![]() The offset, once multiplied by 4 gives the byte count of the TCP header, meaning ((tcp & 0xf0) > 2) provides the size of the TCP header. Tcp means capturing the 13th byte of the tcp packet, corresponding to first half being the offset, second half being reserved. Tcp & 0xf0) > 2)] = 0x16: a bit more tricky, let’s detail this below Tcp port 443: I suppose this is the port your server is listening on, change it if you need Where and are network specifiers, such as 10.0.0.0/8.Tcpdump -ni eth0 “tcp port 443 and (tcp & 0xf0) > 2)] = 0x16)”Įth0: is my network interface, change it if you need You can look for external recursive queries with a filter such as udp port 53 and (udp & 1 = 1) and src net not and src net not On many systems, you can say "port domain" rather than "port 53".ĭNS servers that allow recursive queries from external networks can be used to perform denial of service (DDoS) attacks. However, DNS traffic normally goes to or from port 53, and traffic to and from that port is normally DNS traffic, so you can filter on that port number.Ĭapture only traffic to and from port 53: port 53 You cannot directly filter DNS protocols while capturing if they are going to or from arbitrary ports. Show only the DNS based traffic: dns Capture Filter Display FilterĪ complete list of DNS display filter fields can be found in the display filter reference The SampleCaptures has many DNS capture files. TCP_Reassembly has to be enabled for this feature to work. ![]() As you might have guessed, this takes a DNS request or reply that has been split across multiple TCP segments and reassembles it back into one message. The DNS dissector has one preference: "Reassemble DNS messages spanning multiple TCP segments". Also add info of additional Wireshark features where appropriate, like special statistics of this protocol. ![]() XXX - Add example traffic here (as plain text or Wireshark screenshot). The well known TCP/UDP port for DNS traffic is 53.
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