Cisco securing device
From Ever changing code
- 1 Switching off unnecessary
- 2 Access Control Lists
- 2.1 How to apply
- 2.2 Apply MAC address filtering to Access Point using TFTP
- 2.3 Using object-grup with ACLs
- 2.4 Stateful Filtering
- 3 Safety cushion by scheduled reload
- 4 References
Switching off unnecessary
! ! Special interfaces configuration ! ! ICMP host unreachable messages are sent out if a router receives a nonbroadcast packet that uses an unknown protocol, ! or if the router receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address. ! Because the null interface is a packet sink, packets forwarded there will always be discarded and, unless disabled, will generate host unreachable messages. default interface Null0 interface Null0 no ip unreachables exit ! ! Per-interface security configuration ! ! Enable unicast Reverse Path Forwarding (RPF) on all interfaces that connect to the Internet. IP CEF need to be enabled. ! A feature that causes the router to check the source address of any packet against the interface through which the packet entered the router ! If the input interface is not a feasible path to the source address according to the routing table, the packet will be dropped. ! Use to verify: show ip cef interface atm0/0/0 ip verify unicast reverse-path ntp disable no cdp enable exit ! ! Switch-off unnecessary services in global-configuration-mode ! no ip source-route ip options drop no ip http server no ip http secure-server ! The TCP and UDP small servers are enabled by default on Cisco IOS Software Version 11.2 and earlier. They are disabled by default on Cisco IOS Software Versions 11.3 and later. no service tcp-small-servers no service udp-small-servers service tcp-keepalives-in service tcp-keepalives-out no ip bootp server no ip finger no service finger no ip identd no service config no lldp run no service pad ! ! NetFlow identifies flows of network packets based on the source and destination IP addresses and TCP port numbers. NetFlow then can use just the initial packet of a flow for comparison to ACLs and for other security checks, rather than having to use every packet in the network flow. This can be enabled per also per interface. ip route-cache flow ! ! Setting the TCP synwait time to 20 seconds causes the router to shut down an incomplete connection after 20 ip tcp synwait-time 20 ! ! time-stamps ! service timestamps debug datetime localtime show-timezone msec service timestamps log datetime localtime show-timezone msec service sequence-numbers ! ! add logging buffer ! logging userinfo logging buffered 50000 ip ssh logging events login block-for 300 attempts 3 within 300
- This automatic time access-list will be created after applying
Extended IP access list sl_def_acl 10 deny tcp any any eq telnet log 20 deny tcp any any eq www log 30 deny tcp any any eq 22 log (8015 matches) 40 permit tcp any any eq 22 log
Access Control Lists
How to apply
- First remove ACL from interface
int dialer 0 no ip access-group 90 in exit
- Remove ACL from running-config
default statement to delete ACL, then recreate, see below:
default access-list 90 ip access-list standard 90 remark Block abusive IP addresses deny 18.104.22.168 0.0.0.0 deny 22.214.171.124 0.0.0.0 deny 126.96.36.199 0.0.0.0 permit any exit
- Third re apply ACL to interface
int dialer 0 ip access-group 90 in exit
In the example output below 2 access lists have been applied to Gi0/1 interface INTERNET-OUT and INTERNET-IN. Remember only one access list can be applied to interface per direction in/out.
show ip interface gi 0/1 GigabitEthernet0/1 is up, line protocol is up Internet address is 188.8.131.52/29 Broadcast address is 255.255.255.255 Address determined by non-volatile memory MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Outgoing access list is INTERNET-OUT Inbound access list is INTERNET-IN Proxy ARP is enabled <-- output omitted -->
Apply MAC address filtering to Access Point using TFTP
This method is the most secure way to apply multiple ACL lines. This method is nice because the ACL doesn't get updated until after the TFTP is complete, so it should be safe even if the change will interrupt network connectivity. Another safety measure is to schedule a reload. Please follow these steps:
Verify TFTP server is running
netstat -ap UDP | find 69 UDP 0.0.0.0:69 *:* <-- this is expected output
netstat -ul | grep 69
Schedule a reload
reload in 5 reason Applying ACLs System configuration has been modified. Save? [yes/no]: yes Building configuration... [OK] Reload scheduled for 22:16:38 UTC Sun Nov 24 2013 (in 5 minutes) by dsneteng on vty0 (10.0.99.13) Reload reason: Applying ACLs Proceed with reload? [confirm] *** *** --- SHUTDOWN in 0:05:00 --- ***
Copy ACL to running-config
Copy block-list-apply.txt to tftp root folder. In Windows version of SolarWinds-TFTP-Server its default folder is C:\TFTP-Root.
