Libnids中tcp流处理"tcp.c"

逃离我推掉我的手 2022-08-08 00:49 161阅读 0赞

# 关于libnids的重要参考资料 #

[http://sourcecodebrowser.com/libnids/1.23/tcp\_8c.html][http_sourcecodebrowser.com_libnids_1.23_tcp_8c.html]

这里给出了libnids中函数之间调用关系的详细图解，很不错。例如在tcp.c中添加新的数据流时“add\_new\_tcp”调用其他函数的结构如图所示：

![Center][]

# tcp.c #

/*
      Copyright (c) 1999 Rafal Wojtczuk <nergal@7bulls.com>. All rights reserved.
      See the file COPYING for license details.
     */
    
    #include <config.h>
    #include <sys/types.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>
    #include <netinet/in.h>
    #include <netinet/in_systm.h>
    #include <netinet/ip.h>
    #include <netinet/tcp.h>
    #include <netinet/ip_icmp.h>
    
    #include "checksum.h"
    #include "scan.h"
    #include "tcp.h"
    #include "util.h"
    #include "nids.h"
    #include "hash.h"
    
    #if ! HAVE_TCP_STATES
    enum {
      TCP_ESTABLISHED = 1,
      TCP_SYN_SENT,
      TCP_SYN_RECV,
      TCP_FIN_WAIT1,
      TCP_FIN_WAIT2,
      TCP_TIME_WAIT,
      TCP_CLOSE,
      TCP_CLOSE_WAIT,
      TCP_LAST_ACK,
      TCP_LISTEN,
      TCP_CLOSING			/* now a valid state */
    };
    
    #endif
    
    #define FIN_SENT 120
    #define FIN_CONFIRMED 121
    #define COLLECT_cc 1
    #define COLLECT_sc 2
    #define COLLECT_ccu 4
    #define COLLECT_scu 8
    
    //期待收到的序列号是发送的第一个序列号+收到的数据总和+紧急数据
    #define EXP_SEQ (snd->first_data_seq + rcv->count + rcv->urg_count)
    
    extern struct proc_node *tcp_procs;
    
    static struct tcp_stream **tcp_stream_table;
    static struct tcp_stream *streams_pool;
    static int tcp_num = 0;
    static int tcp_stream_table_size;
    static int max_stream;
    static struct tcp_stream *tcp_latest = 0, *tcp_oldest = 0;
    static struct tcp_stream *free_streams;
    static struct ip *ugly_iphdr;
    struct tcp_timeout *nids_tcp_timeouts = 0;
    
    //删除某个方向的tcp数据流
    static void purge_queue(struct half_stream * h)
    {
      struct skbuff *tmp, *p = h->list;
    
      while (p) {
        free(p->data);
        tmp = p->next;
        free(p);
        p = tmp;
      }
      h->list = h->listtail = 0;
      h->rmem_alloc = 0;
    }
    
    static void
    add_tcp_closing_timeout(struct tcp_stream * a_tcp)
    {
      struct tcp_timeout *to;
      struct tcp_timeout *newto;
    
      if (!nids_params.tcp_workarounds)
        return;
      newto = malloc(sizeof (struct tcp_timeout));
      if (!newto)
          nids_params.no_mem("add_tcp_closing_timeout");
      newto->a_tcp = a_tcp;
      newto->timeout.tv_sec = nids_last_pcap_header->ts.tv_sec + 10;
      newto->prev = 0;
      for (newto->next = to = nids_tcp_timeouts; to; newto->next = to = to->next) {
        if (to->a_tcp == a_tcp) {
          free(newto);
          return;
        }
        if (to->timeout.tv_sec > newto->timeout.tv_sec)
          break;
        newto->prev = to;
      }
      if (!newto->prev)
        nids_tcp_timeouts = newto;
      else
        newto->prev->next = newto;
      if (newto->next)
        newto->next->prev = newto;
    }
    
    static void
    del_tcp_closing_timeout(struct tcp_stream * a_tcp)
    {
      struct tcp_timeout *to;
    
      if (!nids_params.tcp_workarounds)
        return;
      for (to = nids_tcp_timeouts; to; to = to->next)
        if (to->a_tcp == a_tcp)
          break;
      if (!to)
        return;
      if (!to->prev)
        nids_tcp_timeouts = to->next;
      else
        to->prev->next = to->next;
      if (to->next)
        to->next->prev = to->prev;
      free(to);
    }
    
    //删除tcp数据流并释放空间
    void
    nids_free_tcp_stream(struct tcp_stream * a_tcp)
    {
      //在tcp_stream的结构中记录了该tcp流的hash index，这里要使用该值
      int hash_index = a_tcp->hash_index;
      struct lurker_node *i, *j;
    
      del_tcp_closing_timeout(a_tcp);
      //分别删除两个方向上的half_stream
      purge_queue(&a_tcp->server);
      purge_queue(&a_tcp->client);
       
      //从hash表中删除该tcp流并释放空间
      if (a_tcp->next_node)
        a_tcp->next_node->prev_node = a_tcp->prev_node;
      if (a_tcp->prev_node)
        a_tcp->prev_node->next_node = a_tcp->next_node;
      else
        tcp_stream_table[hash_index] = a_tcp->next_node;
      if (a_tcp->client.data)
        free(a_tcp->client.data);
      if (a_tcp->server.data)
        free(a_tcp->server.data);
      if (a_tcp->next_time)
        a_tcp->next_time->prev_time = a_tcp->prev_time;
      if (a_tcp->prev_time)
        a_tcp->prev_time->next_time = a_tcp->next_time;
      if (a_tcp == tcp_oldest)
        tcp_oldest = a_tcp->prev_time;
      if (a_tcp == tcp_latest)
        tcp_latest = a_tcp->next_time;
      
      i = a_tcp->listeners;
      
      while (i) {
        j = i->next;
        free(i);
        i = j;
      }
      a_tcp->next_free = free_streams;
      free_streams = a_tcp;
      tcp_num--; //删除了tcp流后，当前的tcp流数量减1
    }
    
    void
    tcp_check_timeouts(struct timeval *now)
    {
      struct tcp_timeout *to;
      struct tcp_timeout *next;
      struct lurker_node *i;
    
      for (to = nids_tcp_timeouts; to; to = next) {
        if (now->tv_sec < to->timeout.tv_sec)
          return;
        to->a_tcp->nids_state = NIDS_TIMED_OUT;
        for (i = to->a_tcp->listeners; i; i = i->next)
          (i->item) (to->a_tcp, &i->data);
        next = to->next;
        nids_free_tcp_stream(to->a_tcp);
      }
    }
    
