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一. 前言

ICMP即Internet Control Message Protocol因特网控制消息协议。

ICMP是网络层协议，IP不可分割的一部分。

ICMP用于报告数据报处理中的错误，比如以下情况下时发送ICMP消息:当数据报无法到达其目的地时，当网关没有转发数据报的缓冲能力时，以及当网关可以指示主机在较短的路由上发送数据时。

互联网协议的设计并不是绝对可靠的。ICMP这些控制消息的目的是提供有关通信环境中问题的反馈，而不是使IP可靠。不能确保数据报的传递或控制消息的返回，一些数据报可能无法送达时也没有任何丢失报告。如果需要可靠的通信，则使用IP的更高级别协议必须实现其自己的可靠性程序。

ICMP消息通常报告数据报处理中的错误，且不发送关于ICMP消息的ICMP消息，否则消息会无限递归。此外，ICMP消息只发送关于处理分段数据报的零分段时的错误。（片段零的片段offeset等于零）。

参考

RFC 792

https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml

二. ICMP消息格式

ICMP消息格式

从 ICMP 的报文格式来说，ICMP 是 IP 的上层协议，他是在IP报的基础上再添加ICMP报格式。但是 ICMP 是分担了 IP 的一部分功能。所以，他也被认为是与 IP 同层的协议。

我们这里只关注ICMP部分，ICMP由首部和数据两部分组成,如下

Type和Code如下表

使用wirshark协助解析

三. LWIP中ICMP相关代码分析

这里只看IPV4相关的,IPV6下也有对应的实现。

ipv4/icmp.c

icmp.h

3.1调试打印

可以配置宏ICMP_DEBUG,使能调试打印,一遍跟踪对应的程序处理过程。

lwipopts.h中配置

#define ICMP_DEBUG LWIP_DBG_ON

3.2数据结构

实现了以下Type

#define ICMP_ER   0    /* echo reply */


#define ICMP_DUR  3    /* destination unreachable */


#define ICMP_SQ   4    /* source quench */


#define ICMP_RD   5    /* redirect */


#define ICMP_ECHO 8    /* echo */


#define ICMP_TE  11    /* time exceeded */


#define ICMP_PP  12    /* parameter problem */


#define ICMP_TS  13    /* timestamp */


#define ICMP_TSR 14    /* timestamp reply */


#define ICMP_IRQ 15    /* information request */


#define ICMP_IR  16    /* information reply */


#define ICMP_AM  17    /* address mask request */


#define ICMP_AMR 18    /* address mask reply */

定义了头的数据结构

struct icmp_hdr {


PACK_STRUCT_FLD_8(u8_t type);


PACK_STRUCT_FLD_8(u8_t code);


PACK_STRUCT_FIELD(u16_t chksum);


PACK_STRUCT_FIELD(u32_t data);


} PACK_STRUCT_STRUCT;

如果四echo则，ICMP首部后面4字节数据拆分成id和序列号

struct icmp_echo_hdr {


PACK_STRUCT_FLD_8(u8_t type);


PACK_STRUCT_FLD_8(u8_t code);


PACK_STRUCT_FIELD(u16_t chksum);


PACK_STRUCT_FIELD(u16_t id);


PACK_STRUCT_FIELD(u16_t seqno);


} PACK_STRUCT_STRUCT;

3.3输入数据流

关键代码是icmp_input

ethernet_input->Type=0x0800 ip4_input-> Protocol=0x01 icmp_input

通过switch处理各种类型

switch (type) {


  case ICMP_ER:

比如对于收到别人的ping请求就是进入

case ICMP_ECHO:

如果是多播地址不响应

然后检查ICMP头部至少要8字节。

然后检查checksum

最后调用ip4_output_if发送响应包。

3.4输出数据流

icmp_dest_unreach

icmp_time_exceeded

都是调用

icmp_send_response

3.5发送PING

收到响应进入icmp_input的

case ICMP_ER:


    /* This is OK, echo reply might have been parsed by a raw PCB


       (as obviously, an echo request has been sent, too). */


    MIB2_STATS_INC(mib2.icmpinechoreps);


    break

发送ping可以裸机可以使用raw PCB，带OS可以使用socket实现

详见ping.c/h

#include "lwip/opt.h"


#if LWIP_RAW /* don't build if not configured for use in lwipopts.h */


#include "ping.h"


#include "lwip/mem.h"
#include "lwip/raw.h"
#include "lwip/icmp.h"
#include "lwip/netif.h"
#include "lwip/sys.h"
#include "lwip/timeouts.h"
#include "lwip/inet_chksum.h"
#include "lwip/prot/ip4.h"


