Re: issue with -e in IEEE802_11_RADIO mode

[Denis Ovsienko <denis () ovsienko info>]

 ---- On Wed, 04 Apr 2018 21:14:56 +0100 Peter J. Philipp <tcpdump () centroid eu> wrote ---- 
> Hi,
>
> I get a totally bogus output with -e and -X flags set on a wlan0
> interface in monitor mode.  I have spent a lot of time looking into this
> and I have finally figured out what I needed to do here.  Last I'll
> paste my patch, it's easy.  I know it isn't via github but I don't have
> a github login and can't even write an issue wihtout having logged in,
> so I resorted to this sane historical way.

Hello Peter.

Thank you for sending the proposed bug fix. If it requires a specific 802.11 frame to reproduce, could you post a 
sample .pcap file?

> I want to show you the outputs of tcpdump with the -e flag (which is now
> correct):
>
> root@epsilon:/home/pi/tcpdump/tcpdump-4.9.2# ./tcpdump -X -e  -c 1  -n
> -s 100 -i
>  wlan0 -l  'ether[10] == 0x18 and ether[2] == 0x0 and ether[3] ==
> 0x0'         
> tcpdump: verbose output suppressed, use -v or -vv for full protocol
> decode     
> listening on wlan0, link-type IEEE802_11_RADIO (802.11 plus radiotap
> header), ca
> pture size 100
> bytes                                                           
> 21:57:51.598450 1.0 Mb/s 2472 MHz 11b -58dBm signal antenna 0
> BSSID:18:d6:c7:51:
> 55:86 DA:ff:ff:ff:ff:ff:ff SA:18:d6:c7:51:55:86 Beacon () [1.0* 2.0 5.5
> 11.0 6.0
>  9.0 12.0 18.0 Mbit] IBSS CH:
> 13[|802.11]                                      
>         0x0000:  8000 0000 ffff ffff ffff 18d6 c751 5586 
> .............QU.     
>         0x0010:  18d6 c751 5586 c0fa 8061 bb4b 0200 0000 
> ...QU....a.K....    
>         0x0020:  6400 0000 0000 0108 8204 0b16 0c12 1824  d..............$
>         0x0030:  0301 0d05 0401 0200 0032 0430 4860 6c2d  .........2.0H`l-
>         0x0040:  1aee 111b ffff ff00 0000                 ..........
> 1 packet captured
> 1 packet received by filter
> 0 packets dropped by kernel

I understand you are saying the above is now correct.

> and without the -e flag (which is probably everythign after the wlan
> header):
>
> root@epsilon:/home/pi/tcpdump/tcpdump-4.9.2# ./tcpdump -X  -c 1  -n -s
> 100 -i wl
> an0 -l  'ether[10] == 0x18 and ether[2] == 0x0 and ether[3] ==
> 0x0'            
> tcpdump: verbose output suppressed, use -v or -vv for full protocol
> decode     
> listening on wlan0, link-type IEEE802_11_RADIO (802.11 plus radiotap
> header), ca
> pture size 100
> bytes                                                           
> 21:58:05.627380 1.0 Mb/s 2472 MHz 11b -58dBm signal antenna 0 Beacon ()
> [1.0* 2.
> 0 5.5 11.0 6.0 9.0 12.0 18.0 Mbit] IBSS CH:
> 13[|802.11]                        
>         0x0000:  8071 914c 0200 0000 6400 0000 0000 0108 
> .q.L....d.......     
>         0x0010:  8204 0b16 0c12 1824 0301 0d05 0400 0200 
> .......$........    
>         0x0020:  0032 0430 4860 6c2d 1aee 111b ffff ff00  .2.0H`l-........
>         0x0030:  0000                                     ..             
> 1 packet captured                                                  
> 2 packets received by filter
> 0 packets dropped by kernel

I understand the above is now correct as well, right? What is the incorrect version? Is it incorrect for both "-X -e" 
and "-e" or just for one of those cases?

> Lastly as indicated here is my small patch:
>
> --- print-802_11.c.old  2018-04-04 19:40:56.458530097
> +0200                   
> +++ print-802_11.c      2018-04-04 21:44:15.494773874
> +0200                   
> @@ -3349,7 +3360,18 @@
>  ieee802_11_radio_if_print(netdissect_options
> *ndo,                           
>                            const struct pcap_pkthdr *h, const u_char
> *p)      
>  {
> -       return ieee802_11_radio_print(ndo, p, h->len,
> h->caplen);             
> +       const struct ieee80211_radiotap_header
> *hdr;                          
> +       u_int hdrlen, len;
> +
> +        hdr = (const struct ieee80211_radiotap_header
> *)p;                   
> +        len =
> EXTRACT_LE_16BITS(&hdr->it_len);                               
> +
> +       hdrlen = ieee802_11_radio_print(ndo, p, h->len,
> h->caplen);           
> +       if (ndo->ndo_eflag && hdrlen > len)
> {                                 
> +               return len;
> +       }
> +
> +       return hdrlen;
>  }
>
>  /*

-- 
    Denis Ovsienko


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