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[9.20] chg: dev: Implement Fisher-Yates shuffle for nameserver selection

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Replace the two-pass "random start index and wrap around" logic in
fctx_getaddresses_nameservers() with a statistically sound partial
Fisher-Yates shuffle.

The previous implementation picked a random starting node and did two
passes over the linked list to find query candidates. The new logic
introduces fctx_getaddresses_nsorder() to perform an in-place
randomization of indices into a bounded, stack-allocated lookup array
(nsorder) representing the "winning" fetch slots.

The nameserver dataset is now traversed in exactly one sequential pass:
1. Every nameserver is evaluated for local cached data.
2. If the current nameserver's sequential index exists in the randomized
   nsorder array, it is permitted to launch an outgoing network fetch.
3. If not, it is restricted to local lookups via DNS_ADBFIND_NOFETCH.

This guarantees a fair random distribution for outbound queries while
maximizing local cache hits, entirely within O(1) memory and without
the overhead of linked-list pointer shuffling or dynamic allocation.

Closes #5695

Backport of MR !11604

Merge branch 'backport-5695-refactor-the-random-NS-selection-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11606
This commit is contained in:
Ondřej Surý 2026-02-26 09:13:34 +01:00
commit dd453590a0
11 changed files with 425 additions and 64 deletions
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39
bin/tests/system/nsprocessinglimit/ns1/named.conf.j2 Normal file
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@ -0,0 +1,39 @@
/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* SPDX-License-Identifier: MPL-2.0
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
options {
query-source address 10.53.0.1;
notify-source 10.53.0.1;
transfer-source 10.53.0.1;
port @PORT@;
pid-file "named.pid";
listen-on { 10.53.0.1; };
recursion no;
dnssec-validation no;
};
view "default" {
zone "." {
type primary;
file "root.db";
};
};
key rndc_key {
secret "1234abcd8765";
algorithm @DEFAULT_HMAC@;
};
controls {
inet 10.53.0.1 port @CONTROLPORT@ allow { any; } keys { rndc_key; };
};

24
bin/tests/system/nsprocessinglimit/ns1/root.db Normal file
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@ -0,0 +1,24 @@
; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
;
; SPDX-License-Identifier: MPL-2.0
;
; This Source Code Form is subject to the terms of the Mozilla Public
; License, v. 2.0. If a copy of the MPL was not distributed with this
; file, you can obtain one at https://mozilla.org/MPL/2.0/.
;
; See the COPYRIGHT file distributed with this work for additional
; information regarding copyright ownership.
$TTL 300
. IN SOA marka.isc.org. a.root.servers.nil. (
2010 ; serial
600 ; refresh
600 ; retry
1200 ; expire
600 ; minimum
)
. NS a.root-servers.nil.
a.root-servers.nil. A 10.53.0.1
tld. NS ns.tld.
ns.tld. A 10.53.0.2

37
bin/tests/system/nsprocessinglimit/ns2/named.conf.j2 Normal file
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@ -0,0 +1,37 @@
/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* SPDX-License-Identifier: MPL-2.0
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
options {
query-source address 10.53.0.2;
notify-source 10.53.0.2;
transfer-source 10.53.0.2;
port @PORT@;
pid-file "named.pid";
listen-on { 10.53.0.2; };
recursion no;
dnssec-validation no;
};
zone "tld." {
type primary;
file "tld.db";
};
key rndc_key {
secret "1234abcd8765";
algorithm @DEFAULT_HMAC@;
};
controls {
inet 10.53.0.2 port @CONTROLPORT@ allow { any; } keys { rndc_key; };
};

25
bin/tests/system/nsprocessinglimit/ns2/tld.db Normal file
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@ -0,0 +1,25 @@
; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
;
; SPDX-License-Identifier: MPL-2.0
;
; This Source Code Form is subject to the terms of the Mozilla Public
; License, v. 2.0. If a copy of the MPL was not distributed with this
; file, you can obtain one at https://mozilla.org/MPL/2.0/.
;
; See the COPYRIGHT file distributed with this work for additional
; information regarding copyright ownership.
$TTL 300
tld. IN SOA marka.isc.org. ns.tld. (
2010 ; serial
600 ; refresh
600 ; retry
1200 ; expire
600 ; minimum
)
tld. NS ns.tld.
ns.tld. A 10.53.0.2
example.tld. NS ns.example.tld.
ns.example.tld. A 10.53.0.3

