ip_vs实现分析(6)

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8. IPVS的数据包发送

IPVS连接中的数据包的发送方法是由ip_vs_bind_xmit()函数定义的,具体的发送数据包处理函数定义在net/ipv4/ipvs/ip_vs_xmit.c中。

8.1 NAT发送

NAT发送只发送请求方向的数据,因此是进行目的NAT

/*
 *      NAT transmitter (only for outside-to-inside nat forwarding)
 *      Not used for related ICMP
 */
int
ip_vs_nat_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
        struct ip_vs_protocol *pp)
{
 struct rtable *rt;  /* Route to the other host */
 int mtu;
 struct iphdr *iph = skb->nh.iph;

 EnterFunction(10);

 /* check if it is a connection of no-client-port */
 if (unlikely(cp->flags & IP_VS_CONN_F_NO_CPORT)) {
// 如果连接标志了客户端端口为0,将当前skb中的端口填给连接
  __u16 _pt, *p;
  p = skb_header_pointer(skb, iph->ihl*4, sizeof(_pt), &_pt);
  if (p == NULL)
   goto tx_error;
// *p是源端口
  ip_vs_conn_fill_cport(cp, *p);
  IP_VS_DBG(10, "filled cport=%d\n", ntohs(*p));
 }
// 查找路由,找不到的话发ICMP出错包
 if (!(rt = __ip_vs_get_out_rt(cp, RT_TOS(iph->tos))))
  goto tx_error_icmp;

 /* MTU checking */
// 检查路由发出网卡的MTU,如果包长超过MTU又有DF标志,发送ICMP错误信息,而不进行分片操作
 mtu = dst_mtu(&rt->u.dst);
 if ((skb->len > mtu) && (iph->frag_off&__constant_htons(IP_DF))) {
  ip_rt_put(rt);
  icmp_send(skb, ICMP_DEST_UNREACH,ICMP_FRAG_NEEDED, htonl(mtu));
  IP_VS_DBG_RL_PKT(0, pp, skb, 0, "ip_vs_nat_xmit(): frag needed for");
  goto tx_error;
 }

 /* copy-on-write the packet before mangling it */
// 让skb包的IP头部分是可写的
 if (!ip_vs_make_skb_writable(&skb, sizeof(struct iphdr)))
  goto tx_error_put;

// 扩充skb头部空间以容纳硬件MAC头数据
 if (skb_cow(skb, rt->u.dst.dev->hard_header_len))
  goto tx_error_put;

 /* drop old route */
// 释放skb当前的路由cache
 dst_release(skb->dst);
 skb->dst = &rt->u.dst;

 /* mangle the packet */
// 对上层协议(TCP/UDP...)进行目的NAT,因为要发送给实际的目的服务器
 if (pp->dnat_handler && !pp->dnat_handler(&skb, pp, cp))
  goto tx_error;
// 修改目的地址为真实目的服务器地址
 skb->nh.iph->daddr = cp->daddr;
// 计算IP头校验和
 ip_send_check(skb->nh.iph);

 IP_VS_DBG_PKT(10, pp, skb, 0, "After DNAT");

 /* FIXME: when application helper enlarges the packet and the length
    is larger than the MTU of outgoing device, there will be still
    MTU problem. */

 /* Another hack: avoid icmp_send in ip_fragment */
// don't fragment标志
 skb->local_df = 1;
// 发送数据包,实际还是HOOK住netfilter的OUTPUT点,受OUTPUT规则限制
 IP_VS_XMIT(skb, rt);

 LeaveFunction(10);
// 返回STOLEN告诉原来的hook点不用处理该包了
 return NF_STOLEN;

  tx_error_icmp:
 dst_link_failure(skb);
  tx_error:
 LeaveFunction(10);
 kfree_skb(skb);
 return NF_STOLEN;
  tx_error_put:
 ip_rt_put(rt);
 goto tx_error;
}

 

8.2 TUNNEL发送

TUNNEL发送是把原来的IP部分再加在一个IPIP协议(4)头后发出去,新头的目的IP是真实目的服务器,源IP是真实客户端IP,该包是可以路由的,服务器的回应包将直接路由回去而不经过IPVS.

