Deny udp on trying to access internal DNS Servers from outside

Unanswered Question
Nov 26th, 2008

Hi,

I'm trying to setup public DNS Servers in a DMZ using an ASA 5505. The DNS server is also a web host and I can access the web server via port 80 using the static ip.

The issue is that UDP dns requests are dropped:

Deny udp src outside:68.87.71.227/23665 dst Hosting:CommDns1Mail1/53 by access-group "outside_access_in"

the packet trace indicates a inspect-dns-invalid-pak error

Can anyone tell what is wrong with my configuration?

Thanks in for you help!

April

ASA Version 7.2(2)

interface Vlan1

nameif inside

security-level 100

interface Vlan2

nameif outside

security-level 0

interface Vlan22

nameif Hosting

security-level 50

ftp mode passive

dns server-group DefaultDNS

domain-name zzzzzz.com

same-security-traffic permit intra-interface

access-list outside_access_in extended permit tcp any eq domain host CommDns1Mail1 eq domain

access-list outside_access_in extended permit udp any eq domain host CommDns1Mail1 eq domain

access-list outside_access_in extended permit tcp any host CommDns1Mail1 eq www

access-list outside_access_in extended permit tcp any eq imap4 host CommDns1Mail1 eq imap4

access-list outside_access_in extended permit tcp any eq pop3 host CommDns1Mail1 eq pop3

access-list outside_access_in extended permit tcp any eq smtp host CommDns1Mail1 eq smtp

access-list outside_access_in extended permit tcp host CommDns1Mail1 eq www any eq www

mtu inside 1500

mtu outside 1500

mtu dmz 1500

mtu Hosting 1500

icmp unreachable rate-limit 1 burst-size 1

arp timeout 14400

nat-control

global (outside) 1 interface

global (outside) 3 CommDns1Mail1 netmask 255.255.255.255

nat (Hosting) 3 DWDEV01 255.255.255.255

static (Hosting,outside) tcp CommDns1Mail1 www DWDEV01 www netmask 255.255.255.255

static (Hosting,outside) tcp CommDns1Mail1 smtp DWDEV01 smtp netmask 255.255.255.255

static (Hosting,outside) tcp CommDns1Mail1 domain DWDEV01 domain netmask 255.255.255.255

static (Hosting,outside) udp CommDns1Mail1 domain DWDEV01 domain netmask 255.255.255.255 dns

static (Hosting,inside) CommDns1Mail1 DWDEV01 netmask 255.255.255.255

static (inside,Hosting) 192.1.1.0 192.1.1.0 netmask 255.255.255.0

access-group outside_access_in in interface outside

route outside 0.0.0.0 0.0.0.0 10.1.10.1 1

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 192.1.1.0 255.255.255.0 inside

no snmp-server location

no snmp-server contact

snmp-server enable traps snmp telnet timeout 5

ssh timeout 5

console timeout 0

!

class-map inspection_default

match default-inspection-traffic

!

policy-map type inspect dns preset_dns_map

parameters

no dns-guard

no protocol-enforcement

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 esmtp

inspect sqlnet

inspect skinny

inspect sunrpc

inspect xdmcp

inspect sip

inspect netbios

inspect tftp

!

service-policy global_policy global

prompt hostname context

I have this problem too.
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Collin Clark Wed, 11/26/2008 - 10:07

In your ACL you're stating the source port should be UDP 53 which for lookups it's random. For zone transfers it is static ports. Here's the line in the ACL-

access-list outside_access_in extended permit udp any eq domain host CommDns1Mail1 eq domain

Change or add to look like this-

access-list outside_access_in extended permit udp any host CommDns1Mail1 eq domain

Hope that helps.

dalyWebInc Wed, 11/26/2008 - 10:14

Hi,

That seemed to work! Wow, Thanks! (i've been struggling so much to try to understand this stuff - it is really another world!)

Thanks for your help and have a happy Thanksgiving!

Cheers,

April

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