Contents

Introduction

Prerequisites

Requirements

Components Used

Conventions

Configure

Network Diagram

ASA Configuration

ESMTP TLS Configuration

Verify

Troubleshoot

Troubleshooting Commands

Related Information

Introduction

This sample configuration demonstrates how to set up the ASA Security Appliance for access to a Simple Mail Transfer Protocol (SMTP) server located on the Demilitarized Zone (DMZ) network.

Refer to ASA 8.3 and Later: Mail (SMTP) Server Access on Inside Network Configuration Example for more information on how to set up the ASA Security Appliance for access to a mail/SMTP server located on the Inside network.

Refer to ASA 8.3 and Later: Mail (SMTP) Server Access on Outside Network Configuration Example for more information on how to set up the ASA Security Appliance for access to a mail/SMTP server located on the Outside network.

Refer to PIX/ASA 7.x and above: Mail (SMTP) Server Access on the DMZ Configuration Example for identical configuration on Cisco Adaptive Security Appliance (ASA) with versions 8.2 and earlier.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

The information in this document is based on these software and hardware versions:

- Cisco Adaptive Security Appliance (ASA) that runs version 8.3 and later.

- Cisco 1841 Router with Cisco IOS® Software Release 12.4(20)T

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to the Cisco Technical Tips Conventions for more information on document conventions.

Configure

In this section, you are presented with the information to configure the features described in this document.

Note: Use the Command Lookup Tool (registered customers only) to obtain more information on the commands used in this section.

Network Diagram

This document uses this network setup:

Note: The IP addressing schemes used in this configuration are not legally routable on the Internet. They are RFC 1918 addresses that have been used in a lab environment.

The network setup used in this example has the ASA with inside network (10.1.1.0/24) and the outside network (192.168.200.0/27). The mail server with IP address 172.16.31.10 is located in the Demilitarized Zone (DMZ) network. For the Mailserver to be accessed by the inside, users configure the identity NAT. Configure an access list, which is dmz_int in this example, in order to allow the outgoing SMTP connections from the Mailserver to the hosts in the inside network and bind it to the DMZ interface.

Similarly for the outside users to access the Mailserver configure a static NAT and also an access list, which is outside_int in this example, in order to permit outside users to access the Mailserver and bind this access list to the outside interface.

ASA Configuration

This document uses this configuration:

ASA Configuration

ASA#show run

: Saved

:

ASA Version 8.3(1)

!

hostname ASA

enable password 8Ry2YjIyt7RRXU24 encrypted

passwd 2KFQnbNIdI.2KYOU encrypted

names

!

interface Ethernet0

shutdown

no nameif

security-level 0

no ip address

!

interface Ethernet1

shutdown

no nameif

no security-level

no ip address

!

interface Ethernet2

no nameif

no security-level

no ip address

!

!--- Configure the inside interface.

interface Ethernet3

nameif inside

security-level 100

ip address 10.1.1.1 255.255.255.0

!

!--- Configure the outside interface.

interface Ethernet4

nameif outside

security-level 0

ip address  192.168.200.225 255.255.255.224

!

!--- Configure dmz interface.

interface Ethernet5

nameif dmz

security-level 10

ip address 172.16.31.1 255.255.255.0

!

passwd 2KFQnbNIdI.2KYOU encrypted

boot system disk0:/asa831-k8.bin

ftp mode passive

!--- This access list allows hosts to access !--- IP address 192.168.200.227 for the SMTP port.

access-list outside_int extended permit tcp any host  192.168.200.227 eq smtp

!--- Allows outgoing SMTP connections. !--- This access list allows host IP 172.16.31.10 !--- sourcing the SMTP port to access any host.

access-list dmz_int extended permit tcp host 172.16.31.10 eq smtp any

pager lines 24

mtu BB 1500

mtu inside 1500

mtu outside 1500

mtu dmz 1500

no failover

no asdm history enable

arp timeout 14400

object network obj-192.168.200.228-192.168.200.253

range 192.168.200.228-192.168.200.253

object network obj-192.168.200.254

host 192.168.200.254

object-group network nat-pat-group

network-object object obj-192.168.200.228-192.168.200.253

network-object object obj-192.168.200.254

object network obj-10.1.1.0

subnet 10.1.1.0 255.255.255.0

nat (inside,outside) dynamic nat-pat-group

!--- This network static does not use address translation. !--- Inside hosts appear on the DMZ with their own addresses.

object network obj-10.1.1.0

subnet 10.1.1.0 255.255.255.0

nat (inside,dmz) static obj-10.1.1.0

!--- This network static uses address translation. !--- Hosts that access the mail server from the outside !--- use the 192.168.200.227 address.

object network obj-172.16.31.10

host 172.16.31.10

nat (dmz,outside) static 192.168.200.227

access-group outside_int in interface outside

access-group dmz_int in interface dmz

route outside 0.0.0.0 0.0.0.0  192.168.200.226 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

timeout mgcp-pat 0:05:00 sip 0:30:00 sip_media 0:02:00

timeout uauth 0:05:00 absolute

no snmp-server location

no snmp-server contact

telnet timeout 5

ssh timeout 5

console timeout 0

!

class-map inspection_default

match default-inspection-traffic

!

!

!--- The inspect esmtp command (included in the map) allows !--- SMTP/ESMTP to inspect the application.

policy-map global_policy

class inspection_default

inspect dns maximum-length 512

inspect ftp

inspect h323 h225

inspect h323 ras

inspect netbios

inspect rsh

inspect rtsp

inspect skinny

inspect esmtp

inspect sqlnet

inspect sunrpc

inspect tftp

inspect sip

inspect xdmcp

!

!--- The  inspect esmtp  command (included in the map) allows !--- SMTP/ESMTP to inspect the application.

service-policy global_policy global

Cryptochecksum:2653ce2c9446fb244b410c2161a63eda

: end

[OK]

ESMTP TLS Configuration

Note: If you use Transport Layer Security (TLS) encryption for e-mail communication then the ESMTP inspection feature (enabled by default) in the ASA drops the packets. In order to allow the e-mails with TLS enabled, disable the ESMTP inspection feature as this output shows. Refer to Cisco bug ID CSCtn08326 (registered customers only) for more information.

ciscoasa(config)#

policy-map global_policy

ciscoasa(config-pmap)#class inspection_default

ciscoasa(config-pmap-c)#no inspect esmtp

ciscoasa(config-pmap-c)#exit

ciscoasa(config-pmap)#exit

Verify

There is currently no verification procedure available for this configuration.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Troubleshooting Commands

The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.

- debug icmp trace —Shows whether Internet Control Message Protocol (ICMP) requests from the hosts reach the ASA. You need to add the access-list command in order to permit ICMP in your configuration in order to run this debug.

Note: In order to use this debug, make sure you allow ICMP in the access-list outside_int as this output shows:

access-list outside_int extended permit tcp any host  192.168.200.227 eq smtp

access-list outside_int extended permit icmp any any

- logging buffered 7 —Used in global configuration mode to enable the adaptive security appliance to send syslog messages to the log buffer. The contents of the ASA log buffer can be seen with the show logging command.

Refer to Configure Syslog using ASDM for more information on how to set up logging.

Related Information

- Cisco ASA 5500 Series Adaptive Security Appliances

- Requests for Comments (RFCs)

- Technical Support & Documentation - Cisco Systems