Cisco ASA double NAT with DNS translation

ciscocisco-asadnsfirewallnat;

I'm trying to set up double Auto NAT with DNS translation on Cisco ASA 9.0(3), and I'm having a few challenges with the DNS portion. I got double NAT working correctly, such that I have a server in production and in the lab with the same IP address. See b2masd1, nameif INSIDE (production) and masd1, nameif DMZ (lab).

When you ping from DMZ 10.195.18.182 to 1.195.18.182, I see the translations happening correctly in both directions…

D:10.195.18.182      S:192.168.11.101       D:1.195.18.182   S:10.195.18.182
             <-----------                         <-----------
                                           1) echo-request to 1.195.18.182
                                           nat (INSIDE,DMZ) static 1.195.18.182 dns



  S:10.195.18.182    D:192.168.11.101       S:1.195.18.182   D:10.195.18.182
              ------------>                        ------------>
      2) echo-reply to 192.168.11.101
      nat (DMZ,INSIDE) static 192.168.11.101 dns


 b2masd1                   +-----------+              masd1
 10.195.18.182      INSIDE |           | DMZ          10.195.18.182
 Mfg Server   -------------| Cisco ASA |------------  Devel Server
                           |           |
                           +-----------+

 Manufacturing                                        Development
 Network                                              Network
 Security: 100                                        Security: 50

This is what I see on masd1…

masd1$ /usr/sbin/ping 1.195.18.182
PING 1.195.18.182: 64 byte packets
64 bytes from 1.195.18.182: icmp_seq=0. time=0. ms
64 bytes from 1.195.18.182: icmp_seq=1. time=0. ms

----1.195.18.182 PING Statistics----
2 packets transmitted, 2 packets received, 0% packet loss
round-trip (ms)  min/avg/max = 0/0/0
masd1$

The problem is that DNS queries from the DMZ to INSIDE are not getting translated. When I query for b2masd1 from the DMZ, I expect the nameserver on the INSIDE to respond with 10.195.18.182, and then the ASA should translate that to 1.195.18.182. However, that is not happening; as you can see, the DNS entry is not translated.

masd1$ nslookup
Using /etc/hosts on:  masd1

> a2mosd1
Using /etc/hosts on:  masd1

looking up FILES
Trying DNS
Name:    b2masd1.domain.local
Address:  10.195.18.182

> exit
masd1$

Can anyone explain what I need to do to get DNS queries correctly translated? I need a query for b2masd1 in the DMZ to a nameserver on the INSIDE interface to return 1.195.18.182 (because the ASA translates the INSIDE A-Record 10.195.18.182 to DMZ addr 1.195.18.182).

I have set up a chat room to assist with diagnosis

Misc debugging information

This is my config…

!
interface GigabitEthernet0/0
 nameif INSIDE
 security-level 100
 ip address 10.195.2.197 255.255.255.248 standby 10.195.2.198
!
interface GigabitEthernet0/1
 nameif DMZ
 security-level 50
 ip address 10.195.2.201 255.255.255.248 standby 10.195.2.202
!
object network DMZ_NAT_masd1
 host 10.195.18.182
 description xlate masd1 NAT DMZ src 10.195.18.182 to INSIDE src 192.168.11.101
object network INSIDE_NAT_masd1
 host 10.195.18.182
 description xlate masd1 NAT INSIDE src 10.195.18.182 to DMZ src 1.195.18.182
!
object network DMZ_NAT_masd1
 nat (DMZ,INSIDE) static 192.168.11.101 dns
object network INSIDE_NAT_masd1
 nat (INSIDE,DMZ) static 1.195.18.182 dns
!
policy-map type inspect dns DNS_INSPECT_MAP
 parameters
  message-length maximum 512
!
policy-map global_policy
 class inspection_default
  inspect dns DNS_INSPECT_MAP
!
service-policy global_policy global