    //该函数调用mkhash将源/目的IP和端口号hash为一个int数，该数已经
    //具有了足够的随机性，然后在通过取余操作得到其在哈希表中的位置
    static int
    mk_hash_index(struct tuple4 addr)
    {
      int hash=mkhash(addr.saddr, addr.source, addr.daddr, addr.dest);
      return hash % tcp_stream_table_size;
    }
    
    static int get_ts(struct tcphdr * this_tcphdr, unsigned int * ts)
    {
      int len = 4 * this_tcphdr->th_off;
      unsigned int tmp_ts;
      //偏移得到tcp选项options的位置
      unsigned char * options = (unsigned char*)(this_tcphdr + 1);
      int ind = 0, ret = 0;
      while (ind <=  len - (int)sizeof (struct tcphdr) - 10 )
      	switch (options[ind]) {
    		case 0: //TCPOPT_EOL,表示选项结束 
    			return ret;
    		case 1: //TCPOPT_NOP，表示无操作，用NOP填充字段为4字节整数倍
    			ind++;
    			continue;	
      		case 8: // TCPOPT_TIMESTAMP,时间戳选项
    		    //偏移到时间戳值(4字节)的位置并将其拷贝到tmp_ts中
    	  		memcpy((char*)&tmp_ts, options + ind + 2, 4);
      			*ts=ntohl(tmp_ts);
    			ret = 1;
    			/* no break, intentionally */
    		default:	
    			if (options[ind+1] < 2 ) /* "silly option" */
    				return ret;
    			ind += options[ind+1];
    	}			
    			
      return ret;
    }  		
    
    static int get_wscale(struct tcphdr * this_tcphdr, unsigned int * ws)
    {
      int len = 4 * this_tcphdr->th_off;
      unsigned int tmp_ws;
      unsigned char * options = (unsigned char*)(this_tcphdr + 1);
      int ind = 0, ret = 0;
      *ws=1;
      while (ind <=  len - (int)sizeof (struct tcphdr) - 3 )
      	switch (options[ind]) {
    		case 0: /* TCPOPT_EOL */
    			return ret;
    		case 1: /* TCPOPT_NOP */
    			ind++;
    			continue;	
      		case 3: //TCPOPT_WSCALE,窗口扩大因子
    		    //偏移到窗口扩扩大因子中的“位移数”的位置
      			tmp_ws=options[ind+2];
      			if (tmp_ws>14)   //位移数最大为14
      				tmp_ws=14;
    			*ws=1<<tmp_ws;
    			ret = 1;
    			/* no break, intentionally */
    		default:	
    			if (options[ind+1] < 2 ) /* "silly option" */
    				return ret;
    			ind += options[ind+1];
    	}			
    			
      return ret;
    }  		
    
    //如果一个数据报是正常的数据报，但是在哈希表中没有找到该数据报对应的
    //数据流（这是三次握手的第一个数据报，标志着一个新的tcp连接的开始），
    //那么就使用该函数添加该数据报到哈希表中
    //哈希表处理冲突的方法是使用双向链表，如果新来的tcp流发生冲突，
    //将这个新tcp流插入到双向链表的表头
    static void
    add_new_tcp(struct tcphdr * this_tcphdr, struct ip * this_iphdr)
    {
      struct tcp_stream *tolink;
      struct tcp_stream *a_tcp;
      int hash_index;
      struct tuple4 addr;
      
      addr.source = ntohs(this_tcphdr->th_sport);
      addr.dest = ntohs(this_tcphdr->th_dport);
      addr.saddr = this_iphdr->ip_src.s_addr;
      addr.daddr = this_iphdr->ip_dst.s_addr;
      hash_index = mk_hash_index(addr);
      
      //如果该tcp流中数据报数量超过了阈值，需要先释放空间
      if (tcp_num > max_stream) {
        struct lurker_node *i;
        int orig_client_state=tcp_oldest->client.state;
        tcp_oldest->nids_state = NIDS_TIMED_OUT;
        for (i = tcp_oldest->listeners; i; i = i->next)
          (i->item) (tcp_oldest, &i->data);
        nids_free_tcp_stream(tcp_oldest);
        if (orig_client_state!=TCP_SYN_SENT)
          nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_TOOMUCH, ugly_iphdr, this_tcphdr);
      }
      a_tcp = free_streams;
      if (!a_tcp) {
        fprintf(stderr, "gdb me ...\n");
        pause();
      }
      free_streams = a_tcp->next_free;
      
      tcp_num++;  //tcp流数量加1
      //更新tcp流的相关信息
      tolink = tcp_stream_table[hash_index];
      memset(a_tcp, 0, sizeof(struct tcp_stream));
      a_tcp->hash_index = hash_index;
      a_tcp->addr = addr;
      a_tcp->client.state = TCP_SYN_SENT;
      a_tcp->client.seq = ntohl(this_tcphdr->th_seq) + 1;
      a_tcp->client.first_data_seq = a_tcp->client.seq;
      a_tcp->client.window = ntohs(this_tcphdr->th_win);
      a_tcp->client.ts_on = get_ts(this_tcphdr, &a_tcp->client.curr_ts);
      a_tcp->client.wscale_on = get_wscale(this_tcphdr, &a_tcp->client.wscale);
      a_tcp->server.state = TCP_CLOSE;
      a_tcp->next_node = tolink;
      a_tcp->prev_node = 0;
      if (tolink)
        tolink->prev_node = a_tcp;
      tcp_stream_table[hash_index] = a_tcp;
      a_tcp->next_time = tcp_latest; 
      a_tcp->prev_time = 0;
      if (!tcp_oldest)
        tcp_oldest = a_tcp;
      if (tcp_latest)
        tcp_latest->prev_time = a_tcp;
      tcp_latest = a_tcp;  //将最新插入哈希表的流更新为当前流
    }
    
