Re: Running BPF filters on raw packet data (no devices)

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

(The mailing list for libpcap is tcpdump-workers () lists tcpdump org - think of it as also being "libpcap-workers", 
"libpcap-users", and "tcpdump-users".  However, I'll answer this here.

On Jun 16, 2013, at 10:05 AM, Gal Sagie <gal.sagie () gmail com> wrote:

> I want to achieve the following :
>
> 1) I have a raw packet buffer, i want to search if they match a certain BPF filter (i dont care about the device or 
> how i received this packet buffer) just want
>    to know it match or doesn't match.
>
> The code i tried :
>       •  pkt = pointer to packet data
>       •         char errbuf[PCAP_ERRBUF_SIZE];
>       •         pcap_t* pc = pcap_create("any",&errbuf);

That's one thing you're doing wrong.  If you're not going to capture on a device or pseudo-device, don't open it.

If you have a packet with a given type of link-layer headers, there is no guarantee that you will even *have* a device 
that will provide the same type of link-layer headers, and that is what you will need in order to compile a filter with 
pcap_compile() and have it work on your packet.

So:

        pcap_t *pc = pcap_open_dead(linktype, 65536);

        struct bpf_program fp;
        int res = pcap_compile(pc,&fp,"ip",0,0);
        pcap_close(pc);         /* not needed any more */

        struct pcap_pkthdr hdr;
        memset(&hdr,0,sizeof(hdr));
        hdr.caplen = pkt->pkt_len;
        hdr.len = pkt->pkt_len;  
        
        u_char* data = (unsigned char *)pkt->data;

        int match = pcap_offline_filter(&fp, &hdr ,data);
        printf("Packet Match = %d\r\n",match);

You will *HAVE* to choose a value for linktype yourself; there is no value that can possibly work for all packets, 
because the BPF program generated by pcap_compile() *HAS* to know what link-layer headers, if any, are at the beginning 
of the packet - there is none that will simultaneously work on packets with Ethernet headers (DLT_EN10MB) and packets 
with 802.11 headers (DLT_IEEE802_11) and packets with PPP headers (DLT_PPP) and packets with no link-layer headers 
(DLT_RAW, where the packets begin with IPv4 or IPv6 headers) and packets with a "radiotap" header followed by an 802.11 
header (DLT_IEEE802_11_RADIO) and packets with the "fake" headers provided by the "any" device (DLT_LINUX_SLL - packets 
captured on the "any" device have those, rather than the native headers for the particular device from which a 
particular packet was captured).

See

        http://www.tcpdump.org/linktypes.html

for a list of the link-layer header types available.  The DLT_ values are the ones you would use in the call to 
pcap_open_dead().
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