Understanding and Troubleshooting ASA NAT

Created by Oleg Tipisov, Cisco TAC.
Version 1.1. Cisco Public
Understanding and
Troubleshooting ASA NAT
Cisco Support Community
Expert Series Webcasts in
Russian:

© 2010 Cisco and/or its affiliates. All rights reserved. Cisco PublicCisco Support Community 2
Cisco Support Community –
Expert Series Webcasts in Russian
Сегодня на семинаре эксперт Cisco TAC
Олег Типисов
CCIE in Routing and Switching
2
Expert’s photo

Спасибо, что посетили наш семинар
сегодня
Сегодняшняя презентация включает опросы аудитории
Пожалуйста, участвуйте!

Спасибо, что посетили наш семинар
сегодня
Если Вы хотите получить копию слайдов сегодняшнего
семинара, пожалуйста, используйте следующие ссылки:
https://supportforums.cisco.com/community/russian/security
или, https://supportforums.cisco.com/docs/DOC-27135

Опрос #1
Каков уровень ваших знаний о ASA
NAT?
1. Я представляю, что такое NAT, но не работал с
ним
2. Мне приходилось настраивать NAT на ASA с
помощью графического интерфейса ASDM
3. Я владею настройкой NAT из CLI и ASDM и
применяю эту технологию в своей сети
4. Я неоднократно настраивал различные варианты
NAT на ASA во многих версиях ПО

© 2010 Cisco and/or its affiliates. All rights reserved. Cisco Public 6Cisco Support Community
Задавайте Ваши вопросы!
Используйте Q&A панель, чтобы послать вопрос. Наши
эксперты ответят на них

Created by Oleg Tipisov, Cisco TAC.
Version 1.1. Cisco Public
Understanding and
Troubleshooting ASA NAT

Introduction
 This session is mostly about ASA 8.3+ NAT
 ASA 8.2 configuration example is given, but slides are
hidden to save time
 Two real-world troubleshooting scenarios are given
 Students are expected to understand ASA NAT CLI
 We will not discuss:
• 8.2 -> 8.3 Configuration migration
• NAT and Routing integration
• NAT RPF Check and associated issues
 Separate presentation is needed for each of the above

Agenda
 Introduction
 NAT Terminology
 ASA 8.2 Configuration Example
 ASA 8.3+ Configuration Example
 Troubleshooting Scenario #1
 Troubleshooting Scenario #2
 Final Recommendations

Introduction

ASA Features
 Stateful packet filter
 Security policy is based on “interface security levels”
 Application inspection
 NAT/PAT, NAT ALG
 Static & Dynamic IPv4 & IPv6 routing
 Integration with IPS, CSC and CX modules
 L2L & RA VPN (IPSec IKEv1, IPSec IKEv2, SSL)
 Redundancy features and failover

ASA Features
 Virtualization (multiple context mode)
 Transparent mode
 NetFlow v9 for security monitoring
 Botnet traffic filtering (Ironport integration)
 Identity firewall
 ASA Phone Proxy and other UC integration features

Latest Releases
 8.4 – 5505, 5510-5550, 5580, 5585-X
• 8.4(4) is the latest version
 8.5 – ASA SM
• 8.4(1) with few other features
 8.6 – 5500-X
• 8.4(2) with few other features
 8.7 – ASA 1000V
• ASA in a Nexus 1000V switch
 9.0 – To be released soon

NAT Terminology

NAT Terminology
 Real Address vs. Mapped Address
 Connection vs. xlate
 Source Translation vs. Destination Translation (UN-NAT)
 Bidirectional NAT
 Dynamic NAT vs. Static NAT
 NAT vs. PAT
 Identity NAT
 NAT exemption or “NAT 0 ACL” (8.2- only)
 Policy NAT

ASA 8.3+ Configuration Example

NAT in 8.3+
 Completely new implementation
 “NAT simplification”
 Object NAT (Auto NAT)
 Twice NAT (Manual NAT)
 Bidirectional NAT is very easy to configure
 Flexibility is higher, new features are being
implemented

Security Policy Example
ASACorporate Network
DMZ network
Internet
Internet allowed inbound
access to DMZ
Internet traffic blocked
inbound to corporate
network
inside outside
dmz
Corporate network allowed
to access DMZ network
and the Internet