    //该函数用于将已经收集好的tcp数据添加到响应的half_stream中
    static void
    add2buf(struct half_stream * rcv, char *data, int datalen)
    {
      int toalloc;
      
      //如果没有足够的空间存储数据，则先要分配空间
      if (datalen + rcv->count - rcv->offset > rcv->bufsize) {
    	//如果half_stream中还没存过数据（即本次数据是第一次数据）
        if (!rcv->data) {  
    	//如果本次收到的数据小于2048，则分配4096大小的空间
          if (datalen < 2048)  
    	      toalloc = 4096;
          else //否则分配的空间为本次数据大小的2倍
    	     toalloc = datalen * 2;
          rcv->data = malloc(toalloc);
          rcv->bufsize = toalloc;
        }
    	//否则half_stream中已经存储过数据
        else {
    	  //如果本次数据小于可用的空间大小，那么分配可用空间2倍大小的空间
    	  //否则，分配的空间大小是当前的空间大小加上本次数据的2倍
          if (datalen < rcv->bufsize)
          	toalloc = 2 * rcv->bufsize;
          else	
          	toalloc = rcv->bufsize + 2*datalen;
          rcv->data = realloc(rcv->data, toalloc);
          rcv->bufsize = toalloc;
        }
    	//如果分配内存失败
        if (!rcv->data)
          nids_params.no_mem("add2buf");
      }
      //复制数据
      memcpy(rcv->data + rcv->count - rcv->offset, data, datalen);
      rcv->count_new = datalen;
      rcv->count += datalen;
    }
    
    static void
    ride_lurkers(struct tcp_stream * a_tcp, char mask)
    {
      struct lurker_node *i;
      char cc, sc, ccu, scu;
      
      for (i = a_tcp->listeners; i; i = i->next)
        if (i->whatto & mask) {
          cc = a_tcp->client.collect;
          sc = a_tcp->server.collect;
          ccu = a_tcp->client.collect_urg;
          scu = a_tcp->server.collect_urg;
    
          (i->item) (a_tcp, &i->data);
          if (cc < a_tcp->client.collect)
    	i->whatto |= COLLECT_cc;
          if (ccu < a_tcp->client.collect_urg)
    	i->whatto |= COLLECT_ccu;
          if (sc < a_tcp->server.collect)
    	i->whatto |= COLLECT_sc;
          if (scu < a_tcp->server.collect_urg)
    	i->whatto |= COLLECT_scu;
          if (cc > a_tcp->client.collect)
    	i->whatto &= ~COLLECT_cc;
          if (ccu > a_tcp->client.collect_urg)
    	i->whatto &= ~COLLECT_ccu;
          if (sc > a_tcp->server.collect)
    	i->whatto &= ~COLLECT_sc;
          if (scu > a_tcp->server.collect_urg)
    	i->whatto &= ~COLLECT_scu;
        }
    }
    
    static void
    notify(struct tcp_stream * a_tcp, struct half_stream * rcv)
    {
      struct lurker_node *i, **prev_addr;
      char mask;
    
      if (rcv->count_new_urg) {
        if (!rcv->collect_urg)
          return;
        if (rcv == &a_tcp->client)
          mask = COLLECT_ccu;
        else
          mask = COLLECT_scu;
        ride_lurkers(a_tcp, mask);
        goto prune_listeners;
      }
      if (rcv->collect) {
        if (rcv == &a_tcp->client)
          mask = COLLECT_cc;
        else
          mask = COLLECT_sc;
       do {
    	int total;
    		a_tcp->read = rcv->count - rcv->offset;
    		total=a_tcp->read;
      
    	    ride_lurkers(a_tcp, mask);
    	    if (a_tcp->read>total-rcv->count_new)
    	      rcv->count_new=total-a_tcp->read;
    	    
    	    if (a_tcp->read > 0) {
    	      memmove(rcv->data, rcv->data + a_tcp->read, rcv->count - rcv->offset - a_tcp->read);
    	      rcv->offset += a_tcp->read;
    	    }
    	}while (nids_params.one_loop_less && a_tcp->read>0 && rcv->count_new); 
    // we know that if one_loop_less!=0, we have only one callback to notify
       rcv->count_new=0;	    
      }
     prune_listeners:
      prev_addr = &a_tcp->listeners;
      i = a_tcp->listeners;
      while (i)
        if (!i->whatto) {
          *prev_addr = i->next;
          free(i);
          i = *prev_addr;
        }
        else {
          prev_addr = &i->next;
          i = i->next;
        }
    }
    
