draft-ietf-dnsop-respsize-01.txt   draft-ietf-dnsop-respsize-02.txt 
DNSOP Working Group Paul Vixie, ISC (Ed.)
DNSOP Working Group Paul Vixie, ISC
INTERNET-DRAFT Akira Kato, WIDE INTERNET-DRAFT Akira Kato, WIDE
<draft-ietf-dnsop-respsize-01.txt> July, 2004 <draft-ietf-dnsop-respsize-02.txt> July 2005
DNS Response Size Issues DNS Response Size Issues
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003-2004). All Rights Reserved. Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract Abstract
With a mandated default minimum maximum message size of 512 octets, With a mandated default minimum maximum message size of 512 octets,
the DNS protocol presents some special problems for zones wishing to the DNS protocol presents some special problems for zones wishing to
expose a moderate or high number of authority servers (NS RRs). This expose a moderate or high number of authority servers (NS RRs). This
document explains the operational issues caused by, or related to document explains the operational issues caused by, or related to
this response size limit. this response size limit.
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1 - Introduction and Overview 1 - Introduction and Overview
1.1. The DNS standard (see [RFC1035 4.2.1]) limits message size to 512 1.1. The DNS standard (see [RFC1035 4.2.1]) limits message size to 512
octets. Even though this limitation was due to the required minimum UDP octets. Even though this limitation was due to the required minimum UDP
reassembly limit for IPv4, it is a hard DNS protocol limit and is not reassembly limit for IPv4, it is a hard DNS protocol limit and is not
implicitly relaxed by changes in transport, for example to IPv6. implicitly relaxed by changes in transport, for example to IPv6.
1.2. The EDNS0 standard (see [RFC2671 2.3, 4.5]) permits larger 1.2. The EDNS0 standard (see [RFC2671 2.3, 4.5]) permits larger
responses by mutual agreement of the requestor and responder. However, responses by mutual agreement of the requestor and responder. However,
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2.1. A delegation response will include the following elements: 2.1. A delegation response will include the following elements:
Header Section: fixed length (12 octets) Header Section: fixed length (12 octets)
Question Section: original query (name, class, type) Question Section: original query (name, class, type)
Answer Section: (empty) Answer Section: (empty)
Authority Section: NS RRset (nameserver names) Authority Section: NS RRset (nameserver names)
Additional Section: A and AAAA RRsets (nameserver addresses) Additional Section: A and AAAA RRsets (nameserver addresses)
2.2. If the total response size would exceed 512 octets, and if the data 2.2. If the total response size would exceed 512 octets, and if the data
that would not fit was in the question, answer, or authority section, that would not fit belonged in the question, answer, or authority
then the TC bit will be set (indicating truncation) which may cause the section, then the TC bit will be set (indicating truncation) which may
requestor to retry using TCP, depending on what information was present cause the requestor to retry using TCP, depending on what information
and what was omitted. If a retry using TCP is needed, the total cost of was desired and what information was omitted. If a retry using TCP is
the transaction is much higher. needed, the total cost of the transaction is much higher. (See [RFC1123
6.1.3.2] for details on the protocol requirement that UDP be attempted
before falling back to TCP.)
2.3. RRsets are never sent partially, so if truncation occurs, entire 2.3. RRsets are never sent partially unless truncation occurs, in which
RRsets are omitted. Note that the authority section consists of a case the final apparent RRset in the final nonempty section must be
single RRset. It is absolutely essential that truncation not occur in considered "possibly damaged". With or without truncation, the glue
the authority section. present in the additional data section should be considered "possibly
incomplete", and requestors should be prepared to re-query for any
damaged or missing RRsets. For multi-transport name or mail services,
INTERNET-DRAFT July 2005 RESPSIZE
INTERNET-DRAFT June 2003 RESPSIZE this can mean querying for an IPv6 (AAAA) RRset even when an IPv4 (A)
RRset is present.
