Subject: Updates to the rfc2052bis

From: "Levon Esibov (Exchange)" <[email-masked]>
Date: Tue, 12 Oct 1999 16:32:15 -0700

I'd like to encourage the working group to attempt to get the rfc2052bis
published on the standard track (PS). 

To speed up the process I volunteered to updated the draft (attached). If
authors (Paul and Arnt) don't mind, I'd like to ask them to update the
draft, since it expired in June. Hopefully with the changes discussed at the
IETF meeting in Oslo and on this mailing list we'll be able to move on.

 <<rfc2052bis_updated.txt>> 
The changes that I incorporated address all the issues identified in Robert
Elz's e-mail of July 15. I also included the feedback that I received in
response to my message of 8/27/99.

Thanks,
Levon

Below I provide the complete list of the changes.

I. In the Abstract
Remove
	(like a more general form of MX)

II. In the Overview and rationale
Remove
	This has led to, for example, ftp.whatever.com aliases [RFC 2219],
the SMTP-specific MX RR, and using MAC-level broadcasts to locate servers. 

III. In the Introductory example
Replace
	If a SRV-cognizant web-browser wants to retrieve 
		http://www.example.com/ 
	it does a lookup of 
		_http._tcp.www.example.com
	and retrieves the document from one of the servers in the reply. The
example zone file near the end of this memo contains answering RRs for this
query. 
with the following text:
	If a SRV-cognizant LDAP client wants to  identify a LDAP server that
supports TCP protocol and provides LDAP service for the domain example.com.,
it does a lookup of

	      _ldap._tcp.example.com

	as described in [ARM]. The example zone file near the end of this
memo contains answering RRs  for an SRV query.

IV. In the Definitions
Replace
	BCP 14
with
	[BCP 14]

V. In the format of the SRV records
a) Delete in the Priority description
	As for MX, 

b) Replace in the Priority description
	16 bit binary integer 
with
	16 bit unsigned integer 

c) Replace in the Weight description
	A load balancing mechanism. When selecting a target host among the
those that have the same priority, the chance of trying this one first
SHOULD be proportional to its weight, as specified below. Larger weights
lead to a higher probability of being selected. The range of this number is
0-65535. This is a 16 bit binary integer in network byte order. Domain
administrators are urged to use Weight 0 when there isn't any load balancing
to do, to make the RR easier to read for humans (less noisy). In the
presence records containing weights greater than 0, records with weight 0
have a very small chance of being selected. To choose the target, the client
SHOULD implement the effect of this algorithm. This permits administrators
to plan weights to achieve the load distribution desired. Each time a target
is needed, the client should order the remaining (not previously used) SRV
RRs at the current priority in any random fashion, except placing all those
with weight 0 at the beginning of the list. Compute the sum of the weights
of those RRs, and with each RR associate the running sum in the selected
order. Then choose a random number (not necessarily of integral value)
between 0 and the sum computed (inclusive), and select the RR whose running
sum value is the first in the selected order which is greater than or equal
to the random number selected. 
with 
	A server selection mechanism.  The weight field specifies a relative
weight for entries with the same priority. Larger weights SHOULD be given a
proportionately higher probability of being selected. The range of this
number is 0-65535.  This is a 16 bit unsigned integer in network byte order.
Domain administrators SHOULD use Weight 0 when there isn't any server
selection to do, to make the RR easier to read for humans (less noisy).  In
the presence records containing weights greater than 0, records with weight
0 have a very small chance of being selected.
In the absence of a protocol whose specification calls for the use of other
weighting information, implementations SHOULD implement the effect of the
following algorithm:  Each time a target is needed, the client orders the
remaining (not previously used) SRV RRs at the current priority in any
order, except that all those with weight 0 are placed at the 	beginning of
the list.  Compute the sum of the weights of those RRs, and with each RR
associate the running sum in the selected order.  Then choose a uniform
random number between 0 and the sum computed (inclusive), and select the RR
whose running sum value is the first in the selected order which is greater
than or equal to the random number selected.

d) Replace in the Port description
	16 bit binary integer 
with
	16 bit unsigned integer 

e) Delete in the Target description
	As for MX, 

VI. In the Applicability statement
Replace 
	In general, it is expected that SRV records will be used by clients
for applications where the relevant protocol specification indicates that
clients should use the SRV record. The examples in this document use
familiar protocols as an aid in understanding. It is not intended that those
protocols will necessarily use SRV records.
with
	In general, it is expected that SRV records will be used by clients
for applications where the relevant protocol specification indicates that
clients should use the SRV record. Such specification MUST define the
symbolic name to be used in Service field of the SRV record as described
above. It 	also MUST include security considerations. Service SRV
records SHOULD NOT be used in the absence of such specification.