Configuration Migration (8.2 –> 8.3)
INFO: MIGRATION - Saving the startup configuration to file
INFO: MIGRATION - Startup configuration saved to file
'flash:8_2_1_0_startup_cfg.sav'
*** Output from config line 4, "ASA Version 8.2(1) "
.
Cryptochecksum (unchanged): 66abf6f4 1b22b1c8 2f06d057 62b2e46a
NAT migration logs:
The following 'nat' command didn't have a matching 'global' rule on interface
'dmz' and was not migrated.
nat (inside) 1 10.1.1.0 255.255.255.0
INFO: NAT migration completed.
Real IP migration logs:
ACL <outside_in> has been migrated to real-ip version
INFO: MIGRATION - Saving the startup errors to file
'flash:upgrade_startup_errors_200503292016.log'
See this article about 8.2 -> 8.3+ software upgrade and configuration migration:
https://supportforums.cisco.com/docs/DOC-12690

ASA Interface Configuration
interface GigabitEthernet0/0.1
vlan 99
nameif inside
security-level 100
ip address 10.1.1.1 255.255.255.0
vlan 98
nameif dmz
security-level 50
ip address 172.16.1.1 255.255.255.0
interface GigabitEthernet0/1
nameif outside
security-level 0
ip address 194.1.1.1 255.255.255.0
route outside 0.0.0.0 0.0.0.0 194.1.1.2 1

ASA Object NAT – 8.3+
object network obj-10.1.1.0
subnet 10.1.1.0 255.255.255.0
host 172.16.1.2
nat (inside,outside) dynamic interface
nat (dmz,outside) static 194.1.1.254
access-list outside_in extended permit tcp any host 172.16.1.2 eq www
access-group outside_in in interface outside
“host 172.16.1.2 eq www”
real IP in this release!
10.1.1.0/24
172.16.1.0/24
194.1.1.254
.2

ASA Twice NAT – 8.3+
subnet 10.1.1.0 255.255.255.0
nat (inside,outside) source dynamic obj-10.1.1.0 interface
host 172.16.1.2
host 194.1.1.254
nat (dmz,outside) source static obj-172.16.1.2 obj-194.1.1.254
access-list outside_in extended permit tcp any host 172.16.1.2 eq www
access-group outside_in in interface outside
“host 172.16.1.2 eq www”
real IP in this release!
10.1.1.0/24
172.16.1.0/24
194.1.1.254
.2

ASA Dynamic NAT – 8.3+
%ASA-6-305011: Built dynamic TCP translation from inside:10.1.1.2/57126 to
outside:194.1.1.1/57126
%ASA-6-302013: Built outbound TCP connection 3 for outside:207.1.1.2/80
(207.1.1.2/80) to inside:10.1.1.2/57126 (194.1.1.1/57126)
ASA# show conn long
TCP outside:207.1.1.2/80 (207.1.1.2/80) inside:10.1.1.2/57126
(194.1.1.1/57126), flags U, idle 12s, uptime 12s, timeout 1h0m, bytes 0
ASA# show xlate
2 in use, 3 most used
Flags: D - DNS, i - dynamic, r - portmap, s - static, I - identity, T - twice
e - extended
TCP PAT from inside:10.1.1.2/57126 to outside:194.1.1.1/57126 flags ri idle
0:00:23 timeout 0:00:30
Newest software tries to preserve source port if possible

ASA Static NAT – 8.3+
%ASA-6-302013: Built inbound TCP connection 4 for outside:207.1.1.2/41506
(207.1.1.2/41506) to dmz:172.16.1.2/80 (194.1.1.254/80)
ASA# show conn long
TCP outside:207.1.1.2/41506 (207.1.1.2/41506) dmz:172.16.1.2/80
(194.1.1.254/80), flags UB, idle 41s, uptime 43s, timeout 1h0m, bytes 0
ASA# show xlate
e - extended
NAT from dmz:172.16.1.2 to outside:194.1.1.254
flags s idle 0:00:47 timeout 0:00:00
ASA# show access-list
access-list outside_in line 1 extended permit tcp any host 172.16.1.2 eq www
(hitcnt=1) 0xdae674c0
number of connections,
not packets!
public IP

