af-packet: detect MTU mismatch and warn user

If the MTU on the reception interface and the one on the transmission
interface are different, this will result in an error at transmission
when sending packet to the wire.

src/source-af-packet.c

@@ -50,6 +50,7 @@
 #include "util-privs.h"
 #include "util-optimize.h"
 #include "util-checksum.h"
+#include "util-ioctl.h"
 #include "tmqh-packetpool.h"
 #include "source-af-packet.h"
 #include "runmodes.h"
@@ -322,6 +323,7 @@ TmEcode AFPPeersListAdd(AFPThreadVars *ptv)
     SCEnter();
     AFPPeer *peer = SCMalloc(sizeof(AFPPeer));
     AFPPeer *pitem;
+    int mtu, out_mtu;
 
     if (peer == NULL) {
         SCReturnInt(TM_ECODE_FAILED);
@@ -351,6 +353,16 @@ TmEcode AFPPeersListAdd(AFPThreadVars *ptv)
             continue;
         peer->peer = pitem;
         pitem->peer = peer;
+        mtu = GetIfaceMTU(ptv->iface);
+        out_mtu = GetIfaceMTU(ptv->out_iface);
+        if (mtu != out_mtu) {
+            SCLogError(SC_ERR_AFP_CREATE,
+                      "MTU on %s (%d) and %s (%d) are not equal, "
+                      "transmission of packets bigger than %d will fail.",
+                      ptv->iface, mtu,
+                      ptv->out_iface, out_mtu,
+                      (out_mtu > mtu) ? mtu : out_mtu);
+        }
         peerslist.peered += 2;
         break;
     }