2.4. DNS label compression allows a domain name to be instantiated only 2.4. DNS label compression allows a domain name to be instantiated only
once per DNS message, and then referenced with a two-octet "pointer" once per DNS message, and then referenced with a two-octet "pointer"
from other locations in that same DNS message. If all nameserver names from other locations in that same DNS message. If all nameserver names
in a message are similar (for example, all ending in ".ROOT- in a message are similar (for example, all ending in ".ROOT-
SERVERS.NET"), then more space will be available for uncompressable data SERVERS.NET"), then more space will be available for uncompressable data
(such as nameserver addresses). (such as nameserver addresses).
2.5. The query name can be as long as 255 characters of presentation 2.5. The query name can be as long as 255 characters of presentation
data, which can be up to 256 octets of network data. In this worst case data, which can be up to 256 octets of network data. In this worst case
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resources of an authority server since the requestor would have to retry resources of an authority server since the requestor would have to retry
using TCP to complete the attack. An attack that always used TCP would using TCP to complete the attack. An attack that always used TCP would
have a lower cost. have a lower cost.
2.8. The minimum useful number of address records is two, since with 2.8. The minimum useful number of address records is two, since with
only one address, the probability that it would refer to an unreachable only one address, the probability that it would refer to an unreachable
server is too high. Truncation which occurs after two address records server is too high. Truncation which occurs after two address records
have been added to the additional data section is therefore less have been added to the additional data section is therefore less
operationally significant than truncation which occurs earlier. operationally significant than truncation which occurs earlier.
2.9. The best case is no truncation. (This is because many requestors 2.9. The best case is no truncation. This is because many requestors
will retry using TCP by reflex, without considering whether the omitted will retry using TCP by reflex, or will automatically re-query for
data was actually necessary.) RRsets that are "possibly truncated", without considering whether the
omitted data was actually necessary.
INTERNET-DRAFT June 2003 RESPSIZE 2.10. Each added NS RR for a zone will add a minimum of between 16 and
44 octets to every untruncated referral or negative response from the
zone's authority servers (16 octets for an NS RR, 16 octets for an A RR,
and 28 octets for an AAAA RR), in addition to whatever space is taken by
the nameserver name (NS NSDNAME and A/AAAA owner name).
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3 - Analysis 3 - Analysis
3.1. An instrumented protocol trace of a best case delegation response 3.1. An instrumented protocol trace of a best case delegation response
follows. Note that 13 servers are named, and 13 addresses are given. follows. Note that 13 servers are named, and 13 addresses are given.
This query was artificially designed to exactly reach the 512 octet This query was artificially designed to exactly reach the 512 octet
limit. limit.
;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13 ;; flags: qr rd; QUERY: 1, ANS: 0, AUTH: 13, ADDIT: 13
;; QUERY SECTION: ;; QUERY SECTION:
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F.GTLD-SERVERS.NET. 86400 A 192.35.51.30 ;; @400 F.GTLD-SERVERS.NET. 86400 A 192.35.51.30 ;; @400
G.GTLD-SERVERS.NET. 86400 A 192.42.93.30 ;; @416 G.GTLD-SERVERS.NET. 86400 A 192.42.93.30 ;; @416
H.GTLD-SERVERS.NET. 86400 A 192.54.112.30 ;; @432 H.GTLD-SERVERS.NET. 86400 A 192.54.112.30 ;; @432
I.GTLD-SERVERS.NET. 86400 A 192.43.172.30 ;; @448 I.GTLD-SERVERS.NET. 86400 A 192.43.172.30 ;; @448
J.GTLD-SERVERS.NET. 86400 A 192.48.79.30 ;; @464 J.GTLD-SERVERS.NET. 86400 A 192.48.79.30 ;; @464