#if PING_USE_SOCKETS
#include "lwip/sockets.h"
#include "lwip/inet.h"
#include < string.h >
#endif /* PING_USE_SOCKETS */




/**
 * PING_DEBUG: Enable debugging for PING.
 */
#ifndef PING_DEBUG
#define PING_DEBUG     LWIP_DBG_ON
#endif


/** ping receive timeout - in milliseconds */
#ifndef PING_RCV_TIMEO
#define PING_RCV_TIMEO 1000
#endif


/** ping delay - in milliseconds */
#ifndef PING_DELAY
#define PING_DELAY     1000
#endif


/** ping identifier - must fit on a u16_t */
#ifndef PING_ID
#define PING_ID        0xAFAF
#endif


/** ping additional data size to include in the packet */
#ifndef PING_DATA_SIZE
#define PING_DATA_SIZE 32
#endif


/** ping result action - no default action */
#ifndef PING_RESULT
#define PING_RESULT(ping_ok)
#endif


/* ping variables */
static const ip_addr_t* ping_target;
static u16_t ping_seq_num;
#ifdef LWIP_DEBUG
static u32_t ping_time;
#endif /* LWIP_DEBUG */
#if !PING_USE_SOCKETS
static struct raw_pcb *ping_pcb;
#endif /* PING_USE_SOCKETS */


/** Prepare a echo ICMP request */
static void
ping_prepare_echo( struct icmp_echo_hdr *iecho, u16_t len)
{
  size_t i;
  size_t data_len = len - sizeof(struct icmp_echo_hdr);


  ICMPH_TYPE_SET(iecho, ICMP_ECHO);
  ICMPH_CODE_SET(iecho, 0);
  iecho- >chksum = 0;
  iecho- >id     = PING_ID;
  iecho- >seqno  = lwip_htons(++ping_seq_num);


  /* fill the additional data buffer with some data */
  for(i = 0; i < data_len; i++) {
    ((char*)iecho)[sizeof(struct icmp_echo_hdr) + i] = (char)i;
  }


  iecho- >chksum = inet_chksum(iecho, len);
}


#if PING_USE_SOCKETS


/* Ping using the socket ip */
static err_t
ping_send(int s, const ip_addr_t *addr)
{
  int err;
  struct icmp_echo_hdr *iecho;
  struct sockaddr_storage to;
  size_t ping_size = sizeof(struct icmp_echo_hdr) + PING_DATA_SIZE;
  LWIP_ASSERT("ping_size is too big", ping_size <= 0xffff);


#if LWIP_IPV6
  if(IP_IS_V6(addr) && !ip6_addr_isipv4mappedipv6(ip_2_ip6(addr))) {
    /* todo: support ICMP6 echo */
    return ERR_VAL;
  }
#endif /* LWIP_IPV6 */


  iecho = (struct icmp_echo_hdr *)mem_malloc((mem_size_t)ping_size);
  if (!iecho) {
    return ERR_MEM;
  }


  ping_prepare_echo(iecho, (u16_t)ping_size);


#if LWIP_IPV4
  if(IP_IS_V4(addr)) {
    struct sockaddr_in *to4 = (struct sockaddr_in*)&to;
    to4- >sin_len    = sizeof(*to4);
    to4- >sin_family = AF_INET;
    inet_addr_from_ip4addr(&to4- >sin_addr, ip_2_ip4(addr));
  }
#endif /* LWIP_IPV4 */


#if LWIP_IPV6
  if(IP_IS_V6(addr)) {
    struct sockaddr_in6 *to6 = (struct sockaddr_in6*)&to;
    to6- >sin6_len    = sizeof(*to6);
    to6- >sin6_family = AF_INET6;
    inet6_addr_from_ip6addr(&to6- >sin6_addr, ip_2_ip6(addr));
  }
#endif /* LWIP_IPV6 */


  err = lwip_sendto(s, iecho, ping_size, 0, (struct sockaddr*)&to, sizeof(to));


  mem_free(iecho);


  return (err ? ERR_OK : ERR_VAL);
}


static void
ping_recv(int s)
{
  char buf[64];
  int len;
  struct sockaddr_storage from;
  int fromlen = sizeof(from);


  while((len = lwip_recvfrom(s, buf, sizeof(buf), 0, (struct sockaddr*)&from, (socklen_t*)&fromlen)) > 0) {
    if (len >= (int)(sizeof(struct ip_hdr)+sizeof(struct icmp_echo_hdr))) {
      ip_addr_t fromaddr;
      memset(&fromaddr, 0, sizeof(fromaddr));