VII. In the Domain administrator advice
Replace
	("_telnet._tcp.example.com" for instance); 
with
	("_ldap._tcp.example.com" for instance); 

VIII. In The "Weight" field
a) Replace
	Weight, the load balancing field, is not quite satisfactory, but 
with
	Weight, the server selection field, is not quite satisfactory, but 

b) Remove
	like SMTP

c) Remove
	like telnet

d) Add
	Note: There are currently various experiments at providing relative
network proximity estimation, available bandwidth estimation, and similar
services.  Use of the SRV record with such facilities, and in particular the
interpretation of the Weight field when these facilities are used, is for
further study.
Weight is only intended for static, not dynamic, server selection. Using SRV
weight for dynamic server selection would require assigning unreasonably
short TTLs to the SRV RRs, which would limit the usefulness of the DNS
caching mechanism, thus increasing overall network load and decreasing
overall reliability.  Server selection via SRV is only intended to express
static information such as "this server has a faster CPU than that one" or
"this server has a much better network connection than that one".

IX. In the Usage rules
Remove
	else if the service desired is SMTP (and SMTP has been defined
elsewhere to expect SRV lookups)
		skip to RFC 974 (MX).

X. In the Notes
Remove
	- A client MAY use means other than Weight to choose among target
hosts with equal Priority. 

XI. In the Fictional example
a) Add at the beginning
	This example uses fictional service "foobar" as an aid in
understanding SRV records. If ever service "foobar" is implemented, it is
not intended that it will necessarily use SRV records.   

b) Replace
	$ORIGIN example.com.
        @               SOA server.example.com. root.example.com. (
                            1995032001 3600 3600 604800 86400 )
                        NS  server.example.com.
                        NS  ns1.ip-provider.net.
                        NS  ns2.ip-provider.net.
        _ftp._tcp       SRV 0 0 21 server.example.com.
        _finger._tcp    SRV 0 0 79 server.example.com.
        ; telnet - use old-slow-box or new-fast-box if either is
        ; available, make three quarters of the logins go to
        ; new-fast-box.
        _telnet._tcp    SRV 0 1 23 old-slow-box.example.com.
                        SRV 0 3 23 new-fast-box.example.com.
        ; if neither old-slow-box or new-fast-box is up, switch to
        ; using the sysdmin's box and the server
                        SRV 1 0 23 sysadmins-box.example.com.
                        SRV 1 0 23 server.example.com.
        ; HTTP - server is the main server, new-fast-box is the backup
        ; (On new-fast-box, the HTTP daemon runs on port 8000)
        _http._tcp      SRV 0 0 80 server.example.com.
                        SRV 10 0 8000 new-fast-box.example.com.
        ; since we want to support both http://example.com/ and
        ; http://www.example.com/ we need the next two RRs as well
        _http._tcp.www  SRV 0 0 80 server.example.com.
                        SRV 10 0 8000 new-fast-box.example.com.
        ; SMTP - mail goes to the server, and to the IP provider if
        ; the net is down
        _smtp._tcp      SRV 0 0 25 server.example.com.
                        SRV 1 0 25 mailhost.ip-provider.net.
        @               MX  0 server.example.com.
                        MX  1 mailhost.ip-provider.net.
        ; NNTP - use the IP provider's NNTP server
        _nntp._tcp      SRV 0 0 119 nntphost.ip-provider.net.
        ; IDB is an locally defined protocol
        _idb._tcp SRV  0 0 2025 new-fast-box.example.com.
        ; addresses
        server          A   172.30.79.10
        old-slow-box    A   172.30.79.11
        sysadmins-box   A   172.30.79.12
        new-fast-box    A   172.30.79.13
        ; backup address records - new-fast-box and old-slow-box are
        ; included, naturally, and server is too, but might go
        ; if the load got too bad
        @               A   172.30.79.10
                        A   172.30.79.11
                        A   172.30.79.13
        ; backup address record for www.example.com
        www             A       172.30.79.10
        ; NO other services are supported
        *._tcp         SRV  0 0 0 .
        *._udp         SRV  0 0 0 .

