Fix build with old pcap library.

Pcap snaplen related modification broke compilation of Suricata for
system having old pcap library. This patch fixes the issue and allow
old pcap library to honour the snaplen value.

src/source-pcap.c

@@ -83,6 +83,7 @@ typedef struct PcapThreadVars_
 
     /* pcap buffer size */
     int pcap_buffer_size;
+    int pcap_snaplen;
 
     ChecksumValidationMode checksum_mode;
 
@@ -182,7 +183,7 @@ static int PcapTryReopen(PcapThreadVars *ptv)
     ptv->pcap_state = PCAP_STATE_DOWN;
     pcap_close(ptv->pcap_handle);
 
-    ptv->pcap_handle = pcap_open_live((char *)ptv->iface, LIBPCAP_SNAPLEN,
+    ptv->pcap_handle = pcap_open_live((char *)ptv->iface, ptv->pcap_snaplen,
             LIBPCAP_PROMISC, LIBPCAP_COPYWAIT, errbuf);
     if (ptv->pcap_handle == NULL) {
         SCLogError(SC_ERR_PCAP_OPEN_LIVE, "Problem creating pcap handler for live mode, error %s", errbuf);
@@ -354,7 +355,6 @@ TmEcode ReceivePcapLoop(ThreadVars *tv, void *data, void *slot)
 TmEcode ReceivePcapThreadInit(ThreadVars *tv, void *initdata, void **data) {
     SCEnter();
     PcapIfaceConfig *pcapconfig = initdata;
-    int snaplen;
 
     if (initdata == NULL) {
         SCLogError(SC_ERR_INVALID_ARGUMENT, "initdata == NULL");
@@ -403,20 +403,21 @@ TmEcode ReceivePcapThreadInit(ThreadVars *tv, void *initdata, void **data) {
 
     if (pcapconfig->snaplen == 0) {
         /* We set snaplen if we can get the MTU */
-        snaplen = GetIfaceMTU(pcapconfig->iface);
+        ptv->pcap_snaplen = GetIfaceMTU(pcapconfig->iface);
     } else {
-        snaplen = pcapconfig->snaplen;
+        ptv->pcap_snaplen = pcapconfig->snaplen;
     }
-    if (snaplen > 0) {
+    if (ptv->pcap_snaplen > 0) {
         /* set Snaplen. Must be called before pcap_activate */
-        int pcap_set_snaplen_r = pcap_set_snaplen(ptv->pcap_handle, snaplen);
+        int pcap_set_snaplen_r = pcap_set_snaplen(ptv->pcap_handle, ptv->pcap_snaplen);
         if (pcap_set_snaplen_r != 0) {
             SCLogError(SC_ERR_PCAP_SET_SNAPLEN, "Couldn't set snaplen, error: %s", pcap_geterr(ptv->pcap_handle));
             SCFree(ptv);
             pcapconfig->DerefFunc(pcapconfig);
             SCReturnInt(TM_ECODE_FAILED);
         }
-        SCLogInfo("Set snaplen to %d for '%s'", snaplen, pcapconfig->iface);
+        SCLogInfo("Set snaplen to %d for '%s'", ptv->pcap_snaplen,
+                  pcapconfig->iface);
     }
 
     /* set Promisc, and Timeout. Must be called before pcap_activate */
@@ -547,8 +548,19 @@ TmEcode ReceivePcapThreadInit(ThreadVars *tv, void *initdata, void **data) {
     }
     strlcpy(ptv->iface, pcapconfig->iface, PCAP_IFACE_NAME_LENGTH);
 
+    if (pcapconfig->snaplen == 0) {
+        /* We try to set snaplen from MTU value */
+        ptv->pcap_snaplen = GetIfaceMTU(pcapconfig->iface);
+        /* be conservative with old pcap lib to mimic old tcpdump behavior
+           when MTU was not available. */
+        if (ptv->pcap_snaplen <= 0)
+            ptv->pcap_snaplen = LIBPCAP_SNAPLEN;
+    } else {
+        ptv->pcap_snaplen = pcapconfig->snaplen;
+    }
+
     char errbuf[PCAP_ERRBUF_SIZE] = "";
-    ptv->pcap_handle = pcap_open_live(ptv->iface, LIBPCAP_SNAPLEN,
+    ptv->pcap_handle = pcap_open_live(ptv->iface, ptv->pcap_snaplen,
                                         LIBPCAP_PROMISC, LIBPCAP_COPYWAIT, errbuf);
     if (ptv->pcap_handle == NULL) {
         SCLogError(SC_ERR_PCAP_OPEN_LIVE, "Problem creating pcap handler for live mode, error %s", errbuf);

src/source-pcap.h

@@ -32,6 +32,7 @@ int PcapLiveRegisterDevice(char *);
 int PcapLiveGetDeviceCount(void);
 char *PcapLiveGetDevice(int);
 
+#define LIBPCAP_SNAPLEN     1518
 #define LIBPCAP_COPYWAIT    500
 #define LIBPCAP_PROMISC     1