This document describes DHCP relay on the Cisco Adaptive Security Appliance (ASA) with the help of packet captures and debugs, and provides a configuration example.
A DHCP relay agent allows the security appliance to forward DHCP requests from clients to a router or other DHCP server connected to a different interface.
These restrictions apply only to the use of the DHCP relay agent:
The relay agent cannot be enabled if the DHCP server feature is also enabled.
You must be directly connected to the security appliance and cannot send requests through another relay agent or a router.
For multiple context mode, you cannot enable DHCP relay, or configure a DHCP relay server on an interface that is used by more than one context.
DHCP relay services are not available in transparent firewall mode. A security appliance in transparent firewall mode only allows Address Resolution Protocol (ARP) traffic through. All other traffic requires an Access Control List (ACL). In order to allow DHCP requests and replies through the security appliance in transparent mode, you must configure two ACLs:
One ACL that allows DHCP requests from the inside interface to the outside
One ACL that allows the replies from the server in the other direction
Cisco recommends that you have a basic understanding of ASA CLI and Cisco IOS® CLI.
The information in this document is based on these software and hardware versions:
ASA 5500-x Series Security Appliance Release 9.x or later
Cisco 1800 Series Routers
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
The DHCP protocol supplies automatic configuration parameters, such as an IP address with a subnet mask, default gateway, DNS server address, and Wireless Integrated Network Sensor (WINS) address to hosts. Initially, DHCP clients have none of these configuration parameters. In order to obtain this information, they send a broadcast request for it. When a DHCP server sees this request, the DHCP server supplies the necessary information. Due to the nature of these broadcast requests, the DHCP client and server must be on the same subnet. Layer 3 devices such as routers and firewalls do not typically forward these broadcast requests by default.
An attempt to locate DHCP clients and a DHCP server on the same subnet might not always be convenient. In such a situation, you can use DHCP relay. When the DHCP relay agent on the security appliance receives a DHCP request from a host on an inside interface, it forwards the request to one of the specified DHCP servers on an outside interface. When the DHCP server replies to the client, the security appliance forwards that reply back. Thus, the DHCP relay agent acts as a proxy for the DHCP client in its conversation with the DHCP server.
This image illustrates the DHCP packet flow when a DHCP relay agent is not used:
The ASA intercepts these packets and wraps them into DHCP relay format:
DHCP Relay with Packet Captures on the ASA Inside and Outside Interface
Make a note of content highligted in RED, because that is how the ASA modifies various fields.
In order to start the DHCP process, boot the system and send a broadcast message (DHCPDISCOVER) to the destination address 255.255.255.255 - UDP port 67.
Note: If a VPN client requests an IP address, the relay-agent IP address is the first usable IP address that is defined by the dhcp-network-scope command, under the group-policy.
Normally, ASA would drop the broadcast, but because it is configured to act as a DHCP relay, it forwards the DHCPDISCOVER message as a unicast packet to the DHCP server's IP sourcing from the interface IP that faces the server. In this case, it is the outside interface IP address. Notice the change in the IP header and relay agent field:
Note: Due to the fix incorporated in Cisco bug ID CSCuo89924, ASA in Versions 9.1(5.7), 9.3(1), and later will forward the unicast packets to the DHCP sever's IP sourcing from the interface IP address that faces the client (giaddr) where the dhcprelay is enabled. In this case, it will be the inside interface IP address.
The server sends back a DHCPOFFER message as a unicast packet to the ASA, destined to the relay agent IP set up in DHCPDISCOVER- UDP port 67. In this case, it is the IP address of the inside interface (giaddr), where dhcprelay is enabled. Notice the destination IP in the layer 3 header:
ASA sends this packet out of the inside interface - UDP port 68. Notice the change in the IP header while the packet leaves the inside interface:
Once you receive the DHCPOFFER message, send a DHCPREQUEST message in order to indicate that you accept the offer.
ASA passes the DHCPREQUEST to the DHCP server.
Once the server gets the DHCPREQUEST, it sends the DHCPACK back in order to confirm the offered IP.
ASA passes the DHCPACK from the DHCP server to you, and that completes the transaction.
