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

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“No route to host” when i do a telnet in azure

I have two VM's in azure with different public IP's and whose private IP's are : 10.10.1.9 10.10.1.6 When i do a telnet with following command from the server 10.10.1.6, i get the a error: telnet ...
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19 views

Large payload - connection get's Hung with CLOSE_WAIT or LAST_ACK state

We have a webservice client which posts data to a 3rd party Webservice. We are having an issue when the payload is a bit large (Around 600KB) xml. Once we pay the process hangs forever. The Netstat ...
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2answers
32 views

Wireshark Packet Capture Data Data ACK Confusion

I understand how acks work and windowing works. What I am not getting is why am I seeing the following behavior in packet captures Client Server data1-----> data2-----> <--------ack ...
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2answers
65 views

How does TCP offload NIC cause the TCP checksum to be invalid?

A tcpdump pcap exported and being investigated on another machine with wireshark is showing a lot of invalid TCP checksum messages. This is a known and documented phenomenon when using TCP offload ...
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34 views

Linux server failing to send SYN/ACK after receiving SYN remotely, works locally

Problem: A server process can be accessed by a device on the LAN or by the server itself, but it cannot be accessed outside the LAN (port forwarding is configured correctly as the server does receive ...
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2answers
28 views

Count number of incoming connection on a port - Linux

We have a server which listens on port X. The server has a large number of clients, from time to time the process gets hung, I am seeing SYN flooding messages in the log. I have been trying to tune ...
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2answers
35 views

How do I see absolute time stamps in Wireshark?

There is an example of pcap file opened in wireshark The second column is time. Is it possible to see absolute timestamps here instead of relative?
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20 views

Is it possible to specify tcpdump that I want it to interpret packet differently before printing it to STDOUT?

I am using iperf to pinpoint packet reordering issue. Unfortunately, tcpdump does not know how to parse iperf's UDP payload that I could use to pinpoint the issue. I know that iperf stores packet ...
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1answer
41 views

Why are ICMP packets not captured on the target interface?

I have a network setup as in the picture: The central box is a gateway (Ubuntu 15.10) which relays the packets betwen the various networks (only one is shown on the picture - lan0) and Internet. ...
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2answers
24 views

tcpdump http containing non expected payload bytes

I am monitoring an openvpn link with tcpdump a la tcpdump -i tun5 -w capture.dump -W 100 -C 100M -s 0 -n There is http/xml traffic going over that link which is basically caputered just fine. However, ...
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29 views

PCAP filter for first few packets, and packets related to disconnection, TCP-only

I'd like to use tcpdump with a filter for some long-running debugging. Specifically, I only want to capture packets with the following conditions: First 10 packets or so of a new TCP connection ...
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1answer
72 views

Use tcpdump to capture an ICMP destination unreachable message

I am new in this area. I am asked to use tcpdump to capture an ICMP destination unreachable message in my network course. So I am trying to capture one message that is an ICMP destination unreachable ...
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41 views

NATted host cannot talk to itself unless virbr0 is in promiscuous mode

We have the following situation: A hypervisor using KVM-Qemu/libvirt with a single VM. The VM has the internal IP 192.168.122.151 on the default network pool using virbr0 on the hypervisor. We've ...
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1answer
27 views

tcpdump - filter applies after some time?

I have a strange problem with tcpdump - it's like the filter I want to be applied to captured packets doesn't work instantly. Maybe it's perfectly normal but to be honest I've never seen this before. ...
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35 views

Analyzing SSL Encryption Alerts

I have captured the traffic between my machine and my server using Wireshark, trying to find why some SSL requests fail. I've read this and this post about debugging SSL entries, and I'm able to find ...
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2answers
28 views

Ubuntu server - packets from different networks not coming through (flushed iptables)

I have set up a brand new Ubuntu server, and I'm encountering a very strange network related issue - the processes running on the server cannot 'see' packets originating from external networks, but ...
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34 views

Duplicate SYN package forwarded by Docker?

