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Full Discussion: Can't Output Piped Perl In-line command to a File
Top Forums Shell Programming and Scripting Can't Output Piped Perl In-line command to a File Post 302770218 by ConcealedKnight on Thursday 14th of February 2013 08:24:06 PM
Old 02-14-2013
Can't Output Piped Perl In-line command to a File
Hello,

I'm pretty stumped, and I don't know why I am not able to redirect the output to the 'graphme' file with the command below in Fedora 18.

Code:
tcpdump -l -n -t "tcp[13] == 18" | perl -ane '($s,$j)=split(/,/,$F[7],2); print "$s\n";' > graphme

In case you're wondering, I was following the example from the Graphing Initial Sequence Numbers section in Linux Administration: A Beginner's Guide 5th Edition with a little modification as I only wanted to get the first syn value from the tcpdump output.

I'm currently using tcpdump version 4.3, and perl version 5.16.2. I don't know if the version makes a difference here. I would appreciate an explanation why I couldn't redirect the output to the file but was able to redirect it to another tty or current terminal using '> /dev/tty2'.

Thanks in advance for your help.

EDIT: Since there is no response, I thought this may help those who may be afraid to execute the above command, which is pretty harmless since I was only capturing packets. This is the output from the `tcpdump -l -n -t "tcp[13] == 18"' command:
Code:
IP 50.22.206.133.http > 10.0.2.109.44777: Flags [S.], seq 3029681720, ack 19750303, win 11584, options [mss 1460,sackOK,TS val 3311545446 ecr 3361031,nop,wscale 9], length 0
IP 199.38.164.156.http > 10.0.2.109.33959: Flags [S.], seq 4217910485, ack 777386666, win 4380, options [mss 1460,nop,wscale 0,nop,nop,TS val 3690041151 ecr 3361175,sackOK,eol], length 0
IP 74.125.226.249.http > 10.0.2.109.45763: Flags [S.], seq 2645889525, ack 79909840, win 62392, options [mss 1430,sackOK,TS val 1192032006 ecr 3361277,nop,wscale 6], length 0

When I piped it to perl, I expected to extract the following output from the above command:
Code:
3029681720
4217910485
2645889525

Last edited by ConcealedKnight; 02-14-2013 at 11:43 PM..
 

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LEARN ABOUT NETBSD

ipresend

IPRESEND(1)						      General Commands Manual						       IPRESEND(1)

NAME

       ipresend - resend IP packets out to network

SYNOPSIS

       ipresend [ -EHPRSTX ] [ -d <device> ] [ -g <gateway> ] [ -m <MTU> ] [ -r <filename> ]

DESCRIPTION

       ipresend was designed to allow packets to be resent, once captured, back out onto the network for use in testing.  ipresend supports a num-
       ber of different file formats as input, including saved snoop/tcpdump binary data.

OPTIONS

       -d <interface>
	      Set the interface name to be the name supplied.  This is useful with the -P, -S, -T and -E options, where it is not otherwise possi-
	      ble to associate a packet with an interface.  Normal "text packets" can override this setting.

       -g <gateway>
	      Specify  the  hostname of the gateway through which to route packets.  This is required whenever the destination host isn't directly
	      attached to the same network as the host from which you're sending.

       -m <MTU>
	      Specify the MTU to be used when sending out packets.  This option allows you to set a fake MTU, allowing the simulation  of  network
	      interfaces with small MTU's without setting them so.

       -r <filename>
	      Specify the filename from which to take input.  Default is stdin.

       -E     The  input file is to be text output from etherfind.  The text formats which are currently supported are those which result from the
	      following etherfind option combinations:

		 etherfind -n
		 etherfind -n -t

       -H     The input file is to be hex digits, representing the binary makeup of the packet.  No length correction is  made,  if  an  incorrect
	      length is put in the IP header.

       -P     The  input  file specified by -i is a binary file produced using libpcap (i.e., tcpdump version 3).  Packets are read from this file
	      as being input (for rule purposes).

       -R     When sending packets out, send them out "raw" (the way they came in).  The only real significance here is that it  will  expect  the
	      link layer (i.e.	ethernet) headers to be prepended to the IP packet being output.

       -S     The  input  file	is  to be in "snoop" format (see RFC 1761).  Packets are read from this file and used as input from any interface.
	      This is perhaps the most useful input type, currently.

       -T     The input file is to be text output from tcpdump.  The text formats which are currently supported are those which  result  from  the
	      following tcpdump option combinations:

		 tcpdump -n
		 tcpdump -nq
		 tcpdump -nqt
		 tcpdump -nqtt
		 tcpdump -nqte

       -X     The input file is composed of text descriptions of IP packets.

SEE ALSO

       ipftest(1), ipsend(1), iptest(1), bpf(4), ipsend(5), tcpdump(8)

DIAGNOSTICS

       Needs to be run as root.

BUGS

       Not all of the input formats are sufficiently capable of introducing a wide enough variety of packets for them to be all useful in testing.
       If you find any, please send email to me at darrenr@pobox.com

																       IPRESEND(1)
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