Are u an expert in using TCPDUMP command?

binarysoul

If you use the tcpdump command on any Unix-based box, can you give me some specific examples of how you capture traffic to and from hosts?

I've gone through the 'man' page, but find it a big vauge. Any help is appreciated

sprkymrk

In it's most basic level I usuallly use something like this:

tcpdump -i eth0 host 192.168.1.10

This will listen on interface eth0 for any traffic to or from 192.168.1.10.

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tcpdump -i eth0 host 192.168.1.10 and 192.168.1.20

This will only listen for traffic that was between these hosts.

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tcpdump -i eth0 -v -w somefile host 192.168.1.10 and 192.168.1.20

This does the same as above, but gives more verbose output and instead of the output being printed to the screen, it sends it to somefile, which can later be opened in tcpdump for reading by using the -r switch, or even pulled into a program like ethereal/wireshark for analysis.


HTH.

sprkymrk

Thought I would throw in another example for you. You can also specify ports, directions and protocols. If you had a couple of dns servers and you wanted to capture the zone transfers, you could do this:

tcpdump -i eth0 -v -w somefile host 192.168.1.10 and host 192.168.1.20 and tcp port 53

Assuming those 2 ip addresses were both dns servers.

Oh, and just for the record, I'm not an expert (c'mon, you were just thinking to yourself "Wow, he's an expert!").

I just have to use it occasionally. It's a great tool.

ladiesman217

sounds dangerous. is it possible to do that on external networks?

sprkymrk

ladiesman217 wrote:

sounds dangerous. is it possible to do that on external networks?

Your computer has to be able to capture the traffic. By default, when you run tcpdump, it puts your nic into promiscuous mode. The only really reliable way to capture traffic though is to be "inline" or on a span port with the traffic. Basically you would want to run it on a gateway, router or firewall.

liven

Be careful with this command.

I have seen guys run this command. Forget to kill it and it can bring a box down after time.

Also, learn the switches of this tool, you can actually read the entire packet, see the syn, syn-ack, ack etc.

This is by far one of the most useful trouble shooting tools I have EVER used.

And it works best by far on a gateway or router. Especially on the boundary of a network. Like I said combine this tool with good logs getting piped to syslog (pix, cisco routers etc) and you should be able to figure out just about any networking issues.

TCPDUMP don't leave home without it!!!

dtlokee

I prefer Wireshark.

larkspur

windows servers have a similar tool it can be found in the resource kit. You can sav eto a cap file and vie win etherreal.


very unmicrosft to use that extension but it is nice.

binarysoul

Thanks guys for the responses; no I think you guys are expert compared to me

Can someone explain the numbers after port 110, e.g. 18:34. I know win 32768 is window size. Also, why do I see repeated ack 1?


22:47:31.960644 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 18:34(16) ack 1 win 32768
22:47:32.094386 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 34:40(6) ack 1 win 32768
22:47:32.342376 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 40:46(6) ack 1 win 32768
22:47:32.935431 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 46:54( 8 ) ack 1 win 32768
22:47:33.066697 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 54:60(6) ack 1 win 32768
22:47:33.489019 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: . 60:66(6) ack 1 win 32768
22:47:34.702862 IP mytest.2250 > pop1.plus.mail.vip.re2.yahoo.com.110: F 66:66(0) ack 1 win 32768
22:47:34.702876 IP pop1.plus.mail.vip.re2.yahoo.com.110 > mytest.2250: F 1:1(0) ack 67 win 32768

JDMurray

tcptrace is a utility specifically designed for analyzing tcpdump log files.

sprkymrk

binarysoul wrote:

Can someone explain the numbers after port 110, e.g. 18:34.

Sequence number.

I prefer Wireshark.

It can be overkill for some things, also can be considered an invasion of privacy since you can see the data portion. But yeah, I like it too.