Questions tagged [tcpdump]

tcpdump is a CLI tool for capturing and displaying packets sent and received by network devices.

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Site-to-Site VPN from AWS to DO: Unreachable from One Side

I have a new AWS Site-to-Site VPN tunnel. The tunnel is currently "UP". It uses a Customer Gateway that points to an Ubuntu server running strongSwan VPN in DigitalOcean. I'm attempting to ...
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Why is tcpdump not seeing certain traffic

I'm trying to see traffic from a server interface that is receiving port mirrored traffic with tcpdump. Using tcpdump -i ens224 -s65535 -w /tmp/file.pcap I can see that all port mirrored traffic is ...
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Is there a way to disable TCP segmentation offloading to prevent packets greater than the MTU from being captured from the loopback interface?

I'm trying to create some captures on my linux box. The problem I have is that any captures I take from the loopback interface will contain massive packets that are much larger than the MTU. This ...
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Kernel TCP stack not forwarding packets immediately to remote server

I'm trying to figure out why a single HTTP POST request over a re-used TLS connection is still taking roughly twice the roundtrip time (RTT). The client is running on Debian 10.8 within a container on ...
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Can (and do) I use iptables to expose a host directly online (not using NAT)?

I'm using a DSL-5300 VDSL modem/router which is set up with NAT for my house, and works great. Its web interface hasn't been written to support a mix of NAT and no-NAT traffic. I've been assigned 8 ...
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TCP: Why would I get a PUSH+ACK after sending RST

I'm rolling out my own web server and I rely on TCPDUMP to see whats happening behind the scenes. The server address is 192.168.7.2 and the client is 192.168.7.1. I used curl (and also tried wget) to ...
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need help to understand tcpdump

Background: I have a web server hosted on 10.20.30.177:7000 and it should be accessible from 10.20.30.1 (m1) and 172.17.2.88 (m2). I am able to access from m1 and not from m2. I understand there can ...
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tcpdump: invalid cksum from client to localhost interface even after disabling offloading

I'm having a strange issue. In linux I have two windows open. one running tcpdump and one running curl. I used curl to try to make a connection to local host and tcpdump logs invalid checksum errors. ...
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Two clients can connect to OpenVPN server but another cannot

I have an external OpenVPN server running Ubuntu 20.04 and 3 clients: Client1 (Android 11) Client2 (Windows 10 and Virtualbo host) Client3 (Ubuntu 20.04 Virtualbox guest) All 3 clients are able to ...
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openvpn server not showing traffic using tcpdump

I have an OpenVPN server and two clients, I can ping clients from each other, but when I do so, I don't see any packets on the server using tcpdump Server is running on 192.168.0.1 and I try to ...
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Capturing IEEE802.11.x auth and capabilities data with tcpdump?

I'm troubleshooting some issues with my wireless network (OpenWrt 19.x and mobile device) and I came across this: https://dot11.exposed/2017/09/20/violation-of-802-11-standard-intel-wireless-cards-...
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experimenting with vlan setup in terms of inspecting

I initialized vlan test interface over default enp7s0, using ip link add link enp7s0 name test type vlan id 2. As I have very limited experience with the configs of networking, I left things like ...
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connection is paused for long

a redis server is running in a different network namespace. I'm trying to connect a client to the redis server, it keeps waiting but is not getting response, its been hours. Client side code: int main(...
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How to filter out the network timeout cause out of the output of tcpdump, wireshark and such tools?

third party Linux server sending around 15 GB of the data each day via FTP port 21 to my Linux (CentOS) server (headless). Problem is that recent days some of the transferred files timeout: ...
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How can I see the response of the DNS requests that tcpdump is showing me?

I'm using tcpdump (for the first time) to try to debug a DNS issue: tcpdump -n udp port 53 It gives me this output: 10:38:30.431467 IP a.b.c.d.56973 > 8.8.8.8.domain: 49179+ A? ocsp.sectigo.com. (...
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Why echo request doesn't show in tcpdump?