Debugs and Syslogs for DHCP Relay Transactions
This is a DHCP request forwarded to DHCP server interface 198.51.100.2:
DHCPRA: relay binding created for client 0050.5684.396a.DHCPD: setting giaddr to 192.0.2.1.
dhcpd_forward_request: request from 0050.5684.396a forwarded to 198.51.100.2. DHCPD/RA: Punt 198.51.100.2/17152 --> 192.0.2.1/17152 to CP DHCPRA: Received a BOOTREPLY from interface 2 DHCPRA: relay binding found for client 0050.5684.396a. DHCPRA: Adding rule to allow client to respond using offered address 192.0.2.4
After the reply is received from the DHCP server, the security appliance forwards it to the DHCP client with MAC address 0050.5684.396a, and changes the gateway address to its own inside interface.
DHCPRA: forwarding reply to client 0050.5684.396a. DHCPRA: relay binding found for client 0050.5684.396a. DHCPD: setting giaddr to 192.0.2.1. dhcpd_forward_request: request from 0050.5684.396a forwarded to 198.51.100.2. DHCPD/RA: Punt 198.51.100.2/17152 --> 192.0.2.1/17152 to CP DHCPRA: Received a BOOTREPLY from interface 2 DHCPRA: relay binding found for client 0050.5684.396a. DHCPRA: exchange complete - relay binding deleted for client 0050.5684.396a. DHCPD: returned relay binding 192.0.2.1/0050.5684.396a to address pool. dhcpd_destroy_binding() removing NP rule for client 192.0.2.1 DHCPRA: forwarding reply to client 0050.5684.396a.
The same transaction shows up in the syslogs as well:
%ASA-7-609001: Built local-host inside:0.0.0.0 %ASA-7-609001: Built local-host identity:255.255.255.255 %ASA-6-302015: Built inbound UDP connection 13 for inside: 0.0.0.0/68 (0.0.0.0/68) to identity:255.255.255.255/67 (255.255.255.255/67) %ASA-7-609001: Built local-host identity:198.51.100.1 %ASA-7-609001: Built local-host outside:198.51.100.2 %ASA-6-302015: Built outbound UDP connection 14 for outside: 198.51.100.2/67 (198.51.100.2/67) to identity:198.51.100.1/67 (198.51.100.1/67)
%ASA-7-609001: Built local-host inside:192.0.2.4 %ASA-6-302020: Built outbound ICMP connection for faddr 192.0.2.4/0 gaddr 198.51.100.2/1 laddr 198.51.100.2/1 %ASA-7-609001: Built local-host identity:192.0.2.1 %ASA-6-302015: Built inbound UDP connection 16 for outside: 198.51.100.2/67 (198.51.100.2/67) to identity:192.0.2.1/67 (192.0.2.1/67) %ASA-6-302015: Built outbound UDP connection 17 for inside: 192.0.2.4/68 (192.0.2.4/68) to identity:192.0.2.1/67 (192.0.2.1/67) %ASA-6-302021: Teardown ICMP connection for faddr 192.0.2.4/0 gaddr 198.51.100.2/1 laddr 198.51.100.2/1
In this section, you are presented with the information used to configure the features described in this document.
