Naming Scheme for Network Devices Available for Linux on System z 8.5. The DHCP Relay Agent (dhcrelay) enables the relay of DHCP and BOOTP requests from. See Section 9.3.2, “Configure dhcrelay as a DHCPv6 relay agent”. And BOOTP relay agent (by default) or as a DHCPv6 relay agent (with -6 argument). Dec 30, 2019  This guide will walk you through the steps to install DHCP Server & DHCP Client on CentOS 8 / RHEL 8 Linux server. If your question is how do I Install and Configure DHCP Server on CentOS 8 / RHEL 8?, then this guide is for you. A DHCP Server deployed in your organization can.

Asterisk video call. You can configure DHCP relays for IPv4 and IPv6.

To run DHCP for both IPv4 and IPv6, initiate the DHCP relay once forIPv4 and once for IPv6. Following are the configurations on the serverhosts, DHCP relay, and DHCP server using the following topology:

The dhcpd and dhcrelay services are disabled by default. After youfinish configuring the DHCP relays and servers, you need to start thoseservices. If you intend to run these services within aVRF,including the management VRF,follow these steps forconfiguring them. See also the VRF chapter.

Configure IPv4 DHCP Relays

Configure isc-dhcp-relay usingNCLU,specifying the IP addresses to each DHCP server and the interfaces thatare used as the uplinks.

In the examples below, the DHCP server IP address is 172.16.1.102, VLAN1 (the SVI is vlan1) and the uplinks are swp51 and swp52.

You configure a DHCP relay on a per-VLAN basis, specifying the SVI, notthe parent bridge; in our example, you would specify vlan1 as the SVIfor VLAN 1; do not specify the bridge named bridge in this case.

As per RFC 3046, you can specifyas many server IP addresses that can fit in 255 octets, specifying eachaddress only once.

These commands create the following configuration in the/etc/default/isc-dhcp-relay file:

After you finish configuring DHCP relay, restart then enable thedhcrelay service so the configuration persists between reboots:

To see the DHCP relay status, use the systemctl statusdhcrelay.service command:

DHCP Option 8

You can configure DHCP relays to inject the circuit-id field with the-a option, which you add to the OPTIONS line in the/etc/default/isc-dhcp-relay file. By default, the ingress SVIinterface against which the relayed DHCP discover packet is processed isinjected into this field. You can change this behavior by adding the--use-pif-circuit-id option. With this option, the physical switchport (swp) on which the discover packet arrives is placed in thecircuit-id field.

Control the Gateway IP Address with RFC 3527

When DHCP relay is required in an environment that relies on an anycastgateway (such as EVPN), a unique IP address is necessary on each devicefor return traffic. By default, in a BGP unnumbered environment withDHCP relay, the source IP address is set to the loopback IP address andthe gateway IP address (giaddr) is set as the SVI IP address. Howeverwith anycast traffic, the SVI IP address is not unique to each rack; itis typically shared amongst all racks. Most EVPN ToR deployments onlypossess a single unique IP address, which is the loopback IP address.

RFC 3527 enables the DHCP serverto react to these environments by introducing a new parameter to theDHCP header called the link selection sub-option, which is built bythe DHCP relay agent. The link selection sub-option takes on the normalrole of the giaddr in relaying to the DHCP server which subnet iscorrelated to the DHCP request. When using this sub-option, the giaddrcontinues to be present but only relays the return IP address that is tobe used by the DHCP server; the giaddr becomes the unique loopback IPaddress.

When enabling RFC 3527 support, you can specify an interface, such asthe loopback interface or a switchport interface to be used as thegiaddr. The relay picks the first IP address on that interface. If theinterface has multiple IP addresses, you can specify a specific IPaddress for the interface.

The following illustration demonstrates how you can control the giaddrwith RFC 3527.

To enable RFC 3527 support and control the giaddr, run the net add dhcprelay giaddr-interface command with interface/IP address you want touse.

The following example uses the first IP address on the loopbackinterface as the giaddr:

The above command creates the following configuration in the/etc/default/isc-dhcp-relay file:

The first IP address on the loopback interface is typically the127.0.0.1 address; Cumulus Networks recommends that you use morespecific syntax, as shown in the next example.

