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Does anyone know if Linux supports keep-alive socket options on an outgoing connection?

I made an outgoing connection with keep-alive options but netstat --timers shows off (I'm assuming timers are off):

tcp 0 0 localhost.localdomain:44307 172.16.0.15:2717 ESTABLISHED off (0.00/0/0)

Incoming connections with the same socket options applied show:

tcp 0 0 172.16.0.3:8585 localhost.localdomain:21527 ESTABLISHED keepalive (29.26/0/0)

I wish I could see socket options but neither ss or lsof will show me them.

James Hartig
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2 Answers2

7

First you need to make sure that TCP keepalive is enabled on your system. You can check the default settings like this:

# sysctl net.ipv4.tcp_keepalive_time net.ipv4.tcp_keepalive_probes net.ipv4.tcp_keepalive_intvl
net.ipv4.tcp_keepalive_time = 7200
net.ipv4.tcp_keepalive_probes = 9
net.ipv4.tcp_keepalive_intvl = 75

Then make sure you're setting it properly in your code. It should look something like this:

int optval = 1;
if (setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, &optval, sizeof(optval)) < 0) {
    perror("setsockopt()");
    close(s);
    exit(EXIT_FAILURE);
}

On my system when I use the above code to set SO_KEEPALIVE on both sides I see:

tcp        0      0 127.0.0.1:48591         127.0.0.1:5555          ESTABLISHED keepalive (6958.37/0/0)
tcp        0      0 127.0.0.1:5555          127.0.0.1:48591         ESTABLISHED keepalive (6958.37/0/0)

And then I verified with wireshark that the keepalive NOP was being sent.

More details can be found in the TCP Keepalive HOWTO.

aculich
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client-sysctl-setting:

sudo /sbin/sysctl -a|grep keep
net.ipv4.tcp_keepalive_time = 20
net.ipv4.tcp_keepalive_probes = 1
net.ipv4.tcp_keepalive_intvl = 1

client keep tcpdump：

sudo /usr/sbin/tcpdump  -nn -vv -i bond0 tcp and host 10.201.126.72 and port 8001
tcpdump: listening on bond0, link-type EN10MB (Ethernet), capture size 65535 bytes

but not any package capture ， which means tcp_keepalive_time doesn't work

Dennis Nolte
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user393527
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