Re: [libpcap][patch] appending to a capture

[Darren Reed <darren.reed () oracle com>]

Hi Mark,

I must admit that I don't see the point of this patch.

A pcap data file, with packets in it, is something that
I would create using tcpdump over a specific period
of time. The data file is thus associated with a very
specific set of actions. To then append data to that
file without that data being associated with the
original action seems wrong.

That said, I can also imagine people using this function
and running into huge performance problems.

You might be better off spending some time working
on additions to editcap that include concatenating
two or more pcap files.

Darren

On 30/05/11 01:58 PM, Mark Johnston wrote:
> Hello all,
>
> I submitted a patch to the sourceforge tracker a while ago and didn't
> receive any response. It adds a dump append function which verifies that
> we only append if the link-layer type is the same as that in the capture
> to append to. I submitted this based on a (rather old) thread:
>
> http://permalink.gmane.org/gmane.network.tcpdump.devel/1469
>
> My original submission is here:
>
> http://sourceforge.net/tracker/?func=detail&aid=3086711&group_id=53067&atid=469579
>
> I regenerated the patch against 1.1.1 and pasted it inline. I was hoping
> that someone would be interested in reviewing it and giving some feedback,
> and letting me know if there's any interest in having it committed.
>
> Thanks!
> -Mark
>
>
> diff --git a/pcap/pcap.h b/pcap/pcap.h
> index 05ba31f..abf5d5b 100644
> --- a/pcap/pcap.h
> +++ b/pcap/pcap.h
> @@ -337,6 +337,7 @@ int pcap_fileno(pcap_t *);
>
>   pcap_dumper_t *pcap_dump_open(pcap_t *, const char *);
>   pcap_dumper_t *pcap_dump_fopen(pcap_t *, FILE *fp);
> +pcap_dumper_t *pcap_dump_append(pcap_t *, const char *);
>   FILE  *pcap_dump_file(pcap_dumper_t *);
>   long  pcap_dump_ftell(pcap_dumper_t *);
>   int   pcap_dump_flush(pcap_dumper_t *);
> diff --git a/sf-pcap.c b/sf-pcap.c
> index 9d55dae..a3b0757 100644
> --- a/sf-pcap.c
> +++ b/sf-pcap.c
> @@ -56,6 +56,7 @@ static const char rcsid[] _U_ =
>   #include<stdio.h>
>   #include<stdlib.h>
>   #include<string.h>
> +#include<unistd.h>
>
>   #include "pcap-int.h"
>
> @@ -579,6 +580,65 @@ pcap_dump_fopen(pcap_t *p, FILE *f)
>         return (pcap_setup_dump(p, linktype, f, "stream"));
>   }
>
> +pcap_dumper_t *
> +pcap_dump_append(pcap_t *p, const char *fname)
> +{
> +
> +       FILE *f;
> +       int linktype;
> +       int exists = 0, amt_read;
> +       struct pcap_file_header ph;
> +
> +       linktype = dlt_to_linktype(p->linktype);
> +       if (linktype == -1) {
> +               snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
> +                   "%s: link-layer type %d isn't supported in savefiles",
> +                   fname, linktype);
> +               return (NULL);
> +       }
> +       if (fname[0] == '-'&&  fname[1] == '\0') {
> +               sf_write_header(stdout, linktype, p->tzoff, p->snapshot);
> +               return ((pcap_dumper_t *)stdout);
> +       }
> +
> +       exists = !access(fname, R_OK);
> +       f = fopen(fname, "r+");
> +       if (f == NULL) {
> +               snprintf(p->errbuf, PCAP_ERRBUF_SIZE, "%s: %s",
> +                   fname, pcap_strerror(errno));
> +               return (NULL);
> +       }
> +
> +       /* Read the header and make sure it's of the same linktype. */
> +       amt_read = fread(&ph, 1, sizeof (ph), f);
> +       if (amt_read != sizeof (ph)) {
> +               if (ferror(f)) {
> +                       snprintf(p->errbuf, PCAP_ERRBUF_SIZE, "%s: %s",
> +                           fname, pcap_strerror(errno));
> +                       return (NULL);
> +               } else if (feof(f)&&  amt_read>  0) {
> +                       snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
> +                           "%s: truncated pcap file header", fname);
> +                       return (NULL);
> +               }
> +       }
> +
> +       /*
> +        * If a header is already present and doesn't match the linktype,
> +        * return an error.
> +        */
> +       if (amt_read>  0&&  linktype != ph.linktype) {
> +               snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
> +                   "%s: invalid linktype, cannot append to file", fname);
> +               return (NULL);
> +       }
> +
> +       fseek(f, 0, SEEK_END);
> +       if (!exists)
> +               (void)sf_write_header(f, linktype, p->tzoff, p->snapshot);
> +       return ((pcap_dumper_t *)f);
> +}
> +
>   FILE *
>   pcap_dump_file(pcap_dumper_t *p)
>   {
> -
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