Filtering DNS with Tcpdump Port 53

Updated: 7 hours ago

Tcpdump is a very powerful Linux command to capture DNS packets. DNS is a basic part of the Linux admin task. We can use tcpdump to filter DNS query traffic and DNS zone transfer packets to learn more about how DNS works.

How DNS works?

DNS is short for Domain Name System. It is simply a database that links meaningful names (known as host names), such as, to a specific IP address, such as

DNS uses both TCP and UDP port 53. The most frequently used port for DNS is UDP 53. This is used for DNS queries on the client side. Check more info about DNS port here.

Capture DNS zone transfer Packets with Tcpdump

DNS zone transfer uses TCP port 53. We can filter TCP and port 53 in tcpdump command to capture all these packets. In the following example, we use eth0 as the network interface. Please change it to adapt to your environment. Check here to learn more about how to use tcpdump command to capture packets.

# tcpdump -i eth0 tcp port 53

Use tcpdump to filter port 53 for DNS Query packets

We can use this tcpdump command to filter DNS query packets.

# tcpdump -i eth0 udp port 53

We can write these packets to a file with this tcpdump command.

# tcpdump -i eth0  -w /tmp/dns.pcap udp port 53

We can read these packets from dns.pcap file to get more details about the DNS query.

# tcpdump -vvv -r /tmp/dns.pcap port 53 

DNS Packet Analysis

We can get the A record for with the flowing command.

dig +short

This is the output of tcpdump command after we run the above dig command. Check more info about how to use dig command to query DNS records here.

20:11:00.466866 IP > 60712+ [1au] A? (39)

This is the packet we get from the DNS server for this DNS query.

20:11:00.560294 IP > 60712 6/4/1 A, A, A, A, A, A (207)

By default, the dig command query the A record for that domain name with UDP protocol. Check this post to learn more about other DNS records like AAAA, MX, PTR etc.

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