ICMP是（Internet Control Message Protocol）Internet控制报文协议。它是TCP/IP协议族的一个子协议，用于在IP主机、路由器之间传递控制消息。控制消息是指网络通不通、主机是否可达、路由是否可用等网络本身的消息。这些控制消息虽然并不传输用户数据，但是对于用户数据的传递起着重要的作用。
在Linux kernel中密切相关的函数是下面几个：
void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info);
int icmp_rcv(struct sk_buff *skb);
void icmp_err(struct sk_buff *skb, u32 info);
int icmp_init(void);
void icmp_out_count(struct net *net, unsigned char type);相关结构：
static const struct net_protocol icmp_protocol = {
.handler = icmp_rcv,
.err_handler = icmp_err,
.no_policy = 1,
.netns_ok = 1,
};
if (inet_add_protocol(&icmp_protocol, IPPROTO_ICMP) < 0)
pr_crit("%s: Cannot add ICMP protocol\n", __func__);
/*
* Send an ICMP message in response to a situation
*
* RFC 1122: 3.2.2 MUST send at least the IP header and 8 bytes of header.
* MAY send more (we do).
* MUST NOT change this header information.
* MUST NOT reply to a multicast/broadcast IP address.
* MUST NOT reply to a multicast/broadcast MAC address.
* MUST reply to only the first fragment.
*/void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info)
{
struct iphdr *iph;
int room;
struct icmp_bxm *icmp_param;
struct rtable *rt = skb_rtable(skb_in);
struct ipcm_cookie ipc;
struct flowi4 fl4;
__be32 saddr;
u8 tos;
u32 mark;
struct net *net;
struct sock *sk; if (!rt)
goto out;
net = dev_net(rt->dst.dev); /*
* Find the original header. It is expected to be valid, of course.
* Check this, icmp_send is called from the most obscure devices
* sometimes.
*/
iph = ip_hdr(skb_in); if ((u8 *)iph < skb_in->head ||
(skb_network_header(skb_in) + sizeof(*iph)) >
skb_tail_pointer(skb_in))
goto out; /*
* No replies to physical multicast/broadcast
*/
if (skb_in->pkt_type != PACKET_HOST)
goto out; /*
* Now check at the protocol level
*/
if (rt->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
goto out; /*
* Only reply to fragment 0. We byte re-order the constant
* mask for efficiency.
*/
if (iph->frag_off & htons(IP_OFFSET))
goto out; /*
* If we send an ICMP error to an ICMP error a mess would result..
*/
if (icmp_pointers[type].error) {
/*
* We are an error, check if we are replying to an
* ICMP error
*/
if (iph->protocol == IPPROTO_ICMP) {
u8 _inner_type, *itp; itp = skb_header_pointer(skb_in,
skb_network_header(skb_in) +
(iph->ihl << 2) +
offsetof(struct icmphdr,
type) -
skb_in->data,
sizeof(_inner_type),
&_inner_type);
if (!itp)
goto out; /*
* Assume any unknown ICMP type is an error. This
* isn't specified by the RFC, but think about it..
*/
if (*itp > NR_ICMP_TYPES ||
icmp_pointers[*itp].error)
goto out;
}
} icmp_param = kmalloc(sizeof(*icmp_param), GFP_ATOMIC);
if (!icmp_param)
return; sk = icmp_xmit_lock(net);
if (!sk)
goto out_free; /*
* Construct source address and options.
*/ saddr = iph->daddr;
if (!(rt->rt_flags & RTCF_LOCAL)) {
struct net_device *dev = NULL; rcu_read_lock();
if (rt_is_input_route(rt) &&
net->ipv4.sysctl_icmp_errors_use_inbound_ifaddr)
dev = dev_get_by_index_rcu(net, inet_iif(skb_in)); if (dev)
saddr = inet_select_addr(dev, 0, RT_SCOPE_LINK);
else
saddr = 0;
rcu_read_unlock();
} tos = icmp_pointers[type].error ? ((iph->tos & IPTOS_TOS_MASK) |
IPTOS_PREC_INTERNETCONTROL) :
iph->tos;
mark = IP4_REPLY_MARK(net, skb_in->mark); if (ip_options_echo(&icmp_param->replyopts.opt.opt, skb_in))
goto out_unlock; /*
* Prepare data for ICMP header.
*/ icmp_param->data.icmph.type = type;
icmp_param->data.icmph.code = code;
icmp_param->data.icmph.un.gateway = info;
icmp_param->data.icmph.checksum = 0;
icmp_param->skb = skb_in;
icmp_param->offset = skb_network_offset(skb_in);
inet_sk(sk)->tos = tos;
sk->sk_mark = mark;
ipc.addr = iph->saddr;
ipc.opt = &icmp_param->replyopts.opt;
ipc.tx_flags = 0;
ipc.ttl = 0;
ipc.tos = -1; rt = icmp_route_lookup(net, &fl4, skb_in, iph, saddr, tos, mark,
type, code, icmp_param);
if (IS_ERR(rt))
goto out_unlock; if (!icmpv4_xrlim_allow(net, rt, &fl4, type, code))
goto ende; /* RFC says return as much as we can without exceeding 576 bytes. */
room = dst_mtu(&rt->dst);
if (room > 576)
room = 576;
room -= sizeof(struct iphdr) + icmp_param->replyopts.opt.opt.optlen;
room -= sizeof(struct icmphdr); icmp_param->data_len = skb_in->len - icmp_param->offset;
if (icmp_param->data_len > room)
icmp_param->data_len = room;
icmp_param->head_len = sizeof(struct icmphdr); icmp_push_reply(icmp_param, &fl4, &ipc, &rt);
ende:
ip_rt_put(rt);
out_unlock:
icmp_xmit_unlock(sk);
out_free:
kfree(icmp_param);
out:;
} /*
* Deal with incoming ICMP packets.