show run : Saved : ASA Version 9.1(5) ! hostname ASA enable password 8Ry2YjIyt7RRXU24 encrypted passwd 2KFQnbNIdI.2KYOU encrypted names ! interface Ethernet0/0 nameif inside security-level 0 ip address 192.0.2.1 255.255.255.0 ! interface Ethernet0/1 nameif outside security-level 100 ip address 198.51.100.1 255.255.255.0 ! interface Ethernet0/2 no nameif no security-level no ip address ! interface Ethernet0/3 no nameif no security-level no ip address ! interface Management0/0 shutdown no nameif no security-level no ip address ! boot system disk0:/asa825-k8.bin ftp mode passive no pager logging enable logging buffer-size 40960 logging buffered debugging mtu inside 1500 mtu outside 1500 no failover icmp unreachable rate-limit 1 burst-size 1 no asdm history enable arp timeout 14400 timeout xlate 0:30:00 timeout pat-xlate 0:00:30 timeout conn 3:00:00 half-closed 0:30:00 udp 0:15:00 icmp 0:00:02 timeout sunrpc 0:10:00 h323 0:05:00 h225 0:30:00 mgcp 0:05:00 mgcp-pat 0:05:00 timeout sip 0:30:00 sip_media 0:02:00 sip-invite 0:03:00 sip-disconnect 0:02:00 timeout sip-provisional-media 0:02:00 uauth 0:05:00 absolute timeout tcp-proxy-reassembly 0:01:00 timeout floating-conn 0:00:00 dynamic-access-policy-record DfltAccessPolicy http server enable http 0.0.0.0 0.0.0.0 inside no snmp-server location no snmp-server contact crypto ipsec security-association lifetime seconds 28800 crypto ipsec security-association lifetime kilobytes 4608000 telnet timeout 5 ssh timeout 5 console timeout 0 dhcprelay server 198.51.100.2 Outside dhcprelay enable inside dhcprelay setroute inside
//Defining DHCP server IP and interface// //Enables DHCP relay on inside/client facing interface// //Sets ASA inside as DG for clients in DHCP reply packets// dhcprelay timeout 60 threat-detection basic-threat threat-detection statistics access-list no threat-detection statistics tcp-intercept webvpn ! ! prompt hostname context no call-home reporting anonymous call-home profile CiscoTAC-1 no active destination address http https://tools.cisco.com/its/service/oddce/services/DDCEService destination address email firstname.lastname@example.org destination transport-method http subscribe-to-alert-group diagnostic subscribe-to-alert-group environment subscribe-to-alert-group inventory periodic monthly subscribe-to-alert-group configuration periodic monthly subscribe-to-alert-group telemetry periodic daily Cryptochecksum:7ae5f655ffe399c8a88b61cb13425972 : end
DHCP Server Configuration
show run Building configuration...
Current configuration : 1911 bytes ! ! Last configuration change at 18:36:05 UTC Tue May 28 2013 version 15.1 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname Router ! boot-start-marker boot-end-marker ! ! logging buffered 4096 ! no aaa new-model ! crypto pki token default removal timeout 0 ! ! dot11 syslog ip source-route ! ip dhcp excluded-address 192.0.2.1 192.0.2.2 ip dhcp excluded-address 192.0.2.10 192.0.2.254
//IP addresses exluded from DHCP scope// ! ip dhcp pool pool1 import all network 192.0.2.0 255.255.255.0 dns-server 192.0.2.10 192.0.2.11 domain-name cisco.com
//DHCP pool configuration and various parameters// ! ! ! ip cef no ipv6 cef ! multilink bundle-name authenticated ! ! ! license udi pid CISCO1811W-AG-A/K9 sn FCTxxxx ! ! ! interface Dot11Radio0 no ip address shutdown speed basic-1.0 basic-2.0 basic-5.5 6.0 9.0 basic-11.0 12.0 18.0 24.0 36.0 48.0 54.0 station-role root ! interface Dot11Radio1 no ip address shutdown speed basic-6.0 9.0 basic-12.0 18.0 basic-24.0 36.0 48.0 54.0 station-role root ! interface FastEthernet0 ip address 198.51.100.2 255.255.255.0 duplex auto speed auto ! interface FastEthernet1 no ip address duplex auto speed auto ! interface FastEthernet2 no ip address ! interface FastEthernet3 no ip address ! interface FastEthernet4 no ip address ! interface FastEthernet5 no ip address ! interface FastEthernet6 no ip address ! interface FastEthernet7 no ip address ! interface FastEthernet8 no ip address ! interface FastEthernet9 no ip address ! interface Vlan1 no ip address ! interface Async1 no ip address encapsulation slip ! ip forward-protocol nd no ip http server no ip http secure-server ! ! ip route 192.0.2.0 255.255.255.0 198.51.100.1
//Static route to ensure replies are routed to relay agent IP// ! ! ! control-plane ! ! line con 0 line 1 modem InOut stopbits 1 speed 115200 flowcontrol hardware line aux 0 line vty 0 4 login transport input all ! end
Use this section in order to confirm that your configuration works properly.
In order to view the statistical information about the DHCP relay services, enter the show dhcprelay statistics command on the ASA CLI:
ASA# show dhcprelay statistics DHCP UDP Unreachable Errors: 1 DHCP Other UDP Errors: 0