Recently I found an overview of some IOS features that you might encounter on Cisco routers. Some of them are enabled by default and could cause a security risk. It’s a good idea to check if any of these are running on your network and perhaps you want to disable them.
- CDP (Cisco Discovery Protocol) or LLDP (Link Layer Discovery Protocol). You are probably familiar with CDP or LLDP. It’s very useful to discover (Cisco) neighbor devices but it also gives away a lot of information like your (router) model, IP address, IOS version etc. If you don’t use it it’s better to disable it globally or on certain interfaces.
- TCP Small Servers: These are some TCP standard network services like echo. Disable it.
- UDP Small Servers: Same for UDP. Best to disable it.
- Finger: User lookup service, originally for Unix. Can be used remotely to list logged-in users. Nobody needs to know this kind of information remotely…
- HTTP Server: Very nice for in a lab but not a good idea in a production environment.
- BootP Server: Allows other routers to boot from this router. Hardly ever used.
- Configuration auto-loading: Your router will try to boot up from a TFTP, I’ve only used this once, so my regular 2600s could boot the XM image in a lab. Not gonna use it in production.
- PAD Service: Router will support X.25. Not gonna use it.
- IP Source Routing: Allows the creator of an IP packet to choose the route. You don’t want this.
- Proxy ARP: Your router will answer (proxy) for L2 ARP requests, normally you don’t need this.
- IP Directed Broadcasts: Allows you to send packets to the broadcast address of another subnet, and allows “smurf attacks.” Used for DOS attacks…so disable this.
- IP Unreachable Notifications: Your router will notify a sender of incorrect IP addresses, and gives away information.
- IP Mask Reply: Router will send the subnet mask of an interface in response to an ICMP mask request, giving away information.
- IP Redirects: Your router will send an ICMP redirect in response to some router IP packets.
- Maintenance Operations Protocol (MOP): Old management protocol, part of DECnet.
- NTP Service: Your router can become a time server, perhaps unnecessary.
- SNMP: If you don’t use SNMP, I’d suggest disabling/blocking it.
- DNS: Routers can perform DNS lookups. If you don’t use this, I’d disable it.
Is there anything else that you miss on this list? Please let me know!
Good luck securing your routers!
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Thanks for reading: Hardening Cisco IOS Features, Sorry, my English is bad:)