Linux での TCP KeepAlive 有効化方法

import socket
import time

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect(('8.8.8.8', 53))
    time.sleep(5)

$ sudo tcpdump host 8.8.8.8 port 53 -ni eth0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
16:12:47.319493 IP 10.3.0.5.50528 > 8.8.8.8.53: Flags [S], seq 464911782, win 64240, options [mss 1460,sackOK,TS val 3113655258 ecr 0,nop,wscale 7], length 0
16:12:47.321744 IP 8.8.8.8.53 > 10.3.0.5.50528: Flags [S.], seq 2773628164, ack 464911783, win 65535,
options [mss 1430,sackOK,TS val 1138427843 ecr 3113655258,nop,wscale 8], length 0
16:12:47.321781 IP 10.3.0.5.50528 > 8.8.8.8.53: Flags [.], ack 1, win 502, options [nop,nop,TS val 3113655261 ecr 1138427843], length 0
16:12:47.321963 IP 10.3.0.5.50528 > 8.8.8.8.53: Flags [F.], seq 1, ack 1, win 502, options [nop,nop,TS val 3113655261 ecr 1138427843], length 0
16:12:47.323556 IP 8.8.8.8.53 > 10.3.0.5.50528: Flags [F.], seq 1, ack 2, win 256, options [nop,nop,TS val 1138427846 ecr 3113655261], length 0
16:12:47.323571 IP 10.3.0.5.50528 > 8.8.8.8.53: Flags [.], ack 2, win 502, options [nop,nop,TS val 3113655262 ecr 1138427846], length 0

$ strace -ff -e trace=network python3 client.py 2>&1
socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("8.8.8.8")}, 16) = 0
+++ exited with 0 +++

$ python3 client.py

$ ss -tanoe 'dst 8.8.8.8'
State   Recv-Q   Send-Q   Local Address:Port    Peer Address:Port
ESTAB   0        0             10.3.0.5:52686        8.8.8.8:53  
users:(("python3",pid=29357,fd=3)) uid:1000 ino:305411 sk:196 <->

$ curl http://8.8.8.8:53 # --no-keepalive を指定していない (TCP KeepAlive 有効化)

$ ss -tanoe 'dst 8.8.8.8'
State   Recv-Q   Send-Q   Local Address:Port    Peer Address:Port
ESTAB   0        0             10.3.0.5:53326        8.8.8.8:53   \
users:(("curl",pid=30213,fd=3)) timer:(keepalive,1min,0) uid:1000 ino:311003 sk:19a <->

timer:(<timer_name>,<expire_time>,<retrans>)

<timer_name>
        the name of the timer, there are five kind of timer names:
        on : means one of these timers: TCP retrans timer, TCP early retrans timer and tail loss probe timer
        keepalive: tcp keep alive timer
        timewait: timewait stage timer
        persist: zero window probe timer
        unknown: none of the above timers

<expire_time>
        how long time the timer will expire

<retrans>
        how many times the retransmission occurred

$ strace -e trace=network -o on.log curl http://8.8.8.8:53
$ strace -e trace=network -o off.log curl http://8.8.8.8:53 --no-keepalive
$ diff on.log off.log

< setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
< setsockopt(3, SOL_TCP, TCP_KEEPIDLE, [60], 4) = 0
< setsockopt(3, SOL_TCP, TCP_KEEPINTVL, [60], 4) = 0
< getsockname(3, {sa_family=AF_INET, sin_port=htons(57040), sin_addr=inet_addr("10.3.0.5")}, [128->16]) = 0
> getsockname(3, {sa_family=AF_INET, sin_port=htons(57012), sin_addr=inet_addr("10.3.0.5")}, [128->16]) = 0

tcp_keepalive_intvl (integer; default: 75; since Linux 2.4)
    The number of seconds between TCP keep-alive probes.

    (訳) TCP KeepAlive プローブ間の秒数。

tcp_keepalive_probes (integer; default: 9; since Linux 2.2)
    The maximum number of TCP keep-alive probes to send before giving up
    and killing the connection if no response is obtained from the other end.

    (訳) TCP KeepAlive プローブの最大数。他方からの応答が得られない場合、
    接続を切断する前に送信する TCP KeepAlive プローブの最大数。

tcp_keepalive_time (integer; default: 7200; since Linux 2.2)
    The number of seconds a connection needs to be idle before TCP begins
    sending out keep-alive probes. Keep-alives are only sent when the SO_KEEPALIVE
    socket option is enabled.The default value is 7200 seconds (2 hours). An idle
    connection is terminated after approximately an additional 11 minutes
    (9 probes an interval of 75 seconds apart) when keep-alive is enabled.

    (訳) TCP KeepAlive プローブを送信し始めるまでの接続がアイドル状態になる秒数。
    SO_KEEPALIVE ソケットオプションが有効化されている場合のみ、TCP KeepAlive プローブが
    送信されます。デフォルト値は 7200 秒 (2 時間) です。アイドル接続は、KeepAlive が
    有効化されている場合、約 11 分後に追加の 11 分間 (9 回のプローブ、75 秒間隔) で切断されます。
    なお、TCP より下のレイヤーやアプリケーションのタイムアウトは、はるかに短い場合があります。

cat <<EOF >test.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main()
{
   int s;
   int optval;
   socklen_t optlen = sizeof(optval);

   if((s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
      perror("socket()");
      exit(EXIT_FAILURE);
   }

   if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
      perror("getsockopt()");
      close(s);
      exit(EXIT_FAILURE);
   }
   printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));

   optval = 1;
   optlen = sizeof(optval);
   if(setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, optlen) < 0) {
      perror("setsockopt()");
      close(s);
      exit(EXIT_FAILURE);
   }
   printf("SO_KEEPALIVE set on socket\n");

   if(getsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, &optlen) < 0) {
      perror("getsockopt()");
      close(s);
      exit(EXIT_FAILURE);
   }
   printf("SO_KEEPALIVE is %s\n", (optval ? "ON" : "OFF"));

   close(s);
   exit(EXIT_SUCCESS);
}
EOF

$ gcc -o test test.c

$ ./test
SO_KEEPALIVE is OFF
SO_KEEPALIVE set on socket
SO_KEEPALIVE is ON

$ strace -e trace=network ./test
socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
getsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [0], [4]) = 0
SO_KEEPALIVE is OFF
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
SO_KEEPALIVE set on socket
getsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], [4]) = 0
SO_KEEPALIVE is ON

$ wget  https://excellmedia.dl.sourceforge.net/project/libkeepalive/libkeepalive/0.3/libkeepalive-0.3.tar.gz
$ tar -xzvf libkeepalive-0.3.tar.gz
$ cd libkeepalive-0.3
$ make
$ sudo cp libkeepalive.so /usr/lib/

$ LD_PRELOAD=/usr/lib/libkeepalive.so ./test
SO_KEEPALIVE is ON
SO_KEEPALIVE set on socket
SO_KEEPALIVE is ON

$ LD_PRELOAD=/usr/lib/libkeepalive.so strace -e trace=network ./test
socket(AF_INET, SOCK_STREAM, IPPROTO_TCP) = 3
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
getsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], [4]) = 0
SO_KEEPALIVE is ON
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
SO_KEEPALIVE set on socket
getsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], [4]) = 0
SO_KEEPALIVE is ON
+++ exited with 0 +++