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Help Restricting  SMTP traffic

Posted on 2007-07-25
Medium Priority
Last Modified: 2010-04-09
How do I restrict SMTP traffic outbound to only a mail server?  We are being blacklisted due to an infected computer.  Apparently this computer is sending out SMTP traffic.  Until I track down the PC, how do I stop this traffic from getting outbound?

We have a 1751 Cisco router.  Here are some configs:

ip domain name junius. .org
ip cef
ip ips po max-events 100
ip ssh time-out 60
ip ssh authentication-retries 2
vpdn enable
vpdn-group pppoe
  protocol pppoe
no ftp-server write-enable
crypto isakmp policy 1
 encr 3des
 authentication pre-share
crypto isakmp policy 10
 encr 3des
 hash md5
 authentication pre-share
 group 2
crypto isakmp key 6 Organization address x.x.x.x
crypto isakmp identity hostname
crypto isakmp peer address x.x.x.x
crypto ipsec transform-set aptset esp-3des esp-md5-hmac
crypto ipsec df-bit clear
crypto map aptmp 10 ipsec-isakmp
 set peer x.x.x.x
 set transform-set aptset
 match address 103
interface Vif1
 no ip address
interface ATM0/0
 no ip address
 no atm ilmi-keepalive
 dsl operating-mode auto
interface FastEthernet0/0
 ip address 172.x.x.1
 ip nat inside
 ip virtual-reassembly
 ip tcp adjust-mss 1452
 speed auto
 pppoe enable
 pppoe-client dial-pool-number 1
 no cdp enable
interface Virtual-PPP1
 no ip address
interface Virtual-Template1
 no ip address
interface Dialer0
 mtu 1492
 ip address negotiated
 ip nat outside
 ip virtual-reassembly
 encapsulation ppp
 dialer pool 1
 dialer-group 1
 ppp authentication pap chap callin
 ppp chap hostname
 ppp chap password
 ppp pap sent-username
 crypto map aptmp
interface Virtual-TokenRing1
 no ip address
 ring-speed 16
ip classless
ip route Dialer0
no ip http server
no ip http secure-server
ip nat inside source route-map nonat interface Dialer0 overload
ip access-list extended ipsec
access-list 1 permit 172.x.0.0
access-list 102 permit tcp host x.x.x.x any eq 22
access-list 102 permit tcp host x.x.x.x any eq 22
access-list 103 permit ip 172.x.x.0
access-list 130 deny   ip
access-list 130 permit ip any
dialer-list 1 protocol ip permit

route-map nonat permit 10
 match ip address 130
Question by:gdf99
  • 2
LVL 79

Accepted Solution

lrmoore earned 1500 total points
ID: 19569145
access-list 111 permit tcp host 172.16.3.xxx any eq 25   <== xxx= your mail server
access-list 111 deny tcp any any eq 25 log   <== log so you can catch the culprit
access-list 111 permit ip any any
interface fast 0/0
 ip access-group 111 in


Author Comment

ID: 19575233
Lrmoore - Is the following line necessary?  I tried to input the following line and router won't accept it.  
Ip access-group 111 in

LVL 79

Expert Comment

ID: 19575407
Yes, it is necessary. You have to apply the access-list to the interface. This is an interface config:
router(config)#interface fast 0/0
router(config-if)#ip access-group 111 in

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