> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man4/icmp.4.md).

# icmp.4

`icmp` — Internet 控制报文协议

## 名称

`icmp`

## 概要

`#include <sys/types.h>`

`#include <sys/socket.h>`

`#include <netinet/in.h>`

`Ft int Fn socket AF_INET SOCK_RAW proto`

## 描述

ICMP 是 IP 及互联网协议族使用的差错和控制报文协议。可通过“raw socket”访问它，用于网络监控和诊断功能。创建 ICMP 套接字时，socket 调用的 `proto` 参数可从 getprotobyname(3) 获取。ICMP 套接字是无连接的，通常与 sendto(2) 和 recvfrom(2) 调用一起使用，不过也可以使用 connect(2) 调用固定后续数据包的目的地（此时可使用 read(2) 或 recv(2) 以及 write(2) 或 send(2) 系统调用）。

发出的数据包会自动在前面加上 IP 头（基于目的地址）。接收到的数据包保留 IP 头和选项不变。

### 类型

ICMP 报文根据 ICMP 头中的 type 和 code 字段进行分类。类型和代码的缩写可用于 [pf.conf(5)](/man/man5/pf.conf.5.md) 中的规则。已定义以下类型：

| **Num** | **Abbrev.** | **Description**             |
| ------- | ----------- | --------------------------- |
| 0       | echorep     | Echo reply                  |
| 3       | unreach     | Destination unreachable     |
| 4       | squench     | Packet loss, slow down      |
| 5       | redir       | Shorter route exists        |
| 6       | althost     | Alternate host address      |
| 8       | echoreq     | Echo request                |
| 9       | routeradv   | Router advertisement        |
| 10      | routersol   | Router solicitation         |
| 11      | timex       | Time exceeded               |
| 12      | paramprob   | Invalid IP header           |
| 13      | timereq     | Timestamp request           |
| 14      | timerep     | Timestamp reply             |
| 15      | inforeq     | Information request         |
| 16      | inforep     | Information reply           |
| 17      | maskreq     | Address mask request        |
| 18      | maskrep     | Address mask reply          |
| 30      | trace       | Traceroute                  |
| 31      | dataconv    | Data conversion problem     |
| 32      | mobredir    | Mobile host redirection     |
| 33      | ipv6-where  | IPv6 where-are-you          |
| 34      | ipv6-here   | IPv6 i-am-here              |
| 35      | mobregreq   | Mobile registration request |
| 36      | mobregrep   | Mobile registration reply   |
| 39      | skip        | SKIP                        |
| 40      | photuris    | Photuris                    |

已定义以下代码：

| **Num** | **Abbrev.**    | **Type**  | **Description**                     |
| ------- | -------------- | --------- | ----------------------------------- |
| 0       | net-unr        | unreach   | Network unreachable                 |
| 1       | host-unr       | unreach   | Host unreachable                    |
| 2       | proto-unr      | unreach   | Protocol unreachable                |
| 3       | port-unr       | unreach   | Port unreachable                    |
| 4       | needfrag       | unreach   | Fragmentation needed but DF bit set |
| 5       | srcfail        | unreach   | Source routing failed               |
| 6       | net-unk        | unreach   | Network unknown                     |
| 7       | host-unk       | unreach   | Host unknown                        |
| 8       | isolate        | unreach   | Host isolated                       |
| 9       | net-prohib     | unreach   | Network administratively prohibited |
| 10      | host-prohib    | unreach   | Host administratively prohibited    |
| 11      | net-tos        | unreach   | Invalid TOS for network             |
| 12      | host-tos       | unreach   | Invalid TOS for host                |
| 13      | filter-prohib  | unreach   | Prohibited access                   |
| 14      | host-preced    | unreach   | Precedence violation                |
| 15      | cutoff-preced  | unreach   | Precedence cutoff                   |
| 0       | redir-net      | redir     | Shorter route for network           |
| 1       | redir-host     | redir     | Shorter route for host              |
| 2       | redir-tos-net  | redir     | Shorter route for TOS and network   |
| 3       | redir-tos-host | redir     | Shorter route for TOS and host      |
| 0       | normal-adv     | routeradv | Normal advertisement                |
| 16      | common-adv     | routeradv | Selective advertisement             |
| 0       | transit        | timex     | Time exceeded in transit            |
| 1       | reassemb       | timex     | Time exceeded in reassembly         |
| 0       | badhead        | paramprob | Invalid option pointer              |
| 1       | optmiss        | paramprob | Missing option                      |
| 2       | badlen         | paramprob | Invalid length                      |
| 1       | unknown-ind    | photuris  | Unknown security index              |
| 2       | auth-fail      | photuris  | Authentication failed               |
| 3       | decrypt-fail   | photuris  | Decryption failed                   |

