ICMP is a control protocol associated with IP, used in particular by ping and also to report errors and help diagnosis Network. firewall that blocks everything ICMP may mask useful information and, depending on the messages concerned, disrupt certain network mechanisms.
Simple definition
A control protocol used in particular by ping and to report certain network errors.
Technical definition
Internet Control Message Protocol carries diagnostic and control messages to the IP level, for example Echo Request, Echo Reply or Destination UnShapable.
Analyze Message ICMP
Remove the source address, destination address, and type and code ICMP. Then check the equipment that generated the message: the destination itself, a router or a router. firewall This method allows you to distinguish an inaccessible destination, a filtering and a simple refusal to respond to the request. ping.
What is it used for ?
Test for reachability, diagnose routing and report delivery errors.
Practical example
The command ping usually send requests ICMP Echo and expect Echo!
How to use ICMP in a network diagnosis?
ping usually test Echo/Echo Reply messages, but ICMP is not limited to this. Destination A Channelable provide information on delivery failure. To diagnose correctly, compare the ping with the route, an application service test and, if necessary, a network capture to see what type or code ICMP is actually traded.
Common issues
Equipment can filter Echo Request while allowing it to work HTTPS, SSH or another service. Conversely, a ping successful does not guarantee that the application responds. Block all messages ICMP can also remove diagnostic information and hinder mechanisms related to path size. firewall needs and types of ICMP instead of applying a reasoning all or nothing.
Related terms
Ping · Traceroute · Firewall
Key takeaway: This entry explains the general operation of the term. Exact settings may vary depending on software, vendors and environments.
FAQ — ICMP
ICMP Does it use a port TCP or UDP ?
No. ICMP is a separate IP protocol; it does not identify itself by a port number TCP or UDP.
Why a ping fails when the website answers?
Echo’s can be filtered while traffic TCP/HTTPS is authorized.
Should we block everything? ICMP on one firewall ?
Some messages are useful for diagnosis and good network functioning; the policy must be context-specific.