    //该函数调用add2buf函数将收集好的数据进行存储
    static void
    add_from_skb(struct tcp_stream * a_tcp, struct half_stream * rcv,
    	     struct half_stream * snd,
    	     u_char *data, int datalen,
    	     u_int this_seq, char fin, char urg, u_int urg_ptr)
    {
      //比较序列号,如果新到的序列号比期望的大,有可能丢失了部分数据
      u_int lost = EXP_SEQ - this_seq;
      int to_copy, to_copy2;
      
      if (urg && after(urg_ptr, EXP_SEQ - 1) &&
          (!rcv->urg_seen || after(urg_ptr, rcv->urg_ptr))) {
        rcv->urg_ptr = urg_ptr;
        rcv->urg_seen = 1;
      }
      if (rcv->urg_seen && after(rcv->urg_ptr + 1, this_seq + lost) &&
          before(rcv->urg_ptr, this_seq + datalen)) {
        to_copy = rcv->urg_ptr - (this_seq + lost); 
    	//数据中有新的数据到达，将数据进行添加处理，并进行数据的回推处理
        if (to_copy > 0) {
          if (rcv->collect) {
    	    add2buf(rcv, (char *)(data + lost), to_copy);
    	    notify(a_tcp, rcv);
          }
          else {
    	    rcv->count += to_copy;
    	    rcv->offset = rcv->count; /* clear the buffer */
          }
        }
        rcv->urgdata = data[rcv->urg_ptr - this_seq];
        rcv->count_new_urg = 1;
        notify(a_tcp, rcv);
        rcv->count_new_urg = 0;
        rcv->urg_seen = 0;
        rcv->urg_count++;
        to_copy2 = this_seq + datalen - rcv->urg_ptr - 1;
        if (to_copy2 > 0) {
          if (rcv->collect) {
    	    add2buf(rcv, (char *)(data + lost + to_copy + 1), to_copy2);
    	    notify(a_tcp, rcv);
          }
          else {
    	    rcv->count += to_copy2;
    	    rcv->offset = rcv->count; /*clear the buffer*/
          }
        }
      }
      else {
        if (datalen - lost > 0) {
          if (rcv->collect) {
    	    add2buf(rcv, (char *)(data + lost), datalen - lost);
    	    notify(a_tcp, rcv);
          }
          else {
    	    rcv->count += datalen - lost;
    	    rcv->offset = rcv->count; /* clear the buffer */
          }
        }
      }
      //如果发送了fin结束标志，那么发送方将处于FIN_SENT状态
      if (fin) {
        snd->state = FIN_SENT;
    	//若此时接收方状态是TCP_CLOSING，说明接收方之前发送了
    	//fin，但是没有收到对该fin的确认，因此将该数据流加入
    	//关闭超时队列“add_tcp_closing_timeout”
        if (rcv->state == TCP_CLOSING)
          add_tcp_closing_timeout(a_tcp);
      }
    }
    
    //连接的缓冲区维护（主要就是如何更新的）
    static void
    tcp_queue(struct tcp_stream * a_tcp, struct tcphdr * this_tcphdr,
    	  struct half_stream * snd, struct half_stream * rcv,
    	  char *data, int datalen, int skblen
    	  )
    {
      u_int this_seq = ntohl(this_tcphdr->th_seq);
      struct skbuff *pakiet, *tmp;
      
      /*
       * Did we get anything new to ack?
       */
      //EXP_SEQ是目前已集齐的数据流水号，我们希望收到从这里开始的数据  
      //先判断数据是不是在EXP_SEQ之前开始
      if (!after(this_seq, EXP_SEQ)) {
    	//再判断数据长度是不是在EXP_SEQ之后，如果是，说明有新数据，否则是重发的包，无视之
        if (after(this_seq + datalen + (this_tcphdr->th_flags & TH_FIN), EXP_SEQ)) {
          /* the packet straddles our window end */
          get_ts(this_tcphdr, &snd->curr_ts);
    	  //ok，更新集齐的数据区，值得一提的是add_from_skb函数一旦发现集齐了一段数据之后  
          //便立刻调用notify函数，在notify函数里面将数据推给回调方   
          add_from_skb(a_tcp, rcv, snd, (u_char *)data, datalen, this_seq,
    		   (this_tcphdr->th_flags & TH_FIN),
    		   (this_tcphdr->th_flags & TH_URG),
    		   ntohs(this_tcphdr->th_urp) + this_seq - 1);
          /*
           * Do we have any old packets to ack that the above
           * made visible? (Go forward from skb)
           */
    	   //此时EXP_SEQ有了变化了，看看缓冲区里的包有没有符合条件
    	   //能用同样的方法处理掉的,有就处理掉，然后释放  
          pakiet = rcv->list;
          while (pakiet) {
    	if (after(pakiet->seq, EXP_SEQ))
    	  break;
    	if (after(pakiet->seq + pakiet->len + pakiet->fin, EXP_SEQ)) {
    	  add_from_skb(a_tcp, rcv, snd, pakiet->data,
    		       pakiet->len, pakiet->seq, pakiet->fin, pakiet->urg,
    		       pakiet->urg_ptr + pakiet->seq - 1);
            }
    	rcv->rmem_alloc -= pakiet->truesize;
    	if (pakiet->prev)
    	  pakiet->prev->next = pakiet->next;
    	else
    	  rcv->list = pakiet->next;
    	if (pakiet->next)
    	  pakiet->next->prev = pakiet->prev;
    	else
    	  rcv->listtail = pakiet->prev;
    	tmp = pakiet->next;
    	free(pakiet->data);
    	free(pakiet);
    	pakiet = tmp;
          }
        }
        else
          return;
      }
      //这里说明现在这个包是个乱序到达的（数据开始点超过了EXP_SEQ），
      //放到缓冲区等待处理，注意保持缓冲区有序 
      else {
        struct skbuff *p = rcv->listtail;
    