NAT Rules Order (8.3+)
1. Section 1: Twice NAT
• default place for twice NAT rules
• in order, until the first match
1. Section 2: Object NAT
• static NAT (longest match)
• dynamic NAT (longest match)
1. Section 3: Twice NAT
• “after-auto” needs to be specified in “nat” command
• in order, until the first match

Опрос #2
Сталкивались ли Вы с проблемами при
использовании NAT на ASA
1. Нет, никогда. Все отлично работает
2. Иногда бывало, но это были ошибки настройки
3. Проблемы встречались, но они легко решались
переходом на новую версию
4. Проблемы возникали и их могли решить только
инженеры Cisco TAC
5. Сплошные проблемы, не знаю, что делать
6. Я не использую NAT на ASA, потому что он не
работает

Troubleshooting Scenario #1

ASA Internet
Server 172.16.1.2 should be accessible from
both Internet and Remote Office by 194.1.1.254.
Other DMZ servers should be accessible via
VPN without NAT.
VPN is terminated on some other device, such
as perimeter router.
DMZ servers should be able to talk to Internet
DNS servers.
inside outside
dmz
IPSec tunnelIPSec tunnel
172.16.1.0/24
Remote Office
10.2.2.0/24
to
194.1.1.254
172.16.1.2

vlan 99
nameif inside
security-level 100
ip address 10.1.1.1 255.255.255.0
vlan 98
nameif dmz
security-level 50
ip address 172.16.1.1 255.255.255.0
nameif outside
security-level 0
ip address 194.1.1.1 255.255.255.0
route outside 0.0.0.0 0.0.0.0 194.1.1.2 1

ASA NAT Configuration – 8.3+
subnet 172.16.1.0 255.255.255.0
host 172.16.1.2
host 194.1.1.254
object network RemoteOfficeNet
subnet 10.2.2.0 255.255.255.0
object network RemoteOfficeNet-2
subnet 10.2.2.0 255.255.255.0
nat (dmz,outside) source static obj-172.16.1.0 obj-172.16.1.0 destination
static RemoteOfficeNet RemoteOfficeNet-2
nat (dmz,outside) source dynamic obj-172.16.1.0 interface
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24

Customer Symptom
ASA# show conn long
TCP outside:207.1.1.2/80 (207.1.1.2/80) dmz:172.16.1.2/37116
TCP outside:207.1.1.2/16123 (207.1.1.2/16123) dmz:172.16.1.2/80
(194.1.1.254/80), flags UB, idle 12s, uptime 12s, timeout 1h0m, bytes 0
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
Everything works, but 172.16.1.2 gets wrong IP
when goes to the Internet…
At the same time inbound connections to 194.1.1.254
work as expected…

Troubleshooting – Step #1
ASA# show run object
subnet 172.16.1.0 255.255.255.0
host 172.16.1.2
host 194.1.1.254
subnet 10.2.2.0 255.255.255.0
object network RemoteOfficeNet-2
subnet 10.2.2.0 255.255.255.0
ASA# show run nat
nat (dmz,outside) source static obj-172.16.1.0 obj-172.16.1.0 destination
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24

%ASA-6-305011: Built dynamic TCP
translation from dmz:172.16.1.2/37116
to outside:194.1.1.1/23384
%ASA-6-302013: Built outbound TCP connection 54
for outside:207.1.1.2/80 (207.1.1.2/80)
to dmz:172.16.1.2/37116 (194.1.1.1/23384)
ASA# show conn long
TCP outside:207.1.1.2/80 (207.1.1.2/80) dmz:172.16.1.2/37116
ASA# show xlate local 172.16.1.2 global 194.1.1.1
TCP PAT from dmz:172.16.1.2/37116 to outside:194.1.1.1/23384 flags ri idle
0:02:03 timeout 0:00:30
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24

ASA# debug nat 255
nat: policy lock 0x73a1cb40, old count is 1
nat: translation - dmz:172.16.1.2/37116
to outside:194.1.1.1/23384
ASA# show nat detail
Manual NAT Policies (Section 1)
1 (dmz) to (outside) source static obj-172.16.1.2 obj-194.1.1.254
translate_hits = 0, untranslate_hits = 0
Source - Origin: 172.16.1.2/32, Translated: 194.1.1.254/32
2 (dmz) to (outside) source static obj-172.16.1.0 obj-172.16.1.0 destination
Destination - Origin: 10.2.2.0/24, Translated: 10.2.2.0/24
3 (dmz) to (outside) source dynamic obj-172.16.1.0 interface
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
Hmm… This is strange. It seems that dynamic PAT
(rule #3) takes precedence over static NAT (rule #1)

