line: 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. Add a shared-network declaration, such as the following, for IPv4 subnets that are not directly connected to an interface of the server: 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. 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: 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. Optional: Configure that dhcpd starts automatically when the system boots: Start the dhcpd service: For IPv6 networks: Edit the /etc/dhcp/dhcpd6.conf file: Optional: Add global parameters that dhcpd uses as default if no other directives contain these settings: This example sets the default domain name for the connection to example.com , and the default lease time to 86400 seconds (1 day). Add the authoritative statement on a new line: 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. Add a shared-network declaration, such as the following, for IPv6 subnets that are not directly connected to an interface of the server: 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. 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: 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. Optional: Configure that dhcpd6 starts automatically when the system boots: Start the dhcpd6 service: Additional resources The dhcp-options(5) and dhcpd.conf(5) man pages on your system /usr/share/doc/dhcp-server/dhcpd.conf.example file /usr/share/doc/dhcp-server/dhcpd6.conf.example file Setting up a DHCP relay agent 3.10. Assigning a static address to a host using DHCP Using a host declaration, you can configure the DHCP server to assign a fixed IP address to a media access control (MAC) address of a host. For example, use this method to always assign the same IP address to a server or network device. Depending on whether you want to configure fixed addresses for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites The dhcpd service is configured and running. You are logged in as the root user. Procedure For IPv4 networks: Edit the /etc/dhcp/dhcpd.conf file: Add a host declaration: 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. Optional: Add further settings to the host declaration that are specific for this host. Restart the dhcpd service: For IPv6 networks: Edit the /etc/dhcp/dhcpd6.conf file: Add a host declaration: 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. Optional: Add further settings to the host declaration that are specific for this host. Restart the dhcpd6 service: Additional resources dhcp-options(5) man page on your system /usr/share/doc/dhcp-server/dhcpd.conf.example file /usr/share/doc/dhcp-server/dhcpd6.conf.example file 3.11. Using a group declaration to apply parameters to multiple hosts, subnets, and shared networks at the same time Using a group declaration, you can apply the same parameters to multiple hosts, subnets, and shared networks. Note that the procedure describes using a group declaration for hosts, but the steps are the same for subnets and shared networks. Depending on whether you want to configure a group for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites The dhcpd service is configured and running. You are logged in as the root user. Procedure For IPv4 networks: Edit the /etc/dhcp/dhcpd.conf file: Add a group declaration: 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. Optional: Add further settings to the group declaration that are specific for these hosts. Restart the dhcpd service: For IPv6 networks: Edit the /etc/dhcp/dhcpd6.conf file: Add a group declaration: 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. Optional: Add further settings to the group declaration that are specific for these hosts. Restart the dhcpd6 service: Additional resources dhcp-options(5) man page on your system /usr/share/doc/dhcp-server/dhcpd.conf.example file /usr/share/doc/dhcp-server/dhcpd6.conf.example file 3.12. Restoring a corrupt lease database If the DHCP server logs an error that is related to the lease database, such as Corrupt lease file - possible data loss! ,you can restore the lease database from the copy the dhcpd service created. Note that this copy might not reflect the latest status of the database. Warning If you remove the lease database instead of replacing it with a backup, you lose all information about the currently assigned leases. As a consequence, the DHCP server could assign leases to clients that have been previously assigned to other hosts and are not expired yet. This leads to IP conflicts. Depending on whether you want to restore the DHCPv4, DHCPv6, or both databases, see the procedure for: 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: Stop the dhcpd service: Rename the corrupt lease database: Restore the copy of the lease database that the dhcp service created when it refreshed the lease database: Important If you have a more recent backup of the lease database, restore this backup instead. Start the dhcpd service: Restoring the DHCPv6 lease