/*
 *   IP Tunneling transmitter
 *
 *   This function encapsulates the packet in a new IP packet, its
 *   destination will be set to cp->daddr. Most code of this function
 *   is taken from ipip.c.
 *
 *   It is used in VS/TUN cluster. The load balancer selects a real
 *   server from a cluster based on a scheduling algorithm,
 *   encapsulates the request packet and forwards it to the selected
 *   server. For example, all real servers are configured with
 *   "ifconfig tunl0 <Virtual IP Address> up". When the server receives
 *   the encapsulated packet, it will decapsulate the packet, processe
 *   the request and return the response packets directly to the client
 *   without passing the load balancer. This can greatly increase the
 *   scalability of virtual server.
 *
 *   Used for ANY protocol
 */
int
ip_vs_tunnel_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
    struct ip_vs_protocol *pp)
{
 struct rtable *rt;   /* Route to the other host */
 struct net_device *tdev;  /* Device to other host */
 struct iphdr  *old_iph = skb->nh.iph;
 u8     tos = old_iph->tos;
 __be16 df = old_iph->frag_off;
 struct iphdr  *iph;   /* Our new IP header */
 int    max_headroom;   /* The extra header space needed */
 int    mtu;

 EnterFunction(10);
// 只包装IP包，其他协议如ARP，IPX等不管
 if (skb->protocol != __constant_htons(ETH_P_IP)) {
  IP_VS_DBG_RL("ip_vs_tunnel_xmit(): protocol error, "
        "ETH_P_IP: %d, skb protocol: %d\n",
        __constant_htons(ETH_P_IP), skb->protocol);
  goto tx_error;
 }
// 根据连接信息找外出的路由cache
 if (!(rt = __ip_vs_get_out_rt(cp, RT_TOS(tos))))
  goto tx_error_icmp;

// 数据包发出网卡
 tdev = rt->u.dst.dev;

// 检查路径的MTU
 mtu = dst_mtu(&rt->u.dst) - sizeof(struct iphdr);
 if (mtu < 68) {
  ip_rt_put(rt);
  IP_VS_DBG_RL("ip_vs_tunnel_xmit(): mtu less than 68\n");
  goto tx_error;
 }
// 更新路由的MTU
 if (skb->dst)
  skb->dst->ops->update_pmtu(skb->dst, mtu);

// 检查don't fragement标志
 df |= (old_iph->frag_off&__constant_htons(IP_DF));

 if ((old_iph->frag_off&__constant_htons(IP_DF))
     && mtu < ntohs(old_iph->tot_len)) {
// 如果skb包长超过MTU又有DF标志,发送ICMP错误信息,而不进行分片操作
  icmp_send(skb, ICMP_DEST_UNREACH,ICMP_FRAG_NEEDED, htonl(mtu));
  ip_rt_put(rt);
  IP_VS_DBG_RL("ip_vs_tunnel_xmit(): frag needed\n");
  goto tx_error;
 }

 /*
  * Okay, now see if we can stuff it in the buffer as-is.
  */
// 计算需要添加的IP头的最大长度
 max_headroom = LL_RESERVED_SPACE(tdev) + sizeof(struct iphdr);

 if (skb_headroom(skb) < max_headroom
     || skb_cloned(skb) || skb_shared(skb)) {
// 重新分配一个skb包,该skb头部足够大可容纳外部IP头空间
// 分配失败则不发送该包了
  struct sk_buff *new_skb =
   skb_realloc_headroom(skb, max_headroom);
  if (!new_skb) {
   ip_rt_put(rt);
   kfree_skb(skb);
   IP_VS_ERR_RL("ip_vs_tunnel_xmit(): no memory\n");
   return NF_STOLEN;
  }
// 将原来的skb释放掉
  kfree_skb(skb);
// 将skb指向新包,更新ip头指针
  skb = new_skb;
  old_iph = skb->nh.iph;
 }

// skb->h是传输层头,现在要新加个IP头,原来的IP头就升级为传输层头
 skb->h.raw = (void *) old_iph;

 /* fix old IP header checksum */
// 计算老IP头的校验和
 ip_send_check(old_iph);

// skb的data指针前移出IP头长度作为新IP头的起点
 skb->nh.raw = skb_push(skb, sizeof(struct iphdr));
 memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));

 /* drop old route */
// 更新路由cache
 dst_release(skb->dst);
 skb->dst = &rt->u.dst;

// 填写新IP头部信息
 /*
  * Push down and install the IPIP header.
  */
 iph   = skb->nh.iph;
 iph->version  = 4;
 iph->ihl  = sizeof(struct iphdr)>>2;
 iph->frag_off  = df;
// 协议设置为IPIP, 值为4
 iph->protocol  = IPPROTO_IPIP;
 iph->tos  = tos;
 iph->daddr  = rt->rt_dst;
 iph->saddr  = rt->rt_src;
 iph->ttl  = old_iph->ttl;
 iph->tot_len  = htons(skb->len);
// 设置IP头中的ID值
 ip_select_ident(iph, &rt->u.dst, NULL);
// 计算IP头校验和
 ip_send_check(iph);

