问由于使用dpkt解析时使用mergecap生成的pcap导致tcpdump报头无效

EN

import dpkt

f = open('a.pcap')
pcap = dpkt.pcap.Reader(f)

for (src, sport, dst, dport, data) in udp_iterator(pc):

        if dport == 53:
            dns = dpkt.dns.DNS(data)
            if dns.opcode != dpkt.dns.DNS_QUERY:
                print "A DNS packet was sent to the nameserver, but the opcode was %d instead of DNS_QUERY (this is a software error)" % dns.opcode
            if dns.qr != dpkt.dns.DNS_Q:
                print "A DNS packet was sent to the name server, but dns.qr is not 0 and should be.  It is %d" % dns.qr
          print "DNS Query was: ", dns.qd[0].name
            print "ID is: ", dns.id
            print "Hello Dns query is ", dns.qr
            print  "Hello Query Type is ", dns.qd[0].type , type_table[dns.qd[0].type]
            print "Hello DNS Query was: ", dns.qd

File "/usr/local/lib/python2.7/dist-packages/dpkt/pcap.py",
    in __init__
    raise ValueError('invalid tcpdump header')
ValueError: invalid tcpdump header