copy tftp: running-config
or one liner example
copy tftp://10.0.99.11/block-list-apply.txt running-config Destination filename [running-config]? Accessing tftp://10.0.99.11/block-list-apply.txt... Loading block-list-apply.txt from 10.0.99.11 (via BVI1): ! [OK - 3501 bytes] 3501 bytes copied in 1.588 secs (2205 bytes/sec)
Example of block-list-apply.txt file.
! Updated on 24/11/2013@21:00 by user@ ! MAC Vendor Lookup http://www.macvendorlookup.com/list-lookup ! ! Exeption list: ! ! LocationID ! 0022.694C.AABB - Hon Hai Precision - user@ ! ! Apply following to all your APs ! ! interface Dot11Radio0.10 no bridge-group 10 input-address-list 700 exit ! no access-list 700 ! ! LocationID ! access-list 700 deny 0019.7E72.AABB 0000.0000.0000 access-list 700 deny 083E.8E36.CCDD 0000.0000.0000 ! ! Permit all rest ! access-list 700 permit 0000.0000.0000 ffff.ffff.ffff ! interface Dot11Radio0.10 bridge-group 10 input-address-list 700 exit end ! ! MAC filter list has been applied - check for connectivity
reload cancel *** *** --- SHUTDOWN ABORTED --- ***
Using object-grup with ACLs
We can create a group of hosts, range of hosts, nested group-object. Then we can include the object name in ACL. It is
R1(config)#object-group network GoogleServers R1(config-network-group)#? Network object group configuration commands: A.B.C.D Network address of the group members any Any host description Network object group description exit Exit from IP policy-group configuration mode group-object Nested object group host Host address of the object-group member no Negate or set default values of a command range Match only packets in the range of IP address R1(config-network-group)#description Allow to do DNS request to Google Servers R1(config-network-group)#host 184.108.40.206 R1(config-network-group)#host 220.127.116.11 R1(config-network-group)#host 18.104.22.168 R1(config-network-group)#host 22.214.171.124 R1(config-network-group)#host 126.96.36.199 R1(config-network-group)#exit
- Add to access-list
R1(config)#ip access-list extended Only-GoogleServers R1(config-ext-nacl)#remark Allow access google.com server R1(config-ext-nacl)#permit ip any object-group GoogleServers R1(config-ext-nacl)#deny ip any any R1(config-ext-nacl)#exit
R1#sh access-lists Only-GoogleServers Extended IP access list Only-GoogleServers 10 permit ip any object-group GoogleServers 20 deny ip any any
Then you can apply ACL Only-GoogleServers in the standard manner.
Create access-list permitting only tcp, udp, icmp trafic
R1(config)#ip access-list extended GOING-OUT R1(config-ext-nacl)#permit tcp any any reflect REMEMBER R1(config-ext-nacl)#permit udp any any reflect REMEMBER R1(config-ext-nacl)#permit icmp any any reflect REMEMBER R1(config-ext-nacl)#deny ip any any log
Apply to an interface to all traffic leaving interface. The router will create REMEMBER dynamic ACL for all traffic permitting coming back. Therefore reflective ACL makes sense most when you denying access to the router from outside but you want to allow traffic originated from inside network.
