From: Michał Kępień <michal@isc.org>
Date: Sun, 29 Dec 2024 09:22:20 +0000 (+0100)
Subject: Improve reuse of outgoing TCP connections
X-Git-Tag: v9.21.4~11^2
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=086c325ad3f9b54b5db57e58b6e5960e7c6b288e;p=thirdparty%2Fbind9.git

Improve reuse of outgoing TCP connections

The dns_dispatch_gettcp() function is used for finding an existing TCP
connection that can be reused for sending a query from a specified local
address to a specified remote address.  The logic for matching the
provided <local address, remote address> tuple to one of the existing
TCP connections is implemented in the dispatch_match() function:

  - if the examined TCP connection already has a libuv handle assigned,
    it means the connection has already been established; therefore,
    compare the provided <local address, remote address> tuple against
    the corresponding address tuple for the libuv handle associated with
    the connection,

  - if the examined TCP connection does not yet have a libuv handle
    assigned, it means the connection has not yet been established;
    therefore, compare the provided <local address, remote address>
    tuple against the corresponding address tuple that the TCP
    connection was originally created for.

This logic limits TCP connection reuse potential as the libuv handle
assigned to an existing dispatch object may have a more specific local
<address, port> tuple associated with it than the local <address, port>
tuple that the dispatch object was originally created for.  That's
because the local address for outgoing connections can be set to a
wildcard <address, port> tuple (indicating that the caller does not care
what source <address, port> tuple will be used for establishing the
connection, thereby delegating the task of picking it to the operating
system) and then get "upgraded" to a specific <address, port> tuple when
the socket is bound (and a libuv handle gets associated with it).  When
another dns_dispatch_gettcp() caller then tries to look for an existing
TCP connection to the same peer and passes a wildcard address in the
local part of the tuple, the function will not match that request to a
previously-established TCP connection (unless isc_nmhandle_localaddr()
returns a wildcard address as well).

Simplify dispatch_match() so that the libuv handle associated with an
existing dispatch object is not examined for the purpose of matching it
to the provided <local address, remote address> tuple; instead, always
examine the <local address, remote address> tuple that the dispatch
object was originally created for.  This enables reuse of TCP
connections created without providing a specific local socket address
while still preventing other connections (created for a specific local
socket address) from being inadvertently shared.
---

diff --git a/lib/dns/dispatch.c b/lib/dns/dispatch.c
index 5484caffc94..6e71c5c6455 100644
--- a/lib/dns/dispatch.c
+++ b/lib/dns/dispatch.c
@@ -1152,20 +1152,11 @@ static int
 dispatch_match(struct cds_lfht_node *node, const void *key0) {
 	dns_dispatch_t *disp = caa_container_of(node, dns_dispatch_t, ht_node);
 	const struct dispatch_key *key = key0;
-	isc_sockaddr_t local;
-	isc_sockaddr_t peer;
-
-	if (disp->handle != NULL) {
-		local = isc_nmhandle_localaddr(disp->handle);
-		peer = isc_nmhandle_peeraddr(disp->handle);
-	} else {
-		local = disp->local;
-		peer = disp->peer;
-	}
 
-	return isc_sockaddr_equal(&peer, key->peer) &&
-	       disp->transport == key->transport &&
-	       (key->local == NULL || isc_sockaddr_equal(&local, key->local));
+	return disp->transport == key->transport &&
+	       isc_sockaddr_equal(&disp->peer, key->peer) &&
+	       (key->local == NULL ||
+		isc_sockaddr_equal(&disp->local, key->local));
 }
 
 isc_result_t