 /* Another hack: avoid icmp_send in ip_fragment */
// don't fragmemt
 skb->local_df = 1;
// 发送新的skb包
 IP_VS_XMIT(skb, rt);

 LeaveFunction(10);

 return NF_STOLEN;

  tx_error_icmp:
 dst_link_failure(skb);
  tx_error:
 kfree_skb(skb);
 LeaveFunction(10);
 return NF_STOLEN;
}

 

8.3 DR发送

DR发送是将原来的skb包中的目的MAC地址修改为目的服务器的MAC地址后直接发出,因此是不能路由的,IPVS均衡设备和目的服务器物理上必须在同一个二层子网。在DR模式下，IPVS和服务器都配置了相同的对外服务的VIP，服务器也配了自己的真实IP，不过服务器上配VIP的网卡属性中的NOARP信息是打开的，就是在该网卡上不响应ARP信息，但可以接收到达该VIP的数据包，这样外面请求包先是到IPVS均衡器，因为IPVS的VIP是响应ARP的，然后根据调度找一台服务器，用服务器的真实IP来确定路由，然后直接把包发出来，这时包中所有数据都没修改，因为目的服务器上VIP地址符合包中的目的地址，因此是可以接收该包的。

/*
 *      Direct Routing transmitter
 *      Used for ANY protocol
 */
int
ip_vs_dr_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
       struct ip_vs_protocol *pp)
{
 struct rtable *rt;   /* Route to the other host */
 struct iphdr  *iph = skb->nh.iph;
 int    mtu;

 EnterFunction(10);
// 根据连接指定的目的服务器找路由
 if (!(rt = __ip_vs_get_out_rt(cp, RT_TOS(iph->tos))))
  goto tx_error_icmp;

 /* MTU checking */
// 检查MTU
 mtu = dst_mtu(&rt->u.dst);
 if ((iph->frag_off&__constant_htons(IP_DF)) && skb->len > mtu) {
  icmp_send(skb, ICMP_DEST_UNREACH,ICMP_FRAG_NEEDED, htonl(mtu));
  ip_rt_put(rt);
  IP_VS_DBG_RL("ip_vs_dr_xmit(): frag needed\n");
  goto tx_error;
 }

 /*
  * Call ip_send_check because we are not sure it is called
  * after ip_defrag. Is copy-on-write needed?
  */
// 防止skb包是共用的,还被其他地方使用
 if (unlikely((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)) {
  ip_rt_put(rt);
  return NF_STOLEN;
 }
// 重新计算IP头校验和
 ip_send_check(skb->nh.iph);

 /* drop old route */
// 释放原来的路由
 dst_release(skb->dst);
// 指定新路由
 skb->dst = &rt->u.dst;

 /* Another hack: avoid icmp_send in ip_fragment */
 skb->local_df = 1;
// 直接发出了
 IP_VS_XMIT(skb, rt);

 LeaveFunction(10);
 return NF_STOLEN;

  tx_error_icmp:
 dst_link_failure(skb);
  tx_error:
 kfree_skb(skb);
 LeaveFunction(10);
 return NF_STOLEN;
}

 

8.4 NULL发送

啥也没干
/*
 *      NULL transmitter (do nothing except return NF_ACCEPT)
 */
int
ip_vs_null_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
  struct ip_vs_protocol *pp)
{
 /* we do not touch skb and do not need pskb ptr */
 return NF_ACCEPT;
}

8.5 旁路发送

旁路模式,实际数据包不是给IPVS均衡器自己的,由IPVS进行转发
/*
 *      Bypass transmitter
 *      Let packets bypass the destination when the destination is not
 *      available, it may be only used in transparent cache cluster.
 */
int
ip_vs_bypass_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
    struct ip_vs_protocol *pp)
{
 struct rtable *rt;   /* Route to the other host */
 struct iphdr  *iph = skb->nh.iph;
 u8     tos = iph->tos;
 int    mtu;
// 用当前IP包的目的地址作为查路由的key
 struct flowi fl = {
  .oif = 0,
  .nl_u = {
   .ip4_u = {
    .daddr = iph->daddr,
    .saddr = 0,
    .tos = RT_TOS(tos), } },
 };