68
bin/tests/system/nsprocessinglimit/ns3/example.tld.db Normal file
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@ -0,0 +1,68 @@
; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
;
; SPDX-License-Identifier: MPL-2.0
;
; This Source Code Form is subject to the terms of the Mozilla Public
; License, v. 2.0. If a copy of the MPL was not distributed with this
; file, you can obtain one at https://mozilla.org/MPL/2.0/.
;
; See the COPYRIGHT file distributed with this work for additional
; information regarding copyright ownership.
$TTL 300
example.tld. IN SOA marka.isc.org. ns.dnshoster.tld. (
2010 ; serial
600 ; refresh
600 ; retry
1200 ; expire
600 ; minimum
)
example.tld. NS ns.example.tld.
ns.example.tld. A 10.53.0.3
sub.example.tld. NS ns01.sub.example.tld.
sub.example.tld. NS ns02.sub.example.tld.
sub.example.tld. NS ns03.sub.example.tld.
sub.example.tld. NS ns04.sub.example.tld.
sub.example.tld. NS ns05.sub.example.tld.
sub.example.tld. NS ns06.sub.example.tld.
sub.example.tld. NS ns07.sub.example.tld.
sub.example.tld. NS ns08.sub.example.tld.
sub.example.tld. NS ns09.sub.example.tld.
sub.example.tld. NS ns10.sub.example.tld.
sub.example.tld. NS ns11.sub.example.tld.
sub.example.tld. NS ns12.sub.example.tld.
sub.example.tld. NS ns12.sub.example.tld.
sub.example.tld. NS ns12.sub.example.tld.
sub.example.tld. NS ns13.sub.example.tld.
sub.example.tld. NS ns14.sub.example.tld.
sub.example.tld. NS ns15.sub.example.tld.
sub.example.tld. NS ns16.sub.example.tld.
sub.example.tld. NS ns17.sub.example.tld.
sub.example.tld. NS ns18.sub.example.tld.
sub.example.tld. NS ns19.sub.example.tld.
sub.example.tld. NS ns20.sub.example.tld.
sub.example.tld. NS ns21.sub.example.tld.
sub.example.tld. NS ns22.sub.example.tld.
sub.example.tld. NS ns23.sub.example.tld.
ns01.sub.example.tld. A 127.0.0.1
ns02.sub.example.tld. A 127.0.0.2
ns03.sub.example.tld. A 127.0.0.3
ns04.sub.example.tld. A 127.0.0.4
ns05.sub.example.tld. A 127.0.0.5
ns06.sub.example.tld. A 127.0.0.6
ns07.sub.example.tld. A 127.0.0.7
ns08.sub.example.tld. A 127.0.0.8
ns09.sub.example.tld. A 127.0.0.9
ns10.sub.example.tld. A 127.0.0.10
ns11.sub.example.tld. A 127.0.0.11
ns12.sub.example.tld. A 127.0.0.12
ns13.sub.example.tld. A 127.0.0.13
ns14.sub.example.tld. A 127.0.0.14
ns15.sub.example.tld. A 127.0.0.15
ns16.sub.example.tld. A 127.0.0.16
ns17.sub.example.tld. A 127.0.0.17
ns18.sub.example.tld. A 127.0.0.18
ns19.sub.example.tld. A 127.0.0.19
ns20.sub.example.tld. A 127.0.0.20
ns21.sub.example.tld. A 127.0.0.21
ns22.sub.example.tld. A 127.0.0.22
ns23.sub.example.tld. A 127.0.0.23

37
bin/tests/system/nsprocessinglimit/ns3/named.conf.j2 Normal file
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@ -0,0 +1,37 @@
/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* SPDX-License-Identifier: MPL-2.0
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
options {
query-source address 10.53.0.3;
notify-source 10.53.0.3;
transfer-source 10.53.0.3;
port @PORT@;
pid-file "named.pid";
listen-on { 10.53.0.3; };
recursion no;
dnssec-validation no;
};
zone "example.tld." {
type primary;
file "example.tld.db";
};
key rndc_key {
secret "1234abcd8765";
algorithm @DEFAULT_HMAC@;
};
controls {
inet 10.53.0.3 port @CONTROLPORT@ allow { any; } keys { rndc_key; };
};

