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Nameserver lookups from DMZ issue

Posted on 2008-06-11
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Last Modified: 2008-06-20
I am having an issue that I can't seem to figure out with my Pix 515e, mostly because my lack of knowledge with the PIX.  

The web server in our DMZ can ping outside IP addresses, but when you try to do a nslookup, it times out trying to access a DNS server.  I'm testing external DNS using 208.67.222.222.  When I look at the output logs, there are no errors.  It says: Built outbound UDP connection 94067467 for Outside:208.67.222.222/53 (208.67.222.222/53) to DMZ:172.16.10.2/4681 (xxx.xxx.149.54/4681), it shows several lines of this, but on the server, the names are never resolved.

I didn't want to point the DNS to my internal network because I had security concerns doing that.   I'm posting part of my config for the experts to look at.  Feel free to comment on any other mistakes I have made.

PIX Version 7.2(1)19
!
hostname pixfirewall
domain-name hostname
enable password xxx encrypted
names
!
interface Ethernet0
 description Outside Interface
 nameif Outside
 security-level 0
 ip address xxx.xxx.149.2 255.255.255.0
 ospf cost 10
!
interface Ethernet1
 description Inside Interface
 nameif inside
 security-level 100
 ip address 172.16.11.1 255.255.255.0
 ospf cost 10
!
interface Ethernet2
 description DMZ Interface
 nameif DMZ
 security-level 30
 ip address 172.16.10.1 255.255.255.0
 ospf cost 10
!
interface Ethernet3
 shutdown
 no nameif
 no security-level
 no ip address
!
interface Ethernet4
 shutdown
 no nameif
 no security-level
 no ip address
!
interface Ethernet5
 shutdown
 no nameif
 no security-level
 no ip address
!
ftp mode passive
clock timezone CST -6
clock summer-time CDT recurring
dns domain-lookup inside
dns server-group DefaultDNS
 name-server 10.100.0.110
 name-server 10.100.5.15
 domain-name hostname
access-list outbound extended permit icmp host 10.100.1.5 any
access-list outbound extended permit udp host 10.100.1.5 any
access-list outbound extended permit tcp host 10.100.1.5 any
access-list outbound extended permit icmp any any
access-list outbound extended permit icmp host 10.100.1.110 any
access-list outbound extended permit tcp host 10.100.7.245 any eq www
access-list outbound extended permit tcp host 10.100.0.35 any
access-list outbound extended permit tcp host 10.100.0.34 any

access-list outbound extended permit udp host 10.100.0.34 any eq domain
access-list outbound extended permit tcp host 10.100.0.33 any
access-list outbound extended permit tcp host 10.100.3.205 any
access-list outbound extended permit tcp host 10.100.3.206 any

access-list outbound extended permit udp host 10.100.3.205 any eq domain
access-list outbound remark Domain
access-list outbound extended permit udp host 10.100.3.206 any eq domain
access-list outbound extended permit tcp 10.100.0.0 255.255.0.0 172.16.10.0 255.255.255.0 eq www

access-list outbound extended permit tcp 10.100.0.0 255.255.0.0 172.16.10.0 255.255.255.0 eq 5900
access-list inbound extended permit icmp any any
access-list inbound extended permit tcp any host xxx.xxx.149.52 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.35 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.53 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.18 eq https
access-list inbound extended permit tcp any host xxx.xxx.149.30 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.31 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.31 eq https
access-list inbound extended deny tcp any host xxx.xxx.149.30 eq 5900

access-list inbound extended permit tcp any host xxx.xxx.149.16 eq domain
access-list inbound extended permit udp any host xxx.xxx.149.16 eq domain
access-list inbound extended permit tcp any host xxx.xxx.149.16 eq smtp
access-list inbound extended permit tcp any host xxx.xxx.149.16 eq 4443
access-list inbound extended permit tcp any host xxx.xxx.149.16 eq 995
access-list inbound extended permit tcp any host xxx.xxx.149.10 eq domain
access-list inbound extended permit udp any host xxx.xxx.149.10 eq domain
access-list inbound extended permit udp any eq domain host xxx.xxx.149.16 gt 1023
access-list inbound extended permit udp any eq domain host xxx.xxx.149.10 gt 1023
access-list inbound extended permit udp any eq domain host xxx.xxx.149.18 gt 1023
access-list inbound extended permit tcp any host xxx.xxx.149.54 eq www
access-list inbound extended permit tcp any host xxx.xxx.149.54 eq https
access-list dmz extended permit tcp 172.16.10.0 255.255.255.0 any eq https
access-list dmz extended permit tcp 172.16.10.0 255.255.255.0 any eq www
access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any
access-list dmz extended permit tcp 172.16.10.0 255.255.255.0 any eq domain
access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any eq domain
access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any gt 1023
access-list dmz extended permit icmp 172.16.10.0 255.255.255.0 any
access-list inside_nat0_outbound extended permit ip 10.100.0.0 255.255.0.0 172.16.12.0 255.255.255.0
access-list inside_nat0_outbound extended permit ip 10.100.0.0 255.255.0.0 172.16.12.32 255.255.255.248
access-list inside_nat0_outbound extended permit ip 192.168.35.88 255.255.255.248 172.22.12.0 255.255.255.0
access-list Local_Lan_Access standard permit 10.100.0.0 255.255.0.0
mtu Outside 1500
mtu inside 1500
mtu DMZ 1500
no failover
asdm image flash:/asdm-521.bin
no asdm history enable
arp timeout 14400
nat-control
global (Outside) 1 xxx.xxx.149.50 netmask 255.255.255.0
nat (inside) 0 access-list inside_nat0_outbound
nat (inside) 1 0.0.0.0 0.0.0.0
static (inside,Outside) xxx.xxx.149.35 10.100.0.250 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.52 10.100.7.245 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.18 10.100.0.34 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.30 10.100.1.205 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.16 10.100.3.205 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.10 10.100.3.206 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.149.31 10.100.1.5 netmask 255.255.255.255
static (inside,Outside) xxx.xxx.35.89 10.100.0.239 netmask 255.255.255.255 //Site to Site VPN
static (DMZ,Outside) xxx.xxx.149.54 172.16.10.2 netmask 255.255.255.255
static (inside,DMZ) 10.100.0.0 10.100.0.0 netmask 255.255.0.0
access-group inbound in interface Outside
access-group outbound in interface inside
access-group dmz in interface DMZ
route Outside 0.0.0.0 0.0.0.0 xxx.xxx.149.1 1
route inside 10.100.0.0 255.255.0.0 172.16.11.2 1
route inside 10.201.0.0 255.255.0.0 172.16.11.2 1
!
router rip
 network 10.0.0.0
!
timeout xlate 3:00:00
timeout conn 1:00:00 half-closed 0:10:00 udp 0:02:00 icmp 0:00:02
timeout sunrpc 0:10:00 h323 0:05:00 h225 1:00:00 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 uauth 0:05:00 absolute
http server enable
http 172.16.11.0 255.255.255.0 inside
http 10.100.0.0 255.255.0.0 inside
no snmp-server location
no snmp-server contact
snmp-server enable traps snmp authentication linkup linkdown coldstart
telnet timeout 5
ssh 10.100.3.240 255.255.255.255 inside
ssh 172.16.11.245 255.255.255.255 inside
ssh 10.100.7.245 255.255.255.255 inside
ssh 10.100.1.25 255.255.255.255 inside
ssh timeout 30
console timeout 0
!
class-map inspection_default
 match default-inspection-traffic
!
!
policy-map type inspect dns preset_dns_map
 parameters
  message-length maximum 512
policy-map global_policy
 class inspection_default
  inspect dns preset_dns_map
  inspect ftp
  inspect h323 h225
  inspect h323 ras
  inspect rsh
  inspect rtsp
  inspect sqlnet
  inspect skinny
  inspect sunrpc
  inspect xdmcp
  inspect sip
  inspect netbios
  inspect tftp
  inspect pptp
!
service-policy global_policy global
ntp server 10.100.0.110 source inside prefer
smtp-server 10.100.1.5
prompt hostname context
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Question by:zlajoie
  • 2
3 Comments
 
LVL 15

Accepted Solution

by:
Voltz-dk earned 500 total points
Comment Utility
You haven't added NAT on the DMZ, so it will go out with a private address.  Try this:

nat (dmz) 1 0 0
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LVL 15

Expert Comment

by:Voltz-dk
Comment Utility
As for the comment on config part..  You have several shadowing rules, like:

access-list outbound extended permit icmp host 10.100.1.5 any
access-list outbound extended permit icmp any any
access-list outbound extended permit icmp host 10.100.1.110 any

And again:

access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any
access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any eq domain
access-list dmz extended permit udp 172.16.10.0 255.255.255.0 any gt 1023

If I just look at the dmz part, only the first of those 3 rules can ever be used.  If you really NEED complete UDP access, you should just delete the others.  But more likely you should specify what it is that you need.

Even worse, there is a security issue in your general dmz access.  When you use ANY as destination for access-rules in dmz, it's usually done becuz you must be able to access all of the Internet.  But in what you have made, you have also granted full access to the inside network (for the protocols in question).

You should restructure your DMZ access into these 3 parts:

grant specific access to inside as needed
block all other access to inside
Now open for any destinations to the protocols you generally need for Internet

Example:

obj net Inside-Nets
net 172.16.11.0 255.255.255.0
net 10.100.0.0 255.255.0.0
net 10.201.0.0 255.255.0.0
exit
access-list dmz permit tcp host <ip of mailscanner> host <inside mailhost> eq 25
access-list dmz deny ip any obj Inside-Nets
access-list dmz permit tcp 172.16.10.0 255.255.255.0 any eq 80
access-list dmz permit tcp 172.16.10.0 255.255.255.0 any eq 443

(Of cuz I don't know what kinda of access you NEED from dmz to inside)
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Author Comment

by:zlajoie
Comment Utility
Sorry to take so long getting back to this.  It was an issue with our nat and we cleaned up our config based on your suggestions.  Thank you for helping me with this.
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