Cisco IOS features that you should disable or restrict

Recently I found an overview with 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: This is 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 labbut 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 2600’s 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, don’t use this.
  • IP directed broadcasts: Allows you to send packets to the broadcast address of another subnet, allows “smurf attacks”. Used for DOS attacks…so disable this.
  • IP Unreachable notifications: Your router will notify a sender of incorrect IP addresses, gives away information.
  • IP Mask reply: Router will send the subnet mask of an interface in response to a ICMP mask request, gives 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 not needed.
  • SNMP: If you don’t use SNMP, I’d suggest to disable/block it.
  • DNS: Routers can perform DNS lookups, if you don’t use this i’d disable it.

Is there anything else that you miss in this list? please let me know! Good luck securing your routers!

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Forum Replies

  1. Greetings Rene,
    1stly, thanks for the info… truly useful,
    i do have a suggestion: how about “ip domain-lookup” ? we could disable it 2 prevent some domain-server translations (for time saving purposes)…

  2. Hi Reza,

    “no ip domain-lookup” is a nice feature since it can be very annoying when your router tries to do a DNS lookup when you mistype a command. The list above are mostly security-related features however. “Logging synchronous” is also a nice command that most people like to use.

    Rene

  3. Hi Romain,

    Normally CTRL SHIFT 6, X should do the job. If that doesn’t work…try CTRL SHIFT6, 6 and then X.

    Otherwise you can also kill the TCP session like this:

    Switch#show tcp brief
    TCB       Local Address               Foreign Address             (state)
    0DC15ED0  192.168.1.2.23             192.168.1.1.44565           ESTAB
    0B511A08  192.168.1.2.23             192.168.1.1.31365           CLOSED
    0C9FB040  192.168.1.2.23             192.168.1.1.51828           TIMEWAIT
    
    Switch#clear tcp tcb 0DC15ED0
    [confirm]
     [OK]
    

    That will always work :slight_smile:

    Rene

  4. Hi Rene,

    I have 2 questions in shutting down services.
    1.- Which command will allow me to see all the services running on routers and switches?
    2.- I have configured ip source-route and ip cef on a point to point routers. Should I disable it and why?

    Please advise

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