DHCP Relays

You can configure DHCP relays for IPv4 and IPv6.

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

Configuring IPv4 DHCP Relays

Configure isc-dhcp-relay using NCLU, specifying the IP addresses to each DHCP server and the interfaces that are used as the uplinks.

In the examples below, the DHCP server IP address is 172.16.1.102, VLAN 1 (the SVI is vlan1) and the uplinks are swp51 and swp52.

cumulus@leaf01:~$ net add dhcp relay interface swp51
cumulus@leaf01:~$ net add dhcp relay interface swp52
cumulus@leaf01:~$ net add dhcp relay interface vlan1
cumulus@leaf01:~$ net add dhcp relay server 172.16.1.102
cumulus@leaf01:~$ net pending
cumulus@leaf01:~$ net commit

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

cumulus@leaf01:~$ cat /etc/default/isc-dhcp-relay
SERVERS="172.16.1.102"
INTF_CMD="-i vlan1 -i swp51 -i swp52"
OPTIONS=""

After you’ve finished configuring the DHCP relay, restart then enable the dhcrelay service so the configuration persists between reboots:

cumulus@leaf01:~$ sudo systemctl restart dhcrelay.service
cumulus@leaf01:~$ sudo systemctl enable dhcrelay.service

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

cumulus@leaf01:~$ sudo systemctl status dhcrelay.service
● dhcrelay.service - DHCPv4 Relay Agent Daemon
   Loaded: loaded (/lib/systemd/system/dhcrelay.service; enabled)
   Active: active (running) since Fri 2016-12-02 17:09:10 UTC; 2min 16s ago
     Docs: man:dhcrelay(8)
 Main PID: 1997 (dhcrelay)
   CGroup: /system.slice/dhcrelay.service
           └─1997 /usr/sbin/dhcrelay --nl -d -q -i vlan1 -i swp51 -i swp52 172.16.1.102

Using DHCP Option 82

DHCP relays can be configured to inject the circuit-id field with the -a option, which you add to the OPTIONS line in /etc/default/isc-dhcp-relay. By default, the ingress SVI interface that the relayed DHCP discover packet is processed against is injected into this field. You can change this behavior by adding the --use-pif-circuit-id option. With this option, the physical switch port (swp) that the discover packet arrives on is placed in the circuit-id field.

Controlling the Gateway IP Address with RFC 3527

When DHCP relay is required in an environment that relies on an anycast gateway (such as EVPN), a unique IP address is necessary on each device for return traffic. By default in a BGP unnumbered environment with DHCP relay, the source IP address is set to the loopback IP address and the gateway IP address (giaddr) is set as the SVI IP address. However with anycast traffic, the SVI IP address is not unique to each rack; it is typically shared amongst all racks. Most EVPN top of rack deployments only possess a single unique IP address, which is the loopback IP address.

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

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

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

To enable RFC 3527 support and control the giaddr:

1. Edit the /etc/default/isc-dhcp-relay file and provide the -U sub-option with the interface/IP address you want to use, then save the file.

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

   cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay
   SERVERS="172.16.1.102"
   INTF_CMD="-i vlan1 -i swp51 -i swp52 -U lo"
   OPTIONS=""
   

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

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

   cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay
   SERVERS="172.16.1.102"
   INTF_CMD="-i vlan1 -i swp51 -i swp52 -U 10.0.0.1%lo"
   OPTIONS=""
   

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

   cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay
   SERVERS="172.16.1.102"
   INTF_CMD="-i vlan1 -i swp51 -i swp52 -U swp2"
   OPTIONS=""
   

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

   cumulus@leaf01:~$ sudo nano /etc/default/isc-dhcp-relay
   SERVERS="172.16.1.102"
   INTF_CMD="-i vlan1 -i swp51 -i swp52 -U 10.0.0.3%swp2"
   OPTIONS=""
   

2. Restart the dhcrelay service to apply the configuration change, then enable the dhcrelay service so that the configuration persists between reboots:

   cumulus@leaf01:~$ sudo systemctl restart dhcrelay.service
   cumulus@leaf01:~$ sudo systemctl enable dhcrelay.service
   

Using the Gateway IP Address as the Source IP for Relayed DHCP Packets

You can configure the dhcrelay service to forward IPv4 (only) DHCP packets to a server and ensure that the source IP address of the relayed packet is the same as the gateway IP address.