        pakiet = mknew(struct skbuff);
        pakiet->truesize = skblen;
        rcv->rmem_alloc += pakiet->truesize;
        pakiet->len = datalen;
        pakiet->data = malloc(datalen);
        if (!pakiet->data)
          nids_params.no_mem("tcp_queue");
        memcpy(pakiet->data, data, datalen);
        pakiet->fin = (this_tcphdr->th_flags & TH_FIN);
        /* Some Cisco - at least - hardware accept to close a TCP connection
         * even though packets were lost before the first TCP FIN packet and
         * never retransmitted; this violates RFC 793, but since it really
         * happens, it has to be dealt with... The idea is to introduce a 10s
         * timeout after TCP FIN packets were sent by both sides so that
         * corresponding libnids resources can be released instead of waiting
         * for retransmissions which will never happen.  -- Sebastien Raveau
         */
        if (pakiet->fin) {
          snd->state = TCP_CLOSING;
          if (rcv->state == FIN_SENT || rcv->state == FIN_CONFIRMED)
    	    add_tcp_closing_timeout(a_tcp);
        }
        pakiet->seq = this_seq;
        pakiet->urg = (this_tcphdr->th_flags & TH_URG);
        pakiet->urg_ptr = ntohs(this_tcphdr->th_urp);
        for (;;) {
          if (!p || !after(p->seq, this_seq))
    	    break;
          p = p->prev;
        }
        if (!p) {
          pakiet->prev = 0;
          pakiet->next = rcv->list;
          if (rcv->list)
             rcv->list->prev = pakiet;
          rcv->list = pakiet;
          if (!rcv->listtail)
    	    rcv->listtail = pakiet;
        }
        else {
          pakiet->next = p->next;
          p->next = pakiet;
          pakiet->prev = p;
          if (pakiet->next)
    	    pakiet->next->prev = pakiet;
          else
    	    rcv->listtail = pakiet;
        }
      }
    }
    
    static void
    prune_queue(struct half_stream * rcv, struct tcphdr * this_tcphdr)
    {
      struct skbuff *tmp, *p = rcv->list;
    
      nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_BIGQUEUE, ugly_iphdr, this_tcphdr);
      while (p) {
        free(p->data);
        tmp = p->next;
        free(p);
        p = tmp;
      }
      rcv->list = rcv->listtail = 0;
      rcv->rmem_alloc = 0;
    }
    
    static void
    handle_ack(struct half_stream * snd, u_int acknum)
    {
      int ackdiff;
    
      ackdiff = acknum - snd->ack_seq;
      if (ackdiff > 0) {
        snd->ack_seq = acknum;
      }
    }
    #if 0
    static void
    check_flags(struct ip * iph, struct tcphdr * th)
    {
      u_char flag = *(((u_char *) th) + 13);
      if (flag & 0x40 || flag & 0x80)
        nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_BADFLAGS, iph, th);
    //ECN is really the only cause of these warnings...
    }
    #endif
    
    //该函数通过tcp和ip头部来查找是否存在该tcp流，并确定它是来自
    //客户端还是服务器端的数据报
    struct tcp_stream *
    find_stream(struct tcphdr * this_tcphdr, struct ip * this_iphdr,
    	    int *from_client)
    {
      struct tuple4 this_addr, reversed;
      struct tcp_stream *a_tcp;
    
      this_addr.source = ntohs(this_tcphdr->th_sport);
      this_addr.dest = ntohs(this_tcphdr->th_dport);
      this_addr.saddr = this_iphdr->ip_src.s_addr;
      this_addr.daddr = this_iphdr->ip_dst.s_addr;
      a_tcp = nids_find_tcp_stream(&this_addr);
      if (a_tcp) {
        *from_client = 1; //是来自客户端的数据报该字段设置为1
        return a_tcp;
      }
      reversed.source = ntohs(this_tcphdr->th_dport);
      reversed.dest = ntohs(this_tcphdr->th_sport);
      reversed.saddr = this_iphdr->ip_dst.s_addr;
      reversed.daddr = this_iphdr->ip_src.s_addr;
      a_tcp = nids_find_tcp_stream(&reversed);
      if (a_tcp) {
        *from_client = 0; //不是来自客户端的数据报该字段设置为0
        return a_tcp;
      }
      return 0;
    }
    
    //由上一个函数调用。利用tuple4查找哈希表，看是否存在该数据流（如果
    //存在，那么将会在哈希表中找到对应的源/目的IP地址和端口号组成的tuple4）
    //从该函数可以看出，它是使用单链表法来解决哈希碰撞冲突的，tcp_stream_table
    //第一维存储的是直接哈希值，第二维存储的是哈希值相同的不同tuple4的单链表
    //用于处理碰撞
    struct tcp_stream *
    nids_find_tcp_stream(struct tuple4 *addr)
    {
      int hash_index;
      struct tcp_stream *a_tcp;
    
      hash_index = mk_hash_index(*addr);
      for (a_tcp = tcp_stream_table[hash_index];
           a_tcp && memcmp(&a_tcp->addr, addr, sizeof (struct tuple4));
           a_tcp = a_tcp->next_node);
      return a_tcp ? a_tcp : 0;
    }
    
    
    void tcp_exit(void)
    {
      int i;
      struct lurker_node *j;
      struct tcp_stream *a_tcp, *t_tcp;
    
      if (!tcp_stream_table || !streams_pool)
        return;
      for (i = 0; i < tcp_stream_table_size; i++) {
        a_tcp = tcp_stream_table[i];
        while(a_tcp) {
          t_tcp = a_tcp;
          a_tcp = a_tcp->next_node;
          for (j = t_tcp->listeners; j; j = j->next) {
              t_tcp->nids_state = NIDS_EXITING;
    	  (j->item)(t_tcp, &j->data);
          }
          nids_free_tcp_stream(t_tcp);
        }
      }
      free(tcp_stream_table);
      tcp_stream_table = NULL;
      free(streams_pool);
      streams_pool = NULL;
      /* FIXME: anything else we should free? */
      /* yes plz.. */
      tcp_latest = tcp_oldest = NULL;
      tcp_num = 0;
    }
    
    void
    process_tcp(u_char * data, int skblen)
    {
      //处理数据报头部，获取了ip数据报和tcp数据报的头部
      struct ip *this_iphdr = (struct ip *)data;
      //ip首部的“首部长度字段”*4就是tcp首部的偏移位置
      struct tcphdr *this_tcphdr = (struct tcphdr *)(data + 4 * this_iphdr->ip_hl);
      int datalen, iplen;
      int from_client = 1;
      unsigned int tmp_ts;
      struct tcp_stream *a_tcp;
      struct half_stream *snd, *rcv;
      ugly_iphdr = this_iphdr;
      iplen = ntohs(this_iphdr->ip_len);  //ip数据报的总长度(数据加头部)
      
      //如果ip数据报的总长度小于ip首部和tcp首部之和，则通过syslog记录系统日志
      if ((unsigned)iplen < 4 * this_iphdr->ip_hl + sizeof(struct tcphdr)) {
        nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_HDR, this_iphdr,
    		       this_tcphdr);
        return;
      } // ktos sie bawi
      
      //数据报的数据部分长度 = ip数据报总长度 - ip首部长度 - tcp首部长度
      datalen = iplen - 4 * this_iphdr->ip_hl - 4 * this_tcphdr->th_off;
      
      if (datalen < 0) {
        nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_HDR, this_iphdr,
    		       this_tcphdr);
        return;
      } // ktos sie bawi
    
      //源IP或目的IP为空
      if ((this_iphdr->ip_src.s_addr | this_iphdr->ip_dst.s_addr) == 0) {
        nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_HDR, this_iphdr,
    		       this_tcphdr);
        return;
      }
      if (!(this_tcphdr->th_flags & TH_ACK))  //确认号ack无效
        detect_scan(this_iphdr);  //检测是否出现了攻击
    
      //如果存放tcp_stream 数据结构的哈希表长度为0不处理，返回
      if (!nids_params.n_tcp_streams) return;
    
      //进行ip头部校验
      if (my_tcp_check(this_tcphdr, iplen - 4 * this_iphdr->ip_hl,
    		   this_iphdr->ip_src.s_addr, this_iphdr->ip_dst.s_addr)) {
        nids_params.syslog(NIDS_WARN_TCP, NIDS_WARN_TCP_HDR, this_iphdr,
    		       this_tcphdr);
        return;
      }
    #if 0
      check_flags(this_iphdr, this_tcphdr);
    //ECN
    #endif
    	
      //经过上述判断，可以得知数据报正常，因此下面将进行处理
    
      /****************三次握手的第一次握手***************/
      /*如果没有找到该数据报。而且该数据报SYN==1，ack==0，
      rst==0,证明这是tcp连接“三次握手”的第一个数据报，将其添加到tcp流中*/
      if (!(a_tcp = find_stream(this_tcphdr, this_iphdr, &from_client))) {
        if ((this_tcphdr->th_flags & TH_SYN) &&
    	!(this_tcphdr->th_flags & TH_ACK) &&
    	!(this_tcphdr->th_flags & TH_RST))
          add_new_tcp(this_tcphdr, this_iphdr);  //添加新的tcp数据流
        return;
      }
    
      //如果是来自客户端的数据报，那么发送端是客户，接收端是服务器
      //否则，相反
      if (from_client) {
        snd = &a_tcp->client;
        rcv = &a_tcp->server;
      }
      else {
        rcv = &a_tcp->client;
        snd = &a_tcp->server;
      }
    
      /*********************三次握手的第二次*****************/
      //是一个syn包，表示是要新建立一个连接，可断定该数据报来自三次握手
      if ((this_tcphdr->th_flags & TH_SYN)) {
    	//如果该包来自客户，直接返回（因为上面已经处理了三次握手的第一个包）
    	//如果不是三次握手的第二个包都直接返回
        if (from_client || a_tcp->client.state != TCP_SYN_SENT ||
          a_tcp->server.state != TCP_CLOSE || !(this_tcphdr->th_flags & TH_ACK))
          return;
        //如果该数据报的ack不等于找到的tcp流的seq+1（注意a_tcp三次握手第一个数据包
    	//插入到tcp_stream表中时，a_tcp客户端的tcp流的seq：a_tcp->client.set已经加1了），
    	//那么该数据报不属于该tcp流
        if (a_tcp->client.seq != ntohl(this_tcphdr->th_ack))
          return;
    	//收到了server发的三次握手第二个包，那么更新server的初始状态、序列号、确认号、窗口大小
        a_tcp->server.state = TCP_SYN_RECV;
        a_tcp->server.seq = ntohl(this_tcphdr->th_seq) + 1;
        a_tcp->server.first_data_seq = a_tcp->server.seq;
        a_tcp->server.ack_seq = ntohl(this_tcphdr->th_ack);
        a_tcp->server.window = ntohs(this_tcphdr->th_win);
        if (a_tcp->client.ts_on) {  //客户端数据流时间戳选项打开
        	a_tcp->server.ts_on = get_ts(this_tcphdr, &a_tcp->server.curr_ts);
    	if (!a_tcp->server.ts_on)  //如果服务器没发时间戳选项，那么将客户端时间戳关闭
    		a_tcp->client.ts_on = 0;
        } else 
    		a_tcp->server.ts_on = 0;	
    	
    	/*主动发起连接的一方在它的SYN中指定时间戳选项。只有在它从另一方的SYN中收到了
    	这个选项后，该选项才会在以后的报文中进行设置*/
        if (a_tcp->client.wscale_on) {  //客户端窗口扩大选项打开
        	a_tcp->server.wscale_on = get_wscale(this_tcphdr, &a_tcp->server.wscale);
    	if (!a_tcp->server.wscale_on) {
    		a_tcp->client.wscale_on = 0;
    		a_tcp->client.wscale  = 1;
    		a_tcp->server.wscale = 1;
    	}	
        } else {
        	a_tcp->server.wscale_on = 0;	
        	a_tcp->server.wscale = 1;
        }	
        return;
    	/*****************至此，第二次握手完毕，函数返回*************/
      }
    
    
      //如果有数据存在并且该数据报的序列号不等于对端的确认号；并且
      //该数据报的序列号超出了对端确认号以及窗口规模（也就是该数据报在窗口之外），或者
      //这是一个已经收到过的数据报（数据包尾部小于窗口左侧），直接返回
      if (
      	! (  !datalen && ntohl(this_tcphdr->th_seq) == rcv->ack_seq  )
      	&&
      	( !before(ntohl(this_tcphdr->th_seq), rcv->ack_seq + rcv->window*rcv->wscale) ||
              before(ntohl(this_tcphdr->th_seq) + datalen, rcv->ack_seq)  
            )
         )     
         return;
    