I have two network segments connected using a IPSEC-VPN tunnel : NET#1: a.b.0.0/16 with LAN gateway a.b.0.1 NET#2: x.y.0.0/16 with LAN gateway x.y.130.1 I initiate a ping from a.b.0.1 to x.y.130.100 ...
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Can I determine a EWOULDBLOCK/EAGAIN situation from a pcap/tcpdump

Is there a way I can determine if a certain message resulted in an EWOULDBLOCK/EAGAIN return code to the server which sent the packet? The server was sending messages to the client At time 10,the ...
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385 views

Where does tcpdump get the source IP from for a TCP packet?

I've been trying to come up with a solution for a routing problem (multiple interfaces connected to a single Docker container, ensuring response packets go out the right interface), and have come ...
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Intermittent connection failure (TCPDUMP RST)

From a ubuntu 20.04 server, if we curl https://google.com -v a handful of times, it'll get really slow (5seconds - timing out) while establishing the SSL handshake. This is true for any host, not just ...
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Identify Source / Trigger of / for Network Traffic locahost

I want to identify the application trat triggers network traffic like the one below. I have dissabled ethernet and wireless cards, and denied incoming traffic in firewall. $ sudo tcpdump -vv -e -A -i ...
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Nginx post-ssl-termination debug mirror

How can I tell nginx to output all incoming traffic for a given server{} to a specific/additional/redundant location after decrypting it (like port mirroring on routers) so that it can be analyzed for ...
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43 views

How to split pcap per IRC channels

I wonder if there is a way how to split a pcap that contains (only) IRC per channels. I'm trying to compare accuracy of botnet detection in channels using existing IDS (suricata, snort). The only way ...
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Packets dropped somewhere between network interface and iptables when forwarded from router

I have a server configured with multiple interfaces and multiple VLANs. It works perfectly fine for all the local networks, it drops packets forwarded through my router for some reason though. And it'...
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Pcap (tcpdump) filter to match against local port

I'm trying to write a libpcap (tcpdump, iftop) filter that would match packets having a specific local port. That is, I'm interested in traffic that either goes out of port 12345 on the local machine ...
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How to specify both IP address and port in tcpdump?

I want to specify a source IP address, destination IP address and source port in my tcpdump filter. However, I keep getting syntax errors. How do I do that?
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Wireshark TLS conversation for ldaps

I am trying to troubleshoot an issue with delays in LDAPS bind operations with tcpdump/wireshark. Here is what I get from client side on a conversation with delays: I see the following problems: I ...
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203 views

Forwarded Packets are received by namespace's veth0 but not received by application

I use libtins (It uses Pcap) to capture link layer packets and forward to a network namespace where the actual application runs on. Client(Browser) -> Server -> Pcap -> Pcap Send -> br0 (...
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493 views

tcpdump can't open file: permission denied (linux)

I set all filters for Tcpdump in a config file, but when I run this command: sudo tcpdump -i any -w "./mypcap.pcap" -F "myfilters.txt" I get permission denied although my file ...
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How to find out which http/s request is blocked due to hosts file?

I saw many packets on loopback (lo0) interface on wireshark and tcpdump like this 171 1118.753013 127.0.0.1 127.0.0.1 443 TCP 68 54857 → 443 [SYN] Seq=0 Win=65535 Len=0 MSS=16344 WS=32 TSval=...
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communication between 2 servers via NSP

I'm in a situation where one server(tb06) should populate its health status to another server(admin03) via NSP in nsxml format. When I have taken TCPDUMP(.pcap) on admin03, apart from tb06 HTTP header,...
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How can I capture IPSEC packets on my VPN server?

I am running a strongSwan (U5.3.5/K4.4.0-62-generic) VPN server on Ubuntu 16.04. Ususally, when I want to examine traffic on a server, I simply run something like the following: tcpdump -ni eth0 "...
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Display delta timestamp and UDP data payload with tcpdump/tshark

I have a 1.5Gb capture of small UDP packets that I would like to turn into a CSV with just timestamp and UDP payload data in hex format. An example of the desired output would be as follows: % head ...
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661 views

tcpdump, how to capture actual data only?