ASA# packet-tracer input dmz tcp 172.16.1.2 1234 207.1.1.2 80
Phase: 1
Type: ROUTE-LOOKUP
Subtype: input
Result: ALLOW
Config:
Additional Information:
in 0.0.0.0 0.0.0.0 outside
Phase: 2
Type: IP-OPTIONS
Subtype:
Result: ALLOW
Config:
Phase: 3
Type: NAT
Subtype:
Result: ALLOW
Config:
Dynamic translate 172.16.1.2/1234 to 194.1.1.1/40625
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
This again confirms that traffic is processed by
dynamic PAT rule, instead of static NAT rule…

ASA# packet-tracer input dmz tcp 172.16.1.2 1234 207.1.1.2 80 detail
Phase: 1
…
Phase: 2
…
Phase: 3
Type: NAT
Subtype:
Result: ALLOW
Config:
Dynamic translate 172.16.1.2/1234 to 194.1.1.1/12386
Forward Flow based lookup yields rule:
in id=0x73a1de50, priority=6, domain=nat, deny=false
hits=5, user_data=0x73a1cb40, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
src ip/id=172.16.1.0, mask=255.255.255.0, port=0
dst ip/id=0.0.0.0, mask=0.0.0.0, port=0, dscp=0x0
input_ifc=dmz, output_ifc=outside
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
The following command can be used to look at this NAT rule in Accelerated Security Path
(ASP) table:
show asp table classify domain nat

ASA# show asp table classify domain nat
Input Table
in id=0x73984078, priority=6, domain=nat, deny=false
hits=0, user_data=0x73a1ca98, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
input_ifc=outside, output_ifc=dmz
in id=0x739f84d8, priority=6, domain=nat, deny=false
hits=0, user_data=0x73980550, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
hits=0, user_data=0x73a1c608, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24ASP tables are used to classify traffic
in data-path and apply different
security policies to it

Input Table
hits=0, user_data=0x73a1ca98, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
hits=0, user_data=0x73980550, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
hits=0, user_data=0x73a1c608, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
nat (dmz,outside) source static obj-172.16.1.0 obj-172.16.1.0 destination static RemoteOfficeNet RemoteOfficeNet-
2
3
2
1
2
Incorrect order !
Incorrect order !
1
2
3

Troubleshooting – Root Cause
no nat (dmz,outside) source static obj-172.16.1.2 obj-194.1.1.254
nat (dmz,outside) 1 source static obj-172.16.1.2 obj-194.1.1.254
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
It was found that the problem was caused by
editing of NAT lines as shown below
New bug was opened:
CSCtt11890 ASA: Manual NAT rules inserted
above others may fail to match traffic
which was fixed in 8.4(4)

Troubleshooting – Solution
ASA(config)# clear conf nat
ASA(config)# nat (dmz,outside) source static obj-172.16.1.2 obj-194.1.1.254
ASA(config)# nat (dmz,outside) source static obj-172.16.1.0 obj-172.16.1.0 destination
ASA(config)# nat (dmz,outside) source dynamic obj-172.16.1.0 interface
ASA(config)# exit
ASA#
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
Clearing and re-entering configuration resolves
the problem (see next slide)…

Troubleshooting – Solution Verification
Input Table
hits=0, user_data=0x73a1e480, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
in id=0x73982f98, priority=6, domain=nat, deny=false
hits=0, user_data=0x73a1e8f8, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
in id=0x73a1fbd8, priority=6, domain=nat, deny=false
hits=0, user_data=0x73a1e9a0, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
in id=0x739696a0, priority=6, domain=nat, deny=false
nat (dmz,outside) source static obj-172.16.1.0 obj-172.16.1.0 destination static RemoteOfficeNet RemoteOfficeNet-
2
3
2
1
2
Order is now correct!
1
2
3

