通过原始套接字进行TCP握手，为什么recvfrom会挂起？

Question

我正在尝试实现握手功能。我发送SYN数据包，服务器通过ACK数据包响应。为了获得服务器响应，我使用了挂起的recvfrom功能。这是我的代码。

import socket, sys
from struct import *
import codecs

def checksum(msg):
    s = 0 
    for i in range(0, len(msg), 2):
        w = ord(msg[i]) + (ord(msg[i+1]) << 8 )
        s = s + w
    s = (s>>16) + (s & 0xffff);
    s = s + (s >> 16);  
    s = ~s & 0xffff
    return s

try:
    s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)
    s.bind(("", "63798"))
except socket.error , msg:
    print 'Socket could not be created. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
    sys.exit()

packet = ''; 
source_ip = '172.16.87.84'
dest_ip = '172.16.10.1'

# ip header fields
ip_ihl = 5
ip_ver = 4
ip_tos = 0
ip_tot_len = 0
ip_id = 54321
ip_frag_off = 0
ip_ttl = 255
ip_proto = socket.IPPROTO_TCP
ip_check = 0
ip_saddr = socket.inet_aton ( source_ip )
ip_daddr = socket.inet_aton ( dest_ip ) 
ip_ihl_ver = (ip_ver << 4) + ip_ihl

ip_header = pack('!BBHHHBBH4s4s' , ip_ihl_ver, ip_tos, ip_tot_len, ip_id, ip_frag_off, ip_ttl, ip_proto, ip_check, ip_saddr, ip_daddr)

# tcp header fields
tcp_source = 63798   # source port
tcp_dest = 8888   # destination port
tcp_seq = 104
tcp_ack_seq = 0
tcp_doff = 5    #4 bit size of tcp header, 5 * 4 = 20 bytes
#tcp flags
tcp_fin = 0
tcp_syn = 1
tcp_rst = 0
tcp_psh = 0
tcp_ack = 0
tcp_urg = 0
tcp_window = socket.htons (5840)
tcp_check = 0
tcp_urg_ptr = 0

tcp_offset_res = (tcp_doff << 4) + 0
tcp_flags = tcp_fin + (tcp_syn << 1) + (tcp_rst << 2) + (tcp_psh <<3) + (tcp_ack << 4) + (tcp_urg << 5)

tcp_header = pack('!HHLLBBHHH' , tcp_source, tcp_dest, tcp_seq, tcp_ack_seq, tcp_offset_res, tcp_flags,  tcp_window, tcp_check, tcp_urg_ptr) 

source_address = socket.inet_aton( source_ip )
dest_address = socket.inet_aton(dest_ip)
placeholder = 0
protocol = socket.IPPROTO_TCP
tcp_length = len(tcp_header)

psh = pack('!4s4sBBH' , source_address , dest_address , placeholder , protocol , tcp_length);
psh = psh + tcp_header;

tcp_check = checksum(psh)

tcp_header = pack('!HHLLBBH' , tcp_source, tcp_dest, tcp_seq, tcp_ack_seq, tcp_offset_res, tcp_flags,  tcp_window) + pack('H' , tcp_check) + pack('!H' , tcp_urg_ptr)

packet = ip_header + tcp_header
s.sendto(packet, (dest_ip , 8888 ))
s.recvfrom(1024) # Here is recevfrom hangs

程序挂起在recvfrom（）上。怎么做，这个问题的原因是什么？ 实际上服务器发送ACK数据包。它可以在wireshark中看到 这是Wireshark日志

Answer 1

问题出现在这里：s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)，raw(7) man page说：

  

IPPROTO_RAW的协议意味着启用了IP_HDRINCL并且能够   发送传递标头中指定的任何IP协议   使用raw不能通过IPPROTO_RAW接收所有IP协议   插座。当收到数据包时，它会被传递给任何有的数据包   在传递给其他协议处理程序之前已被绑定到其协议   （例如，内核协议模块）   仅发送IPPROTO_RAW套接字。

使用s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_TCP) 代替并启用IP_HDRINCL套接字选项以使用您自己的ip标头： s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)