![]() You may also want to use an ICMP ACL to throttle ICMP traffic that could cause a DoS attack. Of course, the ACL must be applied to your interface in the “in” direction. In the following inbound ACL filtering example, we are filtering ICMP echo, redirect, and mask-requests, while allowing other types: Router(config)# access-list 100 deny icmp any any echo log Router (config)# access-list 100 deny icmp any any redirect log Router (config)# access-list 100 deny icmp any any mask-request log Router (config)# access-list 100 permit icmp any 1.1.1.0 0.0.0.255 #Throttled meaning in nepali how to#I say that because the following ICMP inbound filtering ACLs are examples of how to filter ICMP to block certain traffic – but not necessarily the only ones that will “secure your network.” #Throttled meaning in nepali download#In fact, there are entire books you can buy (like Cisco Press’s Network Security Technologies and Solutions) there are guides you can download (like the NSA Router Security Guide) and there are certifications you can pursue (like the Cisco CCSP). Protecting a network from attack isn’t as simple as adding a few network access-lists. You can see all the ICMP filtering options that can be used with a Cisco IOS ACL by following the link.įiltering ICMP inbound and outbound traffic both to your network and the Internet are important, but the most important of the two is to properly filter ICMP inbound to protect your network. To filter ICMP traffic, you need to use an extended access list and start with something like this: Additionally, no standard access list will work for ICMP specifically. While these may be the two most common ways to filter network traffic with Cisco IOS extended ACLs, neither of these will work to filter ICMP. When creating Cisco IOS ACLs, many admins start out with either: Now, let’s take a look at how Cisco IOS ACLs can be used to filter ICMP traffic. That firewall could be a Cisco PIX, ASA, or a Cisco IOS router. Commonly, ICMP traffic is filtered with a firewall. To prevent these types of attacks, there are various solutions. By not restricting the type and flow of ICMP traffic from the Internet, you increase the potential for a denial of service (DoS) attack by allowing ICMP traffic to flood your network and affect service to all network traffic from the servers. ICMP traffic can be used not only to discover hosts on your network but also to flood your network with traffic. As ICMP traffic from a malicious attacker can be used to bring down your network, ICMP traffic needs to be strictly filtered when coming in from the Internet and, perhaps, when going out to the Internet. Depending on the level of internal network security that you require, you may want to filter ICMP traffic on your LAN between subnets (regardless of the Internet). On the typical LAN with a “soft core,” ICMP traffic is typically unrestricted. What are the security issues with ICMP traffic? Like UDP, ICMP traffic is an unreliable protocol with no guaranteed delivery. For example, for a ping to work, your host needs to be able to send an ICMP echo (type 8) the host you are pinging needs to be able to receive the echo that host needs to be able to send an ICMP echo reply (type 0) and your host needs to be able to receive it for your ping program to be able to respond that the host is alive (and a round trip time for that ping). In fact, there are 42 types of ICMP traffic (you can view each of them at the IANA ICMP parameters site). While many of us just associate ICMP with “ping,” there is actually a lot to know about it. Depending on the version of traceroute used, ICMP may or may not be used for traceroute as well. ICMP is most well known as being used to ping a host on your network. That is basically true as ICMP is used to communicate things like “host unavailable” and other errors. I have heard of ICMP referred to as the management protocol for IP networks. The Internet Control Message Protocol (ICMP) is based on RFC 792 and is used to send IP network errors and diagnostic messages. In this article, learn the basics of ICMP and how to filter it properly in the Cisco IOS. While ICMP is required for IP network traffic redirection and pinging hosts on your LAN or WAN, ICMP can also pose a security concern. However, ICMP is critical to the functionality of any IP network, such as your corporate network or the Internet. The Internet Control Message Protocol (ICMP) is not TCP and it’s not UDP. David Davis tells you about the basics of ICMP and how to filter it properly in the Cisco IOS. ![]()
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