Questions tagged [tcpdump]

a command line packet analyzer

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Why 802.1q protocal packet auto dropped by interface?

I transfer a series of test packet with IEC61850_SV protocal in my host as follows: 01 0C CD 04 00 03 00 0C CD 04 00 00 81 00 80 01 88 BA 40 03 01 24 00 00 00 00 60 82 01 18 80 01 01 A2 82 01 11 30 ...
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Is there an optimized solution for tcpdump pulling a specific IP?

I made a script that runs tcpdump in the background: nohup tcpdump -i $INTERFACENAME dst $IPC >> text.txt And I made another script that pulls any IP address other than $IP from text.txt, but ...
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associate IP with hostname without editing /etc/hosts

I know I can associate hostname with my IP address in /etc/hosts: 1.2.3.4 foo and then, for example in tcpdump output, I will see foo instead of my IP address (if -n was not used) Anyways, can I ...
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tcpdump: always show numerical port numbers

The option -n in tcpdump is useful. Sometimes I prefer to see IP addresses, and sometimes I prefer to have the IP addresses resolved into hostnames. But never do I want to have port numbers translated ...
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System clock not synchronized with NTP server using systemd-timesyncd

I followed this answer here, but it seems that my system clock doesn't synchronize with NTP server: $ cat /etc/debian_version 10.9 $ egrep -v "^$|^#" /etc/systemd/timesyncd.conf [Time] NTP=x....
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TCPDUMP IP UNKNOWN

I am seeing this in the tcpdump output 21:32:51.580969 IP UNKNOWN.9998 > 255.255.255.255.9998: UDP, length 12 What does 'IP UNKNOWN' mean? Is something hiding the ip address? Thanks
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RTS and CTS packets filtered with 8814au

I try to sniff air with Realtek 8814AU. I tried driver from: https://github.com/aircrack-ng/rtl8814au.git https://github.com/morrownr/8814au.git I have compiled source code on Ubuntu 21.10 (5.13.0-...
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netstat output does not match tcpdump output

I have a syslog server that is receiving data on port TCP/514 from a router that has a main and an alternate interface. I have noticed something weird where the output of netstat shows the IP address ...
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tcpdump in Kali Linux VM does not capture scp between two devices

Situation: I'm learning how to develop tests for a hardened server my company is developing for a client. The test configuration will consist of the test target (the server we're developing) and an ...
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315 views

Is possible to capture packets between two hosts from a third host in the same network?

I want to see traffic between two hosts from a third host, there are all on my network I did this on the third host: ip link set eth0 promisc on tcpdump -vv -i eth0 -xX host 192.168.0.2 and host 192....
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Understanding of BPF

When I need to capture some packets using tcpdump, I use command like: tcpdump -i eth0 "dst host 192.168.1.0" I always think the dst host 192.168.1.0 part is something called BPF, Berkeley ...
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Sniffing traffic and promiscuous mode

I've been playing with tcpdump lately and was surprised to see that traffic not sent to/from the host running tcpdump is not picked up. The default tcpdump mode is promiscuous. I've also seen that the ...
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How to log Source IPs of Incoming File Transfers above a Specific Size?

I am trying to create a filter with tcpdump that logs the source IPs of only incoming file transfers above a specific size irrespective of the protocol that is being used for the transfer. For ...
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-Y and read {src,dst} port and tshark

tshark get data from interface or pcap files. When it read data from interface, user has to write filter with -f (accortding to pcap-filter(7)) and when read from file user has to write filter with -Y ...
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how to modify pcap file for Additional Information in packet

I have one Pcap File which consist of Some Information. I need to modify the data in the file. Example: As you can see There are details of packets and in Additional Record section I have TXT value ...
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Why can't assign permission for normal user?

Install tcpdump on my debian10: sudo apt install -y tcpdump which tcpdump /usr/bin/tcpdump groupadd pcap usermod -a -G pcap $USER sudo chgrp pcap /usr/bin/tcpdump sudo chmod 750 /usr/bin/tcpdump sudo ...
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What does double n mean in tcpdump command?

