配置自適應安全裝置(ASA)DHCP中繼

簡介

本文檔通過資料包捕獲和調試的幫助介紹Cisco ASA上的DHCP中繼，並提供配置示例。

必要條件

動態主機配置協定(DHCP)中繼代理允許安全裝置將DHCP請求從客戶端轉發到連線到不同介面的路由器或其他DHCP伺服器。

以下限制僅適用於使用DHCP中繼代理：

• 如果還啟用了DHCP伺服器功能，則無法啟用中繼代理。
• 您必須直接連線到安全裝置，並且不能通過其他中繼代理或路由器傳送請求。
• 對於多情景模式，不能在多個情景使用的介面上啟用DHCP中繼或配置DHCP中繼伺服器。

DHCP中繼服務在透明防火牆模式下不可用。處於透明防火牆模式的安全裝置僅允許地址解析協定(ARP)流量通過。所有其他流量需要存取控制清單(ACL)。為了在透明模式下允許通過安全裝置進行DHCP請求和回覆，您必須配置兩個ACL:

• 一個ACL，它允許從內部介面到外部的DHCP請求。
• 一個ACL，它允許從伺服器向另一個方向進行應答。

需求

Cisco建議您瞭解ASA CLI和Cisco IOS® CLI的基本知識。

採用元件

本文中的資訊係根據以下軟體和硬體版本：

• ASA 5500-x系列安全裝置版本9.x或更高版本
• Cisco 1800系列路由器

本文中的資訊是根據特定實驗室環境內的裝置所建立。文中使用到的所有裝置皆從已清除（預設）的組態來啟動。如果您的網路運作中，請確保您瞭解任何指令可能造成的影響。

背景資訊

DHCP協定向主機提供自動配置引數，如帶有子網掩碼的IP地址、預設網關、DNS伺服器地址和Windows Internet名稱服務(WINS)地址。最初，DHCP客戶端沒有這些配置引數。為了獲取此資訊，它們會傳送廣播請求。當DHCP伺服器看到此請求時，DHCP伺服器會提供必需的資訊。由於這些廣播請求的性質，DHCP客戶端和伺服器必須位於同一子網中。預設情況下，第3層裝置（如路由器和防火牆）通常不會轉發這些廣播請求。

嘗試在同一子網中查詢DHCP客戶端和DHCP伺服器並不總是方便的。在這種情況下，您可以使用DHCP中繼。安全裝置上的DHCP中繼代理收到來自內部介面上的主機的DHCP請求時，會將該請求轉發到外部介面上的指定DHCP伺服器之一。當DHCP伺服器回覆客戶端時，安全裝置會將該回覆轉發回來。因此，DHCP中繼代理在與DHCP伺服器的會話中充當DHCP客戶端的代理。 

封包流量

此圖說明未使用DHCP中繼代理時的DHCP資料包流：

ASA會攔截這些資料包並將其封裝為DHCP中繼格式： 

在ASA內部和外部介面上使用資料包捕獲功能的DHCP中繼

記下以紅色突出顯示的內容，因為ASA就是這樣修改各個欄位的。

1. 要啟動DHCP進程，請啟動系統並向目標地址255.255.255.255 - UDP埠67傳送廣播消息(DHCPDISCOVER)。
   
   

   

   
   
   

   註：如果VPN客戶端請求IP地址，則中繼代理IP地址是通過dhcp-network-scope命令在group-policy下定義的第一個可用IP地址。

   
   
   
2. 通常，ASA會丟棄廣播，但由於它被配置為充當DHCP中繼，因此它會將DHCPDISCOVER消息作為單播資料包轉發到DHCP伺服器的IP源，該源來自面向伺服器的介面IP。在本例中，它是外部介面IP地址。請注意IP報頭和中繼代理欄位的變化：
   
   

   

   
   
   

   註：由於Cisco錯誤ID CSCuo89924中引入的修復程式，版本9.1(5.7)、9.3(1)及更高版本的ASA可以將單播資料包轉發到DHCP伺服器的IP源，該源地址來自啟用了dhcprelay的客戶端(giaddr)。在這種情況下，可以是內部介面IP地址。

   
   
   
3. 伺服器將DHCPOFFER消息作為單播資料包發回ASA，目的地為在DHCPDISCOVER - UDP埠67中設定的中繼代理IP。在本例中，它是內部介面(giaddr)的IP地址，在該介面上啟用了dhcprelay。請注意第3層報頭中的目標IP:
   
   

   

   
   
   
4. ASA將此資料包傳送到內部介面 — UDP埠68。請注意，當封包離開內部介面時，IP標頭會有所變更：
   
   

   

   
   
   
5. 收到DHCPOFFER消息後，傳送DHCPREQUEST消息以指示您接受該提議。
   
   

   

   
   
   
6. ASA將DHCPREQUEST傳遞到DHCP伺服器。
   
   

   

   
   
   
7. 伺服器收到DHCPREQUEST後，會傳送DHCPACK以確認提供的IP。
   
   

   

   
   
   
8. ASA將DHCPACK從DHCP伺服器傳送給您，從而完成事務。
   
   

   

DHCP中繼事務的調試和系統日誌

這是轉發到DHCP伺服器介面198.51.100.2的DHCP請求：

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

從DHCP伺服器收到回覆後，安全裝置會將其轉發到MAC地址為0050.5684.396a的DHCP客戶端，並將網關地址更改為自己的內部介面。

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.

系統日誌中也顯示相同的事務：

%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

設定

本節提供用於設定本檔案中所述功能的資訊。

網路圖表

此文件使用以下網路設定：

組態

本檔案會使用以下設定：

• 使用CLI進行DHCP中繼配置
• DHCP中繼最終配置
• DHCP伺服器配置

使用CLI進行DHCP中繼配置

dhcprelay server 198.51.100.2 outside
dhcprelay enable inside
dhcprelay setroute inside
dhcprelay timeout 60

DHCP中繼最終配置

show run
!
hostname ASA
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
!
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 callhome@cisco.com
  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伺服器配置

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

使用多個DHCP伺服器的DHCP中繼

最多可以定義10個DHCP伺服器。當客戶端傳送DHCP Discover資料包時，該資料包將轉發到所有DHCP伺服器。

以下是範例：

dhcprelay server 198.51.100.2 outside
dhcprelay server 198.51.100.3 outside
dhcprelay server 198.51.100.4 outside
dhcprelay enable inside
dhcprelay setroute inside

使用多個DHCP伺服器進行調試

以下是使用多個DHCP伺服器時的一些調試示例：

DHCP: Received a BOOTREQUEST from interface 2 (size = 300)
DHCPRA: relay binding found for client 000c.291c.34b5.
DHCPRA: setting giaddr to 192.0.2.1.
dhcpd_forward_request: request from 000c.291c.34b5 forwarded to 198.51.100.2.
dhcpd_forward_request: request from 000c.291c.34b5 forwarded to 198.51.100.3.
dhcpd_forward_request: request from 000c.291c.34b5 forwarded to 198.51.100.4.

使用多個DHCP伺服器捕獲

以下是使用多個DHCP伺服器時的封包擷取範例：

ASA# show cap out

3 packets captured

 1: 18:48:41.211628       192.0.2.1.67 > 198.51.100.2.67:  udp 300 
 2: 18:48:41.211689       192.0.2.1.67 > 198.51.100.3.67:  udp 300 
 3: 18:48:41.211704       192.0.2.1.67 > 198.51.100.4.67:  udp 300 

驗證

使用本節內容，確認您的組態是否正常運作。

要檢視有關DHCP中繼服務的統計資訊，請在ASA CLI上輸入show dhcprelay statistics命令：

ASA# show dhcprelay statistics

DHCP UDP Unreachable Errors: 1
DHCP Other UDP Errors: 0

Packets Relayed
BOOTREQUEST          0
DHCPDISCOVER         1
DHCPREQUEST          1
DHCPDECLINE          0
DHCPRELEASE          0
DHCPINFORM           0

BOOTREPLY            0
DHCPOFFER            1
DHCPACK              1
DHCPNAK              0

此輸出提供有關幾種DHCP消息型別的資訊，例如DHCPDISCOVER、DHCP REQUEST、DHCP OFER、DHCP RELEASE和DHCP ACK。

• 顯示ASA CLI上的dhcprelay狀態
• show ip dhcp server statistics on router CLI

疑難排解

本節提供的資訊可用於對組態進行疑難排解。

Router#show ip dhcp server statistics

Memory usage         56637
Address pools        1
Database agents      0
Automatic bindings   1
Manual bindings      0
Expired bindings     0
Malformed messages   0
Secure arp entries   0

Message              Received
BOOTREQUEST          0
DHCPDISCOVER         1
DHCPREQUEST          1
DHCPDECLINE          0
DHCPRELEASE          0
DHCPINFORM           0

Message              Sent
BOOTREPLY            0
DHCPOFFER            1
DHCPACK              1
DHCPNAK              0

ASA# show dhcprelay state
Context  Configured as DHCP Relay
Interface inside, Configured for DHCP RELAY SERVER
Interface outside, Configured for DHCP RELAY

您還可以使用以下debug指令：

• debug dhcprelay packet
• debug dhcprelay event
• 擷取
  
• 系統日誌

附註：使用 debug 指令之前，請先參閱有關 Debug 指令的重要資訊。

相關資訊

• 在ASA上捕獲
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