利用ASA的TwiceNAT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 shut

ip route 0.0.0.0 0.0.0.0 10.1.1.10

B.R2

interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.0

no shut

ip route 0.0.0.0 0.0.0.0 10.1.1.10

line vty 0 4
password cisco
login

C.ASA:

interface GigabitEthernet0
nameif Inside
security-level 100
ip address 10.1.1.10 255.255.255.0

no shut

interface GigabitEthernet1
nameif DMZ
security-level 50
ip address 192.168.1.10 255.255.255.0

no shut

interface GigabitEthernet2
nameif Outside
security-level 0

ip address 202.100.1.10 255.255.255.0

no shut

D.R3:

interface FastEthernet0/0
ip address 192.168.1.3 255.255.255.0

no shut

ip route 0.0.0.0 0.0.0.0 192.168.1.10

line vty 0 4
password cisco
login

E.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 interface

object network DMZ-NET
subnet 192.168.1.0 255.255.255.0

nat (DMZ,Outside) dynamic interface

B.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 Outside

③DMZ口放行策略：

access-list DMZ extended permit tcp any object Inside-R2 eq telnet

access-list DMZ extended permit icmp any any

access-list DMZ extended deny ip any object Inside-NET

access-list DMZ extended permit ip any any

access-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 ms

R2#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 ms

R3#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 ms

②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 tcp23

③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 202.100.1.10

object 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 tcp23

③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 0

④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 tcp23

⑤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 tcp23

③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 tcp23

③允许相同接口的访问：

---前面已经配置:same-security-traffic permit intra-interface

④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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利用ASA的TwiceNAT解决内网无法访问映射后的公网地址

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