AFT Working Group M. Leech
INTERNET DRAFT
Expire in six months M. Ganis
International Business Machines
Y. Lee
NEC Systems Laboratory
R. Kuris
Unify Corporation
D. Koblas
Independent Consultant
L. Jones
Hewlett-Packard Company
D. Blob
NEC USA
W. Lu
NEC USA
March 1997
SOCKS Protocol Version 5
<draft-ietf-aft-socks-pro-v5-00.txt>
Status of this Memo
This document is a submission to the IETF Authenticated Firewall Traversal
(AFT) Working Group. Comments are solicited and should be addressed
to the working group mailing list (aft@unify.com) or to the editor.
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
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Internet-Drafts draft documents are valid for a maximum of six months
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To learn the current status of any Internet-Draft, please check the
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ftp.isi.edu (US West Coast).
Distribution of this memo is unlimited
Acknowledgments
This memo describes a protocol that is an evolution of the previous
version of the protocol, version 4 [1]. This new protocol stems from
active discussions and prototype implementations. The key
contributors are: Marcus Leech: Bell-Northern Research, David Koblas:
Independent Consultant, Ying-Da Lee: NEC Systems Laboratory, LaMont
Jones: Hewlett-Packard Company, Ron Kuris: Unify Corporation, Matt
Ganis: International Business Machines, David Blob: NEC USA, Wei Lu:
NEC USA.
1. Introduction
The use of network firewalls, systems that effectively isolate an
organizations internal network structure from an exterior network,
such as the INTERNET is becoming increasingly popular. These
firewall systems typically act as application-layer gateways between
networks, usually offering controlled TELNET, FTP, and SMTP access.
With the emergence of more sophisticated application layer protocols
designed to facilitate global information discovery, there exists a
need to provide a general framework for these protocols to
transparently and securely traverse a firewall.
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There exists, also, a need for strong authentication of such
traversal in as fine-grained a manner as is practical. This
requirement stems from the realization that client-server
relationships emerge between the networks of various organizations,
and that such relationships need to be controlled and often strongly
authenticated.
The protocol described here is designed to provide a framework for
client-server applications in both the TCP and UDP domains to
conveniently and securely use the services of a network firewall.
The protocol is conceptually a "shim-layer" between the application
layer and the transport layer, and as such does not provide network-
layer gateway services, such as forwarding of ICMP messages.
2. Existing practice
There currently exists a protocol, SOCKS Version 4, that provides for
unsecured firewall traversal for TCP-based client-server
applications, including TELNET, FTP and the popular information-
discovery protocols such as HTTP, WAIS and GOPHER.
This new protocol extends the SOCKS Version 4 model to include UDP,
and extends the framework to include provisions for generalized
strong authentication schemes, and extends the addressing scheme to
encompass domain-name and V6 IP addresses.
The implementation of the SOCKS protocol typically involves the
recompilation or relinking of TCP-based client applications to use
the appropriate encapsulation routines in the SOCKS library.
Note:
Unless otherwise noted, the decimal numbers appearing in packet-
format diagrams represent the length of the corresponding field, in
octets. Where a given octet must take on a specific value, the
syntax X'hh' is used to denote the value of the single octet in that
field. When the word 'Variable' is used, it indicates that the
corresponding field has a variable length defined either by an
associated (one or two octet) length field, or by a data type field.
3. Procedure for TCP-based clients
When a TCP-based client wishes to establish a connection to an object
that is reachable only via a firewall (such determination is left up
to the implementation), it must open a TCP connection to the
appropriate SOCKS port on the SOCKS server system. The SOCKS service
is conventionally located on TCP port 1080. If the connection
request succeeds, the client enters a negotiation for the
authentication method to be used, authenticates with the chosen
method, then sends a relay request. The SOCKS server evaluates the
request, and either establishes the appropriate connection or denies
it.
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The client connects to the server, and sends a version
identifier/method selection message:
+----+----------+----------+
|VER | NMETHODS | METHODS |
+----+----------+----------+
| 1 | 1 | 1 to 255 |
+----+----------+----------+
The VER field is set to X'05' for this version of the protocol. The
NMETHODS field contains the number of method identifier octets that
appear in the METHODS field.
