Description of problem:

rpcbind sockets remain ESTABLISHED indefinitely after port vulnerability scan.

The customer's scan might be keeping the connection open on the client side indefinitely or long enough for a silent firewall to drop the connection and also not send RST to the server.  

In either case, the RHEL7 and RHEL8 test are equal in this behavior yet RHEL7 terminates the connection while RHEL8 does not.  So there is a difference in rpcbind's and seems likely to be in the area referenced by the sourceforge link

https://sourceforge.net/p/libtirpc/mailman/message/34866401/
- removed __svc_clean_idle() from my_svc_run() case when poll() returns "0" (timed out) and current rpcbind only does continue() in my_svc_run() when poll returns 0.

ISSUE:  rpcbind sockets remain ESTABLISHED indefinitely after port scan.
WHERE:  RHEL8  --- didn't happen on RHEL7.

SUMMARY:  It's up to the application, rpcbind in this case, to set use of SO_KEEPALIVE or implement it's own socket timeouts. 
sunrpc uses SO_KEEPALIVE in RHEL7 within xs_tcp_finish_connecting() and RHEL8 does so in xs_tcp_set_socket_timeouts() called from xs_tcp_finish_connecting(), but the kernel sunrpc isn't in control of the rpcbind socket.  

I checked rpcbind source and found no references to KEEPALIVE in RHEL7 nor RHEL8.

Hypothesis:  sunrpc does it's own timeouts to cause shutdown of ESTABLISHED connections in RHEL7 but not in RHEL8, or else there is some configuration variable that needs to be adjusted.

LAB test:

RHEL7:
  [NFS Server]
    kernel 3.10.0-1160.11.1.el7.x86_64
    rpcbind-0.2.0-49.el7.x86_64
    # strace -f -o /tmp/rpcbind.strace -p `pidof rpcbind`
    # iptables -I INPUT 1 --ipv4 -p tcp -s 192.168.122.1 --sport 40052 --dport 111 -j DROP    
  [client] 
    nc -4 192.168.122.18 111
    ... ephemeral port chosen was 40052 ...
    killall nc

  Finding:  rpcbind did not set SO_KEEPALIVE* on the socket, netstat confirmed no keepalive enabled (off) strace showed accept followed by two 30s timed out polls then rpcbind closed the socket,  causing the server to send FIN and initiate closure that went to FIN_WAIT1 then timed out and cleared.

   
RHEL8:
  [NFS server]
    kernel 4.18.0-240.1.1.el8_3.x86_64
    rpcbind-1.2.5-7.el8.x86_64
    # strace -f -o /tmp/rpcbind.strace -p `pidof rpcbind`
  [client]
    nc -4 192.168.122.1 111
    .... ephemeral port chosen 34112 ...
    iptables -I OUTPUT 1 --ipv4 -p tcp -d 192.168.122.1 --sport 34112 --dport 111 -j DROP 
    killall nc

   Finding: rpcbind continues to poll and timeout every 30s but never closes the connection.  Connection remained ESTABLISHED indefinitely.

I have checked with package maintainer and there has been no such change in rpcbind but there might be changes in implementation of the systemd sockets. 
since systemd creates the sockets for rpcbind.

Version-Release number of selected component (if applicable):
rpcbind-1.2.5-7.el8.x86_64

How reproducible:
rpcbind connections are established while scanning the system with vulnerability scanner

Steps to Reproduce:
1. Scan system with vulnerability scanner
2. The scanner opens rpcbind connections to the system
3. Check if rpcbind connections are closed

Actual results:

rpcbind continues to poll and timeout every 30s but never closes the connection.  Connection remained ESTABLISHED indefinitely.

Expected results:

rpcbind established connections should be closed 

Additional info: