Re: Portable way to "block" on pcap_next_ex()

[Fernando Gont <fernando () gont com ar>]

Hi, Guy,

On 01/16/2012 05:25 PM, Guy Harris wrote:
> > > If you have a system where select() works as it should, i.e.:
> > [....]
> > > select() will block until either
> > >
> > > 1) a bufferful of packets arrives
> > >
> > > or
> > >
> > > 2) the timer, started when the select() is done, expires,
> > > regardless of whether any packets are available to read.
> >
> > This doesn't seem to agree with my tests.
> >
> > I've just checked this on FreeBSD-8.2-release and on a current
> > Ubuntu system,
>
> Sorry, I didn't make it clear enough that, when I said that, I was
> speaking only of systems using BPF, so it wouldn't apply to Ubuntu
> (or any other Linux distribution), for example.

But it does work in the same way on FreeBSD and OpenBSD, too. -- just
tested this...


> > and in both cases select() returns "readable" only for each packet
> > that is received.
>
> What do you mean by "only for each packet that is received"?  Do you
> mean that it doesn't return "readable" if there are no packets to
> read?

Both on Linux and on BSDs, each time a packet is received, select()
returns readable (without waiting for the entire buffer to fill up). --
my understanding is that you were expecting select() to block till the
buffer filled up, or when the timer expired. But everytime select()
returns, there's a packet to read, and it seems it never returns when
there are no packets to read.

FWIW, this is my test program:

/*
 * Testing select() and pcap_next_ex()
 *
 * Please send any bug reports to Fernando Gont <fgont () si6networks com>
 */

#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <getopt.h>
#include <unistd.h>
#include <string.h>
#include <pcap.h>
#include <sys/types.h>
#include <sys/param.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <sys/socket.h>
#include <sys/select.h>

/* Constants used with the libpcap functions */
#define PCAP_ICMPV6_FILTER      "icmp6 and ip6[40]==128"
#define PCAP_SNAP_LEN           65535
#define PCAP_TIMEOUT            1
#define PCAP_PROMISC            1
#define PCAP_OPT                1
#ifndef PCAP_NETMASK_UNKNOWN
        #define PCAP_NETMASK_UNKNOWN    0xffffffff
#endif
#define IFACE_LENGTH    255

pcap_t                  *pfd;
bpf_u_int32             my_netmask;
bpf_u_int32             my_ip;
struct bpf_program      pcap_filter;
char                    dev[64], errbuf[PCAP_ERRBUF_SIZE];
char                    iface[IFACE_LENGTH];
char                    *lasts, *rpref, *endptr;
char                    *charptr;


int main(int argc, char **argv){
        extern char             *optarg;        
        extern int              optind;
        int                     r, sel, fd;
        fd_set                  sset, rset;
        struct pcap_pkthdr      *pkthdr;
        const u_char            *pktdata;
        unsigned char           iface_f=0;

        static struct option longopts[] = {
                {"interface", required_argument, 0, 'i'},
        };

        char shortopts[]= "i:";
        char option;

        if(argc<=1){
                puts("Missing parameters");
                exit(1);
        }


        while((option=getopt_long(argc, argv, shortopts, longopts, NULL)) != -1) {
                switch(option) {

                        case 'i':  /* Interface */
                                strncpy(iface, optarg, IFACE_LENGTH-1);
                                iface_f=1;
                                break;

                        default:
                                puts("Unsuported option");
                                exit(1);
                                break;
                        } /* switch */
        } /* while(getopt) */

        if(geteuid()) {
                puts("tool needs root privileges to run.");
                exit(1);
        }

        if(!iface_f){
                puts("Must specify the network interface with the -i option");
                exit(1);
        }

        if( (pfd= pcap_open_live(iface, PCAP_SNAP_LEN, PCAP_PROMISC,
PCAP_TIMEOUT, errbuf)) == NULL){
                printf("pcap_open_live(): %s\n", errbuf);
                exit(1);
        }

        if( pcap_datalink(pfd) != DLT_EN10MB){
                printf("Error: Interface %s is not an Ethernet interface", iface);
                exit(1);
        }

        if(pcap_compile(pfd, &pcap_filter, PCAP_ICMPV6_FILTER, PCAP_OPT,
PCAP_NETMASK_UNKNOWN) == -1){
                printf("pcap_compile(): %s", pcap_geterr(pfd));
                exit(1);
        }

        if(pcap_setfilter(pfd, &pcap_filter) == -1){
                printf("pcap_setfilter(): %s", pcap_geterr(pfd));
                exit(1);
        }

        pcap_freecode(&pcap_filter);


        if( (fd= pcap_fileno(pfd)) == -1){
                puts("Error obtaining descriptor number for pcap_t");
                exit(1);
        }

        FD_ZERO(&sset);
        FD_SET(fd, &sset);

        while(1){
                rset= sset;

                if((sel=select(fd+1, &rset, NULL, NULL, NULL)) == -1){
                        if(errno == EINTR){
                                puts("select() was interrupted");
                                continue;
                        }
                        else{
                                puts("Error in select()");
                                exit(1);
                        }
                }

                puts("Descriptor was readable");

                if((r=pcap_next_ex(pfd, &pkthdr, &pktdata)) == -1){
                        printf("pcap_next_ex(): %s", pcap_geterr(pfd));
                        exit(1);
                }
                else if(r == 0){
                        puts("pcap_next_ex() returned 0");
                        continue;
                }

                puts("pcap_next_ex() returned a packet");
        }

        exit(0);
}


P.S.: The filter above expects ICMPv6 echo requests, so you can test the
program with "ping6 IPV6ADDRESS".

Thanks!

Best regards,
-- 
Fernando Gont
e-mail: fernando () gont com ar || fgont () si6networks com
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1



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