database: Stop the dhcpd6 service: Rename the corrupt lease database: Restore the copy of the lease database that the dhcp service created when it refreshed the lease database: Important If you have a more recent backup of the lease database, restore this backup instead. Start the dhcpd6 service: Additional resources The lease database of the dhcpd service 3.13. Setting up a DHCP relay agent The DHCP Relay Agent ( dhcrelay ) enables the relay of DHCP and BOOTP requests from a subnet with no DHCP server on it to one or more DHCP servers on other subnets. When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified. When a DHCP server returns a reply, the DHCP Relay Agent forwards this request to the client. Depending on whether you want to set up a DHCP relay for IPv4, IPv6, or both protocols, see the procedure for: IPv4 networks IPv6 networks Prerequisites You are logged in as the root user. Procedure For IPv4 networks: Install the dhcp-relay package: Copy the /lib/systemd/system/dhcrelay.service file to the /etc/systemd/system/ directory: Do not edit the /usr/lib/systemd/system/dhcrelay.service file. Future updates of the dhcp-relay package can override the changes. 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: 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 . Reload the systemd manager configuration: Optional: Configure that the dhcrelay service starts when the system boots: Start the dhcrelay service: For IPv6 networks: Install the dhcp-relay package: Copy the /lib/systemd/system/dhcrelay.service file to the /etc/systemd/system/ directory and name the file dhcrelay6.service : Do not edit the /usr/lib/systemd/system/dhcrelay.service file. Future updates of the dhcp-relay package can override the changes. Edit the /etc/systemd/system/dhcrelay6.service file, and append the -l receiving_interface and -u outgoing_interface parameters: With these additional parameters, dhcrelay listens for DHCPv6 requests on the enp1s0 interface and forwards them to the network connected to the enp7s0 interface. Reload the systemd manager configuration: Optional: Configure that the dhcrelay6 service starts when the system boots: Start the dhcrelay6 service: Additional resources dhcrelay(8) man page on your system

Commands:
yum install radvd
interface enp1s0 { AdvSendAdvert on; AdvManagedFlag on; AdvOtherConfigFlag on; prefix 2001:db8:0:1::/64 { }; };
systemctl enable radvd
systemctl start radvd
radvdump
cp /usr/lib/systemd/system/dhcpd.service /etc/systemd/system/
ExecStart=/usr/sbin/dhcpd -f -cf /etc/dhcp/dhcpd.conf -user dhcpd -group dhcpd --no-pid USDDHCPDARGS enp0s1 enp7s0
systemctl daemon-reload
systemctl restart dhcpd.service
cp /usr/lib/systemd/system/dhcpd6.service /etc/systemd/system/
ExecStart=/usr/sbin/dhcpd -f -6 -cf /etc/dhcp/dhcpd6.conf -user dhcpd -group dhcpd --no-pid USDDHCPDARGS enp0s1 enp7s0
systemctl daemon-reload
systemctl restart dhcpd6.service
option domain-name "example.com"; default-lease-time 86400;
authoritative;
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; }
systemctl enable dhcpd
systemctl start dhcpd
option dhcp6.domain-search "example.com"; default-lease-time 86400;
authoritative;
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; }
systemctl enable dhcpd6
systemctl start dhcpd6
option domain-name "example.com"; default-lease-time 86400;
authoritative;
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; } }
subnet 203.0.113.0 netmask 255.255.255.0 { }
systemctl enable dhcpd
systemctl start dhcpd
option dhcp6.domain-search "example.com"; default-lease-time 86400;
authoritative;
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 } }
subnet6 2001:db8:0:1::50:0/120 { }
systemctl enable dhcpd6
systemctl start dhcpd6
host server.example.com { hardware ethernet 52:54:00:72:2f:6e; fixed-address 192.0.2.130; }
systemctl start dhcpd
host server.example.com { hardware ethernet 52:54:00:72:2f:6e; fixed-address6 2001:db8:0:1::200; }
systemctl start dhcpd6
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; } }
systemctl start dhcpd
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; } }
systemctl start dhcpd6
systemctl stop dhcpd
mv /var/lib/dhcpd/dhcpd.leases /var/lib/dhcpd/dhcpd.leases.corrupt
cp -p /var/lib/dhcpd/dhcpd.leases~ /var/lib/dhcpd/dhcpd.leases
systemctl start dhcpd
systemctl stop dhcpd6
mv /var/lib/dhcpd/dhcpd6.leases /var/lib/dhcpd/dhcpd6.leases.corrupt
cp -p /var/lib/dhcpd/dhcpd6.leases~ /var/lib/dhcpd/dhcpd6.leases
systemctl start dhcpd6
yum install dhcp-relay
cp /lib/systemd/system/dhcrelay.service /etc/systemd/system/
ExecStart=/usr/sbin/dhcrelay -d --no-pid -i enp1s0 192.0.2.1
systemctl daemon-reload
systemctl enable dhcrelay.service
systemctl start dhcrelay.service
yum install dhcp-relay
cp /lib/systemd/system/dhcrelay.service /etc/systemd/system/dhcrelay6.service
ExecStart=/usr/sbin/dhcrelay -d --no-pid -l enp1s0 -u enp7s0
systemctl daemon-reload
systemctl enable dhcrelay6.service
systemctl start dhcrelay6.service