io-backend-pcap: Work around a problem with Debian stable pcap / kern…

…el combo

- Reading packets is unreliable and incredibly jittery when TPACKET_V3
  support is on. (Maybe due to
  the-tcpdump-group/libpcap#380). Use a hack
  to force pcap to use TPACKET_V2 instead.

src/io-backend-pcap.cc

@@ -46,6 +46,11 @@ class IoBackendPcap : public IoBackend {
  PCAP(pcap_, pcap_set_snaplen(pcap_, snaplen));
  PCAP(pcap_, pcap_set_timeout(pcap_, 0));
  PCAP(pcap_, pcap_set_promisc(pcap_, 1));
+ // pcap's TPACKET_V3 support doesn't work well in Linux <
+ // 3.19, packet delivery is unreliable and jittery. Luckily
+ // asking for unbuffered packet delivery makes it fall back to
+ // TPACKET_V2.
+ PCAP(pcap_, pcap_set_immediate_mode(pcap_, 1));
 
  // 8MB buffer size. Not a totally arbitrary number. The assumptions:
  //
@@ -61,7 +66,6 @@ class IoBackendPcap : public IoBackend {
  PCAP(pcap_, pcap_set_buffer_size(pcap_, 8*1024*1024));
  PCAP(pcap_, pcap_activate(pcap_));
  PCAP(pcap_, pcap_setnonblock(pcap_, 1, errbuf));
- PCAP(pcap_, pcap_setnonblock(pcap_, 1, errbuf));
 
  return true;
  }
@@ -72,7 +76,7 @@ class IoBackendPcap : public IoBackend {
  }
 
  virtual bool inject(Packet* p) {
- return pcap_inject(pcap_, (char*) p->ethh_, p->length_);
+ return pcap_inject(pcap_, (char*) p->ethh_, p->length_) == p->length_;
  }
 
  virtual bool receive(Packet* p) {