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还在用netstat吗？你OUT了
ss是iproute2工具包中的工具。
ss is used to dump socket statistics. It allows showing information similar to netstat. It can display more TCP and state informations than other tools.
1. 为什么用ss替换netstat?
这要从netstat的先天缺陷说起，netstat通过遍历proc来获取socket信息，当socket数据上万后，netstat的输出就非常耗时（用过的都知道
)
而与之不同的ss使用netlink与内核tcp_diag模块通信获取socket信息。
2.ss命令输出解释之Recv-Q Send-Q
先来看一下ss的输出：
tingw:~ # ss -at
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 :::sunrpc :::*
LISTEN 0 128 *:sunrpc *:*
LISTEN 0 128 :::http :::*
LISTEN 0 128 127.0.0.1:ipp *:*
LISTEN 0 128 ::1:ipp :::*
LISTEN 0 100 ::1:smtp :::*
LISTEN 0 100 127.0.0.1:smtp *:*
LISTEN 0 64 :::59642 :::*
LISTEN 0 128 *:54107 *:*
LISTEN 0 128 :::xinupageserver :::*
LISTEN 0 128 *:xinupageserver *:*
LISTEN 0 64 *:50341 *:*
LISTEN 0 50 *:mysql *:*
ESTAB 0 52 192.168.0.1:xinupageserver 192.168.0.96:50599
输出所有的tcp socket信息。
第一列表示tcp socket的状态，第二列和第三列的内容与socket所处的状态有关，查看tcp_diag代码
-
static void tcp_diag_get_info(struct sock *sk, struct inet_diag_msg *r,
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void *_info)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct tcp_info *info = _info;
-
-
if (sk->sk_state == TCP_LISTEN) {
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r->idiag_rqueue = sk->sk_ack_backlog;
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r->idiag_wqueue = sk->sk_max_ack_backlog;
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} else {
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r->idiag_rqueue = tp->rcv_nxt - tp->copied_seq;
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r->idiag_wqueue = tp->write_seq - tp->snd_una;
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}
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if (info != NULL)
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tcp_get_info(sk, info);
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}
处于LISTEN状态的socket，Recv-Q表示了current listen backlog队列元素数目(等待用户调用accept的完成3次握手的socket），而Send-Q表示了listen socket最大能容纳的backlog。这个数目由listen时指定，且不能大于 /proc/sys/net/ipv4/tcp_max_syn_backlog;对于非LISTEN socket，Recv-Q表示了receive queue中的字节数目（等待接收的下一个tcp段的序号-尚未从内核空间copy到用户空间的段最前面的一个序号）；Send-Q表示发送queue中容纳的字节数（已加入发送队列中最后一个序号-输出段中最早一个未确认的序号）
3.ss命令输出解释之timer
linux-19:~ # ss -a -t -o -4
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 *:sunrpc *:*
LISTEN 0 128 *:47093 *:*
LISTEN 0 3 192.168.86.1:domain *:*
LISTEN 0 3 192.168.0.86:domain *:*
LISTEN 0 3 10.0.64.19:domain *:*
LISTEN 0 3 192.168.100.3:domain *:*
LISTEN 0 3 172.16.132.189:domain *:*
LISTEN 0 3 127.0.0.2:domain *:*
LISTEN 0 3 127.0.0.1:domain *:*
LISTEN 0 128 *:ssh *:*
LISTEN 0 128 127.0.0.1:ipp *:*
LISTEN 0 100 *:smtp *:*
LISTEN 0 128 127.0.0.1:953 *:*
LISTEN 0 64 *:33914 *:*
LISTEN 0 64 *:nfs *:*
LISTEN 0 128 *:35659 *:*
LISTEN 0 128 *:remotefs *:*
LISTEN 0 128 10.0.64.19:6380 *:*
LISTEN 0 128 *:openvms-sysipc *:*
ESTAB 0 0 172.16.132.189:ssh 172.16.132.93:hs-port timer:(keepalive,65min,0)
ESTAB 0 0 10.0.64.19:35225 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:46617 10.0.64.107:61616
ESTAB 0 0 10.0.64.19:openvms-sysipc 10.0.64.2:videotex
ESTAB 0 0 10.0.64.19:49462 10.0.64.107:61616
ESTAB 0 0 172.16.132.189:ssh 172.16.132.85:63934 timer:(keepalive,38min,0)
ESTAB 0 0 10.0.64.19:60569 10.0.64.107:61616
ESTAB 0 0 10.0.64.19:52745 10.0.64.129:61616
ESTAB 0 52 172.16.132.189:ssh 172.16.132.92:50598 timer:(on,476ms,0)
ESTAB 0 0 10.0.64.19:56401 10.0.64.107:61616
ESTAB 0 0 10.0.64.19:54805 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:60772 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:55510 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:45663 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:39262 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:57775 10.0.64.129:61616
ESTAB 0 0 10.0.64.19:52205 10.0.64.18:6379
ESTAB 0 0 10.0.64.19:remotefs 10.0.64.12:sentinel
这个输出更上次输出相比，多了一个timer输出。这个输出描述的是tcp socket上的定时器，在说明这个之前先了解一下linux对一个tcp socket可能设置的定时器。
tcp socket总共有7个定时器，通过4个timer实现。分别是
通过icsk_retransmit_timer实现的重传定时器、零窗口探测定时器；通过sk_timer实现的连接建立定时器、保活定时器和FIN_WAIT_2定时器；通过icsk_delack_timer实现的延时ack定时器；以及TIME_WAIT定时器。
我们看一下ss的代码
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static const char *tmr_name[] = {
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"off",
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"on",
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"keepalive",
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"timewait",
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"persist",
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"unknown"