with
	        $ORIGIN example.com.
        @               SOA server.example.com. root.example.com. (
                            1995032001 3600 3600 604800 86400 )
                        NS  server.example.com.
                        NS  ns1.ip-provider.net.
                        NS  ns2.ip-provider.net.
        ; foobar - use old-slow-box or new-fast-box if either is
        ; available, make three quarters of the logins go to
        ; new-fast-box.
        _ foobar._tcp    SRV 0 1 9 old-slow-box.example.com.
                        SRV 0 3 9 new-fast-box.example.com.
        ; if neither old-slow-box or new-fast-box is up, switch to
        ; using the sysadmin's box and the server
                        SRV 1 0 9 sysadmins-box.example.com.
                        SRV 1 0 9 server.example.com.
        server          A   172.30.79.10
        old-slow-box    A   172.30.79.11
        sysadmins-box   A   172.30.79.12
        new-fast-box    A   172.30.79.13
        ; NO other services are supported
        *._tcp         SRV  0 0 0 .
        *._udp         SRV  0 0 0 .

c) Replace
	In this example, a telnet connection to "example.com." needs an SRV
lookup of "_telnet._tcp.example.com." and 
with
	In this example, a client of the "foobar" service in the
"example.com." domain needs an SRV lookup of "_foobar._tcp.example.com." and
...

d) Replace
	"_telnet._tcp.example.com."
with
	"_foobar._tcp.example.com."

XII. In the Changes from RFC 2052
add at the end
	Among such modification, this specification clarifies the use of the
Weight field of the SRV records.

XIII. In the Security Considerations
a) replace
	The authors believes
with
	The authors believe 

b) remove
	(as, indeed, some sites become unwilling secondary MXes today). 

XIV. In the References
add
   BCP 14: Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
   ARM: Armijo, M., Esibov, L., Leach, P., "Discovering LDAP Services with
DNS", Internet Draft draft-armijo-ldap-locate-00.txt, June 1999.
     Altman, J., "Distributing Kerberos KDC and Realm Information with DNS",
Internet Draft draft-ietf-cat-krb-dns-locate-00.txt, June 1999.

Although on some issues we didn't reach a consensus, I've tried to
accommodate requests/wishes/comments to satisfy the majority.

Here is the list of controversial once:
1. I kept the following statement in the text:
"it is expected that SRV records will be used by clients for applications
where the relevant protocol specification indicates that clients should use
the SRV record...  ...Service SRV records SHOULD NOT be used in the absence
of such specification."

Jeffrey Altman opposed this statement with the following arguments:
a) It puts extra work on the Applications Area Directors
b) this will not prevent anyone from using SRV records.

In response, Thomas Narten indicated that
a) the Applications Area Directors requested "Service SRV records SHOULD NOT
be used in the absence of such specification" statement themselves.
b) of course this statement will not prevent anybody from using SRV records
for some services without specifying it, but it will at least prevent them
from saying that they follow "standard" behavior.

------_=_NextPart_000_01BF150A.08B43ACE
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	name="rfc2052bis_updated.txt"
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Applications Area                                       Arnt =
Gulbrandsen
INTERNET-DRAFT                                        Troll =
Technologies
                         				      Paul Vixie
Obsoletes: RFC 2052                         Internet Software =
Consortium
                                                            October =
1999

       A DNS RR for specifying the location of services (DNS SRV)

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six =
months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

Abstract

   This document describes a DNS RR which specifies the location of the
   server(s) for a specific protocol and domain.

Overview and rationale

   Currently, one must either know the exact address of a server to
   contact it, or broadcast a question. =20

   The SRV RR allows administrators to use several servers for a single
   domain, to move services from host to host with little fuss, and to
   designate some hosts as primary servers for a service and others as
   backups.