Troubleshooting – Solution Verification
ASA# packet-tracer input dmz tcp 172.16.1.2 1234 207.1.1.2 80 detail
Phase: 1
Type: ROUTE-LOOKUP
Subtype: input
Result: ALLOW
Config:
in 0.0.0.0 0.0.0.0 outside
…
Phase: 3
Type: NAT
Subtype:
Result: ALLOW
Config:
Static translate 172.16.1.2/1234 to 194.1.1.254/1234
172.16.1.0/2
4
to 194.1.1.254
172.16.1.2
10.2.2.0/24
Now the correct rule is hit and static NAT works as
expected for traffic DMZ:172.16.1.2 -> Internet
1

Troubleshooting Scenario #2

Remote office server 10.2.2.2 needs to talk to inside server 10.1.1.2, but 10.1.1.2 has default route via
some internal router 10.1.1.x and cannot have routes through the ASA due to security reasons.
So, the decision was made to NAT all incoming requests, coming from 10.2.2.2, to 10.1.1.254.
Other hosts in 10.1.1.0/24 and 10.2.2.0/24 should be able to communicate without NAT.
ASA
Internet
inside outside
dmz
IPSec tunnelIPSec tunnel
Remote Office
10.2.2.0/24
Destination IP is
NATed to 10.1.1.254Corporate Network
10.1.1.0/24
.2
.2

vlan 99
nameif inside
security-level 100
ip address 10.1.1.1 255.255.255.0
vlan 98
nameif dmz
security-level 50
ip address 172.16.1.1 255.255.255.0
nameif outside
security-level 0
ip address 194.1.1.1 255.255.255.0
route outside 0.0.0.0 0.0.0.0 194.1.1.2 1

ASA NAT Configuration – 8.3+
subnet 10.1.1.0 255.255.255.0
host 10.1.1.254
subnet 10.2.2.0 255.255.255.0
object network RemoteServer
host 10.2.2.2
nat (outside,inside) source static RemoteServer obj-10.1.1.254
nat (inside,outside) source static obj-10.1.1.0 obj-10.1.1.0 destination
static RemoteOfficeNet RemoteOfficeNet
nat (inside,outside) source dynamic any interface
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2

Customer Symptom
ASA# show conn long
TCP outside:10.2.2.2/31444 (10.2.2.2/31444) inside:10.1.1.2/80 (10.1.1.2/80),
flags SaAB, idle 6s, uptime 12s, timeout 30s, bytes 0
Remote office server 10.2.2.2 cannot
access inside server 10.1.1.2.
Connection is created, but 10.2.2.2 is
not NATed to 10.1.1.254
%ASA-6-302013: Built inbound TCP connection 198 for outside:10.2.2.2/31444
(10.2.2.2/31444) to inside:10.1.1.2/80 (10.1.1.2/80)
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2

ASA# debug nat 255
nat: untranslation - outside:10.1.1.2/80 to inside:10.1.1.2/80
1 (outside) to (inside) source static RemoteServer obj-10.1.1.254
2 (inside) to (outside) source static obj-10.1.1.0 obj-10.1.1.0 destination static
RemoteOfficeNet RemoteOfficeNet
3 (inside) to (outside) source dynamic any interface
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
It seems that first NAT rule is not hit

ASA# packet-tracer input outside tcp 10.2.2.2 1234 10.1.1.2 80 detail
Phase: 1
Type: UN-NAT
Subtype: static
Result: ALLOW
Config:
nat (inside,outside) source static obj-10.1.1.0 obj-10.1.1.0 destination static
NAT divert to egress interface inside
Untranslate 10.1.1.2/80 to 10.1.1.2/80
…
Phase: 4
Type: NAT
Subtype:
Result: ALLOW
Config:
…
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
UN-NAT chose another rule at step #1,
so static NAT rule was ignored and
NAT at step #4 wasn't performed

ASA# show nat divert-table
Divert Table
id=0x73a92fc0, domain=divert-route
type=static, hits=0, flags=0x1, protocol=0
src ip/id=0.0.0.0, mask=0.0.0.0 port=0-0
dst ip/id=10.1.1.254, mask=255.255.255.255 port=0-0
input_ifc=inside, output_ifc=outside
id=0x73a944a0, domain=divert-route
input_ifc=outside, output_ifc=inside
ASA# show run nat
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
There is only one rule for outside interface in “NAT divert table”.
This rule was created from NAT rule #2 and blocked execution of NAT rule #1…

