What is ICMP type?

Updated: Aug 11

ICMP type is the first 8 bits in the ICMP message header. It provides a brief explanation of what the message is for so the receiving network device knows why it is getting the message and how to treat it.


For example, a Type 8 Echo is a query a host sends to see if a potential destination system is available. Upon receiving an Echo message, the receiving device might send back an Echo Reply (Type 0), indicating it is available.



Common ICMP Types

Some common message types include the following:

  • Type 0 -- Echo reply

  • Type 3 -- Destination unreachable

  • Type 8 -- Echo

  • Type 5 -- Redirect


Internet Assigned Numbers Authority (IANA) provides a list of all message types ICMP packets use.

ICMP Type and ICMP Codes

Each ICMP Type can have 1 or more Codes related to it. For example the Type 0 has only 1 Code, but Type 3 has 16 Codes — Type 3 is Destination Unreachable, Destination could be unreachable due to any of the reasons mentioned in the 16 Codes corresponding to this Type starting from 0 to 15.





Capture ICMP Type Echo Request Packets

we can use this tcpdump command to filter all ICMP packets. We use eth0 network interface in all our examples. Please change it based on the environment.


# tcpdump -i eth0 icmp

To filter ICMP echo-requests, we can use this tcpdump command.


# tcpdump -i eth0 "icmp[0] == 8"

These are the packets we get captured with tcpdump command.


14:37:14.555295IP10.79.101.23>108.177.125.101:ICMP echo request, id 61205, seq 0, length 64



Capture ICMP Type Echo Reply Packets

To filter ICMP echo reply requests, we can use this tcpdump command.


# tcpdump -i eth0 "icmp[0] == 0"

These are the packets we get captured with tcpdump command.


21:05:51.164467 IP 66.114.168.201 > 10.79.102.71: ICMP echo reply, id 16790, seq 203, length 64

Related:

Exploring ICMP Protocol with Examples

Understanding Ping Command and ICMP with Examples

Exploring ICMP Port Number with Example

Capture ICMP Packets With Tcpdump

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