Added -o <file_offset> option to start tcpdump resuming log rotation

from a given index

tcpdump.c

@@ -604,6 +604,7 @@ main(int argc, char **argv)
 	register int cnt, op, i;
 	bpf_u_int32 localnet, netmask;
 	register char *cp, *infile, *cmdbuf, *device, *RFileName, *WFileName;
+        int WFileNameOffset;
 	pcap_handler callback;
 	int type;
 	struct bpf_program fcode;
@@ -640,6 +641,7 @@ main(int argc, char **argv)
 	infile = NULL;
 	RFileName = NULL;
 	WFileName = NULL;
+        WFileNameOffset = 0;
 	if ((cp = strrchr(argv[0], '/')) != NULL)
 		program_name = cp + 1;
 	else
@@ -653,7 +655,7 @@ main(int argc, char **argv)
 #endif
 
 	while (
-	    (op = getopt(argc, argv, "aAb" B_FLAG "c:C:d" D_FLAG "eE:fF:G:hHi:" I_FLAG j_FLAG J_FLAG "KlLm:M:nNOpqr:Rs:StT:u" U_FLAG "vw:W:xXy:Yz:Z:")) != -1)
+	    (op = getopt(argc, argv, "aAb" B_FLAG "c:C:d" D_FLAG "eE:fF:G:hHi:" I_FLAG j_FLAG J_FLAG "KlLm:M:nNo:Opqr:Rs:StT:u" U_FLAG "vw:W:xXy:Yz:Z:")) != -1)
 		switch (op) {
 
 		case 'a':
@@ -867,6 +869,12 @@ main(int argc, char **argv)
 			++Nflag;
 			break;
 
+                case 'o':
+                        WFileNameOffset = atoi(optarg);
+                        if (WFileNameOffset < 0)
+                                error("invalid output file offset %s", optarg);
+                        break;
+
 		case 'O':
 			Oflag = 0;
 			break;
@@ -1298,10 +1306,13 @@ main(int argc, char **argv)
 			error("malloc of dumpinfo.CurrentFileName");
 
 		/* We do not need numbering for dumpfiles if Cflag isn't set. */
-		if (Cflag != 0)
-		  MakeFilename(dumpinfo.CurrentFileName, WFileName, 0, WflagChars);
+		if (Cflag != 0) {
+		  MakeFilename(dumpinfo.CurrentFileName, WFileName, WFileNameOffset, WflagChars);
+                  /* Offset the count too so rotated names are continuous */
+                  Cflag_count = WFileNameOffset;
+                }
 		else
-		  MakeFilename(dumpinfo.CurrentFileName, WFileName, 0, 0);
+		  MakeFilename(dumpinfo.CurrentFileName, WFileName, WFileNameOffset, 0);
 
 		p = pcap_dump_open(pd, dumpinfo.CurrentFileName);
 		if (p == NULL)
@@ -1902,7 +1913,7 @@ usage(void)
 	(void)fprintf(stderr,
 "\t\t[ -C file_size ] [ -E algo:secret ] [ -F file ] [ -G seconds ]\n");
 	(void)fprintf(stderr,
-"\t\t[ -i interface ]" j_FLAG_USAGE " [ -M secret ]\n");
+"\t\t[ -i interface ]" j_FLAG_USAGE " [ -M secret ] [ -o file_offset ]\n");
 	(void)fprintf(stderr,
 "\t\t[ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]\n");
 	(void)fprintf(stderr,