Add comments telling people not to gratuitously change the capture file

format (file header or per-packet header format, or interpretation of
any of the fields in those headers) without getting a new magic number
from "tcpdump-workers@tcpdump.org", and to make sure that libpcap can
still read files with the existing magic numbers, not just files with
the new magic number and record formats.

(There have been at least three libpcap changes I know of that have
changed the header formats, or the interpretation of fields in those
headers, without changing the magic number.  I would like not to ever
have any other such changes happen ever again.)

pcap-int.h

@@ -30,7 +30,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- * @(#) $Header: /tcpdump/master/libpcap/pcap-int.h,v 1.26 2000-07-29 07:36:41 guy Exp $ (LBL)
+ * @(#) $Header: /tcpdump/master/libpcap/pcap-int.h,v 1.27 2000-09-14 09:49:28 guy Exp $ (LBL)
  */
 
 #ifndef pcap_int_h
@@ -116,6 +116,28 @@ struct pcap_timeval {
 
 /*
  * How a `pcap_pkthdr' is actually stored in the dumpfile.
+ *
+ * Do not change the format of this structure, in any way (this includes
+ * changes that only affect the length of fields in this structure),
+ * and do not make the time stamp anything other than seconds and
+ * microseconds (e.g., seconds and nanoseconds).  Instead:
+ *
+ *	introduce a new structure for the new format;
+ *
+ *	send mail to "tcpdump-workers@tcpdump.org", requesting a new
+ *	magic number for your new capture file format, and, when
+ *	you get the new magic number, put it in "savefile.c";
+ *
+ *	use that magic number for save files with the changed record
+ *	header;
+ *
+ *	make the code in "savefile.c" capable of reading files with
+ *	the old record header as well as files with the new record header
+ *	(using the magic number to determine the header format).
+ *
+ * Then supply the changes to "patches@tcpdump.org", so that future
+ * versions of libpcap and programs that use it (such as tcpdump) will
+ * be able to read your new capture file format.
  */
 
 struct pcap_sf_pkthdr {
@@ -128,6 +150,10 @@ struct pcap_sf_pkthdr {
  * How a `pcap_pkthdr' is actually stored in dumpfiles written
  * by some patched versions of libpcap (e.g. the ones in Red
  * Hat Linux 6.1 and 6.2).
+ *
+ * Do not change the format of this structure, in any way (this includes
+ * changes that only affect the length of fields in this structure).
+ * Instead, introduce a new structure, as per the above.
  */
 
 struct pcap_sf_patched_pkthdr {

pcap.h

@@ -30,7 +30,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- * @(#) $Header: /tcpdump/master/libpcap/pcap.h,v 1.24 2000-07-29 07:36:43 guy Exp $ (LBL)
+ * @(#) $Header: /tcpdump/master/libpcap/pcap.h,v 1.25 2000-09-14 09:49:29 guy Exp $ (LBL)
  */
 
 #ifndef lib_pcap_h
@@ -69,6 +69,27 @@ typedef struct pcap_dumper pcap_dumper_t;
  * of the flags used in the printout phases of tcpdump.
  * Many fields here are 32 bit ints so compilers won't insert unwanted
  * padding; these files need to be interchangeable across architectures.
+ *
+ * Do not change the format of this structure, in any way (this includes
+ * changes that only affect the length of fields in this structure).
+ * Instead:
+ *
+ *	introduce a new structure for the new format;
+ *
+ *	send mail to "tcpdump-workers@tcpdump.org", requesting a new
+ *	magic number for your new capture file format, and, when
+ *	you get the new magic number, put it in "savefile.c";
+ *
+ *	use that magic number for save files with the changed file
+ *	header;
+ *
+ *	make the code in "savefile.c" capable of reading files with
+ *	the old file header as well as files with the new file header
+ *	(using the magic number to determine the header format).
+ *
+ * Then supply the changes to "patches@tcpdump.org", so that future
+ * versions of libpcap and programs that use it (such as tcpdump) will
+ * be able to read your new capture file format.
  */
 struct pcap_file_header {
 	bpf_u_int32 magic;