K.GTLD-SERVERS.NET. 86400 A 192.52.178.30 ;; @480 K.GTLD-SERVERS.NET. 86400 A 192.52.178.30 ;; @480
L.GTLD-SERVERS.NET. 86400 A 192.41.162.30 ;; @496 L.GTLD-SERVERS.NET. 86400 A 192.41.162.30 ;; @496
M.GTLD-SERVERS.NET. 86400 A 192.55.83.30 ;; @512 M.GTLD-SERVERS.NET. 86400 A 192.55.83.30 ;; @512
;; MSG SIZE sent: 80 rcvd: 512 ;; MSG SIZE sent: 80 rcvd: 512
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3.2. For longer query names, the number of address records supplied will 3.2. For longer query names, the number of address records supplied will
be lower. Furthermore, it is only by using a common parent name (which be lower. Furthermore, it is only by using a common parent name (which
is GTLD-SERVERS.NET in this example) that all 13 addresses are able to is GTLD-SERVERS.NET in this example) that all 13 addresses are able to
fit. The following output from a response simulator demonstrates these fit. The following output from a response simulator demonstrates these
properties: properties:
% perl respsize.pl 13 13 0 % perl respsize.pl a.dns.br b.dns.br c.dns.br d.dns.br
common name, average case: msg:303 nsaddr#13 (green) a.dns.br requires 10 bytes
common name, worst case: msg:495 nsaddr# 1 (red) b.dns.br requires 4 bytes
uncommon name, average case: msg:457 nsaddr# 3 (orange) c.dns.br requires 4 bytes
uncommon name, worst case: msg:649(*) nsaddr# 0 (red) d.dns.br requires 4 bytes
% perl respsize.pl 13 13 2 # of NS: 4
common name, average case: msg:303 nsaddr#11 (orange) For maximum size query (255 byte):
common name, worst case: msg:495 nsaddr# 1 (red) if only A is considered: # of A is 4 (green)
uncommon name, average case: msg:457 nsaddr# 2 (orange) if A and AAAA are condered: # of A+AAAA is 3 (yellow)
uncommon name, worst case: msg:649(*) nsaddr# 0 (red) if prefer_glue A is assumed: # of A is 4, # of AAAA is 3 (yellow)
For average size query (64 byte):
if only A is considered: # of A is 4 (green)
if A and AAAA are condered: # of A+AAAA is 4 (green)
if prefer_glue A is assumed: # of A is 4, # of AAAA is 4 (green)
% perl respsize.pl ns-ext.isc.org ns.psg.com ns.ripe.net ns.eu.int
ns-ext.isc.org requires 16 bytes
ns.psg.com requires 12 bytes
ns.ripe.net requires 13 bytes
ns.eu.int requires 11 bytes
# of NS: 4
For maximum size query (255 byte):
if only A is considered: # of A is 4 (green)
if A and AAAA are condered: # of A+AAAA is 3 (yellow)
if prefer_glue A is assumed: # of A is 4, # of AAAA is 2 (yellow)
For average size query (64 byte):
if only A is considered: # of A is 4 (green)
if A and AAAA are condered: # of A+AAAA is 4 (green)
if prefer_glue A is assumed: # of A is 4, # of AAAA is 4 (green)
(Note: The response simulator program is shown in Section 5.) (Note: The response simulator program is shown in Section 5.)
Here we use the term "green" if all address records could fit, or Here we use the term "green" if all address records could fit, or
"orange" if two or more could fit, or "red" if fewer than two could fit. "orange" if two or more could fit, or "red" if fewer than two could fit.
It's clear that without a common parent for nameserver names, much space It's clear that without a common parent for nameserver names, much space
would be lost. would be lost. For these examples we use an average/common name size of
15 octets, befitting our assumption of GTLD-SERVERS.NET as our common
parent name.
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We're assuming an average query name size of 64 since that is the We're assuming an average query name size of 64 since that is the
typical average maximum size seen in trace data at the time of this typical average maximum size seen in trace data at the time of this
writing. If Internationalized Domain Name (IDN) or any other technology writing. If Internationalized Domain Name (IDN) or any other technology
which results in larger query names be deployed significantly in advance which results in larger query names be deployed significantly in advance
of EDNS, then more new measurements and new estimates will have to be of EDNS, then new measurements and new estimates will have to be made.
made.