   Clients ask for a specific service/protocol for a specific domain
   (the word domain is used here in the strict RFC 1034 sense), and get
   back the names of any available servers.

   Note that where this document refers to "address records", it means =
A
   RR's, AAAA RR's, or their most modern equivalent.

Gulbrandsen and Vixie           Proposed                        [Page =
1]

RFC 2052bis                    DNS SRV RR                   October =
1999

Introductory example

   If a SRV-cognizant LDAP client wants to discover a LDAP server that
   supports TCP protocol and provides LDAP service for the domain
   example.com., it does a lookup of

      _ldap._tcp.example.com

   as described in [ARM].  The example zone file near the end of this
   memo contains answering RRs for an SRV query.

Definitions

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
   used in this document are to be interpreted as specified in [BCP =
14].
   Other terms used in this document are defined in the DNS
   specification, RFC 1034.

The format of the SRV RR

   Here is the format of the SRV RR, whose DNS type code is 33:

        _Service._Proto.Name TTL Class SRV Priority Weight Port Target

        (There is an example near the end of this document.)

   Service
        The symbolic name of the desired service, as defined in =
Assigned
        Numbers [STD 2] or locally.  An underscore (_) is prepended to
        the service identifier to avoid collisions with DNS labels that
        occur in nature.

        Some widely used services, notably POP, don't have a single
        universal name.  If Assigned Numbers names the service
        indicated, that name is the only name which is legal for SRV
        lookups.  Only locally defined services may be named locally.
        The Service is case insensitive.

   Proto
        The symbolic name of the desired protocol, with an underscore
        (_) prepended to prevent collisions with DNS labels that occur
        in nature.  _TCP and _UDP are at present the most useful values
        for this field, though any name defined by Assigned Numbers or
        locally may be used (as for Service).  The Proto is case
        insensitive.

Gulbrandsen and Vixie           Proposed                        [Page =
2]

RFC 2052bis                    DNS SRV RR                   October =
1999

   Name
        The domain this RR refers to.  The SRV RR is unique in that the
        name one searches for is not this name; the example near the =
end
        shows this clearly.

   TTL
        Standard DNS meaning [RFC 1035].

   Class
        Standard DNS meaning [RFC 1035].   SRV records occur in the IN
        Class.

   Priority
        The priority of this target host.  A client MUST
        attempt to contact the target host with the lowest-numbered
        priority it can reach; target hosts with the same priority
        SHOULD be tried in an order defined by the weight field.  The
        range is 0-65535.  This is a 16 bit unsigned integer in network
        byte order.

   Weight
	A server selection mechanism.  The weight field specifies a
	relative weight for entries with the same priority. Larger
	weights SHOULD be given a proportionately higher probability of
	being selected. The range of this number is 0-65535.  This is a
	16 bit unsigned integer in network byte order.  Domain
	administrators SHOULD use Weight 0 when there isn't any server
	selection to do, to make the RR easier to read for humans (less
	noisy).  In the presence records containing weights greater than
	0, records with weight 0 have a very small chance of being
	selected.

	In the absence of a protocol whose specification calls for the
	use of other weighting information, implementations SHOULD
	implement the effect of the following algorithm:  Each time a
	target is needed, the client orders the remaining (not previously
	used) SRV RRs at the current priority in any order, except that
	all those with weight 0 are placed at the beginning of the list.
	Compute the sum of the weights of those RRs, and with each RR
	associate the running sum in the selected order.  Then choose a
	uniform random number between 0 and the sum computed (inclusive),
	and select the RR whose running sum value is the first in the
	selected order which is greater than or equal to the random
	number selected.

       =20

Gulbrandsen and Vixie           Proposed                        [Page =
3]

RFC 2052bis                    DNS SRV RR                   October =
1999

   Port
        The port on this target host of this service.  The range is
        0-65535.  This is a 16 bit unsigned integer in network byte
	order. This is often as specified in Assigned Numbers but need
	not be.

   Target
        The domain name of the target host.  There MUST be
        one or more address records for this name, the name MUST NOT be
        an alias (in the sense of RFC 1034 or RFC 2181).  Implementors
        are urged, but not required, to return the address record(s) in
        the Additional Data section.  Unless and until permitted by
        future standards action, name compression is not to be used for
        this field.

