Hardening a router means that the router is secured against attacks as best as possible. This article discusses various means of making sure your routers are set up with maximum security, including manually hardening the router and router hardening with Cisco SDM.

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We don’t tend to like people that are hardened. They can be cold and unsympathetic, and generally morose. We do, on the other hand, love it when our Cisco routers are hardened, especially when these routers live at the perimeter of our internal networks.

What exactly do we mean by router hardening? It means that the router is secured against attacks as best as possible. From secure, tough-to-crack passwords that are encrypted in the configurations, to the shutting down of unnecessary ports and services, the router has few vulnerabilities for would-be attackers to exploit.

For example, the router has built-in web server capabilities. That’s right. Your Cisco router can actually function as a web server on your network. Don’t plan on using the router as a web server? Be sure that this functionality is disabled.

In addition to web services, the Cisco router of today’s networks can provide many, many other potential services to the network. A key element to hardening the router is to find all of these services you are not using, and to disable them.
Manually Hardening the Router

Feel like going “old school” with your perimeter router security? Well, here is what you can do for configurations manually on the device in order to help secure (harden) it:

Disable unused router interfaces—that’s right, find ANY interface that is not in use and make sure you issue the shutdown command
Disable unused services—these typically include:
BOOTP
CDP
Configuration autoloading
FTP
TFTP
PAD
TCP and UDP minor services
DEC MOP
Disable management protocols that you are not using—these typically include:
SNMP
HTTP or HTTPS
DNS
Disable features that are techniques for re-directing your traffic:
ICMP Redirects
IP Source routing
Disable features that are techniques for probes and scans in reconnaissance attacks:
Finger
ICMP unreachables
ICMP mask reply
Ensure security of terminal connections:
IP identification service
TCP keepalives
Disable gratuitous ARP and proxy ARP
Disable IP-directed broadcasts

You also should inspect the network management protocols in use in your network infrastructure. Remember the following:

SNMP version 1 and 2c transfer passwords (called community strings) in clear text—if security is required—consider SNMP version 3.
HTTP authentication also sends clear text passwords—when needed, use HTTPS instead.
Attackers can respond to broadcast DNS lookups—disable DNS when not in use or configure securely.
Telnet is a clear text protocol—do not use it; instead, use SSH.

Router Hardening with the Cisco Router and Security and Device Manager (SDM)

Now one of the reasons that we love Cisco is that they are always trying to make it easy on us. We see this in the area of router hardening. In the Graphical User Interface for managing your perimeter routers, Cisco provides a Security Audit feature. This feature provides two “modes” of operation. The first is the Security Audit Wizard and the second is the One-step Lockdown mode.

The Security Audit Wizard examines your router and then lets you choose which potential security flaws you want to correct. The One-step Lockdown mode automatically makes the router hardening configurations that Cisco would recommend.

Here are the steps of the Security Audit Wizard:

Within the Cisco SDM, choose Configure then Security Audit.
Click the Perform Security Audit button.
Click Next and the Security Audit Interface Configuration page appears —here you are presented with your router interfaces. It is up to you to select which of the interfaces connect to your internal networks, and which of the interfaces connect to your external networks. After you select these options, choose Next.
The security audit now runs. When it is complete, the SDM presents you with a report of potential security vulnerabilities on your device. You can click Save Report in order to save it. If you select Close, the Security Audit Wizard continues to the next phase.
In the final phase of the Security Audit Wizard, you can check or uncheck the vulnerabilities that you want the wizard to automatically repair. Notice there is even an option for Fix All.

As you might guess, using the One-step Lockdown mode is even easier!

Within the Cisco SDM, choose Configure then Security Audit.
Click the One-step Lockdown button.
Click the Yes button that you want to continue and the SDM goes about its business of locking down the device for you.

What exactly does the One-step lockdown do on your device? Well, it is very busy indeed. Here is a list of the changes made by this GUI:

Disable Finger Service
Disable PAD Service
Disable TCP Small Servers Service
Disable UDP Small Servers Service
Disable IP BOOTP Server Service
Disable IP Identification Service
Disable CDP
Disable IP Source Route
Enable Password Encryption Service
Enable TCP Keepalives for Inbound Telnet Sessions
Enable TCP Keepalives for Outbound Telnet Sessions
Enable Sequence Numbers and Time Stamps on Debugs
Enable IP CEF
Disable IP Gratuitous ARPs
Set Minimum Password Length to Less Than 6 Characters
Set Authentication Failure Rate to Less Than 3 Retries
Set TCP Synwait Time
Set Banner
Enable Logging
Set Enable Secret Password
Disable SNMP
Set Scheduler Interval
Set Scheduler Allocate
Set Users
Enable Telnet Settings
Enable NetFlow Switching
Disable IP Redirects
Disable IP Proxy ARP
Disable IP Directed Broadcast
Disable MOP Service
Disable IP Unreachables
Disable IP Mask Reply
Disable IP Unreachables on NULL Interface
Enable Unicast RPF on Outside Interfaces
Enable Firewall on All of the Outside Interfaces
Set Access Class on HTTP Server Service
Set Access Class on VTY Lines
Enable SSH for Access to the Router

Router Hardening with the Cisco’s AutoSecure

Cisco also provides a One-step lockdown-like feature at the command line! This feature is called AutoSecure. It uses the command shown below:

auto secure [management | forwarding] [no-interact | full]
[ntp | login | ssh | firewall | tcp-intercept]

Notice that this command can run fully automated like the One-step Lockdown mode of the Security Audit feature in SDM. You would issue the command auto secure no-interact. You should notice also that you can run “subsets” of the command’s full capabilities. For example, you could run auto secure management in order to just harden the network management capabilities of the router.

You might not be surprised to learn that the Command Line AutoSecure feature is capable of doing a bit more than the graphical user interface counterpart. Specifically, Cisco SDM does not implement these Cisco AutoSecure features:

Disabling NTP
Configuring AAA
Setting SPD values
Enabling TCP intercepts
Configuring antispoofing ACLs on outside interfaces

The Cisco SDM also implements some of the Cisco AutoSecure features differently. For example:

The SDM disables SNMP but does not configure SNMPv3 (on some routers).
The SDM enables and configures SSH on crypto Cisco IOS images, but does not enable SCP or disable other access and file transfer services, such as FTP for example.

Conclusion

As we have seen, there is more than one option when it comes to ensuring that your Cisco router is well protected against security vulnerabilities. Essentially, two GUI options exist within the Cisco SDM, and a very powerful command line option exists. Of course, administrators can always walk through configuration manually. Whatever you choose, progress with a plan of carefully testing to ensure your router hardening configurations do not cause disruptions within your production network. It is wonderful to be secure, but you do not want this security at the risk of a lack of functionality. We love the help desk phones to remain quiet!