This option impacts all relayed packets globally.

To enable this feature:

1. Edit the /etc/default/isc-dhcp-relay file adding OPTIONS=-giaddr_src, setting it to the gateway IP address:

   cumulus@leaf:~$ sudo nano /etc/default/isc-dhcp-relay
   SERVERS="172.16.1.102"
   INTF_CMD="-i vlan1 -i swp51 -i swp52 -U swp2"
   OPTIONS="-giaddr_src=10.0.0.1"
   

2. Restart the dhcrelay service:

   cumulus@leaf:~$ sudo systemctl restart dhcrelay.service
   

Configuring IPv6 DHCP Relays

If you’re configuring IPv6, the /etc/default/isc-dhcp-relay6 variables file has a different format than the /etc/default/isc-dhcp-relay file for IPv4 DHCP relays. Make sure to configure the variables appropriately by editing this file.

cumulus@leaf01:$ sudo nano /etc/default/isc-dhcp-relay6 
SERVERS=" -u 2001:db8:100::2%swp51 -u 2001:db8:100::2%swp52"
INTF_CMD="-l vlan1"

After you’ve finished configuring the DHCP relay, save your changes, restart the dhcrelay6 service, then enable the dhcrelay6 service so the configuration persists between reboots:

cumulus@leaf01:~$ sudo systemctl restart dhcrelay6.service
cumulus@leaf01:~$ sudo systemctl enable dhcrelay6.service

To see the status of the IPv6 DHCP relay, use the systemctl status dhcrelay6.service command:

cumulus@leaf01:~$ sudo systemctl status dhcrelay6.service
● dhcrelay6.service - DHCPv6 Relay Agent Daemon
   Loaded: loaded (/lib/systemd/system/dhcrelay6.service; disabled)
   Active: active (running) since Fri 2016-12-02 21:00:26 UTC; 1s ago
     Docs: man:dhcrelay(8)
 Main PID: 6152 (dhcrelay)
   CGroup: /system.slice/dhcrelay6.service
           └─6152 /usr/sbin/dhcrelay -6 --nl -d -q -l vlan1 -u 2001:db8:100::2 swp51 -u 2001:db8:100::2 swp52

Configuring Multiple DHCP Relays

Cumulus Linux supports configuring multiple DHCP relay daemons on a switch, to enable relaying of packets from different bridges to different upstreams.

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

2. Run the following command to reload the systemd files:

   cumulus@switch:~$ sudo systemctl daemon-reload
   

3. Create a config file in /etc/default using the following format for each dhcrelay: isc-dhcp-relay-<dhcp-name>. An example file can be seen below:

   # Defaults for isc-dhcp-relay initscript# sourced by /etc/init.d/isc-dhcp-relay
   # installed at /etc/default/isc-dhcp-relay by the maintainer scripts
   #
   # This is a POSIX shell fragment
   #
   # What servers should the DHCP relay forward requests to?
   SERVERS="102.0.0.2"
   # On what interfaces should the DHCP relay (dhrelay) serve DHCP requests?
   # Always include the interface towards the DHCP server.
   # This variable requires a -i for each interface configured above.
   # This will be used in the actual dhcrelay command
   # For example, "-i eth0 -i eth1"
   INTF_CMD="-i swp2s2 -i swp2s3"
   # Additional options that are passed to the DHCP relay daemon?
   OPTIONS=""
   

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

   cumulus@switch:~$ sudo systemctl start dhcrelay@<dhcp-name>
   

Configuring a DHCP Relay with VRR

If a DHCP relay is configured and you want to enable virtual router redundancy (VRR) on the SVI, then you must include the VRR interface in the INTF_CMD field in the /etc/default/isc-dhcp-relay file. For example:

cumulus@switch:~$ net add bridge
cumulus@switch:~$ net add vlan 500 ip address 192.0.2.252/24
cumulus@switch:~$ net add vlan 500 ip address-virtual 00:00:5e:00:01:01 192.0.2.254/24
cumulus@switch:~$ net add dhcp relay interface vlan500
cumulus@switch:~$ net add dhcp relay server 172.16.1.102
cumulus@switch:~$ net pending
cumulus@switch:~$ net commit