Troubleshooting – Workaround
subnet 10.1.1.0 255.255.255.0
host 10.1.1.254
subnet 10.2.2.0 255.255.255.0
object network RemoteServer
host 10.2.2.2
object network LocalServer
host 10.1.1.2
object network LocalServer-2
host 10.1.1.2
nat (outside,inside) source static RemoteServer obj-10.1.1.254 destination static
LocalServer LocalServer-2
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
The root cause of the problem is that “NAT divert table”
is not populated with an entry for 1st NAT rule.
So, we can try to reconfigure 1st NAT rule and correct
entry will be installed into the “NAT divert table”.
Note that it is necessary to use two different object names here: LocalServer and
LocalServer-2, otherwise “NAT divert” rule will not be installed

Troubleshooting – Verification
Divert Table
id=0x739f6ea8, domain=divert-route
id=0x73a92fc0, domain=divert-route
id=0x72f21768, domain=divert-route
ASA# show run nat
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2

ASA# packet-tracer input outside tcp 10.2.2.2 1234 10.1.1.2 80 detail
Phase: 1
Type: UN-NAT
Subtype: static
Result: ALLOW
Config:
Untranslate 10.1.1.2/80 to 10.1.1.2/80
…
Phase: 4
Type: NAT
Subtype:
Result: ALLOW
Config:
in id=0x6dec91a0, priority=6, domain=nat, deny=false
hits=1, user_data=0x739f6ea8, cs_id=0x0, use_real_addr, flags=0x0, protocol=0
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
Correct rule is hit at both UN-NAT and
NAT steps

1 (outside) to (inside) source static RemoteServer obj-10.1.1.254 destination static
2 (inside) to (outside) source static obj-10.1.1.0 obj-10.1.1.0 destination static
3 (inside) to (outside) source dynamic any interface
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
NAT steps

Troubleshooting – Permanent Fix
ASA# show run nat
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
Permanent fix is available in 8.4(4.2)
CSCtq47028 ASA: Manual NAT rules
are not processed in order
The CSCtq47028 fix made above workaround unnecessary. NAT rules are installed
automatically into both “NAT divert” and NAT tables. This is a huge change in NAT
implementation!
1

Input Table
in id=0x6decad08, priority=6, domain=nat, deny=false
hits=0, user_data=0x6deca528, cs_id=0x0, flags=0x0, protocol=0
in id=0x6decaf80, priority=6, domain=nat, deny=false
hits=0, user_data=0x6deca5d0, cs_id=0x0, flags=0x0, protocol=0
in id=0x739c64f8, priority=6, domain=nat, deny=false
hits=0, user_data=0x739c0378, cs_id=0x0, flags=0x0, protocol=0
in id=0x739c6770, priority=6, domain=nat, deny=false
hits=0, user_data=0x6decd9f8, cs_id=0x0, flags=0x0, protocol=0
in id=0x6decc5f0, priority=6, domain=nat, deny=false
hits=0, user_data=0x7307e530, cs_id=0x0, flags=0x0, protocol=0
in id=0x6decc9d8, priority=6, domain=nat, deny=false
hits=0, user_data=0x7307e3e0, cs_id=0x0, flags=0x0, protocol=0
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
1
Rule (1) in ASP NAT table
is used to classify traffic,
coming from 10.2.2.2, and
translate source IP from
10.2.2.2 to 10.1.1.254

Divert Table
id=0x6deca528, domain=divert-route
id=0x6deca5d0, domain=divert-route
id=0x739c0378, domain=divert-route
id=0x6decd9f8, domain=divert-route
id=0x7307e530, domain=divert-route
type=dynamic, hits=0, flags=0x1, protocol=0
id=0x7307e3e0, domain=divert-route
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
1
Rule (1) in ASP “NAT
divert” table is used to
classify traffic, coming
from 10.2.2.2, and
translate destination IP
from a.b.c.d (i.e. any IP)
to itself

ASA# packet-tracer input outside tcp 10.2.2.2 1234 10.1.1.2 80 detailed
…
Phase: 2
Type: UN-NAT
Subtype: static
Result: ALLOW
Config:
Untranslate 10.1.1.2/80 to 10.1.1.2/80
…
Phase: 5
Type: NAT
Subtype:
Result: ALLOW
Config:
in id=0x6decad08, priority=6, domain=nat, deny=false
hits=1, user_data=0x6deca528, cs_id=0x0, flags=0x0, protocol=0
10.1.1.0/24
.2
10.2.2.0/24
IPSec
Internet
.2
NAT steps