I'm experiencing a strange interaction between a CISCO firewall and my Docker host connected to it: the CISCO periodically marks my host as a SYN attacker and shuts my ethernet port down. I've been ...
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1answer
133 views

Server send TCP ACK with wrong number after receiving SYN

I have a client establish new TCP connection then sends request to a server per second. Both client and server are on the same linux box (via loopback interface). The client periodically reports ...
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2answers
35 views

Monitor networking between two interfaces

I need to monitor TCP networking between two hosts which does not have tcpdump installed, and I cannot install on them. Is it possible to do so with tcpdump using some known rule ?
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69 views

Xen networking - dom0 receives ARP requests but no reachability

Both dom0 and domU are Debian 8. Dom0 network configuration is: auto xenbr0 iface xenbr0 inet manual bridge_ports eth1 bridge_stp off bridge_waitport 0 bridge_fd 0 auto xenbr0.4 ...
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2answers
3k views

TCP Messages Merged?

I have an application that sends 100 of 186-byte (excluding headers) TCP messages back to back without gap from host A to host B. I ran tcpdump to capture the packets on host A (where the sender is), ...
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2answers
49 views

How to send captured packets to a different destination?

I have some data packets captured using tcpdump in a pcap file. Now I want to send those packets to a another destination. how I can achieve this?
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119 views

tcpdump on bridge interface (virbr) does not receive any packets destined for one of its addresses

Here's the configuration for the interface: # ifconfig virbr0 virbr0: flags=4355 mtu 1500 inet 192.168.11.1 netmask 255.255.255.255 broadcast 255.255.255.255 ether ...
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1answer
51 views

How can tcpdump monitor TCP disconnect and log timedate stamp?

I have two servers each running a Java application that creates two separate TCP connections, one from HostA -> HostB and one from HostB -> HostA on separate ports. I am told the TCP connection is ...
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31 views

How does TCP PSH and HTTPS works?

Right now I monitor traffic on my VPN server. I'm using tcpdump for it and it works pretty good. sudo tcpdump -n host SENDER_IP In this example you can see one HTTPS request from SENDER_IP to ...
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46 views

Server socket sitting in FIN_WAIT_1 whilst network doesn't see its traffic

We've been trying to get a grip on a really weird problem where we can wget a page from Apache 2.2.19 on solaris 10 and some permutations of requests reliably take various fixed lengths of time to ...
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51 views

Monitor outgoing web traffic sources on CentOS [duplicate]

I received a report today that a server I manage (Centos + Apache) is launching a bruteforce attack against wordpress websites: hacked-joomla/brobot The requests sent look like this: x.x.x.x - - ...
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25 views

Not getting TCP Update Window packet after TCP Zero Window begins

Once I get TCP Zero Window during data transmission, I can't send data anymore in that desktop sharing session. Even I stopped the data sending to check if it can resume but no benefit! In my test ...
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1answer
192 views

Wireshark shows “TCP Dup Ack” on SACK after each regular ACK

I have a TCP session captured via switch port mirroring and tcpdump. When viewing it (in Wireshark), I see the same pattern whenever I send a message; here's an excerpt of the outbound packets (I ...
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61 views

wget only works when tcpdump is listening

I am running two Virtual Machines over the XEN hypervisor both with Ubuntu trusty (Linaro-Developer version) with the following simple bridge networking setup: Backend domain (dom0): br0 ...
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62 views

“Filtered” port when accessing server

I'm having periodic trouble accessing one of my DigitalOcean servers from Azure machines. I have isolated a test that - I believe - demonstrates the issue and captured a tcpdump from the server for ...
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1answer
87 views

On-the-fly decoding HTTP bodies on Linux?