      //如果是rst包，ok，关闭连接  
      //将现有数据推给注册的回调方，然后销毁这个会话
      if ((this_tcphdr->th_flags & TH_RST)) {
        if (a_tcp->nids_state == NIDS_DATA) {
          struct lurker_node *i;
          a_tcp->nids_state = NIDS_RESET;
    	  //回调所用的钩子
          for (i = a_tcp->listeners; i; i = i->next)
    	      (i->item) (a_tcp, &i->data);
        }
        nids_free_tcp_stream(a_tcp);
        return;
      }
    
      /* PAWS check */
      //防止回绕的序号，如果收到的数据报的时间戳小于发送方
      //当前的时间戳，证明当前的报文段是丢失的报文，PAWS算法将其丢弃
      if (rcv->ts_on && get_ts(this_tcphdr, &tmp_ts) && 
      	before(tmp_ts, snd->curr_ts))
      return; 	
      
      /***********第三次握手*******************/
      if ((this_tcphdr->th_flags & TH_ACK)) { //如果该数据报设置了ack
    	//检查主动连接一方和被动连接一方状态，确定其属于第三次握手数据报
        if (from_client && a_tcp->client.state == TCP_SYN_SENT &&
    	a_tcp->server.state == TCP_SYN_RECV) {
    	  //在此情况下，序列号对得上，说明是第三次握手包，连接建立
          if (ntohl(this_tcphdr->th_ack) == a_tcp->server.seq) {
    	    a_tcp->client.state = TCP_ESTABLISHED;  //更新客户状态
    		//更新客户的序列号
    	    a_tcp->client.ack_seq = ntohl(this_tcphdr->th_ack); 
    	{
    	  struct proc_node *i;
    	  struct lurker_node *j;
    	  void *data;
    	  
    	  //更新服务器状态
    	  a_tcp->server.state = TCP_ESTABLISHED;
    	  a_tcp->nids_state = NIDS_JUST_EST;
    	  for (i = tcp_procs; i; i = i->next) {
    		//此处根据调用者的设定来判断哪些数据需要在回调时返回  
    	    char whatto = 0;
    	    char cc = a_tcp->client.collect;
    	    char sc = a_tcp->server.collect;
    	    char ccu = a_tcp->client.collect_urg;
    	    char scu = a_tcp->server.collect_urg;  
    
    	    /*进入回调函数处理*/  
    	    (i->item) (a_tcp, &data);
    	    if (cc < a_tcp->client.collect)
    	      whatto |= COLLECT_cc;
    	    if (ccu < a_tcp->client.collect_urg)
    	      whatto |= COLLECT_ccu;
    	    if (sc < a_tcp->server.collect)
    	      whatto |= COLLECT_sc;
    	    if (scu < a_tcp->server.collect_urg)
    	      whatto |= COLLECT_scu;
    	    if (nids_params.one_loop_less) {
    	    		if (a_tcp->client.collect >=2) {
    	    			a_tcp->client.collect=cc;
    	    			whatto&=~COLLECT_cc;
    	    		}
    	    		if (a_tcp->server.collect >=2 ) {
    	    			a_tcp->server.collect=sc;
    	    			whatto&=~COLLECT_sc;
    	    		}
    	    }  
    	    if (whatto) {
    	      j = mknew(struct lurker_node);
    	      j->item = i->item;
    	      j->data = data;
    	      j->whatto = whatto;
    	      j->next = a_tcp->listeners;
    	      a_tcp->listeners = j;
    	    }
    	  }
    	  if (!a_tcp->listeners) {
    	    nids_free_tcp_stream(a_tcp);
    	    return;
    	  }
    	  a_tcp->nids_state = NIDS_DATA;
    	}
          }
          // return;
        }
      }
      //自此，握手包处理完毕  
    
      //下面是四次断开的处理
      if ((this_tcphdr->th_flags & TH_ACK)) {
    	//先调用handle_ack更新ack序号  
        handle_ack(snd, ntohl(this_tcphdr->th_ack));
    	//更新状态，回调告知连接关闭，然后释放连接  
        if (rcv->state == FIN_SENT)
          rcv->state = FIN_CONFIRMED;
        if (rcv->state == FIN_CONFIRMED && snd->state == FIN_CONFIRMED) {
          struct lurker_node *i;
    
          a_tcp->nids_state = NIDS_CLOSE;
          for (i = a_tcp->listeners; i; i = i->next)
    	      (i->item) (a_tcp, &i->data);
            nids_free_tcp_stream(a_tcp);
          return;
        }
      }
    
      //下面处理数据包，和初始的fin包
      /*  
        a_tcp -----a_tcp 客户端连接包 
        this_tcphdr tcp 包头 
        snd-----发送包 
        rcv -----接收包 
        (char *) (this_tcphdr) + 4 * this_tcphdr->th_off -----数据包内容 
        datalen---------数据包长度 
      */  
      if (datalen + (this_tcphdr->th_flags & TH_FIN) > 0)
        tcp_queue(a_tcp, this_tcphdr, snd, rcv,
    	      (char *) (this_tcphdr) + 4 * this_tcphdr->th_off,
    	      datalen, skblen);
      snd->window = ntohs(this_tcphdr->th_win);     //更新窗口大小
      //如果缓存溢出（说明出了问题），果断释放连接
      if (rcv->rmem_alloc > 65535)
        prune_queue(rcv, this_tcphdr);
      if (!a_tcp->listeners)
        nids_free_tcp_stream(a_tcp);
    }
    
    void
    nids_discard(struct tcp_stream * a_tcp, int num)
    {
      if (num < a_tcp->read)
        a_tcp->read = num;
    }
    
    void
    nids_register_tcp(void (*x))
    {
      register_callback(&tcp_procs, x);
    }
    
    void
    nids_unregister_tcp(void (*x))
    {
      unregister_callback(&tcp_procs, x);
    }
    
    int
    tcp_init(int size)
    {
      int i;
      struct tcp_timeout *tmp;
    
      if (!size) return 0;
      //tcp_stream_table_size默认大小为1040
      tcp_stream_table_size = size;  //初始化全局tcp会话哈希表
      tcp_stream_table = calloc(tcp_stream_table_size, sizeof(char *));
      if (!tcp_stream_table) {
        nids_params.no_mem("tcp_init");
        return -1;
      }
    
