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Does the tcpdump you're using support -U or some other to disable buffering?

-U --packet-buffered If the -w option is not specified, make the printed packet out- put packet-buffered''; i.e., as the description of the con- tents of each packet is printed, it will be written to the stan- dard 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() func-
tion.


Does the tcpdump you're using support -U or some other to disable buffering?

 -U
--packet-buffered
If  the -w option is not specified, make the printed packet out-
put packet-buffered''; i.e., as the description  of  the  con-
tents of each packet is printed, it will be written to the stan-
dard output, rather than, when not writing to a terminal,  being
written only when the output buffer fills. 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() func-
tion.
  

Does the tcpdump you're using support -U or some other option to disable buffering?

   -U
--packet-buffered
If  the -w option is not specified, make the printed packet out-
put packet-buffered''; i.e., as the description  of  the  con-
tents of each packet is printed, it will be written to the stan-
dard 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() func-
tion.