== How to do TCP keepalives in Python

TCP keepalives are do-nothing packets the TCP layer can send to see if
a connection is still alive or if the remote end has gone unreachable
(due to a machine crash, a network problem, or whatever). Keepalives
are not default TCP behavior (at least not in any TCP stack that
conforms to the RFCs), so you have to specifically turn them on.
(There are various reasons why this is sensible.)

In Python you can do this with the _.setsockopt()_ socket method,
using the ((socket.SO_KEEPALIVE)) option and setting a value of
integer 1. The only mystery is what the level parameter should be;
despite what you might guess, it is ((socket.SOL_SOCKET)). So a
complete code example is:

>  import socket
>
>  def setkeepalives(sck):
>    sck.setsockopt(socket.SOL_SOCKET, \
>                   socket.SO_KEEPALIVE, 1)

Various sources recommend turning keepalives on as soon as possible
after you have the socket.

(Keepalives are only applicable to TCP sockets, so one might expect
((SOL_TCP)) or at least ((SOL_IP)), but no; they are a generic socket
level option. Go figure.)

On Linux, you can control various bits of keepalive behavior by
setting the additional ((SOL_TCP)) integer parameters
((TCP_KEEPIDLE)), ((TCP_KEEPINTVL)), and ((TCP_KEEPCNT)); Python
defines them all in the _socket_ module. See the tcp(7) manpage for
details. The default values are found in _/proc/sys/net/ipv4_ in the
files ((tcp_keepalive_time)), ((tcp_keepalive_intvl)), and
((tcp_keepalive_probes)), and are fairly large.