Show xlate, in case this helps…

B2-DEV-FW1/DEVELOPMENT# sh xlate local 10.195.18.182
121 in use, 126 most used
Flags: D - DNS, e - extended, I - identity, i - dynamic, r - portmap,
       s - static, T - twice, N - net-to-net
NAT from DMZ:10.195.18.182 to INSIDE:192.168.11.101
    flags sD idle 0:00:01 timeout 0:00:00
NAT from INSIDE:10.195.18.182 to DMZ:1.195.18.182
    flags sD idle 0:03:55 timeout 0:00:00
B2-DEV-FW1/DEVELOPMENT#

Show service-policy inspect dns…

B2-DEV-FW1/DEVELOPMENT# sh service-policy inspect dns

Global policy:
  Service-policy: global_policy
    Class-map: inspection_default
      Inspect: dns DNS_INSPECT_MAP, packet 15302, drop 0, reset-drop 0, v6-fail-close 0
        message-length maximum 512, drop 0
        dns-guard, count 7649
        protocol-enforcement, drop 0
        nat-rewrite, count 139
B2-DEV-FW1/DEVELOPMENT#

Captures showing queries from b2masd1 to the name server (10.195.18.201). NOTE the double DNS queries sent on the INSIDE interface, but we don't seem to get those on the DMZ interface.

B2-DEV-FW1/DEVELOPMENT# capture FOO interface DMZ real-time match udp host 10.195.18.182 host 10.195.18.201

Warning: using this option with a slow console connection may
         result in an excessive amount of non-displayed packets
         due to performance limitations.

Use ctrl-c to terminate real-time capture


   1: 09:54:35.994730       10.195.18.182.52639 > 10.195.18.201.53:  udp 45
   2: 09:54:35.995218       10.195.18.201.53 > 10.195.18.182.52639:  udp 83
   3: 09:54:47.875076       10.195.18.182.52644 > 10.195.18.201.53:  udp 53
   4: 09:54:47.875549       10.195.18.201.53 > 10.195.18.182.52644:  udp 136
   5: 09:54:47.875854       10.195.18.182.52645 > 10.195.18.201.53:  udp 51
   6: 09:54:47.876297       10.195.18.201.53 > 10.195.18.182.52645:  udp 138
   7: 09:54:47.876648       10.195.18.182.52646 > 10.195.18.201.53:  udp 35
   8: 09:54:47.877075       10.195.18.201.53 > 10.195.18.182.52646:  udp 35

B2-DEV-FW1/DEVELOPMENT# capture FOO interface INSIDE real-time match udp host 192.168.11.101 host 10.195.18.201

Warning: using this option with a slow console connection may
         result in an excessive amount of non-displayed packets
         due to performance limitations.

Use ctrl-c to terminate real-time capture


   1: 09:56:27.282608       10.195.18.182.52742 > 10.195.18.201.53:  udp 43
   2: 09:56:27.282684       192.168.11.101.52742 > 10.195.18.201.53:  udp 43
   3: 09:56:27.283081       10.195.18.201.53 > 192.168.11.101.52742:  udp 59
   4: 09:56:27.283096       10.195.18.201.53 > 10.195.18.182.52742:  udp 59

Best Answer

Answering my own question to help future googlers. I spent about 3 hours on the phone with TAC; we finally got to the root cause of the issue.

The solution is to add a special NAT entry, which matches the IP address in the DNS A-Record when it arrives on the INSIDE interface.

object network DNS_NAT_masd1
 description xlate A-Record DMZ src 1.195.18.182 to INSIDE src 10.195.18.182
 host 1.195.18.182
 nat (DMZ,INSIDE) static 10.195.18.182

When I asked for a pointer to documentation that describes why DNS translation works this way, the TAC lead said that he didn't know of any that described this behavior. The TAC lead also mentioned that with more code, the ASA would know to automatically translate the DNS A-Record without explicitly adding object network DNS_NAT_masd1; however, that is not how the dns keyword for ASA NAT works today. For reasons that still are not completely clear yet, the ASA requires the DNS A-Record IP to match the <proxy_addr> in the NAT statement, using syntax similar to this...

object network obj-EXAMPLE
 description NAT object explicitly for translating DNS A-Records
 host <proxy_addr>
 nat (<REAL_INTF>,<PROXY_INTF>) static <real_addr> dns

The difficulty is that this configuration is exactly backwards for what you need to do if you're going to nat regular "data plane" IP traffic through the firewall.