R1(config)#interface dialer 1 R1(config-if)#ip access-group GOING-OUT out
Block all trafic comming in
R1(config)#ip access-list extended COMING-IN R1(config-ext-nacl)#evaluate REMEMBER R1(config-ext-nacl)#deny ip any any log R1(config-ext-nacl)#exit R1(config)#interface dialer 1 R1(config-if)#ip access-group COMING-IN in
Verify, by opening https://www.google.co.uk
R1#show access-lists Extended IP access list COMING-IN 10 evaluate REMEMBER 20 deny ip any any log (77 matches) Extended IP access list GOING-OUT 10 permit tcp any any reflect REMEMBER (79 matches) 20 permit udp any any reflect REMEMBER 30 permit icmp any any reflect REMEMBER 40 deny ip any any log Reflexive IP access list REMEMBER permit tcp host 188.8.131.52 eq 443 host 10.161.93.20 eq 52767 (10 matches) (time left 296) permit tcp host 184.108.40.206 eq 443 host 10.161.93.20 eq 41852 (9 matches) (time left 296) permit tcp host 220.127.116.11 eq 443 host 10.161.93.20 eq 56001 (37 matches) (time left 296) permit tcp host 18.104.22.168 eq www host 10.161.93.20 eq 37352 (4 matches) (time left 295) permit tcp host 22.214.171.124 eq www host 10.161.93.20 eq 43924 (6 matches) (time left 294) permit tcp host 126.96.36.199 eq www host 10.161.93.20 eq 43923 (5 matches) (time left 294)
Even all incoming traffic on Dialer1 interface is denied we EVALUATE access-list REMEMBER for remembered traffic that left out network. Therefore all traffic originated form our network will be permitted.
Example working reflective ACL allowing only ssh and inside originated traffic
! ! Remember all traffic orginated from inside ! ip access-list extended INTERNET-OUT permit tcp any any reflect REMEMBER timeout 300 permit udp any any reflect REMEMBER timeout 300 permit icmp any any reflect REMEMBER timeout 300 deny ip any any log ! ! Block all except dns(udp53) and ssh(tcp22) ! ip access-list extended INTERNET-IN ! permit udp any eq 53 any permit udp any eq domain any permit tcp any any eq 22 evaluate REMEMBER deny ip any any log ! ! Apply access lists to Internet-edge intreface ! interface Dialer0 ip access-group INTERNET-IN in ip access-group INTERNET-OUT out
Just a note, not existing ACL applied to an interface have no affect on the traffic - all is allowed.
CBAC - Context Based Access Control is the next generation of stateful filtering, superseding the reflective access lists. We are going to use inspection rules to remember kind of traffic (aka legacy reflective ACL)
This feature is available only with security or data&security technology feature enabled. Please refer to Cisco Feature Toolkit.
Safety cushion by scheduled reload
When applying ACLs or adding extra security it possible to lock out yourself. Therefore before you apply any changes you can schedule reload and do not save running-config to startup-config until you 100% sure all is working correctly. Then cancel scheduled reload and save changes.
- Schedule reload in 5 minutes
reload in 5 Reload scheduled for 19:57:04 GMT Sun Nov 17 2013 (in 5 minutes) by dsneteng on console Reload reason: Reload Command Proceed with reload? [confirm] *** *** --- SHUTDOWN in 0:05:00 --- ***
- Cancel reload
reload cancel *** *** --- SHUTDOWN ABORTED --- ***
- Other options
reload ? at Reload at a specific time/date cancel Cancel pending reload in Reload after a time interval reason Reload reason warm Reload should be warm
- Use copy tftp run when applying ACLs
Then to a
copy tftp run to install the updated ACL into the running config. This method is nice because the ACL doesn't get updated until after the TFTP is complete, so it should be safe even if the change will interrupt network connectivity.
- Best Practices for Inbound Cisco Access Lists 2007
- Best Practices and Securing Cisco IOS 2011
- Cisco Guide to Harden Cisco IOS Devices 2011, nice good list explained
- How do I secure a Cisco router from the Internet? 2009
- DNS Best Practices, Network Protections, and Attack Identification
- ststeful filtering