Source and destination port of ICMP packet
I'm writing a packet filter in netfilter. Is there a way I can get the source and destination port of a ICMP packet?
I have extracted icmp_hdr from the sk_buff structure. But I don't see any property from source and destination address?
Answer
Port numbers are the way the TRANSPORT layer recognizes which packet belongs to what process at the end systems.
They're used to let the process-to-process delivery work; but ICMP, from a functional point of view, is not a transport layer protocol.
ICMP is a messaging protocol at the Network layer(on top of the IP; but not really in the transport layer), it's got a lot of responsibilities but none of them has anything to do with process-to-process delivery, so having a port number there wouldn't make any sense.
Take an example, when you ping(ping uses ICMP echo messages) an IP address, which port are you really pinging?
The answer is: no port, you're pinging the whole station to see if it's alive.
Now, ICMP has many types of messages; if you want to filter out, for example, ping messages, you should check the field type and if it equals ICMP_ECHO, you can return NF_DROP.