The tcpdump manual shows: -n Don't convert addresses (i.e., host addresses, port numbers, etc.) to names. I have seen double n in a tcpdump command: https://danielmiessler.com/study/tcpdump/ ...
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Is it possible to regenerate a TCP request with the help of tcpdump

I'm developing a backend server and here is how the architecture of the backend looks like: APP --> Server1 --> Server2 --> Server3 --> MyServer APP sends a HTTP request to the Server1, ...
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IPTables issue between proxmox and pfsense VM

I'm kind of desperate with an IPTables issue. I started to setup my own dedicated server which is a proxmox server. I have a pfSense VM to which all traffic is routed. I secured the LAN access by ...
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tcpdump isn't storing packets to file upon power loss

I'm working on a pi device that I can plug into the network and it will automatically start packet capturing to a rotating file. The idea behind the device is that when one of my clients is having a ...
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Does writing tcpdump output to a file on ramdisk have effect on the number of packets dropped?

When running tcpdump, I see that sometimes number of packets dropped by kernel is greater than zero (0). I was told to write to a file on ramdisk instead of a file on harddisk. I am not expert on ...
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What is exactly the timestamp measured by tcpdump?

We have a server that communicates over UDP with two clients (client1 and client2), via the same NIC through a switch. The server runs on debian 9 standard kernel (no real time patch). The system is ...
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Snort not detecting 3306 traffic, but tcpdump on port 3306 DOES show traffic

My scenario is: Server with Snort, mariadb, etc Attacker (client) I'm connected to the DB from the attacker, and whenever I execute regular SQL commands while in the server this tcpdump command is ...
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tcpdump, determine transport protocol

Is there a easy way to determine the transport protocol of a packet by looking at the output of a TCP dump trace? I'd just like to be able to tell if a packet is UDP or TCP. I've tried using the -x ...
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tcpdump to monitor snmp from subnet causing syntax error

I'm trying to monitor SNMP from 2 hosts on the same subnet; 10.52.17.31 and 10.52.17.32. They are on the only machines on that subnet. Using the following commands I receive a syntax error: tcpdump -...
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Why tcpdump does not work with -w flag?

So I have a network that I want to scan with tcpdump and dump the captured traffic into a file. Let's say I try: sudo tcpdump net 192.168.0.0/24 tcpdump will start and will show the traffic in the ...
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How does tcpdump choose the default NIC to listen on?

From TCPDUMP(1) -i interface --interface=interface Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up ...
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tcpdump to perform wireless capture with advanced filters

I’m trying to use tcpdump to perform realtime wireless captures (without the option to save the file) on a MAC. My wireless interface is en0. I want to use similar filters as I would in Wireshark. My ...
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Tcp connection not seen on peer node

I got several nodes to run my software. After startup, I found tcp connection is abnormal between two nodes. Assume A is the leader which is listening on port 1234, B should connect A. On node B, use ...
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Is tcpdump a client of rpcpad and implemented in pcap?

https://www.tcpdump.org/index.html#documentation has manpages for pcap, tcpdump, and rpcapd, but I don't find what relation is between the three. Is pcap a C library for implementing a client of ...
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How do I filter "BOOTP/DHCP, Request" out in tcpdump?

There are a lot of "BOOTP/DHCP, Request" in tcpdump and I would like to filter it out. It's easy to filter arp out. tcpdump -nni eth0 not arp What about BOOTP/DHCP, Request? I've tried the ...
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tcpdump: why ack number greater than sack number?

I use tcpdump to troubleshot network problem, and found one record that ack number is greater than sack number: Why ack number greater than sack number?
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What is Fedora doing in this TCPdump?

So I decided to have tcpdump listen on port 80 because I use HTTPS whenever possible, and thought there shouldn't be anything there. Well, there was. I can see that one of these connections is the ...
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How to use tcpdump safely (-Z option vs file capabilities / setcap)?