The following example uses IP address 10.0.0.1 on the loopback interfaceas the giaddr:

The above command creates the following configuration in the/etc/default/isc-dhcp-relay file:

The following example uses the first IP address on swp2 as the giaddr:

The above command creates the following configuration in the/etc/default/isc-dhcp-relay file:

The following example uses IP address 10.0.0.3 on swp2 as the giaddr:

The above command creates the following configuration in the/etc/default/isc-dhcp-relay file:

Configure IPv6 DHCP Relays

If you are configuring IPv6, the /etc/default/isc-dhcp-relay6variables file has a different format than the/etc/default/isc-dhcp-relay file for IPv4 DHCP relays. Make sure toconfigure the variables appropriately by editing this file.

After you finish configuring the DHCP relay, save your changes, restartthe dhcrelay6 service, then enable the dhcrelay6 service so theconfiguration persists between reboots:

To see the status of the IPv6 DHCP relay, use the systemctl statusdhcrelay6.service command:

Configure Multiple DHCP Relays

Cumulus Linux supports multiple DHCP relay daemons on a switch to enablerelaying of packets from different bridges to different upstreams.

To configure multiple DHCP relay daemons on a switch:

1. As the sudo user, open the /etc/vrf/systemd.conf file in a texteditor and remove dhcrelay.

2. To reload the systemd files, run the following command:

3. Create a config file in /etc/default using the following formatfor each dhcrelay: isc-dhcp-relay-<dhcp-name>. An example file isshown below:

4. Run the following command to start a dhcrelay instance. Replacedhcp-name with the instance name or number:

Configure a DHCP Relay with VRR

The configuration procedure for DHCP relay with VRR is the same asdocumented above. Note that DHCP relaymust run on the SVI and not on the -v0 interface.

Configure the DHCP Relay Service Manually (Advanced)

By default, Cumulus Linux configures the DHCP relay serviceautomatically. However, in older versions of Cumulus Linux, you neededto edit the dhcrelay.service file as described below. The IPv4dhcrelay.serviceUnit script calls /etc/default/isc-dhcp-relay tofind launch variables.

The /etc/default/isc-dhcp-relay variables file needs to reference bothinterfaces participating in DHCP relay (facing the server and facing theclient) and the IP address of the server. If the client-facing interfaceis a bridge port, specify the switch virtual interface (SVI) name if youare using aVLAN-aware bridge(for example, vlan100), or the bridge name if you are using traditionalbridging (for example, br100).

Use the Gateway IP Address as the Source IP for Relayed DHCP Packets (Advanced)

You can configure the dhcrelay service to forward IPv4 (only) DHCPpackets to a server and ensure that the source IP address of the relayedpacket is the same as the gateway IP address. You do this by enablingthe giaddr-src option; when set, dhcrelay attempts to set the sourceIP address of the packet to be the gateway IP address.

This option impacts all relayed packets globally.

To enable this feature:

These commands create the following configuration in the/etc/default/isc-dhcp-relay file:

Troubleshooting

If you are experiencing issues with the DHCP relay, run the followingcommands to determine if the issue is with systemd. The followingcommands manually activate the DHCP relay process and they do notpersist when you reboot the switch:

For example:

See man dhcrelay for more information.

Use the journalctl command to look at the behavior on the CumulusLinux switch that is providing the DHCP relay functionality:

You can run the journalctl command with the --since flag to specifya time period:

Configuration Errors

If you configure DHCP relays by editing the/etc/default/isc-dhcp-relay filemanually instead of running NCLU commands, you might introduceconfiguration errors that can cause the switch to crash.

For example, if you see an error similar to the following, there mightbe a space between the DHCP server address and the interface used as theuplink.

To resolve the issue, manually edit the /etc/default/isc-dhcp-relayfile to remove the space, then run thesystemctl restart dhcrelay.service command to restart the dhcrelayservice and apply the configuration change.

Caveats and Errata

Interface Names Cannot Be Longer than 14 Characters

The dhcrelay command does not bind to an interface if the interface’s name islonger than 14 characters. To work around this issue, change the interface nameto be 14 or fewer characters if dhcrelay is required to bind to it.

This is a known limitation in dhcrelay.