This is the whole configuration that works...

object network DMZ_NAT_masd1
 host 10.195.18.182
 description xlate masd1 NAT DMZ src 10.195.18.182 to INSIDE src 192.168.11.101
object network INSIDE_NAT_masd1
 host 10.195.18.182
 description xlate masd1 NAT INSIDE src 10.195.18.182 to DMZ src 1.195.18.182
!!! DNS_NAT_masd1 is new
object network DNS_NAT_masd1
 host 1.195.18.182
 description xlate A-Record DMZ src 1.195.18.182 to INSIDE src 10.195.18.182
!
object network DMZ_NAT_masd1
 nat (DMZ,INSIDE) static 192.168.11.101
object network INSIDE_NAT_masd1
 nat (INSIDE,DMZ) static 1.195.18.182
!!! DNS_NAT_masd1 is new
object network DNS_NAT_masd1
 nat (DMZ,INSIDE) static 10.195.18.182 dns

Your ASA configuration:

route outside 0.0.0.0 0.0.0.0 172.16.1.1 1

This will provide a default route via the outside interface, but how will the ASA know how to reach subnets residing behind the Layer 3 Distribution Switch?

You'll need to add routes to the internal subnets via the inside interface using the Layer 3 Distribution Switch as the next-hop IP address.

ASA static routing example:

route inside 172.19.12.0 255.255.255.240 172.16.2.2
route inside 172.19.3.0 255.255.255.0 172.16.2.2
route inside 172.20.100.0 255.255.255.224 172.16.2.2

Your Cisco Router's configuration:

ip route 0.0.0.0 0.0.0.0 200.200.200.200

Additionally, how will your border router know how to reach subnets other than it's connected routes, and the catch all default route via the outside interface's next-hop address 200.200.200.200?

Router static routing example:

ip route 172.19.12.0 255.255.255.240 172.16.1.10
ip route 172.19.3.0 255.255.255.0 172.16.1.10
ip route 172.19.100.0 255.255.255.224 172.16.1.10
ip route 172.16.2.0 255.255.255.224 172.16.1.10

Vlan – ASA 5505 Assign Outbound IP to VLAN

This is an old question, but I recently ran into this same issue, and after some trial and error, I was able to come up with a solution. The following applies to ASA Version 9.1. I have two VLANs on our network: one for PCs, and one for VoIP. I wanted the VoIP VLAN to use its own outside address, separate from the outside address used by the PCs. Here's what the configuration ended up looking like (IP addresses have been obfuscated slightly). These are only the pertinent parts of the configuration, not a complete dump:

: ASA Version 9.1(2) 

interface Ethernet0/0
!
interface Ethernet0/1
 switchport access vlan 2
!
interface Ethernet0/6
 switchport access vlan 50
!
interface Vlan1
  nameif outside
  security-level 0
  ip address 65.65.65.10 255.255.255.192 
!
interface Vlan2
 nameif net-inside
 security-level 100
 ip address 10.1.10.1 255.255.248.0 
!
interface Vlan50
 nameif net-voip
 security-level 100
 ip address 172.16.0.1 255.255.255.0 
!
object network net-voip
 subnet 172.16.0.0 255.255.255.0
 nat (net-voip,outside) dynamic 65.65.65.20
!
nat (net-inside,outside) after-auto source dynamic any interface
!
dhcpd ping_timeout 750
dhcpd auto_config outside
dhcpd address 172.16.0.100-172.16.0.127 net-voip
dhcpd dns 8.8.8.8 interface net-voip
dhcpd enable net-voip

I also included the part of my configuration which sets up DHCP on the VoIP VLAN for the phones. One snag I ran into (which should've been obvious in retrospect), the net-voip VLAN needs to have a security level higher than the outside VLAN, otherwise it won't be allowed out.

Best Answer

Related Solutions

Cisco ASA Routing Issue

Your ASA configuration:

ASA static routing example:

Your Cisco Router's configuration:

Router static routing example:

Vlan – ASA 5505 Assign Outbound IP to VLAN

Related Topic