I am running the following command in my server(GNU/LINUX 2.6.27.5):
tcpdump -i eth0 -w test.pcap
After stopping the packet capture manually it shows the number of packets captured & received by filter but the file test.pcap does not contain anything, means its size is 0.
tcpdump version used is tcpdump-3.9.8
Related
I am trying to capture the DHCP frames for analysis using the following command in my mac book.
sudo tshark -i en0 -f "port 67 or port 68" -a duration:300 -w /tmp/dump.pcap
I use the following command to get all the fields of all protocols in the packet but it is not printing any value. Is the capture filter option for the DHCP frame is correct? Any help is appreciated?
sudo tshark -T text -r /tmp/dump.pcap -V
Answer
Yes, your commands are OK. Maybe no DHCP packets arrived and therefore not captured. Try to force a DHCP activity by commands in second teminal window of the same device:
sudo dhclient -r
sudo dhclient
Warning: Do not apply these commands if you are connected remotely. First command releases the IP address and your connection will be interrupted without a possibility to put second command and get address back remotely.
Some details concerning data capture
The thsark filters have the same syntax as Wireshark.
Threre exist 2 (or 3) filter types:
capture filter, -f tshark option: It selects which packets will be captured and which not. This is useful e.g. for getting lower capture file size.
display filter, -Y tshark option: It selects which packets will be displayed from all captured ones.
You can combine both types.
Examples:
tshark -i eth0 -n -Y "ip.addr==8.8.8.8"
tshark -i eth0 -n -Y "ip.addr==8.8.8.8" -f "udp port 53"
tshark -i eth0 -n -Y "ip.addr==8.8.8.8 and udp.port==53"
All packets are captured, but only the 8.8.8.8 IP address packets are displayed.
Only the DNS packets are captured, and only the 8.8.8.8 IP address packets from captured are displayed.
All packets are captured, but only the 8.8.8.8 IP address packets having UDP port 53 (i.e. DNS) are displayed. Compare different syntax of the port filtering between the display and the capture filters in line above.
All other options like -a, -b, -w, -s can be applied too.
The tcpdump application is usefull too. It is available in most Linux systems even very small or special. It does not have a display filter option. Only capture filters can be applied. Other options are missing: -a, -b ...
sudo tcpdump -i eth0 -w /tmp/dhcp.pcap "udp port 67 or udp port 68"
I have the following command:
sudo tcpdump -ni enp0s3 -W 1 -C 1 -w file.cap
with this command I say: "listen on the network interface enp0s3 and capture all packets in a file whose maximum size must be 1 mb". It works, however the problem is that when the file reaches the size of 1mb, it is reset and the capture starts all over again from 0 kb, deleting all the packets.
I want that when the file is 1MB, only the older packages are deleted and the new ones are added replacing them. I don't want all packets to be deleted and acquisition restarts at 0kb. In other words, I want the file to always be around 1mb, adding the new incoming packets in place of the oldest ones.
You can use -U -W 2 with the -C size limit. It will then alternate between two files and you can concatenate them (or work on the older one).
Alternatives would be to write to a stream or pipe and not to files, at all.
I'm trying to run tcpdump on linux machine, which needs to write pcap on the remote server with file rotation every 10 seconds.
tcpdump -s0 -i eth0 -G 10 -w - | ssh {remote_ip} "cat > capture_%d-%m_%Y__%H_%M.pcap"
The file gets return on the remote server for first cycle (10 seconds) and then I'm getting the following error.
tcpdump: listening on ens224, link-type EN10MB (Ethernet), capture size 262144 bytes
tcpdump: Can't write to standard output: Bad file descriptor
I'm using -G for time based rotation, if I remove -G, then i'm able to write to remote server continuously.
My remote server is configured with password-less login form this host.
You can pipe tcpdump to another tcpdump so in your case :
tcpdump -i eth0 -w - not port 22 | \
ssh my.remote.host tcpdump -r - -w /tmp/capture_%d-%m_%Y__%H_%M_%S.pcap -G 2 -C 100
I am capturing network traffic by using tcpdump. The problem is: I can't see all capture data when the package is too long. For example, when the tcp frame length is more than 500, I just see 100-200 or less. How to display all frame data(500+)?
I have tried add -vv and -vvv parameter. This is my current command:
tcpdump -i eth1 tcp and host 10.27.13.14 and port 6973 -vv -X -c 1000
Add -s0 parameter:
tcpdump -i eth1 tcp and host 10.27.13.14 and port 6973 -s0 -vv -X -c 1000
Running linux ubuntu.
Essentially, why is this command a syntax error: -
tcpdump -i eth0 -n udp -X -v -s 1514 'tcp[40:4] = 0x31323334'
Which should show udp packets with '1234' at the 40th byte.
I mean, I get that udp isn't a tcp packet, but the logic should still work. Given it doesn't how can I write this?
try the following:
tcpdump -i eth0 -X -v -s 1514 'udp[40:4] = 0x31323334'
Afaik, proto relop filters should match only the protocol you specify, -n udp should not be needed.
tcpdump is confused what to take as filtering parameter. When you've explicitly used udp, then it captures all the udp packets or if you want particular udp packet then you can specify the offset. So, based on what you need either specify udp with offset or simply udp if you want to capture all the udp packets. Something like below should meet your requirement:
tcpdump -i eth0 -n -X -v -s 1514 'udp[40:4] = 0x31323334'