4 - Conclusions 4 - Conclusions
4.1. The current practice of giving all nameserver names a common parent 4.1. The current practice of giving all nameserver names a common parent
(such as GTLD-SERVERS.NET or ROOT-SERVERS.NET) saves space in DNS (such as GTLD-SERVERS.NET or ROOT-SERVERS.NET) saves space in DNS
responses and allows for more nameservers to be enumerated than would responses and allows for more nameservers to be enumerated than would
otherwise be possible. (Note that in this case it is wise to serve the otherwise be possible. (Note that in this case it is wise to serve the
common parent domain's zone from the same servers that are named within common parent domain's zone from the same servers that are named within
it, in order to limit external dependencies when all your eggs are in a it, in order to limit external dependencies when all your eggs are in a
single basket.) single basket.)
4.2. Thirteen (13) seems to be the effective maximum number of 4.2. Thirteen (13) seems to be the effective maximum number of
nameserver names usable traditional (non-extended) DNS, assuming a nameserver names usable traditional (non-extended) DNS, assuming a
common parent domain name, and assuming that additional-data truncation common parent domain name, and given that response truncation is
is undesirable in the average case. undesirable as an average case, and assuming mostly IPv4-only
reachability (only A RRs exist, not AAAA RRs).
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4.3. Adding two to five IPv6 nameserver address records (AAAA RRs) to a 4.3. Adding two to five IPv6 nameserver address records (AAAA RRs) to a
prototypical delegation that currently contains thirteen (13) IPv4 prototypical delegation that currently contains thirteen (13) IPv4
nameserver addresses (A RRs) for thirteen (13) nameserver names under a nameserver addresses (A RRs) for thirteen (13) nameserver names under a
common parent, would not have a significant negative operational impact common parent, would not have a significant negative operational impact
on the domain name system. on the domain name system.
5 - Source Code 5 - Source Code
#!/usr/bin/perl -w #!/usr/bin/perl
#
$asize = 2+2+2+4+2+4; # SYNOPSIS
$aaaasize = 2+2+2+4+2+16; # repsize.pl [ -z zone ] fqdn_ns1 fqdn_ns2 ...
($nns, $na, $naaaa) = @ARGV; # if all queries are assumed to have zone suffux, such as "jp" in
test("common", "average", common_name_average($nns), # JP TLD servers, specify it in -z option
$na, $naaaa); #
test("common", "worst", common_name_worst($nns), use strict;
$na, $naaaa); use Getopt::Std;
test("uncommon", "average", uncommon_name_average($nns), my ($sz_msg) = (512);
$na, $naaaa); my ($sz_header, $sz_ptr, $sz_rr_a, $sz_rr_aaaa) = (12, 2, 16, 28);
test("uncommon", "worst", uncommon_name_worst($nns), my ($sz_type, $sz_class, $sz_ttl, $sz_rdlen) = (2, 2, 4, 2);
$na, $naaaa); my (%namedb, $name, $nssect, %opts, $optz);
exit 0; my $n_ns = 0;
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sub test { my ($namekind, $casekind, $msg, $na, $naaaa) = @_;
my $nglue = numglue($msg, $na, $naaaa);
printf "%8s name, %7s case: msg:%3d%s nsaddr#%2d (%s)\n",
$namekind, $casekind,
$msg, ($msg > 512) ? "(*)" : " ",
$nglue, ($nglue == $na + $naaaa) ? "green"
: ($nglue >= 2) ? "orange"
: "red";
}
sub pnum { my ($num, $tot) = @_; getopt('z', opts);
return sprintf "%3d%s", if (defined($opts{'z'})) {
server_name_len($opts{'z'}); # just register it
} }
sub numglue { my ($msg, $na, $naaaa) = @_; foreach $name (@ARGV) {
my $space = ($msg > 512) ? 0 : (512 - $msg); my $len;
my $num = 0; $n_ns++;
$len = server_name_len($name);
while ($space && ($na || $naaaa )) { print "$name requires $len bytes\n";
if ($na) { $nssect += $sz_ptr + $sz_type + $sz_class + $sz_ttl + $sz_rdlen + $len;
if ($space >= $asize) {
$space -= $asize;
INTERNET-DRAFT June 2003 RESPSIZE
$num++;
}
$na--;
}
if ($naaaa) {
if ($space >= $aaaasize) {
$space -= $aaaasize;
$num++;
}
$naaaa--;
}
}
return $num;
} }
print "# of NS: $n_ns\n";
arsect(255, $nssect, $n_ns, "maximum");
arsect(64, $nssect, $n_ns, "average");
sub msgsize { my ($qname, $nns, $nsns) = @_; sub server_name_len {
return 12 + # header my ($name) = @_;
$qname+2+2 + # query my (@labels, $len, $n, $suffix);
0 + # answer
$nns * (4+2+2+4+2+$nsns); # authority
}
sub average_case { my ($nns, $nsns) = @_; $name =~ tr/A-Z/a-z/;
return msgsize(64, $nns, $nsns); @labels = split(/./, $name);
$len = length(join('.', @labels)) + 2;
for ($n = 0; $#labels >= 0; $n++, shift @labels) {
$suffix = join('.', @labels);
return length($name) - length($suffix) + $sz_ptr
if (defined($namedb{$suffix}));
$namedb{$suffix} = 1;
} }
return $len;
sub worst_case { my ($nns, $nsns) = @_;
return msgsize(256, $nns, $nsns);
} }
sub common_name_average { my ($nns) = @_; sub arsect {
return 15 + average_case($nns, 2); my ($sz_query, $nssect, $n_ns, $cond) = @_;
} my ($space, $n_a, $n_a_aaaa, $n_p_aaaa, $ansect);
$ansect = $sz_query + 1 + $sz_type + $sz_class;
$space = $sz_msg - $sz_header - $ansect - $nssect;
$n_a = atmost(int($space / $sz_rr_a), $n_ns);
$n_a_aaaa = atmost(int($space / ($sz_rr_a + $sz_rr_aaaa)), $n_ns);
$n_p_aaaa = atmost(int(($space - $sz_rr_a * $n_ns) / $sz_rr_aaaa), $n_ns);
printf "For %s size query (%d byte):\n", $cond, $sz_query;
printf "if only A is considered: ";
printf "# of A is %d (%s)\n", $n_a, &judge($n_a, $n_ns);
printf "if A and AAAA are condered: ";
printf "# of A+AAAA is %d (%s)\n", $n_a_aaaa, &judge($n_a_aaaa, $n_ns);
INTERNET-DRAFT July 2005 RESPSIZE
sub common_name_worst { my ($nns) = @_; printf "if prefer_glue A is assumed: ";
return 15 + worst_case($nns, 2); printf "# of A is %d, # of AAAA is %d (%s)\n",
$n_a, $n_p_aaaa, &judge($n_p_aaaa, $n_ns);
} }
sub uncommon_name_average { my ($nns) = @_; sub judge {
return average_case($nns, 15); my ($n, $n_ns) = @_;
return "green" if ($n >= $n_ns);
return "yellow" if ($n >= 2);
return "orange" if ($n == 1);
return "red";
} }
sub uncommon_name_worst { my ($nns) = @_; sub atmost {
return worst_case($nns, 15); my ($a, $b) = @_;
return 0 if ($a < 0);
return $b if ($a > $b);
return $a;
} }
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Security Considerations Security Considerations
The recommendations contained in this document have no known security The recommendations contained in this document have no known security
implications. implications.
IANA Considerations IANA Considerations
This document does not call for changes or additions to any IANA This document does not call for changes or additions to any IANA
registry. registry.
IPR Statement IPR Statement
Copyright (C) The Internet Society (2003-2004). This document is Copyright (C) The Internet Society (2005). This document is subject to
subject to the rights, licenses and restrictions contained in BCP 78, the rights, licenses and restrictions contained in BCP 78, and except as
and except as set forth therein, the authors retain all their rights. set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Authors' Addresses Authors' Addresses
Paul Vixie Paul Vixie
950 Charter Street 950 Charter Street
Redwood City, CA 94063 Redwood City, CA 94063
+1 650 423 1301 +1 650 423 1301
vixie@isc.org vixie@isc.org
Akira Kato Akira Kato
University of Tokyo, Information Technology Center University of Tokyo, Information Technology Center
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