Fix: add keep alive request duration to configs

[therealak12]

Trying to solve this.

[xiaokangwang]

If I understand this correctly, TCP_KEEPINTVL is a TCP specific option. You should not set it without checking the type of the socket.

You can either change the code, let me to fix this and merge, or justify your design.

[therealak12]

You're right. I'll try to find a more general approach for setting the keep-alive duration.
If you are going to fix it before me, I would appreciate (solving this problem is by far more important than my contribution).

[xiaokangwang]

I will make necessary modification and merge it.

[ValdikSS]

Is this modification only for Linux sockets, not for other OS?

[chromer030]

how we should set it in configs ?

[Loyalsoldier]

Is this modification only for Linux sockets, not for other OS?

Only for Linux.

[Loyalsoldier]

how we should set it in configs ?

https://www.v2fly.org/config/transport.html#sockoptobject

[ValdikSS]

Does not seem to be fixed in v2fly v4.41.1.

Test 1:

1. Run v2fly v4.41.1 with default config.json (remove geoip:private block first)
2. start nc -l -p 4141
3. execute curl --proxy socks5://127.0.0.1:1080 127.0.0.1:4141

Result: 15 second keep-alive from v2fly to socks client (1080 → client source port)

Adding "streamSettings": {"sockopt": {"tcpKeepAliveInterval": 900}}, to both inbounds and outbounds directives did not change anything.

[ValdikSS]

This change is for applyOutboundSocketOptions, however the main issue is with inbound (listening) sockets which are used for client connections and client-v2fly transmission from the server standpoint.

[ValdikSS]

This commit changes the wrong parameter. TCP_KEEPIDLE should be changed, not TCP_KEEPINTVL.

TCP_KEEPIDLE (since Linux 2.4)
The time (in seconds) the connection needs to remain idle before TCP starts sending keepalive probes, if the socket option SO_KEEPALIVE has been set on this socket. This option should not be used in code intended to be portable.

TCP_KEEPINTVL (since Linux 2.4)
The time (in seconds) between individual keepalive probes. This option should not be used in code intended to be portable.

Go sets both TCP_KEEPINTVL and TCP_KEEPIDLE to 15. Leaving TCP_KEEPINTVL to 15 looks fine to me (Linux default is 75). TCP_KEEPCNT is not changed and kept to standard 9, so 15*9 = 135 seconds before connection teardown, after TCP_KEEPIDLE had an effect.