Socat not closing tcp connection - sockets

Question

I use socat 1.7.3.1-r0 and run following command on an alpine 3.3 linux server:
socat -d -d -d PTY,link=/dev/ttyFOOBAR,echo=0,raw,unlink-close=0 TCP-LISTEN:7000,forever,reuseaddr
Socat will listen for clients and create a bidirectional communication by passing data from the virtual serial port /dev/ttyFOOBAR to the client and back again over TCP. Once the client disconnects socat should exit.
When such a connection is established socat logs the following:
I socat by Gerhard Rieger - see www.dest-unreach.org
I This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit. (http://www.openssl.org/)
I This product includes software written by Tim Hudson (tjh#cryptsoft.com)
I setting option "symbolic-link" to "/dev/ttyFOOBAR"
I setting option "echo" to 0
I setting option "raw"
I setting option "unlink-close" to 0
I openpty({5}, {6}, {"/dev/pts/3"},,) -> 0
N PTY is /dev/pts/3
I setting option "forever" to 1
I setting option "so-reuseaddr" to 1
I socket(2, 1, 6) -> 7
I starting accept loop
N listening on AF=2 0.0.0.0:7000
I accept(7, {2, AF=2 CLIENT_IP:PORT}, 16) -> 8
N accepting connection from AF=2 CLIENT_IP:PORT on AF=2 172.20.0.2:7000
I permitting connection from AF=2 CLIENT_IP:PORT
I close(7)
I resolved and opened all sock addresses
N starting data transfer loop with FDs [5,5] and [8,8]
ss command on the server prints:
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
tcp ESTAB 0 0 172.20.0.2:7000 CLIENT_IP:PORT
The problem is, that when I disconnect the client (by switching it off), the tcp connection is still established and no addition logging is coming from socat. ss still shows the connection as ESTAB. Any ideas why? When I again connect the client following appears in the logs:
W read(8, 0x7fa8f48c4020, 8192): Connection reset by peer
N socket 2 to socket 1 is in error
N socket 2 (fd 8) is at EOF
I poll timed out (no data within 0.500000 seconds)
I close(5)
I shutdown(8, 2)
I shutdown(8, 2): Socket not connected
N exiting with status 0
But why does this happen on connect instead of disconnect?

Answer 1

If there is no data to send or receive on a socket and you cut the underlying connection neither side is aware until it attempts to send data. Normally, that would be application level data, but at the protocol level you can enable TCP keep alives to emulate flowing data whenever there is no real data.
According to the socat manpage you could try something like:
socat -d -d -d PTY,link=/dev/ttyFOOBAR,echo=0,raw,unlink-close=0 TCP-LISTEN:7000,forever,reuseaddr,keepalive,keepidle=10,keepintvl=10,keepcnt=2
(keepalive actually looks like the essential option but it is unclear what the defaults will be for the tuning options if unset.)