The server selects from one of the methods given in METHODS, and
sends a METHOD selection message:
+----+--------+
|VER | METHOD |
+----+--------+
| 1 | 1 |
+----+--------+
If the selected METHOD is X'FF', none of the methods listed by the
client are acceptable, and the client MUST close the connection.
The values currently defined for METHOD are:
o X'00' NO AUTHENTICATION REQUIRED
o X'01' GSSAPI
o X'02' USERNAME/PASSWORD
o X'03' to X'7F' IANA ASSIGNED
o X'80' to X'FE' RESERVED FOR PRIVATE METHODS
o X'FF' NO ACCEPTABLE METHODS
The client and server then enter a method-specific sub-negotiation.
Descriptions of the method-dependent sub-negotiations appear in
separate memos.
Developers of new METHOD support for this protocol should contact
IANA for a METHOD number. The ASSIGNED NUMBERS document should be
referred to for a current list of METHOD numbers and their
corresponding protocols.
Compliant implementations MUST support GSSAPI and SHOULD support
USERNAME/PASSWORD authentication methods.
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4. Requests
Once the method-dependent subnegotiation has completed, the client
sends the request details. If the negotiated method includes
encapsulation for purposes of integrity checking and/or
confidentiality, these requests MUST be encapsulated in the method-
dependent encapsulation.
The SOCKS request is formed as follows:
+----+-----+------+------+----------+----------+
|VER | CMD | FLAG | ATYP | DST.ADDR | DST.PORT |
+----+-----+------+------+----------+----------+
| 1 | 1 | 1 | 1 | Variable | 2 |
+----+-----+------+------+----------+----------+
Where:
o VER protocol version: X'05'
o CMD
o CONNECT X'01'
o BIND X'02'
o UDP ASSOCIATE X'03'
o FLAG command dependent flag
o ATYP address type of following address
o IP V4 address: X'01'
o DOMAINNAME: X'03'
o IP V6 address: X'04'
o DST.ADDR desired destination address
o DST.PORT desired destination port in network octet
order
The SOCKS server will typically evaluate the request based on source
and destination addresses, and return one or more reply messages, as
appropriate for the request type.
5. Addressing
In an address field (DST.ADDR, BND.ADDR), the ATYP field specifies
the type of address contained within the field:
o X'01'
the address is a version-4 IP address, with a length of 4 octets
o X'03'
the address field contains a fully-qualified domain name. The first
octet of the address field contains the number of octets of name that
follow, there is no terminating NUL octet.
o X'04'
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the address is a version-6 IP address, with a length of 16 octets.
6. Replies
The SOCKS request information is sent by the client as soon as it has
established a connection to the SOCKS server, and completed the
authentication negotiations. The server evaluates the request, and
returns a reply formed as follows:
+----+-----+------+------+----------+----------+
|VER | REP | FLAG | ATYP | BND.ADDR | BND.PORT |
+----+-----+------+------+----------+----------+
| 1 | 1 | 1 | 1 | Variable | 2 |
+----+-----+------+------+----------+----------+
Where:
o VER protocol version: X'05'
o REP Reply field:
o X'00' succeeded
o X'01' general SOCKS server failure
o X'02' connection not allowed by ruleset
o X'03' Network unreachable
o X'04' Host unreachable
o X'05' Connection refused
o X'06' TTL expired
o X'07' Command not supported
o X'08' Address type not supported
o X'09' to X'FF' unassigned
o FLAG command dependent flag
o ATYP address type of following address
o IP V4 address: X'01'
o DOMAINNAME: X'03'
o IP V6 address: X'04'
o BND.ADDR server bound address
o BND.PORT server bound port in network octet order
Fields marked RESERVED (RSV) must be set to X'00'.
If the chosen method includes encapsulation for purposes of
authentication, integrity and/or confidentiality, the replies are
encapsulated in the method-dependent encapsulation.
