Enable TCP KeepAlive by default

[zonyitoo]

ref #490

TCP Keep Alive is useful for purging dead connections. Since many users run shadowsocks with mobile connection, there should be higher chance to leave dead connections on servers.

Further more, Go already have set 15 seconds keep alive timeout on all TCP connections. (Go's source code can prove that: net/tcpsock_posix.go:accept. And here is the issue golang/go#31510 that introduced this feature into the standard net library.)

SO_KEEPALIVE is not enabled by default for most operating systems.

Introducing this feature shouldn't break anything. If you have encountered anything related to this, please comment.

Adding a new configuration field for customizing the keep alive timeout

{
    "keep_alive": 15
}

[om2mo]

15秒可能消耗过多的电量在移动平台设备，如何在命令行中配置它？

[zonyitoo]

Try with this commit and specify keep alive timeouts with command line options like --tcp-keep-alive 300.

[om2mo]

➜ ~ sslocal -U --protocol redir --tcp-fast-open --tcp-no-delay --tcp-keep-alive 300 -s server -m aes-256-gcm -k -b 0.0.0.0:12345
error: Found argument '--tcp-keep-alive' which wasn't expected, or isn't valid in this context
Did you mean --tcp-redir?

[zonyitoo]

You should either compile from the latest master branch, or download the nightly build of v1.11.2.

[zonyitoo]

@madeye @Mygod Consider adding --tcp-keep-alive or keep_alive configuration if shadowsocks-android upgrades the underlying shadowsocks-rust, if this related to battery consumption.

But v2ray-plugin will still have the default 15 seconds set by Go's net library.

[Mygod]

According to this, Android will not respect TCP keep alive settings when device goes to sleep.

[zonyitoo]

So, @om2mo your assumption doesn't exist. What's your mobile platform?

[zonyitoo]

But on the other hand, what if system is not going to sleep? Would this increase battery consumption while system is awake?

[Mygod]

Setting it to 28 minutes as default should alleviate he problem.

[zonyitoo]

Why 28 minutes?

[Mygod]

It is the timeout for GMS heartbeat on mobile network.