Questions tagged [tcpdump]
For questions about traffic dumps from the packet analyzer tcpdump. This tag should only be used in relation to troubleshooting an issue with a network device. If the host that runs tcpdump has been converted to a network device (eg. firewall on Linux) then this tag is also on topic.
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How does a client end the TCP connection when SYN+ACK is not received after multiple retransmissions? [closed]
I am trying to establish a TCP connection with a server. I added a firewall rule in the server to drop the TCP packets in order to check how the TCP client terminates.
Client retransmits SYN multiple ...
1
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0
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393
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How I can filter only quic packets with `tcpdump` command?
I want to capture only QUIC UDP datagrams on 443 port.
With following command tcpdump reports all udp packets that arrive to the host on port 443.
tcpdump -n udp -SX -i any port 443
How I can set it ...
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0
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10
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why is the length same with the MSS value? [duplicate]
I have read this I have issue with MSS and window size and what is difference between MSS and Window Size?
I'm confused.
I have a setup like this: my host machine (Macbook) and a virtual machine (...
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2
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1
answer
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tcpdump output with bad length indicator present
Could someone help me interpret this tcpdump udp packet output? We are getting out image so I'm wondering why we are seeing this message. Is the packet length indicated by "length 1500)" or ...
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2
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160
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Detecting Ethernet over power by packets
If someone plugs an Ethernet over power (EOP, Homeplug) into the switch and then connect elsewhere in the building, can I detect this device by sniffing the packets with tools like Wireshark or ...
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2
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1
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961
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How to capture mixture of packets with and without VLAN tag over LACP with tcpdump?
Given following configuration in Linux:
bond0 in LACP (802.3ad) mode with two VLAN networks running over it.
bond0.111 configured with static IP
bond0.222 configured with DHCP
How to capture packets ...
1
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1
answer
91
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Does tcpdump work differently from Omnipeek?
I have been trying to analyze packets using tcpdump. I am trying to use a wireless card and capture packets through monitor mode, instead of promiscuous mode, since I want to see what kind of traffic ...
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1
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2
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230
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tcpdump find source ip with most bytes
some times we receive volumetric traffic from single source IP (out of our networks) towards one destination in our network,
is there anyway to find it with tcp dump ?
thank you.
- 565
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1
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tcpdump: cksum incorrect [closed]
When I run tcpdump on my machine (here I use 1.2.3.4)
tcpdump -i eth0 -n dst host 1.2.3.4 -v
roughly 90% of incoming packets have incorrect checksum:
cksum 0xc25b (correct), seq 101134607:101136035
...
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2
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1
answer
479
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Nmap differences between scanning with -Pn or without
I'm trying to understand the difference in NMAP between put -Pn or not. I know that if I'm not mistaken -Pn skip ICMP (ping) scan. Ok.. so I try scan a random VM and intercept the packets with tcpdump....
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2
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1k
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tcpdump filter for ERSPAN session ID value
I am using rcdcap to decapsulate ERSPAN on a Linux host. I would like to be able to split ERSPAN session IDs out to different logical capture interfaces. My current ERSPAN session IDs are 10 and 20. ...
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1
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57
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Packet analyze on router does not match client and server
thank you for your time.
My (virtual) network has 3 objects : a client, a server and a router between them.
I'm sniffing the traffic on the 3 machine with the command
tcpdump -i any -s 0 -w [path]
...
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0
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1
answer
66
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Is it possible to re-transmit only the last packet at TCP flow?
I try to understand the TCP behavior, especially re-transmission.
I did a small experiment and find the curious thing.
The only last packet is re-transmitted. Why do this happen?
I think if the ...
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0
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140
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Why wifi and ethernet use the same frame format from tcpdump -d output?
If I execute
tcpdump -i wlan0 tcp -d
or
tcpdump -i eth0 tcp -d
I get the same output
(000) ldh [12]
(001) jeq #0x86dd jt 2 jf 7
(002) ldb [20]
(003) jeq #0x6 ...
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10
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1
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Is there a way to capture only gratuitous ARP's using tcpdump?
I'm trying to set up tcpdump to filter only gratuitous ARP's. I know that I need to search for packets with a host set to ff:ff:ff:ff:ff:ff. And I found the arp.opcode parameter, but I can't seem to ...