CONNECT
In the reply to a CONNECT, BND.PORT contains the port number that the
server assigned to connect to the target host, while BND.ADDR
contains the associated IP address. The supplied BND.ADDR is often
different from the IP address that the client uses to reach the SOCKS
server, since such servers are often multi-homed. It is expected
that the SOCKS server will use DST.ADDR and DST.PORT, and the
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client-side source address and port in evaluating the CONNECT
request.
BIND
The BIND request is used in protocols which require the client to
accept connections from the server. FTP is a well-known example,
which uses the primary client-to-server connection for commands and
status reports, but may use a server-to-client connection for
transferring data on demand (e.g. LS, GET, PUT).
It is expected that the client side of an application protocol will
use the BIND request only to establish secondary connections after a
primary connection is established using CONNECT. In is expected that
a SOCKS server will use DST.ADDR and DST.PORT in evaluating the BIND
request.
Two replies are sent from the SOCKS server to the client during a
BIND operation. The first is sent after the server creates and binds
a new socket. The BND.PORT field contains the port number that the
SOCKS server assigned to listen for an incoming connection. The
BND.ADDR field contains the associated IP address. The client will
typically use these pieces of information to notify (via the primary
or control connection) the application server of the rendezvous
address. The second reply occurs only after the anticipated incoming
connection succeeds or fails.
In the second reply, the BND.PORT and BND.ADDR fields contain the
address and port number of the connecting host.
UDP ASSOCIATE
The UDP ASSOCIATE request is used to establish an association within
the UDP relay process to handle UDP datagrams. The DST.ADDR and
DST.PORT fields contain the address and port that the client expects
to use to send UDP datagrams on for the association. The server MAY
use this information to limit access to the association. If the
client is not in possesion of the information at the time of the UDP
ASSOCIATE, the client MUST use address type X'01' with a port number
and address of all zeros.
A UDP association terminates when the TCP connection that the UDP
ASSOCIATE request arrived on terminates.
In the reply to a UDP ASSOCIATE request, the BND.PORT and BND.ADDR
fields indicate the port number/address where the client MUST send
UDP request messages to be relayed (unless the UDP relaying is done
in the TCP channel as specified by the TCP RELAY flag).
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Reply Processing
When a reply (REP value other than X'00') indicates a failure, the
SOCKS server MUST terminate the TCP connection shortly after sending
the reply. This must be no more than 10 seconds after detecting the
condition that caused a failure.
If the reply code (REP value of X'00') indicates a success, and the
request was either a BIND or a CONNECT, the client may now start
passing data. If the selected authentication method supports
encapsulation for the purposes of integrity, authentication and/or
confidentiality, the data are encapsulated using the method-dependent
encapsulation. Similarly, when data arrives at the SOCKS server for
the client, the server MUST encapsulate the data as appropriate for
the authentication method in use.
UDP Control Channel
Following UDP association, the tcp channel remains unused until
termination unless the client and server use it in accordance with a
FLAG setting. After the initial negotiation, the client and the server
MUST use this format to send any data on the control channel:
+----+-----+------+------+----------+------+------+----------+
|RSV | SUB | FLAG | ATYP | ADDR | PORT | SIZE | DATA |
+----+-----+------+------+----------+------+------+----------+
| 1 | 1 | 1 | 1 | Variable | 2 | 4 | Variable |
+----+-----+------+------+----------+------+------+----------+
The fields in the CONTROL CHANNEL packet are:
o RSV Reserved X'00'
o SUB Subcommand
o INTERFACE DATA: X'01'
o SENDTO: X'03'
o FLAG A subcommand dependent flag
o ATYP address type of following addresses:
o IP V4 address: X'01'
o DOMAINNAME: X'03'
o IP V6 address: X'04'
o ADDR any address information
o PORT any port information
o SIZE the size (in octets) of data in network order
o DATA user data
Flags
UDP ASSOCIATE request flags enable optional features in UDP ASSOCIATE.
All flags are optional and XOR the flags to combine them. Clients
that demand a feature be set must terminate the connection when they
receive a response that does not confirm the feature setting.