My wish is to monitor HTTP exchanges between my server and a browser. At present time, I use the localhost because I'm developping the app. I installed and used tcpflow and found it quite useful but ...
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25 views

What does “-tnn” mean in “tcpdump -i eth0 -tnn dst port 80 -c 1000”

What does -tnn mean in tcpdump -i eth0 -tnn dst port 80 -c 1000. I didn't find -tnn on the man page, and I didn't find it on the Internet. Can anyone explain it to me?
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45 views

Packet capture on ESXi host

I have an issue I am trying to track down and I believe the problem is with physical networking hardware. I have read the VMWare documentation on the pktcap-uw command and I know I can use it to dump ...
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89 views

Wireshark RST against TCP Zero Window

During application sharing with Microsoft Lync Client (Mac OS X), TCP ACK with RST flag is sent from my application end to Lync end against TCP Zero Window packets and call gets dropped. Image Link. ...
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1answer
190 views

Finding google unusual traffic

We are a small Internet provider. In order to get Internet access we are using NAT (10-20 users per one public IP). And lately we've met with Google blocking services (captcha and full block) and we ...
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330 views

Sometimes unable to tune Linux TCP stack for improved high-latency performance

I have been unsuccessfully trying to apply the TCP Tuning techniques discussed at http://fasterdata.es.net/host-tuning/ As you'll see when you read through the entire question, sometimes the es.net ...
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48 views

pfsend does not send a pcap file at specified rate

I'm replaying traffic using pfsend application that comes from PF_RING. I'm using SMIA pcap file to evaluate Bro and Snort on a multi-core machine. Using pfsend, we can specify at which rate the pcap ...
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3answers
551 views

How would a PCAP filter look like to capture all DHCP related traffic?

As I understand it, for IPv4 I would need to capture UDP port 67 and 68, ARP, ICMP echo request and reply, and for IPv6 I would need UDP port 546 and 547, all DHCP-related multicast addresses, ...
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50 views

OOM Killer when tcpdump is run in system with low memory

I am working on an embedded system with limited memory. Here is the dump from my vmstat: procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu----- r b swpd free buff ...
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1answer
94 views

Is it possible to run 2 concurrent tcpdump with different options?

I need to run 2 concurrent tcpdump commands with different arguments/options. Why ? Because we wrote some long long scripts compatible with following options : tcpdump -ixenbr0 -s 400 -n -A 'port sip ...
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91 views

Capture all mysql requests/responses and caculate the response time using tcpdump

I need to capture all mysql requests/responses using tcpdump and after that need to find out which response related to which request in order to caculate the response time of each request. I used the ...
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1answer
341 views

TLS Error: Handshake failed. Openvpn Server is behind NAT on dynamic ip

this is a common problem that appears easily in searches and seems to be related to the firewall , yet I can't seem to figure out how to fix it. I have actually asked before, but this time I come back ...
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1answer
85 views

Tcp connection breaking, too fast for serial?

I've been having real difficulties getting a tcp connection to work with my web server. I think the issue might have something to do with the speed or ordering of the packets as the connection seems ...
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1answer
108 views

Sniff outgoing traffic on my server on a SSL channel

Im executing a CURL command on a php application running on my nginx server (centOS). The end point is a https address so the outgoing traffic is of course encrypted. I want to see the network ...
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34 views

tcpdump returns no multicast traffic when a DNS entry is being used in resolv.conf

I have noticed on a 6.5 Centos host that when I am using tcpdump to determine if a multicast source is being received/ sent that it does appear to get any results when I have a DNS registered in the ...
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40 views

Remote OSX Server does not accept TCP connections from abroad?

Long story short: I am able to connect to the box (which is located in my office) from home. Two different ISP providers. On the contrary, my friend from the US is not able to access it - the ...
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154 views

how to use tcpdump on interface with link down

A set of daemon applications communicating over UDP broadcasts on the same machine give me trouble capturing their traffic. The network interface is eth0, it's configured to a static IP and there's no ...
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65 views

Two processes stuck on network communication

We have had hang on our Oracle extraction process very intermittantly. Nothing obvious from the Oracle side, and starting to think it is more network related. There are two Oracle processes ...
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1answer
241 views

tcpdump freezes and not capturing properly without internet connection

When I don't have any internet conncetion and I want to capture network between local computers, tcpdump acts like this: 1.I start tcpdump using following command: tcpdump -ixenbr0 2.After I run ...