### MIB (sysctl) 变量

ICMP 协议在 sysctl(3) MIB 的 `net.inet.icmp` 分支中实现了若干变量，也可通过 [sysctl(8)](/man/man8/sysctl.8.md) 读取或修改。

**`bmcastecho`** (`boolean`) 启用/禁用对通过广播或多播接收到的 ICMP 报文的回复。默认为 false。

**`drop_redirect`** (`boolean`) 启用/禁用丢弃 ICMP Redirect 数据包。默认为 false。

**`icmplim`** (`unsigned integer`) 报文的平均速率限制，单位为包/秒。实际限制为 `icmplim` 加上由 `icmplim_jitter` 限定的随机抖动。设为零则不进行限制。默认为 200。

**`icmplim_jitter`** (`unsigned integer`) 在 `icmplim_jitter` 的负值与正值之间取一个随机抖动，应用于 `icmplim`，用于限制报文发送速率。`icmplim` 不为零时，`icmplim_jitter` 必须小于 `icmplim`。设为零则不应用抖动。默认为 16。

**`icmplim_output`** (`boolean`) 启用/禁用 ICMP 报文带宽限制的日志记录。默认为 true。

**`log_redirect`** (`boolean`) 启用/禁用 ICMP Redirect 数据包的日志记录。默认为 false。

**`maskfake`** (`unsigned integer`) 当 `maskrepl` 已设置且此值非零时，系统回复 ICMP Address Mask Request 数据包时将使用此值替代真实地址掩码。默认为 0。

**`maskrepl`** (`boolean`) 启用/禁用对 ICMP Address Mask Request 数据包的回复。默认为 false。

**`quotelen`** (`integer`) 在 ICMP 回复中引用原始数据包的字节数。此数字在内部强制为至少 8 字节（依据 RFC792），至多为 ICMP 回复 mbuf 中剩余的最大空间。

**`redirtimeout`** (`integer`) 由 ICMP redirect 创建的路由过期前的延迟（秒）。

**`reply_from_interface`** (`boolean`) 对并非直接发往本机的数据包，使用数据包进入接口的 IP 地址作为响应源。若启用，此规则先于其他所有规则处理。默认情况下继续进行正常的源地址选择。在路由器上启用此选项特别有用，因为这样可使外部 traceroute 显示数据包经过的实际路径，而非可能不同的返回路径。

**`reply_src`** (`str`) 用于响应并非直接发往本机的数据包的 ICMP 回复源的接口名。默认情况下继续进行正常的源地址选择。

**`tstamprepl`** (`boolean`) 启用/禁用对 ICMP Timestamp 数据包的回复。默认为 true。

## 错误

套接字操作可能失败并返回以下错误之一：

**\[EISCONN]** 当尝试在已建立连接的套接字上再次建立连接，或尝试在套接字已连接时指定目的地址发送数据报时；

**\[ENOTCONN]** 当尝试发送数据报，但未指定目的地址且套接字未连接时；

**\[ENOBUFS]** 当系统内存不足以分配内部数据结构时；

**\[EADDRNOTAVAIL]** 当尝试创建套接字时，其网络地址没有对应的网络接口。

## 参见

recv(2), send(2), sysctl(3), [dtrace\_mib(4)](/man/man4/dtrace_mib.4.md), [inet(4)](/man/man4/inet.4.md), [intro(4)](/man/man4/intro.4.md), [ip(4)](/man/man4/ip.4.md), [pf.conf(5)](/man/man5/pf.conf.5.md), [sysctl(8)](/man/man8/sysctl.8.md)

## 历史

`icmp` 协议实现出现于 4.2BSD。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://book.bsdcn.org/man/man4/icmp.4.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.