前段時(shí)間在解決celery和rabbitmq之間的TCP連接問題，今天剛好看到有關(guān)于TCP keeplive的東西，覺得非常棒，就記錄下來。
摘抄來自http://www.firefoxbug.com/index.php/archives/2805/

理解Keepalive(1)

大家都聽過keepalive，但是其實(shí)對(duì)于keepalive這個(gè)詞還是很晦澀的，至少我一直都只知道一個(gè)大概，直到之前排查線上一些問題，發(fā)現(xiàn)keepalive還是有很多玄機(jī)的。其實(shí)keepalive有兩種，一種是TCP層的keepalive，另一種是HTTP層的Keep-Alive。這篇文章先說說tcp層的keepalive

tcp keepalive

設(shè)想有一種場景：A和B兩邊通過三次握手建立好TCP連接，然后突然間B就宕機(jī)了，之后時(shí)間內(nèi)B再也沒有起來。如果B宕機(jī)后A和B一直沒有數(shù)據(jù)通信的需求，A就永遠(yuǎn)都發(fā)現(xiàn)不了B已經(jīng)掛了，那么A的內(nèi)核里還維護(hù)著一份關(guān)于A&B之間TCP連接的信息，浪費(fèi)系統(tǒng)資源。于是在TCP層面引入了keepalive的機(jī)制，A會(huì)定期給B發(fā)空的數(shù)據(jù)包，通俗講就是心跳包，一旦發(fā)現(xiàn)到B的網(wǎng)絡(luò)不通就關(guān)閉連接。這一點(diǎn)在LVS內(nèi)尤為明顯，因?yàn)長VS維護(hù)著兩邊大量的連接狀態(tài)信息，一旦超時(shí)就需要釋放連接。

Linux內(nèi)核對(duì)于tcp keepalive的調(diào)整主要有以下三個(gè)參數(shù)
1. tcp_keepalive_time

the interval between the last data packet sent (simple ACKs are not considered data) and the first keepalive probe; after the connection is marked to need keepalive, this counter is not used any further

tcp_keepalive_intvl

the interval between subsequential keepalive probes, regardless of what the connection has exchanged in the meantime

tcp_keepalive_probes

the number of unacknowledged probes to send before considering the connection dead and notifying the application layer

Example

cat/proc/sys/net/ipv4/tcpkeepalivetime7200 cat /proc/sys/net/ipv4/tcp_keepalive_intvl
75
$ cat /proc/sys/net/ipv4/tcp_keepalive_probes
9

當(dāng)tcp發(fā)現(xiàn)有tcp_keepalive_time(7200)秒未收到對(duì)端數(shù)據(jù)后，開始以間隔tcp_keepalive_intvl(75)秒的頻率發(fā)送的空心跳包，如果連續(xù)tcp_keepalive_probes(9)次以上未響應(yīng)代碼對(duì)端已經(jīng)down了，close連接

在socket編程時(shí)候，可以調(diào)用setsockopt指定不同的宏來更改上面幾個(gè)參數(shù)

TCP_KEEPCNT: tcp_keepalive_probes

TCP_KEEPIDLE: tcp_keepalive_time

TCP_KEEPINTVL: tcp_keepalive_intvl
Nginx配置tcp keepalive

Nginx對(duì)于keepalive的配置有一大堆，大伙每次看都迷茫了，其實(shí)Nginx涉及到tcp層面的keepalive只有一個(gè)：so_keepalive。它屬于listen指令的配置參數(shù)，具體配置

so_keepalive=on|off|[keepidle]:[keepintvl]:[keepcnt]
this parameter (1.1.11) configures the “TCP keepalive” behavior for the listening socket. If this parameter is omitted then the operating system’s settings will be in effect for the socket. If it is set to the value “on”, the SO_KEEPALIVE option is turned on for the socket. If it is set to the value “off”, the SO_KEEPALIVE option is turned off for the socket. Some operating systems support setting of TCP keepalive parameters on a per-socket basis using the TCP_KEEPIDLE, TCP_KEEPINTVL, and TCP_KEEPCNT socket options. On such systems (currently, Linux 2.4+, NetBSD 5+, and FreeBSD 9.0-STABLE), they can be configured using the keepidle, keepintvl, and keepcnt parameters. One or two parameters may be omitted, in which case the system default setting for the corresponding socket option will be in effect.

Example
so_keepalive=30m::10
will set the idle timeout (TCP_KEEPIDLE) to 30 minutes, leave the probe interval (TCP_KEEPINTVL) at its system default, and set the probes count (TCP_KEEPCNT) to 10 probes.
在Nginx的代碼里可以看到

./src/http/ngx_http_core_module.c

static ngx_command_t ngx_http_core_commands[] = {
…
// listen 指令解析 –>> call ngx_http_core_listen()
{ ngx_string(“l(fā)isten”),
NGX_HTTP_SRV_CONF|NGX_CONF_1MORE,
ngx_http_core_listen,
NGX_HTTP_SRV_CONF_OFFSET,
0,
NULL },
…
}

static char *
ngx_http_core_listen(ngx_conf_t *cf, ngx_command_t *cmd, void *conf){

…
// 下面就是 so_keepalive 后面的參數(shù)解析
if (ngx_strncmp(value[n].data, “so_keepalive=”, 13) == 0) {

if (ngx_strcmp(&value[n].data[13], "on") == 0) {lsopt.so_keepalive = 1;} else if (ngx_strcmp(&value[n].data[13], "off") == 0) {lsopt.so_keepalive = 2;} else {// 自定義系統(tǒng)keepalive的相關(guān)設(shè)置...}if (ngx_http_add_listen(cf, cscf, &lsopt) == NGX_OK) {return NGX_CONF_OK; }

}

./src/core/ngx_connection.c

if (ls[i].keepidle) {value = ls[i].keepidle;// 設(shè)置 tcp_keepalive_time if (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPIDLE,(const void *) &value, sizeof(int))== -1){ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,"setsockopt(TCP_KEEPIDLE, %d) %V failed, ignored",value, &ls[i].addr_text);}}if (ls[i].keepintvl) {value = ls[i].keepintvl;// 設(shè)置 tcp_keepalive_intvlif (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPINTVL,(const void *) &value, sizeof(int))== -1){ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,"setsockopt(TCP_KEEPINTVL, %d) %V failed, ignored",value, &ls[i].addr_text);}}if (ls[i].keepcnt) {// 設(shè)置 tcp_keepalive_intvlif (setsockopt(ls[i].fd, IPPROTO_TCP, TCP_KEEPCNT,(const void *) &ls[i].keepcnt, sizeof(int))== -1){ngx_log_error(NGX_LOG_ALERT, cycle->log, ngx_socket_errno,"setsockopt(TCP_KEEPCNT, %d) %V failed, ignored",ls[i].keepcnt, &ls[i].addr_text);}}

總結(jié)

這篇文章說了TCP層面的keepalive相關(guān)知識(shí)以及Nginx的支持tcp keepalive的配置。tcp層面的keepalive存在更多意義上是為了檢測兩端連接是否正常，注重點(diǎn)是在于連接的本身！要和HTTP層面的keepaplive區(qū)分開來，明白這點(diǎn)很重要。

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