#if LWIP_IPV4
      if(from.ss_family == AF_INET) {
        struct sockaddr_in *from4 = (struct sockaddr_in*)&from;
        inet_addr_to_ip4addr(ip_2_ip4(&fromaddr), &from4- >sin_addr);
        IP_SET_TYPE_VAL(fromaddr, IPADDR_TYPE_V4);
      }
#endif /* LWIP_IPV4 */


#if LWIP_IPV6
      if(from.ss_family == AF_INET6) {
        struct sockaddr_in6 *from6 = (struct sockaddr_in6*)&from;
        inet6_addr_to_ip6addr(ip_2_ip6(&fromaddr), &from6- >sin6_addr);
        IP_SET_TYPE_VAL(fromaddr, IPADDR_TYPE_V6);
      }
#endif /* LWIP_IPV6 */


      LWIP_DEBUGF( PING_DEBUG, ("ping: recv "));
      ip_addr_debug_print_val(PING_DEBUG, fromaddr);
      LWIP_DEBUGF( PING_DEBUG, (" %"U32_F" msn", (sys_now() - ping_time)));


      /* todo: support ICMP6 echo */
#if LWIP_IPV4
      if (IP_IS_V4_VAL(fromaddr)) {
        struct ip_hdr *iphdr;
        struct icmp_echo_hdr *iecho;


        iphdr = (struct ip_hdr *)buf;
        iecho = (struct icmp_echo_hdr *)(buf + (IPH_HL(iphdr) * 4));
        if ((iecho- >id == PING_ID) && (iecho- >seqno == lwip_htons(ping_seq_num))) {
          /* do some ping result processing */
          PING_RESULT((ICMPH_TYPE(iecho) == ICMP_ER));
          return;
        } else {
          LWIP_DEBUGF( PING_DEBUG, ("ping: dropn"));
        }
      }
#endif /* LWIP_IPV4 */
    }
    fromlen = sizeof(from);
  }


  if (len == 0) {
    LWIP_DEBUGF( PING_DEBUG, ("ping: recv - %"U32_F" ms - timeoutn", (sys_now()-ping_time)));
  }


  /* do some ping result processing */
  PING_RESULT(0);
}


static void
ping_thread(void *arg)
{
  int s;
  int ret;


#if LWIP_SO_SNDRCVTIMEO_NONSTANDARD
  int timeout = PING_RCV_TIMEO;
#else
  struct timeval timeout;
  timeout.tv_sec = PING_RCV_TIMEO/1000;
  timeout.tv_usec = (PING_RCV_TIMEO%1000)*1000;
#endif
  LWIP_UNUSED_ARG(arg);


#if LWIP_IPV6
  if(IP_IS_V4(ping_target) || ip6_addr_isipv4mappedipv6(ip_2_ip6(ping_target))) {
    s = lwip_socket(AF_INET6, SOCK_RAW, IP_PROTO_ICMP);
  } else {
    s = lwip_socket(AF_INET6, SOCK_RAW, IP6_NEXTH_ICMP6);
  }
#else
  s = lwip_socket(AF_INET,  SOCK_RAW, IP_PROTO_ICMP);
#endif
  if (s < 0) {
    return;
  }


  ret = lwip_setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout));
  LWIP_ASSERT("setting receive timeout failed", ret == 0);
  LWIP_UNUSED_ARG(ret);


  while (1) {
    if (ping_send(s, ping_target) == ERR_OK) {
      LWIP_DEBUGF( PING_DEBUG, ("ping: send "));
      ip_addr_debug_print(PING_DEBUG, ping_target);
      LWIP_DEBUGF( PING_DEBUG, ("n"));


#ifdef LWIP_DEBUG
      ping_time = sys_now();
#endif /* LWIP_DEBUG */
      ping_recv(s);
    } else {
      LWIP_DEBUGF( PING_DEBUG, ("ping: send "));
      ip_addr_debug_print(PING_DEBUG, ping_target);
      LWIP_DEBUGF( PING_DEBUG, (" - errorn"));
    }
    sys_msleep(PING_DELAY);
  }
}