        A Target of "." means that the service is decidedly not
        available at this domain.

Applicability Statement

   In general, it is expected that SRV records will be used by clients
   for applications where the relevant protocol specification indicates
   that clients should use the SRV record. Such specification MUST
   define the symbolic name to be used in Service field of the SRV
   record as described above. It also MUST include security
   considerations. Service SRV records SHOULD NOT be used in the =
absence
   of such specification.

Domain administrator advice

   Expecting everyone to update their client applications when the =
first
   server publishes a SRV RR is futile (even if desirable).  Therefore
   SRV would have to coexist with address record lookups for existing
   protocols, and DNS administrators should try to provide address
   records to support old clients:

      - Where the services for a single domain are spread over several
        hosts, it seems advisable to have a list of address records at
        the same DNS node as the SRV RR, listing reasonable (if perhaps
        suboptimal) fallback hosts for Telnet, NNTP and other protocols
        likely to be used with this name.  Note that some programs only
        try the first address they get back from e.g. gethostbyname(),
        and we don't know how widespread this behavior is.

      - Where one service is provided by several hosts, one can either
        provide address records for all the hosts (in which case the
        round-robin mechanism, where available, will share the load
        equally) or just for one (presumably the fastest).

Gulbrandsen and Vixie           Proposed                        [Page =
4]

RFC 2052bis                    DNS SRV RR                   October =
1999

      - If a host is intended to provide a service only when the main
        server(s) is/are down, it probably shouldn't be listed in
        address records.

      - Hosts that are referenced by backup address records must use =
the
        port number specified in Assigned Numbers for the service.

      - Designers of future protocols for which "secondary servers" is
        not useful (or meaningful) may choose to not use SRV's support
        for secondary servers.  Clients for such protocols may use or
        ignore SRV RRs with Priority higher than the RR with the lowest
        Priority for a domain.

   Currently there's a practical limit of 512 bytes for DNS replies.
   Until all resolvers can handle larger responses, domain
   administrators are strongly advised to keep their SRV replies below
   512 bytes.

   All round numbers, wrote Dr. Johnson, are false, and these numbers
   are very round: A reply packet has a 30-byte overhead plus the name
   of the service ("_ldap._tcp.example.com" for instance); each SRV RR
   adds 20 bytes plus the name of the target host; each NS RR in the NS
   section is 15 bytes plus the name of the name server host; and
   finally each A RR in the additional data section is 20 bytes or so,
   and there are A's for each SRV and NS RR mentioned in the answer.
   This size estimate is extremely crude, but shouldn't underestimate
   the actual answer size by much.  If an answer may be close to the
   limit, using a DNS query tool (e.g. "dig") to look at the actual
   answer is a good idea.

The "Weight" field

   Weight, the server selection field, is not quite satisfactory, but =
the
   actual load on typical servers changes much too quickly to be kept
   around in DNS caches.  It seems to the authors that offering
   administrators a way to say "this machine is three times as fast as
   that one" is the best that can practically be done.

   The only way the authors can see of getting a "better" load figure =
is
   asking a separate server when the client selects a server and
   contacts it.  For short-lived services an extra step in the
   connection establishment seems too expensive, and for long-lived
   services, the load figure may well be thrown off a minute
   after the connection is established when someone else starts or
   finishes a heavy job.

   Note: There are currently various experiments at providing relative
   network proximity estimation, available bandwidth estimation, and

Gulbrandsen and Vixie           Proposed                        [Page =
5]

RFC 2052bis                    DNS SRV RR                   October =
1999

   similar services.  Use of the SRV record with such facilities, and =
in
   particular the interpretation of the Weight field when these
   facilities are used, is for further study.
   Weight is only intended for static, not dynamic, server selection.
   Using SRV weight for dynamic server selection would require =
assigning
   unreasonably short TTLs to the SRV RRs, which would limit the
   usefulness of the DNS caching mechanism, thus increasing overall
   network load and decreasing overall reliability.  Server selection
   via SRV is only intended to express static information such as "this
   server has a faster CPU than that one" or "this server has a much
   better network connection than that one".