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};
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if (show_options) {
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if (r->idiag_timer) {
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if (r->idiag_timer > 4)
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r->idiag_timer = 5;
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printf(" timer:(%s,%s,%d)",
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tmr_name[r->idiag_timer],
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print_ms_timer(r->idiag_expires),
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r->idiag_retrans);
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}
-
}
对应的内核代码是
-
if (icsk->icsk_pending == ICSK_TIME_RETRANS) {
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r->idiag_timer = 1;
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r->idiag_retrans = icsk->icsk_retransmits;
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r->idiag_expires = EXPIRES_IN_MS(icsk->icsk_timeout);
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} else if (icsk->icsk_pending == ICSK_TIME_PROBE0) {
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r->idiag_timer = 4;
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r->idiag_retrans = icsk->icsk_probes_out;
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r->idiag_expires = EXPIRES_IN_MS(icsk->icsk_timeout);
-
} else if (timer_pending(&sk->sk_timer)) {
-
r->idiag_timer = 2;
-
r->idiag_retrans = icsk->icsk_probes_out;
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r->idiag_expires = EXPIRES_IN_MS(sk->sk_timer.expires);
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} else {
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r->idiag_timer = 0;
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r->idiag_expires = 0;
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}
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static int inet_twsk_diag_fill(struct inet_timewait_sock *tw,
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struct sk_buff *skb, int ext, u32 pid,
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u32 seq, u16 nlmsg_flags,
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const struct nlmsghdr *unlh)
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{
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long tmo;
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struct inet_diag_msg *r;
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const unsigned char *previous_tail = skb_tail_pointer(skb);
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struct nlmsghdr *nlh = NLMSG_PUT(skb, pid, seq,
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unlh->nlmsg_type, sizeof(*r));
-
-
r = NLMSG_DATA(nlh);
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BUG_ON(tw->tw_state != TCP_TIME_WAIT);
-
-
nlh->nlmsg_flags = nlmsg_flags;
-
-
tmo = tw->tw_ttd - jiffies;
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if (tmo < 0)
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tmo = 0;
-
-
r->idiag_family = tw->tw_family;
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r->idiag_retrans = 0;
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r->id.idiag_if = tw->tw_bound_dev_if;
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r->id.idiag_cookie[0] = (u32)(unsigned long)tw;
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r->id.idiag_cookie[1] = (u32)(((unsigned long)tw >> 31) >> 1);
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r->id.idiag_sport = tw->tw_sport;
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r->id.idiag_dport = tw->tw_dport;
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r->id.idiag_src[0] = tw->tw_rcv_saddr;
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r->id.idiag_dst[0] = tw->tw_daddr;
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r->idiag_state = tw->tw_substate;
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r->idiag_timer = 3;
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r->idiag_expires = DIV_ROUND_UP(tmo * 1000, HZ);
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r->idiag_rqueue = 0;
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r->idiag_wqueue = 0;
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r->idiag_uid = 0;
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r->idiag_inode = 0;
这样timer的输出含义就是（类型，过期时间，重试次数），这里说一下类型的含义：
off: 当前socket没有timer
on: 重传timer
keepalive：连接建立timer or fin_wait_2 timer or 保活timer；具体是那个timer，可以根据连接的状态来确定。
timewait: TIME_WAITtimer
persist：零窗口探测timer
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