#else /* PING_USE_SOCKETS */


/* Ping using the raw ip */
static u8_t
ping_recv(void *arg, struct raw_pcb *pcb, struct pbuf *p, const ip_addr_t *addr)
{
  struct icmp_echo_hdr *iecho;
  LWIP_UNUSED_ARG(arg);
  LWIP_UNUSED_ARG(pcb);
  LWIP_UNUSED_ARG(addr);
  LWIP_ASSERT("p != NULL", p != NULL);


  if ((p- >tot_len >= (PBUF_IP_HLEN + sizeof(struct icmp_echo_hdr))) &&
      pbuf_remove_header(p, PBUF_IP_HLEN) == 0) {
    iecho = (struct icmp_echo_hdr *)p- >payload;


    if ((iecho- >id == PING_ID) && (iecho- >seqno == lwip_htons(ping_seq_num))) {
      LWIP_DEBUGF( PING_DEBUG, ("ping: recv "));
      ip_addr_debug_print(PING_DEBUG, addr);
      LWIP_DEBUGF( PING_DEBUG, (" %"U32_F" msn", (sys_now()-ping_time)));


      /* do some ping result processing */
      PING_RESULT(1);
      pbuf_free(p);
      return 1; /* eat the packet */
    }
    /* not eaten, restore original packet */
    pbuf_add_header(p, PBUF_IP_HLEN);
  }


  return 0; /* don't eat the packet */
}


static void
ping_send(struct raw_pcb *raw, const ip_addr_t *addr)
{
  struct pbuf *p;
  struct icmp_echo_hdr *iecho;
  size_t ping_size = sizeof(struct icmp_echo_hdr) + PING_DATA_SIZE;


  LWIP_DEBUGF( PING_DEBUG, ("ping: send "));
  ip_addr_debug_print(PING_DEBUG, addr);
  LWIP_DEBUGF( PING_DEBUG, ("n"));
  LWIP_ASSERT("ping_size <= 0xffff", ping_size <= 0xffff);


  p = pbuf_alloc(PBUF_IP, (u16_t)ping_size, PBUF_RAM);
  if (!p) {
    return;
  }
  if ((p- >len == p- >tot_len) && (p- >next == NULL)) {
    iecho = (struct icmp_echo_hdr *)p- >payload;


    ping_prepare_echo(iecho, (u16_t)ping_size);


    raw_sendto(raw, p, addr);
#ifdef LWIP_DEBUG
    ping_time = sys_now();
#endif /* LWIP_DEBUG */
  }
  pbuf_free(p);
}


static void
ping_timeout(void *arg)
{
  struct raw_pcb *pcb = (struct raw_pcb*)arg;


  LWIP_ASSERT("ping_timeout: no pcb given!", pcb != NULL);


  ping_send(pcb, ping_target);


  sys_timeout(PING_DELAY, ping_timeout, pcb);
}


static void
ping_raw_init(void)
{
  ping_pcb = raw_new(IP_PROTO_ICMP);
  LWIP_ASSERT("ping_pcb != NULL", ping_pcb != NULL);


  raw_recv(ping_pcb, ping_recv, NULL);
  raw_bind(ping_pcb, IP_ADDR_ANY);
  sys_timeout(PING_DELAY, ping_timeout, ping_pcb);
}


void
ping_send_now(void)
{
  LWIP_ASSERT("ping_pcb != NULL", ping_pcb != NULL);
  ping_send(ping_pcb, ping_target);
}


#endif /* PING_USE_SOCKETS */


void
ping_init(const ip_addr_t* ping_addr)
{
  ping_target = ping_addr;


#if PING_USE_SOCKETS
  sys_thread_new("ping_thread", ping_thread, NULL, DEFAULT_THREAD_STACKSIZE, DEFAULT_THREAD_PRIO);
#else /* PING_USE_SOCKETS */
  ping_raw_init();
#endif /* PING_USE_SOCKETS */
}


#endif /* LWIP_RAW */

#ifndef LWIP_PING_H
#define LWIP_PING_H


#include "lwip/ip_addr.h"


/**
 * PING_USE_SOCKETS: Set to 1 to use sockets, otherwise the raw api is used
 */
#ifndef PING_USE_SOCKETS
#define PING_USE_SOCKETS    LWIP_SOCKET
#endif


void ping_init(const ip_addr_t* ping_addr);


#if !PING_USE_SOCKETS
void ping_send_now(void);
#endif /* !PING_USE_SOCKETS */


#endif /* LWIP_PING_H */

如下是带OS的测试

ip_addr_t ping_addr;


  IP4_ADDR(&ping_addr, 192, 168, 1, 9);


  ping_init(&ping_addr);

打印如下，(这里printf不支持某些格式所以IP地址打印显示不对)

四. 总结

了解ICMP包的格式，了解LWIP发送ping和对堆ping请求响应的过程。

审核编辑 黄宇