The Port number

   Currently, the translation from service name to port number happens
   at the client, often using a file such as /etc/services.

   Moving this information to the DNS makes it less necessary to update
   these files on every single computer of the net every time a new
   service is added, and makes it possible to move standard services =
out
   of the "root-only" port range on unix.

Usage rules

   A SRV-cognizant client SHOULD use this procedure to locate a list of
   servers and connect to the preferred one:

        Do a lookup for QNAME=3D_service._protocol.target, QCLASS=3DIN,
        QTYPE=3DSRV.

        If the reply is NOERROR, ANCOUNT>0 and there is at least one =
SRV
        RR which specifies the requested Service and Protocol in the
        reply:

             If there is precisely one SRV RR, and its Target is "."
             (the root domain), abort.

             Else, for all such RR's, build a list of (Priority, =
Weight,
             Target) tuples

             Sort the list by priority (lowest number first)

             Create a new empty list

             For each distinct priority level
                  While there are still elements left at this priority
                  level

Gulbrandsen and Vixie           Proposed                        [Page =
6]

RFC 2052bis                    DNS SRV RR                   October =
1999

                       Select an element randomly, with probability
                       Weight, as specified above, and move it to the
                       tail of the new list

             For each element in the new list

                  query the DNS for address records for the Target or
                  use any such records found in the Additional Data
                  section of the earlier SRV response.

                  for each address record found, try to connect to the
                  (protocol, address, service).

        else

             Do a lookup for QNAME=3Dtarget, QCLASS=3DIN, QTYPE=3DA

             for each address record found, try to connect to the
             (protocol, address, service)

   Notes:

      - Port numbers SHOULD NOT be used in place of the symbolic =
service
        or protocol names (for the same reason why variant names cannot
        be allowed: Applications would have to do two or more lookups).

      - If a truncated response comes back from an SRV query, the rules
        described in [RFC2181] shall apply.

      - A client MUST parse all of the RR's in the reply.

      - If the Additional Data section doesn't contain address records
        for all the SRV RR's and the client may want to connect to the
        target host(s) involved, the client MUST look up the address
        record(s).  (This happens quite often when the address record
        has shorter TTL than the SRV or NS RR's.)

      - Future protocols could be designed to use SRV RR lookups as the
        means by which clients locate their servers.

Fictional example

   This example uses fictional service =93foobar=94 as an aid in
   understanding SRV records. If ever service =93foobar=94 is =
implemented,
   it is not intended that it will necessarily use SRV records.  This
   is (part of) the zone file for example.com, a still-unused domain:

Gulbrandsen and Vixie           Proposed                        [Page =
7]

RFC 2052bis                    DNS SRV RR                   October =
1999

        $ORIGIN example.com.
        @               SOA server.example.com. root.example.com. (
                            1995032001 3600 3600 604800 86400 )
                        NS  server.example.com.
                        NS  ns1.ip-provider.net.
                        NS  ns2.ip-provider.net.
        ; foobar - use old-slow-box or new-fast-box if either is
        ; available, make three quarters of the logins go to
        ; new-fast-box.
        _ foobar._tcp    SRV 0 1 9 old-slow-box.example.com.
                         SRV 0 3 9 new-fast-box.example.com.
        ; if neither old-slow-box or new-fast-box is up, switch to
        ; using the sysdmin's box and the server
                         SRV 1 0 9 sysadmins-box.example.com.
                         SRV 1 0 9 server.example.com.
        server           A   172.30.79.10
        old-slow-box     A   172.30.79.11
        sysadmins-box    A   172.30.79.12
        new-fast-box     A   172.30.79.13
        ; NO other services are supported
        *._tcp          SRV  0 0 0 .
        *._udp          SRV  0 0 0 .

   In this example, a client of the =93foobar=94 service in the
   "example.com." domain needs an SRV lookup of
   "_foobar._tcp.example.com." and possibly A lookups of "new-
   fast-box.example.com." and/or the other hosts named.  The size of =
the
   SRV reply is approximately 365 bytes:

      30 bytes general overhead
      20 bytes for the query string, "_foobar._tcp.example.com."
      130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new-
        fast-box", "old-slow-box", "server" and "sysadmins-box" -
        "example.com" in the query section is quoted here and doesn't
        need to be counted again.
      75 bytes for 3 NS RRs, 15 bytes each plus the lengths of =
"server",
        "ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is
        quoted and only needs to be counted once.
      120 bytes for the 6 address records (assuming IPv4 only) =
mentioned
        by the SRV and NS RR's.

