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利用ASA的Twice NAT解决内网无法访问映射后的公网地址

发表于：2025-02-03 作者：千家信息网编辑

千家信息网最后更新 2025年02月03日，一.概述：默认情况下，不管是Inside还是DMZ区映射到Outside区的地址或服务，Inside和DMZ区都无法通过映射后地址来访问内部服务器。ASA8.3版本之后有一种新的NAT叫Twice-

千家信息网最后更新 2025年02月03日利用ASA的Twice NAT解决内网无法访问映射后的公网地址一.概述： 默认情况下，不管是Inside还是DMZ区映射到Outside区的地址或服务，Inside和DMZ区都无法通过映射后地址来访问内部服务器。ASA8.3版本之后有一种新的NAT叫Twice-NAT,它可以在一个NAT语句中既匹配源地址，又匹配目标地址，并且可以对源地址、目标地址，端口号，三个参数中一~三个参数的转换。二.基本思路：A.Inside区映射到Outside区①Outside区访问Inside区映射后的地址没有问题②Inside区访问Inside区映射后的地址，通过Twice NAT做源地址和目标地址转换---将源地址转换为Inside接口地址，目标地址转换为Inside设备实际地址---如果不做源地址，因为访问的数据流来回路径不同，无法建立会话③DMZ区访问Inside区映射后的地址，通过Twice NAT做目标地址转换---将目标地址转换为Inside设备的实际地址---因为两边都是实际地址，因此需要Inside和DMZ互相有对方的路由---也可以同时做源地址转换，将源地址转换为Iniside接口地址，为了便于审计，不建议这样做
B.DMZ区映射到Outside区①Outside区访问DMZ区映射后的地址没有问题②Inside区访问DMZ区映射后的地址，通过Twice NAT做目标地址转换---将目标地址转换为DMZ区设备的实际地址---因为两边都是实际地址，因此需要Inside和DMZ互相有对方的路由---也可以同时做源地址转换，将源地址转换为DMZ接口地址，为了便于审计，不建议这样做---实际用模拟器测试，却发现不做源地址转换，经常报 -- bad sequence number的错误---想不来为什么会报序列号错误，即使用MPF设置set connection random-sequence-number disable，也只是缓解，还是会偶尔出现，觉得可能是模拟器的缘故③DMZ区访问DMZ区映射后的地址，通过Twice NAT做源地址和目标地址转换---将源地址转换为DMZ接口地址，目标地址转换为DMZ区设备实际地址---如果不做源地址，因为访问的数据流来回路径不同，无法建立会话三.测试拓扑：R1-----SW--(Inside)----ASA---（Outside）----R4 | | | | R2 R3（DMZ)四.测试需求：A.将R2的TCP23端口映射到ASA的Outside接口的23端口上----要求R1~R4都能通访问ASA的Outside接口的23端口访问到R2的23端口B.将R3的TCP23端口映射到ASA的Outside接口的2323端口上----要求R1~R4都能通访问ASA的Outside接口的2323端口访问到R3的23端口五.基本配置：A.R1：interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.0 no shutip route 0.0.0.0 0.0.0.0 10.1.1.10B.R2interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.0 no shutip route 0.0.0.0 0.0.0.0 10.1.1.10line vty 0 4
password cisco
loginC.ASA:interface GigabitEthernet0
nameif Inside
security-level 100
ip address 10.1.1.10 255.255.255.0
no shutinterface GigabitEthernet1
nameif DMZ
security-level 50
ip address 192.168.1.10 255.255.255.0
no shutinterface GigabitEthernet2
nameif Outside
security-level 0 ip address 202.100.1.10 255.255.255.0 no shutD.R3:interface FastEthernet0/0
ip address 192.168.1.3 255.255.255.0 no shutip route 0.0.0.0 0.0.0.0 192.168.1.10line vty 0 4
password cisco
loginE.R4:interface FastEthernet0/0
ip address 202.100.1.4 255.255.255.0 no shut五.ASA的NAT及策略配置：A.动态PAT允许Inside和DMZ区能访问公网object network Inside-NET
subnet 10.1.1.0 255.255.255.0 nat (Inside,Outside) dynamic interfaceobject network DMZ-NET
subnet 192.168.1.0 255.255.255.0 nat (DMZ,Outside) dynamic interfaceB.静态PAT将R2和R3映射出去：object network Inside-R2 host 10.1.1.2
nat (Inside,Outside) static interface service tcp 23 23
object network DMZ-R3 host 192.168.1.3
nat (DMZ,Outside) static interface service tcp 23 2323 C.防火墙策略：①开启icmp审查：policy-map global_policy
class inspection_default
inspect icmp ②Outside口放行策略：access-list OUTSIDE extended permit tcp any object Inside-R2 eq telnet
access-list OUTSIDE extended permit tcp any object DMZ-R3 eq telnet access-group OUTSIDE in interface Outside③DMZ口放行策略：access-list DMZ extended permit tcp any object Inside-R2 eq telnetaccess-list DMZ extended permit icmp any anyaccess-list DMZ extended deny ip any object Inside-NETaccess-list DMZ extended permit ip any anyaccess-group DMZ in interface DMZ---备注：这里只是测试，实际除非必要，尽量不要放行DMZ到Inside的访问，要放行也需要针对主机放行D.测试：①Inside区和DMZ区能访问公网：R1#PING 202.100.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.100.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/80/168 msR2#ping 202.100.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.100.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/67/156 msR3#ping 202.100.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 202.100.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/120/204 ms②被映射后的端口只能被Outside区访问：R4#TELNET 202.100.1.10 23
Trying 202.100.1.10 ... Open

User Access Verification

Password:
R2>show users
Line User Host(s) Idle Location
0 con 0 idle 00:55:06
* 66 vty 0 idle 00:00:00 202.100.1.4

Interface User Mode Idle Peer Address

R2>
R4#TELNET 202.100.1.10 2323
Trying 202.100.1.10, 2323 ... Open

User Access Verification

Password:
R3>show users
Line User Host(s) Idle Location
0 con 0 idle 00:51:12
* 66 vty 0 idle 00:00:00 202.100.1.4

Interface User Mode Idle Peer Address

R3>R1#telnet 202.100.1.10
Trying 202.100.1.10 ...
% Connection timed out; remote host not responding六.ASA的Twice NAT配置：A.使得Inside区访问Inside区映射后的地址时，既做源地址转换，又做目标地址转换①配置对象：object network Public-R2
host 202.100.1.10
object service tcp23
service tcp destination eq telnet
②配置twice-NAT:nat (Inside,Inside) source static any interface destination static Public-R2 Inside-R2 service tcp23 tcp23③允许相同接口的访问：same-security-traffic permit intra-interface---因为访问时，既从Inside口进，又从Inside出，所以需要这条④测试：R1#telnet 202.100.1.10
Trying 202.100.1.10 ... Open

User Access Verification

Password:
R2>show users
Line User Host(s) Idle Location
0 con 0 idle 01:07:35
* 66 vty 0 idle 00:00:0010.1.1.10

Interface User Mode Idle Peer Address

R2>
R2#telnet 202.100.1.10
Trying 202.100.1.10 ... Open

User Access Verification

Password:
R2>show users
Line User Host(s) Idle Location
0 con 0 202.100.1.10 00:00:00
* 66 vty 0 idle 00:00:0010.1.1.10

Interface User Mode Idle Peer Address

B.使得Inside区访问DMZ区映射后的地址时，只做目标地址转换①配置对象：object network Public-R3
host 202.100.1.10object network DMZ-R3
host 192.168.1.3
object service tcp2323
service tcp destination eq 2323 ②配置twice-NAT:nat (Inside,DMZ) source static any any destination static Public-R3 DMZ-R3 service tcp2323 tcp23③测试：R1#telnet 202.100.1.10 2323
Trying 202.100.1.10, 2323 ...
% Connection timed out; remote host not responding

R3#debug ip tcp packet port 23
TCP Packet debugging is on for port number 23
R3#
*Mar 1 13:18:25.648: tcp0: I LISTEN 10.1.1.1:17155 192.168.1.3:23 seq 1568429504
OPTS 4 SYN WIN 4128
*Mar 1 13:18:25.652: tcp0: O SYNRCVD 10.1.1.1:17155 192.168.1.3:23 seq 1603796811
OPTS 4 ACK 1568429505 SYN WIN 4128
*Mar 1 13:18:25.656: tcp0: I SYNRCVD 10.1.1.1:17155 192.168.1.3:23 seq 4193850862
OPTS 4 SYN WIN 4128
*Mar 1 13:18:25.660: tcp0: O SYNRCVD 10.1.1.1:17155 192.168.1.3:23 seq 1603796811
ACK 1568429505 WIN 4128
*Mar 1 13:18:25.660: TCP0: bad seg from 10.1.1.1 -- bad sequence number: port 23 seq 4193850862 ack 0 rcvnxt 1568429505 rcvwnd 4128 len 0④解决方法：---修改NAT，做源地址转换nat (Inside,DMZ) source static any interface destination static Public-R3 DMZ-R3 service tcp2323 tcp23⑤再次测试：R1#telnet 202.100.1.10 2323
Trying 202.100.1.10, 2323 ... Open

User Access Verification

Password:
R3>show users
Line User Host(s) Idle Location
0 con 0 idle 00:02:15
* 66 vty 0 idle 00:00:00 192.168.1.10

Interface User Mode Idle Peer Address

R3>R2#telnet 202.100.1.10 2323
Trying 202.100.1.10, 2323 ... Open

User Access Verification

Password:
R3>show users
Line User Host(s) Idle Location
0 con 0 idle 00:03:13
66 vty 0 idle 00:00:58 192.168.1.10
* 67 vty 1 idle 00:00:00 192.168.1.10

Interface User Mode Idle Peer Address

R3>C.使得DMZ区访问Inside区映射后的地址时，只做目标地址转换①配置对象：---前面已经定义②配置twice-NAT:nat (DMZ,Inside) source static any any destination static Public-R2 Inside-R2 service tcp23 tcp23③测试：R3#telnet 202.100.1.10
Trying 202.100.1.10 ... Open

User Access Verification

Password:
R2>show users
Line User Host(s) Idle Location
0 con 0 202.100.1.10 00:02:49
* 66 vty 0 idle 00:00:00192.168.1.3

Interface User Mode Idle Peer Address

R2>D.使得DMZ区访问DMZ区映射后的地址时，既做源地址转换，又做目标地址转换①配置对象：---前面已经定义②配置twice-NAT:nat (DMZ,DMZ) source static any interface destination static Public-R3 DMZ-R3 service tcp2323 tcp23③允许相同接口的访问：---前面已经配置:same-security-traffic permit intra-interface④测试：R3#telnet 202.100.1.10 2323
Trying 202.100.1.10, 2323 ... Open

User Access Verification

Password:
R3>show users
Line User Host(s) Idle Location
0 con 0 202.100.1.10 00:00:00
66 vty 0 idle 00:07:01 192.168.1.10
67 vty 1 idle 00:06:02 192.168.1.10
* 68 vty 2 idle 00:00:00 192.168.1.10

Interface User Mode Idle Peer Address

R3>

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