rpcbind: rpcbind.socket listens on IPv6

[pmenzel]

@geniux$ sudo lsof -i6TCP
COMMAND     PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
systemd       1 root  248u  IPv6  29701      0t0  TCP *:sunrpc (LISTEN)
rpc.mount   596 root    7u  IPv6  33876      0t0  TCP *:59967 (LISTEN)
rpc.mount   596 root   11u  IPv6  33884      0t0  TCP *:45737 (LISTEN)
rpc.mount   596 root   15u  IPv6  33892      0t0  TCP *:52663 (LISTEN)
rpcbind     603 root    6u  IPv6  29701      0t0  TCP *:sunrpc (LISTEN)
rpc.statd 11065 root   10u  IPv6 923143      0t0  TCP *:52357 (LISTEN)

@geniux$ systemctl cat rpcbind.socket
# /lib/systemd/system/rpcbind.socket
[Unit]
Description=RPCbind Server Activation Socket
DefaultDependencies=no
Wants=rpcbind.target
Before=rpcbind.target

[Socket]
ListenStream=/run/rpcbind.sock

# RPC netconfig can't handle ipv6/ipv4 dual sockets
BindIPv6Only=ipv6-only
ListenStream=0.0.0.0:111
ListenDatagram=0.0.0.0:111
ListenStream=[::]:111
ListenDatagram=[::]:111

[Install]
WantedBy=sockets.target

@geniux$ systemctl status rpcbind.socket
● rpcbind.socket - RPCbind Server Activation Socket
   Loaded: loaded (/lib/systemd/system/rpcbind.socket; enabled; vendor pre
   Active: active (running) since Wed 2019-08-14 17:30:31 CEST; 9 months 2
   Listen: /run/rpcbind.sock (Stream)
           0.0.0.0:111 (Stream)
           0.0.0.0:111 (Datagram)
           [::]:111 (Stream)
           [::]:111 (Datagram)
    Tasks: 0 (limit: 7372)
   Memory: 28.0K
   CGroup: /system.slice/rpcbind.socket

I think, /etc/sysctl.conf is parsed after the socket is started.

We could add a drop-in to override the properties.

[Socket]
ListenStream=
BindIPv6Only=
ListenDatagram=

ListenStream=/run/rpcbind.sock
ListenStream=0.0.0.0:111
ListenDatagram=0.0.0.0:111

Investigating how systemd socket units deal with disabled IPv6 support would be useful too.