1
bin/tests/system/nsprocessinglimit/ns4/named.args Normal file
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@ -0,0 +1 @@
-D nsprocessinglimit-ns4 -m record -c named.conf -d 99 -g -T maxcachesize=2097152 -4

39
bin/tests/system/nsprocessinglimit/ns4/named.conf.j2 Normal file
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@ -0,0 +1,39 @@
/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* SPDX-License-Identifier: MPL-2.0
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
options {
query-source address 10.53.0.4;
notify-source 10.53.0.4;
transfer-source 10.53.0.4;
port @PORT@;
pid-file "named.pid";
listen-on { 10.53.0.4; };
recursion yes;
dnssec-validation no;
dnstap { resolver query; };
dnstap-output file "dnstap.out";
};
zone "." {
type hint;
file "root.hint";
};
key rndc_key {
secret "1234abcd8765";
algorithm @DEFAULT_HMAC@;
};
controls {
inet 10.53.0.4 port @CONTROLPORT@ allow { any; } keys { rndc_key; };
};

14
bin/tests/system/nsprocessinglimit/ns4/root.hint Normal file
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@ -0,0 +1,14 @@
; Copyright (C) Internet Systems Consortium, Inc. ("ISC")
;
; SPDX-License-Identifier: MPL-2.0
;
; This Source Code Form is subject to the terms of the Mozilla Public
; License, v. 2.0. If a copy of the MPL was not distributed with this
; file, you can obtain one at https://mozilla.org/MPL/2.0/.
;
; See the COPYRIGHT file distributed with this work for additional
; information regarding copyright ownership.
$TTL 999999
. IN NS a.root-servers.nil.
a.root-servers.nil. IN A 10.53.0.1

74
bin/tests/system/nsprocessinglimit/tests_nsprocessinglimit.py Normal file
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@ -0,0 +1,74 @@
# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
#
# SPDX-License-Identifier: MPL-2.0
#
# This Source Code Form is subject to the terms of the Mozilla Public
# License, v. 2.0. If a copy of the MPL was not distributed with this
# file, you can obtain one at https://mozilla.org/MPL/2.0/.
#
# See the COPYRIGHT file distributed with this work for additional
# information regarding copyright ownership.
import os
import isctest
import isctest.mark
pytestmark = [isctest.mark.with_dnstap]
def line_to_ips_and_queries(line):
# dnstap-read output line example
# 05-Feb-2026 11:00:57.853 RQ 10.53.0.4:38507 -> 10.53.0.3:22047 TCP 56b sub.example.tld/IN/NS
_, _, _, _, _, dst, _, _, query = line.split(" ", 9)
ip, _ = dst.split(":", 1)
return (ip, query)
def extract_dnstap(ns, expectedlen):
ns.rndc("dnstap -roll 1")
path = os.path.join(ns.identifier, "dnstap.out.0")
dnstapread = isctest.run.cmd(
[isctest.vars.ALL["DNSTAPREAD"], path],
)
lines = dnstapread.out.splitlines()
assert expectedlen == len(lines)
return map(line_to_ips_and_queries, lines)
def expect_query(expected_query, expected_query_count, ips_and_queries):
count = 0
for _, query in ips_and_queries:
if query == expected_query:
count += 1
assert count == expected_query_count
def expect_next_ip_and_query(expected_ips_and_queries, ips_and_queries):
for expected_ip, expected_query in expected_ips_and_queries:
ip, query = next(ips_and_queries)
assert ip == expected_ip
assert query == expected_query
def test_selfpointedglue_nslimit(ns4):
msg = isctest.query.create("a.sub.example.tld.", "A")
res = isctest.query.tcp(msg, ns4.ip)
isctest.check.servfail(res)
# The 4 formers lines are request to find sub.example2.tld NSs.
# The latest 20 are queries to sub.example2.tld NSs.
ips_and_queries = extract_dnstap(ns4, 24)
# Checking the begining of the resulution
expect_next_ip_and_query(
[
("10.53.0.1", "./IN/NS"),
("10.53.0.1", "tld/IN/NS"),
("10.53.0.2", "example.tld/IN/NS"),
("10.53.0.3", "sub.example.tld/IN/NS"),
],
ips_and_queries,
)
expect_query("a.sub.example.tld/IN/A", 20, ips_and_queries)

131
lib/dns/resolver.c
View file

@ -3474,6 +3474,8 @@ fctx_getaddresses(fetchctx_t *fctx) {
bool have_address = false;
unsigned int ns_processed = 0;
size_t fetches_allowed = 0;
dns_rdata_t nameservers_s[NS_PROCESSING_LIMIT];
dns_rdata_t *nameservers[NS_PROCESSING_LIMIT];
FCTXTRACE5("getaddresses", "fctx->depth=", fctx->depth);
@ -3657,73 +3659,74 @@ normal_nses:
break;
}
for (result = dns_rdataset_first(&fctx->nameservers);
result == ISC_R_SUCCESS;
result = dns_rdataset_next(&fctx->nameservers))
{
dns_rdata_t *rdata = nameservers[ns_processed] =
&nameservers_s[ns_processed];
dns_rdata_init(rdata);
dns_rdataset_current(&fctx->nameservers, rdata);
if (++ns_processed >= NS_PROCESSING_LIMIT) {
break;
}
}
if (ns_processed > 1 && ns_processed > fetches_allowed) {
/*
* Skip the shuffle if:
* - there's nothing to shuffle (no or one nameserver)
* - there are less nameserver than allowed fetches as
* we are going to start fetches for all of them.
*/
for (size_t i = 0; i < ns_processed - 1; i++) {
size_t j = i + isc_random_uniform(ns_processed - i);
ISC_SWAP(nameservers[i], nameservers[j]);
}
}
for (;;) {
size_t nscount = dns_rdataset_count(&fctx->nameservers);
size_t maxstartns = nscount > NS_PROCESSING_LIMIT
? NS_PROCESSING_LIMIT
: nscount;
size_t startns = isc_random_uniform(maxstartns);
for (size_t i = 0; i < ns_processed; i++) {
bool overquota = false;
unsigned int static_stub = 0;
unsigned int no_fetch = 0;
dns_rdata_t *rdata = nameservers[i];
for (size_t pass = 0; pass < 2; pass++) {
size_t curns = 0;
for (result = dns_rdataset_first(&fctx->nameservers);
result == ISC_R_SUCCESS;
result = dns_rdataset_next(&fctx->nameservers))
{
dns_rdata_t rdata = DNS_RDATA_INIT;
bool overquota = false;
unsigned int static_stub = 0;
unsigned int no_fetch = 0;
if (pass == 0 && curns++ < startns) {
continue;
}
if (pass == 1 && curns++ >= startns) {
break;
}
dns_rdataset_current(&fctx->nameservers,
&rdata);
/*
* Extract the name from the NS record.
*/
result = dns_rdata_tostruct(&rdata, &ns, NULL);
if (result != ISC_R_SUCCESS) {
continue;
}
if (STATICSTUB(&fctx->nameservers) &&
dns_name_equal(&ns.name, fctx->domain))
{
static_stub = DNS_ADBFIND_STATICSTUB;
}
/*
* Make sure we only launch a limited number of
* outgoing fetches.
*/
if (fctx->pending_running >= fetches_allowed) {
no_fetch = DNS_ADBFIND_NOFETCH;
}
findname(fctx, &ns.name, 0,
stdoptions | static_stub | no_fetch, 0,
now, &overquota, &need_alternate,
&have_address);
if (!overquota) {
all_spilled = false;
}
dns_rdata_reset(&rdata);
dns_rdata_freestruct(&ns);
if (++ns_processed >= NS_PROCESSING_LIMIT) {
result = ISC_R_NOMORE;
break;
}
/*
* Extract the name from the NS record.
*/
result = dns_rdata_tostruct(rdata, &ns, NULL);
if (result != ISC_R_SUCCESS) {
continue;
}
if (STATICSTUB(&fctx->nameservers) &&
dns_name_equal(&ns.name, fctx->domain))
{
static_stub = DNS_ADBFIND_STATICSTUB;
}
/*
* Make sure we only launch a limited number of
* outgoing fetches.
*/
if (fctx->pending_running >= fetches_allowed) {
no_fetch = DNS_ADBFIND_NOFETCH;
}
findname(fctx, &ns.name, 0,
stdoptions | static_stub | no_fetch, 0, now,
&overquota, &need_alternate, &have_address);
if (!overquota) {
all_spilled = false;
}
dns_rdata_freestruct(&ns);
}
/*