These commands create the following configuration in the /etc/network/interfaces file:

cumulus@switch:~$ cat /etc/network/interfaces
 
...
 
auto bridge
iface bridge
    bridge-vids 500
    bridge-vlan-aware yes
 
auto vlan500
iface vlan500
    address 192.0.2.252/24
    address-virtual 00:00:5e:00:01:01 192.0.2.254/24
    vlan-id 500
    vlan-raw-device bridge

They also create the following configuration in the /etc/default/isc-dhcp-relay file:

cumulus@leaf02:mgmt-vrf:~$ cat /etc/default/isc-dhcp-relay
# Defaults for isc-dhcp-relay initscript
# sourced by /etc/init.d/isc-dhcp-relay
# installed at /etc/default/isc-dhcp-relay by the maintainer scripts
#
# This is a POSIX shell fragment
#
 
# What servers should the DHCP relay forward requests to?
SERVERS="172.16.1.102"
 
# On what interfaces should the DHCP relay (dhrelay) serve DHCP requests?
# Always include the interface towards the DHCP server.
# This variable requires a -i for each interface configured above.
# This will be used in the actual dhcrelay command
# For example, "-i eth0 -i eth1"
INTF_CMD="-i vlan500"
 
# Additional options that are passed to the DHCP relay daemon?
OPTIONS=""

Configuring the DHCP Relay Service Manually (Advanced)

cumulus@switch:~$ cat /lib/systemd/system/dhcrelay.service 
[Unit]
Description=DHCPv4 Relay Agent Daemon
Documentation=man:dhcrelay(8)
After=network-oneline.target networking.service syslog.service
 
[Service]
Type=simple
EnvironmentFile=-/etc/default/isc-dhcp-relay
# Here, we are expecting the INTF_CMD to contain
# the -i for each interface specified,
#     e.g. "-i eth0 -i swp1"
ExecStart=/usr/sbin/dhcrelay -d -q $INTF_CMD $SERVERS $OPTIONS
 
[Install]
WantedBy=multi-user.target

Troubleshooting the DHCP Relays

If you are experiencing issues with the DHCP relay, you can run the following commands to determine whether or not the issue is with systemd. The following commands manually activate the DHCP relay process, and they do not persist when you reboot the switch:

cumulus@switch:~$ /usr/sbin/dhcrelay -4 -i <interface_facing_host> <ip_address_dhcp_server> -i <interface_facing_dhcp_server>
cumulus@switch:~$ /usr/sbin/dhcrelay -6 -l <interface_facing_host> -u <ip_address_dhcp_server>%<interface_facing_dhcp_server>

For example:

cumulus@leaf01:~$ /usr/sbin/dhcrelay -4 -i vlan1 172.16.1.102 -i swp51
cumulus@leaf01:~$ /usr/sbin/dhcrelay -6 -l vlan1 -u 2001:db8:100::2%swp51

See man dhcrelay for more information.

Looking at the Log on Switch where DHCP Relay Is Configured

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

cumulus@leaf01:~$ sudo journalctl -l -n 20 | grep dhcrelay
Dec 05 20:58:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 20:58:55 leaf01 dhcrelay[6152]: sending upstream swp51
Dec 05 20:58:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 20:58:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Renew from fe80::4638:39ff:fe00:3 port 546 going up.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 21:03:55 leaf01 dhcrelay[6152]: sending upstream swp51
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.
Dec 05 21:03:55 leaf01 dhcrelay[6152]: Relaying Reply to fe80::4638:39ff:fe00:3 port 546 down.

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

cumulus@leaf01:~$ sudo journalctl -l --since "2 minutes ago" | grep dhcrelay
Dec 05 21:08:55 leaf01 dhcrelay[6152]: Relaying Renew from fe80::4638:39ff:fe00:3 port 546 going up.
Dec 05 21:08:55 leaf01 dhcrelay[6152]: sending upstream swp52
Dec 05 21:08:55 leaf01 dhcrelay[6152]: sending upstream swp51