HTTP server example and DHCP Server rewrite (#38)

* improvements to dhcpv4 server

* begin adding examples/httpserver

* add better VLAN tagging methods

* pcap: add error printing; fix bug in CRC

* ipv4: ToS and Flags construction and flag manipulation improvements

* add dhcp interception to httptap and improve httpserver example

internet/pcap/capture.go

@@ -244,10 +244,12 @@ func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) (
 				println("BAD UDP")
 				return dst, pc.vld.ErrPop()
 			}
-			frameLen := ufrm.Length()
-			ifrm4.CRCWriteUDPPseudo(&crc, frameLen)
-			if crc.PayloadSum16(ufrm.RawData()[:frameLen]) != 0 {
-				protoErrs = append(protoErrs, lneto.ErrBadCRC)
+			if ufrm.CRC() != 0 {
+				frameLen := ufrm.Length()
+				ifrm4.CRCWriteUDPPseudo(&crc, frameLen)
+				if crc.PayloadSum16(ufrm.RawData()[:frameLen]) != 0 {
+					protoErrs = append(protoErrs, lneto.ErrBadCRC)
+				}
 			}
 		}
 	case lneto.IPProtoICMP:

internet/pcap/format.go

@@ -80,6 +80,16 @@ func (f *Formatter) FormatFrame(dst []byte, frm Frame, pkt []byte) (_ []byte, er
 			return dst, err
 		}
 	}
+	if len(frm.Errors) > 0 {
+		dst = append(dst, " errs=("...)
+		for i, err := range frm.Errors {
+			if i != 0 {
+				dst = append(dst, ';')
+			}
+			dst = append(dst, err.Error()...)
+		}
+		dst = append(dst, ')')
+	}
 	return dst, nil
 }