*/
int icmp_rcv(struct sk_buff *skb)
{
struct icmphdr *icmph;
struct rtable *rt = skb_rtable(skb);
struct net *net = dev_net(rt->dst.dev);
bool success; if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb)) {
struct sec_path *sp = skb_sec_path(skb);
int nh; if (!(sp && sp->xvec[sp->len - 1]->props.flags &
XFRM_STATE_ICMP))
goto drop; if (!pskb_may_pull(skb, sizeof(*icmph) + sizeof(struct iphdr)))
goto drop; nh = skb_network_offset(skb);
skb_set_network_header(skb, sizeof(*icmph)); if (!xfrm4_policy_check_reverse(NULL, XFRM_POLICY_IN, skb))
goto drop; skb_set_network_header(skb, nh);
} __ICMP_INC_STATS(net, ICMP_MIB_INMSGS);
if (skb_checksum_simple_validate(skb))
goto csum_error; if (!pskb_pull(skb, sizeof(*icmph)))
goto error; icmph = icmp_hdr(skb);
ICMPMSGIN_INC_STATS(net, icmph->type);
/*
* 18 is the highest 'known' ICMP type. Anything else is a mystery
*
* RFC 1122: 3.2.2 Unknown ICMP messages types MUST be silently
* discarded.
*/
if (icmph->type > NR_ICMP_TYPES)
goto error; /*
* Parse the ICMP message
*/ if (rt->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST)) {
/*
* RFC 1122: 3.2.2.6 An ICMP_ECHO to broadcast MAY be
* silently ignored (we let user decide with a sysctl).
* RFC 1122: 3.2.2.8 An ICMP_TIMESTAMP MAY be silently
* discarded if to broadcast/multicast.
*/
if ((icmph->type == ICMP_ECHO ||
icmph->type == ICMP_TIMESTAMP) &&
net->ipv4.sysctl_icmp_echo_ignore_broadcasts) {
goto error;
}
if (icmph->type != ICMP_ECHO &&
icmph->type != ICMP_TIMESTAMP &&
icmph->type != ICMP_ADDRESS &&
icmph->type != ICMP_ADDRESSREPLY) {
goto error;
}
} success = icmp_pointers[icmph->type].handler(skb);
if (success) {
consume_skb(skb);
return 0;
}drop:
kfree_skb(skb);
return 0;
csum_error:
__ICMP_INC_STATS(net, ICMP_MIB_CSUMERRORS);
error:
__ICMP_INC_STATS(net, ICMP_MIB_INERRORS);
goto drop;
}#define ICMP_ECHOREPLY 0 /* Echo Reply */
#define ICMP_DEST_UNREACH 3 /* Destination Unreachable */
#define ICMP_SOURCE_QUENCH 4 /* Source Quench */
#define ICMP_REDIRECT 5 /* Redirect (change route) */
#define ICMP_ECHO 8 /* Echo Request */
#define ICMP_TIME_EXCEEDED 11 /* Time Exceeded */
#define ICMP_PARAMETERPROB 12 /* Parameter Problem */
#define ICMP_TIMESTAMP 13 /* Timestamp Request */
#define ICMP_TIMESTAMPREPLY 14 /* Timestamp Reply */
#define ICMP_INFO_REQUEST 15 /* Information Request */
#define ICMP_INFO_REPLY 16 /* Information Reply */
#define ICMP_ADDRESS 17 /* Address Mask Request */
#define ICMP_ADDRESSREPLY 18 /* Address Mask Reply */
#define NR_ICMP_TYPES 18
/*
* This table is the definition of how we handle ICMP.
*/
static const struct icmp_control icmp_pointers[NR_ICMP_TYPES + 1] = {
[ICMP_ECHOREPLY] = {
.handler = ping_rcv,
},
[1] = {
.handler = icmp_discard,
.error = 1,
},
[2] = {
.handler = icmp_discard,
.error = 1,
},
[ICMP_DEST_UNREACH] = {
.handler = icmp_unreach,
.error = 1,
},
[ICMP_SOURCE_QUENCH] = {
.handler = icmp_unreach,
.error = 1,
},
[ICMP_REDIRECT] = {
.handler = icmp_redirect,
.error = 1,
},
[6] = {
.handler = icmp_discard,
.error = 1,
},
[7] = {
.handler = icmp_discard,
.error = 1,
},
[ICMP_ECHO] = {
.handler = icmp_echo,
},
[9] = {
.handler = icmp_discard,
.error = 1,
},
[10] = {
.handler = icmp_discard,
.error = 1,
},
[ICMP_TIME_EXCEEDED] = {
.handler = icmp_unreach,
.error = 1,
},
[ICMP_PARAMETERPROB] = {
.handler = icmp_unreach,
.error = 1,
},
[ICMP_TIMESTAMP] = {
.handler = icmp_timestamp,
},
[ICMP_TIMESTAMPREPLY] = {
.handler = icmp_discard,
},
[ICMP_INFO_REQUEST] = {
.handler = icmp_discard,
},
[ICMP_INFO_REPLY] = {
.handler = icmp_discard,
},
[ICMP_ADDRESS] = {
.handler = icmp_discard,
},
[ICMP_ADDRESSREPLY] = {
.handler = icmp_discard,
},
};