Re: is tcpdump supposed to behave like this?

[Guy Harris <guy () alum mit edu>]

On Jul 1, 2013, at 6:32 AM, Téssio Fechine <tessiof () gmail com> wrote:

> Hello,
> I was trying to analyze the traffic generated by this command:
>
> root@atena:~# dig  rt-dq.quimica.ufpb.br @150.165.145.1
>
> But I noticed that when the option '-w file' was not used, the tcpdump
> capture changed:
>
> ** WITH -w (2 packets captured):
> root@atena:~# tcpdump -pi eth0 port 53 -w dns.dump
> tcpdump: WARNING: eth0: no IPv4 address assigned
> tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535
> bytes
> ^C2 packets captured
> 4 packets received by filter
> 0 packets dropped by kernel
> root@atena:~# tcpdump -r dns.dump
> reading from file dns.dump, link-type EN10MB (Ethernet)
> 09:27:36.961325 IP atena.nti.ufpb.br.53124 > rt-dq.quimica.ufpb.br.domain: 47498+ A? rt-dq.quimica.ufpb.br. (39)
> 09:27:36.964252 IP rt-dq.quimica.ufpb.br.domain > atena.nti.ufpb.br.53124: 47498*- 1/3/0 A 150.165.145.1 (107)
>
> ** WITHOUT -w (8 packets captured):
> root@atena:~# tcpdump -pi eth0 port 53
> tcpdump: WARNING: eth0: no IPv4 address assigned
> tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
> listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
> 09:28:46.192113 IP atena.nti.ufpb.br.44510 > rt-dq.quimica.ufpb.br.domain: 43490+ A? rt-dq.quimica.ufpb.br. (39)
> 09:28:46.193493 IP atena.nti.ufpb.br.48548 > dns-cache-2.bbn.ufpb.br.domain: 13528+ PTR? 1.145.165.150.in-addr.arpa. 
> (44)
> 09:28:46.193851 IP rt-dq.quimica.ufpb.br.domain > atena.nti.ufpb.br.44510: 43490*- 1/3/0 A 150.165.145.1 (107)
> 09:28:46.194279 IP dns-cache-2.bbn.ufpb.br.domain > atena.nti.ufpb.br.48548: 13528 1/2/3 PTR rt-dq.quimica.ufpb.br. 
> (198)
> 09:28:46.194540 IP atena.nti.ufpb.br.41682 > dns-cache-2.bbn.ufpb.br.domain: 33671+ PTR? 13.250.165.150.in-addr.arpa. 
> (45)
> 09:28:46.195187 IP dns-cache-2.bbn.ufpb.br.domain > atena.nti.ufpb.br.41682: 33671 1/2/3 PTR atena.nti.ufpb.br. (195)
> 09:28:46.195462 IP atena.nti.ufpb.br.51372 > dns-cache-2.bbn.ufpb.br.domain: 36444+ PTR? 3.255.165.150.in-addr.arpa. 
> (44)
> 09:28:46.196094 IP dns-cache-2.bbn.ufpb.br.domain > atena.nti.ufpb.br.51372: 36444 1/2/3 PTR dns-cache-2.bbn.ufpb.br. 
> (200)

As Michael Tuexen noted, those PTR queries and replies come from tcpdump *itself* trying to look up the source and 
destination addresses of packets it's seen and is trying to print - and, yes, it *is* supposed to behave like that, if 
you're printing packets rather than writing their raw binary content to a file, and if you haven't specified the "-n" 
flag to tell it *not* to translate IP addresses to host names before printing packet contents.

> I also tried with tshark, and got the same 2 packets as when using tcpdum
> with -w:
>
> root@atena:~# tshark -pf "port 53" -i eth0
> tshark: Lua: Error during loading:
> [string "/usr/share/wireshark/init.lua"]:45: dofile has been disabled
> Running as user "root" and group "root". This could be dangerous.
> Capturing on eth0
>  0.000000 150.165.250.13 -> 150.165.145.1 DNS 85 Standard query A rt-dq.quimica.ufpb.br
>  0.002514 150.165.145.1 -> 150.165.250.13 DNS 153 Standard query response A 150.165.145.1
> ^C2 packets captured

You may have configured Wireshark not to do name resolution for network-layer addresses; if so, that configuration also 
applies to TShark.  With that option, TShark will behave like "tcpdump -n" rather than like "tcpdump" without "-n".  
(And, in fact, you can run TShark with a "-n" flag to suppress name resolution even if your Wireshark configuration 
hasn't disabled it.)

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