 EnterFunction(10);
// 查找当前数据包的目的IP地址对应的路由,而不是是IPVS连接的信息找路由
 if (ip_route_output_key(&rt, &fl)) {
  IP_VS_DBG_RL("ip_vs_bypass_xmit(): ip_route_output error, "
        "dest: %u.%u.%u.%u\n", NIPQUAD(iph->daddr));
  goto tx_error_icmp;
 }

// MTU检查
 /* MTU checking */
 mtu = dst_mtu(&rt->u.dst);
 if ((skb->len > mtu) && (iph->frag_off&__constant_htons(IP_DF))) {
  ip_rt_put(rt);
  icmp_send(skb, ICMP_DEST_UNREACH,ICMP_FRAG_NEEDED, htonl(mtu));
  IP_VS_DBG_RL("ip_vs_bypass_xmit(): frag needed\n");
  goto tx_error;
 }

 /*
  * Call ip_send_check because we are not sure it is called
  * after ip_defrag. Is copy-on-write needed?
  */
// 防止skb包是共用的,还被其他地方使用
 if (unlikely((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)) {
  ip_rt_put(rt);
  return NF_STOLEN;
 }
// 计算IP头校验和
 ip_send_check(skb->nh.iph);

 /* drop old route */
// 释放老路由,更新路由
 dst_release(skb->dst);
 skb->dst = &rt->u.dst;

 /* Another hack: avoid icmp_send in ip_fragment */
 skb->local_df = 1;
// 发送
 IP_VS_XMIT(skb, rt);

 LeaveFunction(10);
 return NF_STOLEN;

 tx_error_icmp:
 dst_link_failure(skb);
 tx_error:
 kfree_skb(skb);
 LeaveFunction(10);
 return NF_STOLEN;
}

 

8.6 ICMP发送

发送各种ICMP错误信息包
/*
 * ICMP packet transmitter
 * called by the ip_vs_in_icmp
 */
int
ip_vs_icmp_xmit(struct sk_buff *skb, struct ip_vs_conn *cp,
  struct ip_vs_protocol *pp, int offset)
{
 struct rtable *rt; /* Route to the other host */
 int mtu;
 int rc;

 EnterFunction(10);

 /* The ICMP packet for VS/TUN, VS/DR and LOCALNODE will be
    forwarded directly here, because there is no need to
    translate address/port back */
 if (IP_VS_FWD_METHOD(cp) != IP_VS_CONN_F_MASQ) {
// TUNNEL和DR模式下直接调用连接的发送方法发送即可
  if (cp->packet_xmit)
   rc = cp->packet_xmit(skb, cp, pp);
  else
   rc = NF_ACCEPT;
  /* do not touch skb anymore */
  atomic_inc(&cp->in_pkts);
  goto out;
 }

 /*
  * mangle and send the packet here (only for VS/NAT)
  */
// 根据连接的信息找路由
 if (!(rt = __ip_vs_get_out_rt(cp, RT_TOS(skb->nh.iph->tos))))
  goto tx_error_icmp;

 /* MTU checking */
 mtu = dst_mtu(&rt->u.dst);
 if ((skb->len > mtu) && (skb->nh.iph->frag_off&__constant_htons(IP_DF))) {
  ip_rt_put(rt);
  icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
  IP_VS_DBG_RL("ip_vs_in_icmp(): frag needed\n");
  goto tx_error;
 }

 /* copy-on-write the packet before mangling it */
// 使skb包可写
 if (!ip_vs_make_skb_writable(&skb, offset))
  goto tx_error_put;

// 扩充skb头部空间以容纳硬件MAC头数据
 if (skb_cow(skb, rt->u.dst.dev->hard_header_len))
  goto tx_error_put;

// 路由更新
 /* drop the old route when skb is not shared */
 dst_release(skb->dst);
 skb->dst = &rt->u.dst;

// 对ICMP包进行地址转换
 ip_vs_nat_icmp(skb, pp, cp, 0);

 /* Another hack: avoid icmp_send in ip_fragment */
 skb->local_df = 1;

 IP_VS_XMIT(skb, rt);

 rc = NF_STOLEN;
 goto out;

  tx_error_icmp:
 dst_link_failure(skb);
  tx_error:
 dev_kfree_skb(skb);
 rc = NF_STOLEN;
  out:
 LeaveFunction(10);
 return rc;
  tx_error_put:
 ip_rt_put(rt);
 goto tx_error;
}