For example I have a server listening on port 8001, a client programe open a tcp socket connect to that port, send some binary data, I want to capture the actual data only without any TCP/IP header ...
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63 views

Re-routing DNS queries with iptables properly without resolv.conf

I run a custom DNS service on 127.0.0.1:53 and I dont just want all queries to go through it, I also want only that service to respond back. Which is where the problem arises. My NAT iptables setup: ...
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494 views

Trying to capture 3-way handshake in linux

I am trying to capture 3-way handshake using the following in linux, it does not seems to work... sudo tcpdump "tcp[tcpflags] & (tcp-syn) !=0" -w filename.pcap -i eth0 Could you someone suggest ...
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50 views

Proxy service only responding on some IP addresses (others suck on SYN_RECV)

I have a server using a proxy service (WAF etc) which forwards packets to my server. I can see established SSL conenctions from all proxy netstat -an and the rest stuck in SYN_RECV: tcp 0 ...
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How we have differentiate the packet capture based on the RX & TX

I am working in the security industry. We have taken packet capture on the firewall multiple times. Can we take Pcap based on RX and & tx and force the tcpdump command to write the output in two ...
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TCP Window Scaling Not Working Between Windows 10 and Linux Server

We are having some issues with several clients running Windows 10 downloading files from our HTTP file delivery server. We can't reproduce the error on our end but notice that TCP window scaling is ...
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SSH connection not established, but standard TCP/IP connection works

I'm working on a custom yocto Linux for a Raspberry PI 3 and try to get the WIFI connection working with SSH. However when trying to connect from my PC (Ubuntu 19.10, SSH OpenSSH_8.0p1 Ubuntu-6build1, ...
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tcpdump not capturing packets but Apache log shows the request

I'm trying to debug a client side javascript application that sends a XMLHttpRequest to a php script but I don't get any output from tcpdump when this happens. Apache logs the request, the script ...
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Does tcpdump guarantee packet integrity?

In normal use of tcpdump without any parameter, does it guarantee? tcpdump -i eth0 -w file.pcap In using tcpdump rotating by time with -G parameter, does it guarantee? tcpdump -i eth0 -w file.pcap -...
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Duplicate ACK and TCP Retransmissions

I'm doing some scp transfers between an AIX and a Linux in two separated LAN (I have a VPN between them). Sometimes the connection ends well, so I get 50MB/s. In many other cases, instead, the ...
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89 views

Why is an interface ignored by -i any with tcpdump/wireshark?

Using tcpdump and wireshark, if I capture with -i enp1s0f0, no problem I can see the packets. If I capture with -i any, I don't see the enp1s0f0's packets. Why? Is there a NetworkManager option or ...
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144 views

How do I capture packets on a Linux VM in a VMware environment?

I've got a Linux server that is a VM running on ESXi 6.7. We're running into some network trouble with it, and I'm trying to use tcpdump to capture traffic on the VM itself, but I'm getting only the ...
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138 views

Monitor MySQL Connection and Request

I'm trying to monitor all incoming connections to a mysql database. I've used tcpdump port 3306 and '(tcp-syn|tcp-ack)!=0' currently which is showing connections but is there a way to also see what ...
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25 views

Why does one of four-way handshake miss with the command nc

I'm using the command nc to simulate the TCP connection with a client and a server. I execute the command nc -l 1234 and tcpdump -i eth0 port 1234 at the server and I execute the command nc ...
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139 views

Tcpdump logfile

the thing is I am trying transfer log file from one server to another dividing by size. I have dump such as: tcpdump -i any tcp x.x.x.x -s 0 -G 86400 -w /var/log/%F.zip Questions are: The output ...
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315 views

Sending tcpdump/pcap files to a remote server

I want to get all traffic on an embedded system, using tcpdump. I will send those files via sftp or ssh to my server. Can it happen, that tcpdump "sees" the transfer of the pcap files to my server? ...
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241 views

Traffic on port 80 although Http-Server is not running

Good'day Just out of curiosity I did a tcpdump port 80 on one of my remote virtual machines. I see a huge amount of requests to this http-port, 95% going to some AWS EC2 instance. Strange thing: The ...
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Strange (mail related) traffic on my server

I'm noticing strange traffic on my webserver which is around 50kbps constantly. Doing a (few seconds worth of) tcpdump on the specific ip address, I get this (pardon the huge list): 09:33:02.385238 ...

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