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1
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Dump traffic on routing-instance with a Juniper SRX firewall
A Juniper firewall can display data with the tcpdump command:
% tcpdump -i ge-0/0/0
% tcpdump -i vlan10
However, if the interface or the VLAN is assigned to a routing instance, the traffic is not ...
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1
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345
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tcpdump - bogus ethernet address
I'm trying to capture packets on an interface.
The command is as follows
tcpdump -ni any port 1812 or port 1813 or port 3799 or port 8080 or port 15001 -s 0 -w /var/tmp/lb01_trace_$( date +'%Y-%m-%d %...
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1
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0
answers
46
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Remote monitoring using a non root user account with Sudo Privilege [closed]
I want to remotely take tcpdump on my server with a non root user account? I tried it but doesnt seems to be working? Can someone guide me on this?
plink.exe -ssh -pw abc123 iamuser@192.168.1.1 "sudo ...
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2
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2
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351
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tcp connection and stream flow question
Could anyone explain the main principe of tcp flow when making a session ?
I have take a tcp dump when i'm trying to browse a site. when I analyse the packet by wireshark, i have found many Three way ...
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3
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2
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438
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Which tcpdump metrics reveal server connectivity problems vs client problems?
I am attempting to diagnose a ~45 sec drop (to 0 Kbps) in the reported RTMP streaming bitrate in OBS while publishing a live stream. I had been running a tcpdump the entire time using Apple's ...
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2
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What does the output of this tcpdump mean?
Bellow I show the output of my tcpdump
What does the first two mean lines means(what are all the question-marks meaning is this a sign of a intruder)?
I am especially interested in the meaning of what ...
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1
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962
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Wireshark capture on LoopBack interface [closed]
We have two applications running on same machine, both of them are listening on the loopback interface on two different ports. Client Application is sending data to server, but receiving Application ...
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1
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697
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What is "cflow" in ipfix？
I am learning ipfix and found a cmd like this:
tshark -r ipfix.cap -n -d udp.port==4379，cflow -O cflow
What does the cflow part mean here?
I was also told to filter cflow if I use Wireshark in ...
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1
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721
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TCP close 2-way handshake with tcpdump package
Well, I got a pcap package that uses the tcpdump command. In this pcap, the TCP closes in 2-way handshake!
Questions：
In this picture, can anyone determine who initiated the signal to
close the ...
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1
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569
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tcpdump: out of memory when running big filter file
i am currently running tcpdump with the following command
tcpdump -r input.pcap -w output.pcap -F /filter_file
the problem is that tcpdump throws this error
tcpdump: out of memory
and i am ...
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-1
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2
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4k
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read filters from a text file with tcpdump
i would like to know as the title describes if there is a way read filters from a file in tcpdump
currently i use
tcpdump -r input.pcap -w output.pcap src host 1.1.1.1
and what i wan to do is make ...
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2
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1
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241
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Analysing a HTTP request with tcpdump
I am trying to understand the acks from the sequence numbers in a TCP/IP packet. AFAIK, an ack from a transmitted package is calculated by the following [1].
In [2] I have an HTTP request to a site.
...
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1
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How can I filter out particular MAC addresses when running tshark on a pcap trace?
I've tried a number of things, but none of them seem to do the trick. I want to run this command:
tshark -r data.pcap -T fields -e frame.time_epoch -e frame.len
but to have it ignore any packets ...
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2
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481
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Why does it show LAN address when NAT is enabled on cisco?
I have following network connections
LAN -> Cisco Router -> 16 port switch -> Server (netflow)
-> ISP
And i have following config on Cisco router
...
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3
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3
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783
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What more information can I get from filtering by MAC address rather than IP?
I want to do some very simple traffic analysis for a single device. I'm using tcpdump to collect this data. Is there an advantage to using a MAC address capture filter over a (static) IP address for ...
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1
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5k
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RST after an out of order packet
I've encountered an force termination by a host, even after a successful (at least it seems to be successful; correct me if I'm wrong) FIN ACK close of connection. Packet trace at the moment is ...
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1
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Where in PCAP-NG does the Source/Destination IP address information reside?
I'm studying the pcapng file format, and I know that when wireshark reads a pcapng file, it tells you all sorts of information. I've been reading the pcap-ng format and I don't see anywhere in pcapng ...
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3
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who determines whether should ping send a ARP request?
