Page 5 of 8

Now that we are familiar with nslookup and ipconfig, we're ready to get down to some serious business. Let's first capture the DNS packets that are generated by ordinary Web-surfing activity.

If you are on a windows host, do the following:

Use ipconfig to empty the DNS cache in your host.
Open your browser and empty your browser cache. (With Internet Explorer, go to Tools menu and select Internet Options; then in the General tab select Delete Files.)
Open Wireshark and enter "dns && ip.addr == your_IP_address” into the filter, where you obtain your_IP_address (the IP address for the computer on which you are running Wireshark) with ipconfig. This filter removes all packets that neither originate nor are destined to your host or are not dns packets.
Start packet capture in Wireshark.
With your browser, visit the Web page: http://www.ietf.org
Stop packet capture.

If you are on a Linux/Unix machine and do not have privileges to clear your host cache (or don't know how to) you can download this dns-tracefile that was captured while following the steps above. Or you could give it a try if you haven't been to www.ietf.org in a few days (it may not be cached). But if you don't see a DNS request, then download the tracefile.

Answer the following questions

What is the destination port for the DNS query message for www.ietf.org?

enter a number

Question

Page 5 of 8

Now that we are familiar with nslookup and ipconfig, we're ready to get down to some serious business. Let's first capture the DNS packets that are generated by ordinary Web-surfing activity.

If you are on a windows host, do the following:

Use ipconfig to empty the DNS cache in your host.
Open your browser and empty your browser cache. (With Internet Explorer, go to Tools menu and select Internet Options; then in the General tab select Delete Files.) 
Open Wireshark and enter "dns && ip.addr == your_IP_address” into the filter, where you obtain your_IP_address (the IP address for the computer on which you are running Wireshark) with ipconfig. This filter removes all packets that neither originate nor are destined to your host or are not dns packets. 
Start packet capture in Wireshark. 
With your browser, visit the Web page: http://www.ietf.org
Stop packet capture.

If you are on a Linux/Unix machine and do not have privileges to clear your host cache (or don't know how to) you can download this dns-tracefile that was captured while following the steps above. Or you could give it a try if you haven't been to www.ietf.org in a few days (it may not be cached). But if you don't see a DNS request, then download the tracefile.

Answer the following questions

What is the destination port for the DNS query message for www.ietf.org?

enter a number

BlackTom AI · Accepted Answer

The task asks for the destination port used by a DNS query message to www.ietf.org.
First, recall how DNS operates: standard, traditional DNS traffic uses UDP (and sometimes TCP) on port 53 for queries from clients to DNS servers. In most typical client scenarios—such as when a web browser looks up a domain name—the query is sent over UDP to port 53 on the DNS server, unless the response is too large or certain conditions require TCP, in which case a fallback to TCP on port 53 occurs. 
Next, in the provided lab steps, you are filtering for DNS traffic and capturing packets while visiting a domain. The question specifically asks for the destination port of the DNS query message for www.ietf.org. Since the client initiates the lookup by sending a DNS query to a DNS server, the destination port is the well-known DNS port, which is 53. 
Consider potential misinterpretations: some might think to look at the source port, which would be a high-numbered ephemeral port assigned by the client, not 53. Others might wonder about the transport protocol differences or historical variations; however, the standard and most common destination port for DNS queries is indeed 53. In rare cases where DNS-over-HTTPS or DNS-over-TLS is used, the port would differ, but those are explicit alternate protocols and not implied by the classic DNS query captured in a typical DNS trace file. While analyzing a trace, ensure you’re looking at the DNS protocol and identify the destination port field in the DNS query frame. 
Therefore, the correct destination port for the DNS query message to www.ietf.org is 53.

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