The router doesn't quote the whole packet, usually just the IPv4 and TCP/UDP/ICMP header. In that case, the router with the missing path sends an ICMP Destination Unreachable, Network Unreachable message quoting the original packet. Suppose a network engineer sends an ICMP Echo Request to a network that can not be routed. The highlighted packets are destination unreachable packets ( icmp.type =35) that should not be removed from the packet list in this case. But unfortunately, some other ICMP messages that indicate errors are filtered, too.
You will notice that this naive filter does not quite work. So the filter should be icmp and not (icmp.type = 0 or icmp.type = 8) Echo Request has the type field in the ICMP header set to 8, whereas an Echo Reply sets it to 0. The ICMP Echo Request / Reply messages, better known as Ping, don't indicate a problem and are common background noise in networks network engineers use them for troubleshooting or monitoring. In practice, you will notice that we get way too many packets, and you will quickly find yourself narrowing down the filter a bit more. So we might use a display filter like icmp. ICMP is an excellent protocol for network analysts because, correctly interpreted, we can diagnose a problem right away. This means that the original packet was encapsulated in a new packet, with a new IP header added on top. If you see a packet that has two IP headers, it's likely that the packet has been tunneled or quoted. When troubleshooting network issues, it's important to be able to read a PCAP and understand what's going on. This can be confusing for people who are trying to read a PCAP, because they might not expect to see a header twice in the same packet. Packets can get a lot more complex, including repeating the same protocol twice (tunneling) or repeating the same protocol field twice within the same packet layer. Not every packet in a PCAP is just a simple Ethernet / IPv4 / TCP packet. Matching a specific layer in the protocol stack That's where these enhancements make your filtering job easier.
If you analyze network protocols like IPv4, ICMP, IPv6, ICMPv6, TLS, and GRE, this article is for you.
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