CSCtq47028 Fix – More Info
 In many customer cases problems were seen when
asymmetric or overlapping NAT rules were configured
 Definition of “asymmetric”
• If an outbound packet matches a specific NAT rule and the return packet
matches a different NAT rule in the table, then they are called asymmetric
NAT rules. Most common with usage of 'dynamic' or 'unidirectional' NAT.
 Definition of “overlapping”
• If two or more NAT rules matches both source and destination (ports
included) in the table, then they are called overlapping rules. This also
involves usage of 'any' keyword as source or destination network.
 In this case Twice NAT rules may not be processed in
order

CSCtq47028 Fix – More Info
 This behavior is fixed by CSCtq47028 in 8.4(4.2)
 For a Twice NAT rule, if the destination is not explicitly
specified, ASA implicitly adds “destination static any
any”
 This populates “NAT divert” table with required rules
and NAT rules order is strictly enforced
This change may affect existing configurations!

Final Recommendations

 KISS: Keep It Simple, Stupid
 Use Object NAT whenever possible, use Twice NAT
when Policy NAT is really needed
 Design your network carefully, don’t use NAT to
workaround routing problems
 Don’t use NAT for policy control, use ACLs instead
 Remember that ASA is not a IOS router; learn how
NAT and routing interoperate on this platform

 Always issue “no nat-control” prior to upgrading to 8.3+
 Don’t upgrade to 8.3 on a Friday night just as you are
getting ready to go out of town for the weekend
 Test upgrade in a lab first (if you have one)
 Read documentation and Cisco Support Community
documents with ‘ASA’ and ‘nat’ tags
 Call in to the TAC in case of a trouble

FIN, ACK [ Thank you ! ]

Опрос #3
Какие темы семинаров по безопасности
Вам интересны?
1. Построение Remote Access VPN с помощью ASA
2. Построение Site-to-Site VPN на маршрутизаторах
3. DMVPN и GETVPN на маршрутизаторах Cisco
4. Использование платформы ASR1k для VPN
5. Любые вопросы использования Cisco ASA в
качестве межсетевого экрана
6. IOS Zone-based Firewall
7. Система обнаружения атак IPS 4200
8. Продукты IronPort

Q & A
Эксперт ответит на некоторые Ваши вопросы. Используйте
Q&A панель, чтобы задать еще вопросы

Вторник, 9 октября
10:00 по московскому времени
Эксперт Cisco:
Amit Singh
Расскажет об архитектуре лицензирования Cisco
Unified Call Manager (CUCM) версии 9.0х.
Регистрация:
http://tools.cisco.com/gems/cust/customerSite.do?METHOD=
Тема: Licensing Architecture: Cisco Unified Call
Manager Version 9.x

Вторник, 30 октября
18:00 по московскому времени
Эксперт Cisco:
Jazib Frahim
Расскажет о том как решать проблемы с SSL VPN на
Cisco Adaptive Security Appliance (ASA)
Регистрация
https://supportforums.cisco.com/community/netpro/expert-corner
Тема: Troubleshooting SSL VPN on ASA

Приглашаем Вас активно участвовать в Cisco
Support Community и социальных сетях
https://supportforms.cisco.com/community/russian
http://www.facebook.com/CiscoRu
http://twitter.com/CiscoRussia
http://www.youtube.com/user/CiscoRussiaMedia
http://itunes.apple.com/us/app/cisco-technical-support/id398104252?mt=8
http://www.linkedin.com/groups/CSC-Cisco-Support-Community-3210019
Newsletter Subscription: https://tools.cisco.com/gdrp/coiga/showsurvey.do?
surveyCode=589&keyCode=146298_2&PHYSICAL%20FULFILLMENT
%20Y/N=NO&SUBSCRIPTION%20CENTER=YES

Спасибо за
Ваше время
Пожалуйста, участвуйте в опросе

Understanding and Troubleshooting ASA NAT

Understanding and Troubleshooting ASA NAT

More Related Content

Understanding and Troubleshooting ASA NAT

Editor's Notes