I am evaluating, how tcpdump can be used in a safer manner with as few as possible privileges. Two possibilities: 1. -Z option If tcpdump is running as root, after opening the capture device or input ...
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tcpdump -G option for rotation

I would like to rotate the tcp dump saving every 100mS. Hence I am trying the following option: tcpdump -i eth0 -G 0.1 -w '.pcap' This dumps all the incoming data into file and do not rotate every ...
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tcpdump displaying no output

I'm currently learning tcpdump and having trouble understanding it. When I type: sudo tcpdump It has all traffic going in and out of my machine. How would I apply this to listening to everything on my ...
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What is the filter expression to only sniff ICMP packets?

What is the filter expression to only sniff ICMP packets given the source IP address is 18.23.54.78 and the destination IP address is 12.67.24.89? The filter order must start with the protocol name ...
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Filter tcpdump output (PCAP)

The following is the output from my PCAP file. 20:02:52.306161 192.162.70.150.58078 > 192.179.91.61.1194: P 635362993:635363048(55) ack 2046024708 win 4373 <nop,nop,timestamp 52993632 1059054949&...
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filter by incoming ip v4 packets with tcpdump

I want to filter with TCPDUMP only incoming and ip v4 packets. tcpdump -i eth0 inbound and ip -w res.pcap give me incoming packets with ipv4 and ipv6 How can I filter by ipv4 only?
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Using tcpdump to trace IP of strangers(predators) on omegle

I am using tcpdump to capture packets while being on omegle. However, I have been fairly unsuccessful to get strangers IP. All the IP I see are traced back to either 172...* which is for google 104....
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Why do my Send-Q keep getting loaded but nothing come out of it?

I'm currently diagnosing a issue related to packet forwarding. I forward link layer packets to B network namespace and I found that B network namespace didn't reply to them, but I could curl and view ...
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How to extract domains related to "Client Hello" in a pcap file

I have a tcpdump (pcap) capture file and want to extract all domains. I do it with the following command: strings capture_file | grep -oiE '([a-zA-Z0-9][a-zA-Z0-9-]{1,61}\.){1,}(\.?[a-zA-Z]{2,}){1,}' |...
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non promiscuous option in tcpdump

Strangely, depending on the program in which it is included, the tcpdump command used with the -p option (which should say "don't use promiscuous mode") nevertheless switches it on during ...
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capturing and analyzing packages through Raspberry Pi 4

So I am a beginner to RP4. I am trying to learn more about network security using the Raspberry Pi 4. I managed to set up the raspberry pi 4 as an access point. I am able to connect my personal laptop ...
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How can I forward output from one terminal to another server where I can use grep?

I need to run tcpdump on some switch and pass it through grep with advanced regexp. That switch shell ist quite stupid and there is no grep with regexp but there is a tcpdump with the options I need. ...
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I installed tcpdump, but it is showing command not found while using it

Also I used whereis & which command to check if the package exists and it does exist.
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Timed tcpdump: pipe is slow

What I actually want to do is measure tcpdump for 10s. $ timeout 10s tcpdump > my.log but I found there has only a short period actually being logged, instead of whole 10s. therefore I did this ...
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Why there are a lot of packets not related to my IP been captured with tcpdump ? (Centos 7)

Tcpdump running by : /usr/sbin/tcpdump -U -n -i eth0 icmp or tcp dst port 22 or tcp dst port When I review the captured data , it shows : 132.132.211.221.40372 > 89.31.125.17.ssh: Flags [S], ...
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Can "dumb" ethernet hub device be used for breaking security on Ethernet cable connection? [closed]

Many people will still know about those old ethernet "hub"-devices - also called "dumb" device or "Ethernet signal concentrator". Did anyone ever try to "listen to ANY traffic" on such port through an ...
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Get the user-agent from tcpdump

I'm working on a monitoring application for my home gateway and I want to know the device type of all connected clients. For that purpose I'm using tcpdump to extract the user-agent of all devices ...

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