In my Fedora linux, My eno1's ip is 192.168.1.105 and my gateway's ip is 192.168.1.1, and netmask netmask 255.255.255.0
For monitoring ARP request, in one terminal, I used sudo tcpdump -i eno1 -v "...
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1
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`tcpdump bootp.type == 2` tcpdump: syntax error
tcpdump -i eth0 'bootp.type == 2'
Results in a:
tcpdump: syntax error
However, bootp.type == 2 works fine in Wireshark.
Also tcpdump -i eth0 'tcp[13] & 16!=0' works fine, no syntax error....
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1
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994
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How to Capture packets generated from linux pktgen traffic generator? [closed]
I am testing an openflow switch for its performance. I am using "pktgen" kernel module available in linux to generate packets at different flow rates. I have a host with two interfaces eth1 and eth2. ...
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1
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5k
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capturing specific BFD packets using tcpdump
I'm troubleshooting a problem with my site-to-site VPN at one location. I've narrowed it down to the link being up, but traffic dropping due to routing problems. I'm running ospf with over ipsec ...
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2
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1
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678
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Wireshark - How to find cause of incomplete download
I captured a 70MB tcpdump of a linux client that downloads many big files with curl. At some point curl aborts the download with the errror
transfer closed with 3304876121 bytes remaining to read
(...
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1
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2k
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debugging arp requests over vlan
We have a chain of switches configured for vlan 13, let's call them switch1, switch2 and switch3
No, somewhere on this chain the arp requests get lost. I can run tcpdump on these switches. Running ...
- 131
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1
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40
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Which terminal based tool can I use to display only the source and destination IPs of all packets [closed]
I am currently using ngrep or tcpdumpin conjunction to grep to display destinations and sources of all incoming and outgoing packets.
Would you suggest a better way to achieve this?
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why I didn't get three way handshake by tcpdump command?
That's what I do .
1、open tcpdump command sudo tcpdump -i lo0 tcp port 20009
2、start a sample server nc -l 20009
3、connect 20009 port telnet localhost 20009
4、tcpdump command got :
○ → sudo ...
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6
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2
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587
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TCP Flow-control questions that drives me mad
This is a capture of a TCP session when I was downloading the 10Mb file from an FTP server. The server has an ICW of 10x MSS, so this means that the server can initially send 10 segments without ...
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0
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65
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How to trap message between two servers? [closed]
I have two server one of them sends message in xml format to another server. The number of message varies i.e from 10000/sec to 1 million per second. The other end receives and stores it as file. Both ...
- 111
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1
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207
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Is it possible to capture ieee802.11 management and control frames from the same phone using android APP or tcpdump on root device?
I am trying to study the traffic between the mobile and access points. So I have phone Galaxy Nexus which does not support monitoring mode but still support promiscuous mode. Unfortunately, I try many ...
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2
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Why only after 3-duplicate ACK retransmission takes place in TCP?
Is there any reason of choosing "3" only for retransmission. why not 2 or 4 or any other number?
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Extract packets with ECN notification set.
I am trying to understand DCTCP behavior. For that I am using RED queue discipline on switch port. I am generating traffic using iperf3 for 30 seconds. To capture the packets i am using tcpdump. ...
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1
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10k
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tcpdump not capturing all TCP packets when capturing whole packets
I have a server and a client on different machines and on different locations.
I'm trying to capture TCP packets with tcpdump on an interface of the server machine. I've injected an identifier in the ...
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2
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624
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Seeing traffic not addressed to me using tcpdump, how does that work?
I did a tcpdump capture, and when inspecting the reults, I saw some data I couldn't understand.
Say the hostname of the box I did the capture on was box1.blah
I saw traffic like this:
box2.blah:...
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1
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503
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Different types of ICMP [closed]
I have a task on which I have spent a lot of time. I am not fluent in Linux, but I can manage basic things.
The task is to gather different types of ICMP packets. I can harvest them by tcpdump (which ...
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6
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2
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13k
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How to check if a NIC supports hardware timestamps [closed]
I want to capture packets using tcpdump and get as precise timestamps as possible. The latest version of tcpdump has the -j adapter command-line option, which can be used for this.
I tested the ...
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2
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3k
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How to interpret this `tcpdump` output
this tcpdump output is the result of sending a single ping package from host 1 to host 2 through a simple hub connection:
root@mininet-vm:~# tcpdump -XX -n -i h2-eth0
tcpdump: verbose output ...
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