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 socketdef 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.
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