IANA Considerations

   The IANA has assigned RR type value 33 to the SRV RR.  No other IANA
   services are required by this document.

Gulbrandsen and Vixie           Proposed                        [Page =
8]

RFC 2052bis                    DNS SRV RR                   October =
1999

Changes from RFC 2052

   This document obsoletes RFC 2052.   The major change from that
   previous, experimental, version of this specification is that now =
the
   protocol and service labels are prepended with an underscore, to
   lower the probability of an accidental clash with a similar name =
used
   for unrelated purposes.  Aside from that, changes are only intended
   to increase the clarity and completeness of the document. This
   document especially clarifies the use of the Weight field of the SRV
   records.

Security Considerations

   The authors believe this RR to not cause any new security problems.
   Some problems become more visible, though.

      - The ability to specify ports on a fine-grained basis obviously
        changes how a router can filter packets.  It becomes impossible
        to block internal clients from accessing specific external
        services, slightly harder to block internal users from running
        unauthorized services, and more important for the router
        operations and DNS operations personnel to cooperate.

      - There is no way a site can keep its hosts from being referenced
        as servers.  This could lead to denial of service.

      - With SRV, DNS spoofers can supply false port numbers, as well =
as
        host names and addresses.   Because this vunerability exists
        already, with names and addresses, this is not a new
        vunerability, merely a slightly extended one, with little
        practical effect.

References

   STD 2: Reynolds, J., Postel, J., "Assigned Numbers", STD 2, RFC =
1700,
        October 1994 (as currently updated by the IANA).

   RFC 1034: Mockapetris, P., "Domain names - concepts and facilities",
        STD 13, RFC 1034, November 1987.

   RFC 1035: Mockapetris, P., "Domain names - Implementation and
        Specification", STD 13, RFC 1035, November 1987.

   RFC 974: Partridge, C., "Mail routing and the domain system", RFC
        974, January 1986.

   BCP 14: Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", BCP 14, RFC 2119, March 1997.

Gulbrandsen and Vixie           Proposed                        [Page =
9]

RFC 2052bis                    DNS SRV RR                   October =
1999

   RFC 2181: Elz, R., Bush, R., "Clarifications to the DNS
        Specification", RFC 2181, July 1997

   RFC 2219: Hamilton, M., Wright, R., "Use of DNS Aliases for Network
        Services", BCP 17, RFC 2219, October 1997
   BCP 14: Bradner, S., =93Key words for use in RFCs to Indicate =
Requirement
	Levels=94, RFC 2119, March 1997.
   ARM: Armijo, M., Esibov, L., Leach, P., =93Discovering LDAP Services
	with DNS=94, Internet Draft draft-armijo-ldap-locate-00.txt,
	June 1999.
   Altman, J., =93Distributing Kerberos KDC and Realm Information with
	DNS=94, Internet Draft draft-ietf-cat-krb-dns-locate-00.txt,=20
	June 1999.

Acknowledgements

   The algorithm used to select from the weighted SRV RRs of equal
   priority is adapted from one supplied by Dan Bernstein.

Authors' Addresses

   Arnt Gulbrandsen              Paul Vixie
      Troll Tech                    Internet Software Consortium
      Postboks 6133 Etterstad            950 Charter Street
      N-0602 Oslo, Norway                Redwood City, CA 94063
      +47 22646966                       +1 650 779 7001
                      =20

Gulbrandsen and Vixie           Proposed                       [Page =
10]

---

From: Paul A Vixie <[email-masked]>
Date: Wed, 13 Oct 1999 10:10:58 -0700

so you really want the _ removed and kept removed?

---

From: "Matt Crawford" <[email-masked]>
Date: Wed, 13 Oct 1999 15:41:31 -0500

> so you really want the _ removed and kept removed?

I say: Huh?  I re-read the referenced message and see nothing about
removing the underscores!  There's one place where an unintended
space crept in after an underscore: