Re: [PATCH v2] tcp: Enable SO_KEEPALIVE if we see keep-alive segments from container / guest

On Fri, Dec 05, 2025 at 03:36:23PM +0100, Stefano Brivio wrote:

This is an approximation, as enabling SO_KEEPALIVE doesn't mean that a keep-alive segment will be sent right away, rather that keep-alive segments will start being sent if the connection is idle.

On the other hand, we don't have direct control over the host-side TCP, so this is probably the best approximation we can get.

By default, namespaces inherit keep-alive parameters from their parent namespace, so we can assume that, in case of a container, we'll wait for the same interval it took for the container to start sending us keep-alives, effectively doubling that interval.

To keep this simple, set SO_KEEPALIVE whenever we see a keep-alive segment, instead of tracking its state. Keep-alive segments are relatively infrequent, so we don't expect any substantial cost from doing that.

Reported-by: Dominic Kohls Signed-off-by: Stefano Brivio

Reviewed-by: David Gibson

--- v2: Coverity Scan just reminded me that we shouldn't ignore setsockopt() return codes. It doesn't really matter if this fails as we can't do much else in that case, but a trace-level message is a good idea anyway (not that I've ever seen this failing).

tcp.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/tcp.c b/tcp.c index 8357c0e..d6a5337 100644 --- a/tcp.c +++ b/tcp.c @@ -1838,6 +1838,10 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn, tcp_send_flag(c, conn, ACK); tcp_timer_ctl(c, conn);

+ if (setsockopt(conn->sock, SOL_SOCKET, SO_KEEPALIVE, + &((int){ 1 }), sizeof(int))) + flow_trace(conn, "failed to set SO_KEEPALIVE"); + if (p->count == 1) { tcp_tap_window_update(c, conn, ntohs(th->window)); -- 2.43.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson