Reading this blog entry I realised that very likely I have never tried forward tcpdump to a wireshark. How many times I have taken a pcap in a switch and then I need to download to see the details in wireshark…
I guess you can find some blocking points in firewalls (at least for 2-steps option)
So I tried the single command with a switch in my ceoslab and it works!
Why it works?
ssh <username>@<switch> "bash tcpdump -s 0 -U -n -w - -i <interface>" | wireshark -k -i -
The ssh command is actually executing the “bash tcpdump…” remotely. But the key is the “-U” and “-w -” flags. “-U” in conjunction with “-w” sends the packet without waiting for the buffer to fill. Then “-w -” says that it writes the output to stdout instead of a file. If you run the command without -U, it would work but it will update a bit slower as it needs to fill the buffers.
From tcpdump manual:
-U
--packet-buffered
If the -w option is not specified, make the printed packet output ``packet-buffered''; i.e., as the description of the contents of each packet is printed, it will be written to the standard output, rather than, when not writing to a terminal, being written only when the output buffer fills.
If the -w option is specified, make the saved raw packet output ``packet-buffered''; i.e., as each packet is saved, it will be written to the output file, rather than being written only when the output buffer fills.
The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_dump_flush() function.
...... -w file
Write the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option. Standard output is used if file is ``-''.
This output will be buffered if written to a file or pipe, so a program reading from the file or pipe may not see packets for an arbitrary amount of time after they are received. Use the -U flag to cause packets to be written as soon as they are received.
And the stdout of that process is the ssh command so we redirect that outout via a pipe “|” and it is sent as input for wireshark thanks to “-i -” that makes wireshark to read from stdin (that is the stdout from the tcpdump in the switch!)
The wireshark manual:
-i|--interface <capture interface>|-
Set the name of the network interface or pipe to use for live packet capture.
Network interface names should match one of the names listed in "wireshark -D" (described above); a number, as reported by "wireshark -D", can also be used. If you're using UNIX, "netstat -i", "ifconfig -a" or "ip link" might also work to list interface names, although not all versions of UNIX support the -a flag to ifconfig.
If no interface is specified, Wireshark searches the list of interfaces, choosing the first non-loopback interface if there are any non-loopback interfaces, and choosing the first loopback interface if there are no non-loopback interfaces. If there are no interfaces at all, Wireshark reports an error and doesn't start the capture.
Pipe names should be either the name of a FIFO (named pipe) or "-" to read data from the standard input. On Windows systems, pipe names must be of the form "\\pipe\.\pipename". Data read from pipes must be in standard pcapng or pcap format. Pcapng data must have the same endianness as the capturing host.
This option can occur multiple times. When capturing from multiple interfaces, the capture file will be saved in pcapng format.
....
-k Start the capture session immediately. If the -i flag was specified, the capture uses the specified interface. Otherwise, Wireshark searches the list of interfaces, choosing the first non-loopback interface if
there are any non-loopback interfaces, and choosing the first loopback interface if there are no non-loopback interfaces; if there are no interfaces, Wireshark reports an error and doesn't start the capture.
The two-steps option relies on “nc” to send/receive the data, but it is the same idea regarding the tcpdump/wireshark flags using “-“
On switch: tcpdump -s 0 -U -n -w - -i <interface> | nc <computer-ip> <port> On PC: netcat -l -p <port> | wireshark -k -S -i -