      //设置同一时刻捕获tcp数据的最大数量，为了哈希效率，哈希元素个数为
      //哈希表大小的3/4
      max_stream = 3 * tcp_stream_table_size / 4;
      //先将max_stream个tcp会话结构体申请好，放着（避免后面陆陆续续申请浪费时间）
      streams_pool = (struct tcp_stream *) malloc((max_stream + 1) * sizeof(struct tcp_stream));
      if (!streams_pool) {
        nids_params.no_mem("tcp_init");
        return -1;
      }
      //将这个数组初始化为链表
      for (i = 0; i < max_stream; i++)
        streams_pool[i].next_free = &(streams_pool[i + 1]);
      streams_pool[max_stream].next_free = 0;
      free_streams = streams_pool;
      init_hash();
      while (nids_tcp_timeouts) {
        tmp = nids_tcp_timeouts->next;
        free(nids_tcp_timeouts);
        nids_tcp_timeouts = tmp;
      }
      return 0;
    }
    
    #if HAVE_ICMPHDR
    #define STRUCT_ICMP struct icmphdr
    #define ICMP_CODE   code
    #define ICMP_TYPE   type
    #else
    #define STRUCT_ICMP struct icmp
    #define ICMP_CODE   icmp_code
    #define ICMP_TYPE   icmp_type
    #endif
    
    #ifndef ICMP_DEST_UNREACH
    #define ICMP_DEST_UNREACH ICMP_UNREACH
    #define ICMP_PROT_UNREACH ICMP_UNREACH_PROTOCOL
    #define ICMP_PORT_UNREACH ICMP_UNREACH_PORT
    #define NR_ICMP_UNREACH   ICMP_MAXTYPE
    #endif				
    
    
    void
    process_icmp(u_char * data)
    {
      struct ip *iph = (struct ip *) data;
      struct ip *orig_ip;
      STRUCT_ICMP *pkt;
      struct tcphdr *th;
      struct half_stream *hlf;
      int match_addr;
      struct tcp_stream *a_tcp;
      struct lurker_node *i;
    
      int from_client;
      /* we will use unsigned, to suppress warning; we must be careful with
         possible wrap when substracting 
         the following is ok, as the ip header has already been sanitized */
      unsigned int len = ntohs(iph->ip_len) - (iph->ip_hl << 2);
      
      if (len < sizeof(STRUCT_ICMP))
        return;
      pkt = (STRUCT_ICMP *) (data + (iph->ip_hl << 2));
      if (ip_compute_csum((char *) pkt, len))
        return;
      if (pkt->ICMP_TYPE != ICMP_DEST_UNREACH)
        return;
      /* ok due to check 7 lines above */  
      len -= sizeof(STRUCT_ICMP);
      // sizeof(struct icmp) is not what we want here
      
      if (len < sizeof(struct ip))
        return;
    
      orig_ip = (struct ip *) (((char *) pkt) + 8);
      if (len < (unsigned)(orig_ip->ip_hl << 2) + 8)
         return;
      /* subtraction ok due to the check above */
      len -= orig_ip->ip_hl << 2;
      if ((pkt->ICMP_CODE & 15) == ICMP_PROT_UNREACH ||
          (pkt->ICMP_CODE & 15) == ICMP_PORT_UNREACH)
        match_addr = 1;
      else
        match_addr = 0;
      if (pkt->ICMP_CODE > NR_ICMP_UNREACH)
        return;
      if (match_addr && (iph->ip_src.s_addr != orig_ip->ip_dst.s_addr))
        return;
      if (orig_ip->ip_p != IPPROTO_TCP)
        return;
      th = (struct tcphdr *) (((char *) orig_ip) + (orig_ip->ip_hl << 2));
      if (!(a_tcp = find_stream(th, orig_ip, &from_client)))
        return;
      if (a_tcp->addr.dest == iph->ip_dst.s_addr)
        hlf = &a_tcp->server;
      else
        hlf = &a_tcp->client;
      if (hlf->state != TCP_SYN_SENT && hlf->state != TCP_SYN_RECV)
        return;
      a_tcp->nids_state = NIDS_RESET;
      for (i = a_tcp->listeners; i; i = i->next)
        (i->item) (a_tcp, &i->data);
      nids_free_tcp_stream(a_tcp);
    }

# tcp.h #

/*
      Copyright (c) 1999 Rafal Wojtczuk <nergal@7bulls.com>. All rights reserved.
      See the file COPYING for license details.
    */
    #ifndef _NIDS_TCP_H
    #define _NIDS_TCP_H
    #include <sys/time.h>
    
    //双向链表，是linux在其他协议栈中传送的数据结构
    //也就是“包”
    struct skbuff {
      struct skbuff *next;  //双链表的指针
      struct skbuff *prev;
    
      void *data;
      u_int len;
      u_int truesize;
      u_int urg_ptr;
      
      char fin;
      char urg;
      u_int seq;
      u_int ack;
    };
    
    int tcp_init(int);
    void tcp_exit(void);
    void process_tcp(u_char *, int);
    void process_icmp(u_char *);
    void tcp_check_timeouts(struct timeval *);
    
    #endif /* _NIDS_TCP_H */

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