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Valid flags for use with UDP ASSOCIATE are:
o INTERFACE REQUEST: X'01'
o TCP RELAY: X'02'
o USE PORT: X'04'
Interface Requests
When the INTERFACE REQUEST flag is set in the UDP ASSOCIATE request
and also in the reply, the client may use the CONTROL CHANNEL to send
interface requests and the server uses the CONTROL CHANNEL to receive
interface requests. Clients use interface requests to determine the
interface address and port that the server uses to send data to a
destination.
In an interface request, the client sets SUB to INTERFACE DATA X'01',
sets FLAGS to X'00', and leaves DATA empty. The interface request
should specify the destination address using ATYPE, ADDR, and PORT.
When the server receives an INTERFACE REQUEST, the server checks to
determine if a bound UDP socket exists to send a datagram to the
destination. If a UDP socket does not exist, the server creates and
binds a UDP socket.
The server's reply to the client on the CONTOL CHANNEL sets SUB to
INTERFACE DATA X'01', sets FLAGS to X'00', and leaves DATA empty. The
server determines the remaining fields in the packet by the existence
of a bound UDP socket. When a bound socket does not exist and the
server fails to create or bind a socket to send data to the
destination, it sets ATYPE to X'01, and sets ADDR and PORT to zeroes.
When a bound UDP socket exists or the server successfully creates and
binds a UDP socket, the server sets the PORT field to the port number
that the SOCKS server assigned to the socket, and sets the ADDR field
to the associated IP address. The client will typically use this
information to notify the application server of the rendezvous address
(through the primary or control connection).
Whenever possible, the server should bind to the same port on all
outgoing UDP sockets so that the client may effectively consider
itself bound to a given port.
TCP relay server
The client can request that the server not set up a UDP relay server,
and that all communication between the client and the SOCKS server
occur on the TCP CONTROL CHANNEL. To do so, the client uses CONTROL
CHANNEL packets with the SUB command set to SENDTO, X'03'.
Fragmentation is not necessary and so not supported with TCP relay
sendtos.
As with the UDP relay, a TCP relay server relays a UDP datagram
silently. Similarly, it disregards packets with datagrams it cannot
or will not relay.
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Use Port
When the USE PORT flag is set in the UDP associate request, the
server MUST bind all UDP sockets associated with this session to the
same port as the client. When the relay is TCP based and there is
no client UDP socket, the server should use the port the client
specified in the initial request. When the client omits a port, the
server can choose any port. When the SOCKS server can not bind to
the client requested port, it should terminate the connection by
closing the TCP connection.
7. Procedure for UDP-based clients
A UDP-based client MUST send its datagrams to the UDP relay server at
the UDP port indicated by BND.PORT in the reply to the UDP ASSOCIATE
request. If the selected authentication method provides
encapsulation for the purposes of authenticity, integrity, and/or
confidentiality, the datagram MUST be encapsulated using the
appropriate encapsulation. Each UDP datagram carries a UDP request
header with it:
+----+------+------+----------+----------+----------+
|RSV | FRAG | ATYP | DST.ADDR | DST.PORT | DATA |
+----+------+------+----------+----------+----------+
| 2 | 1 | 1 | Variable | 2 | Variable |
+----+------+------+----------+----------+----------+
The fields in the UDP request header are:
o RSV Reserved X'0000'
o FRAG Current fragment number
o ATYP address type of following addresses:
o IP V4 address: X'01'
o DOMAINNAME: X'03'
o IP V6 address: X'04'
o DST.ADDR desired destination address
o DST.PORT desired destination port
o DATA user data
When a UDP relay server decides to relay a UDP datagram, it does so
silently, without any notification to the requesting client.
Similarly, it will drop datagrams it cannot or will not relay. When
a UDP relay server receives a reply datagram from a remote host, it
MUST encapsulate that datagram using the above UDP request header,
and any authentication-method-dependent encapsulation.
The UDP relay server MUST acquire from the SOCKS server the expected
IP address of the client that will send datagrams to the BND.PORT
given in the reply to UDP ASSOCIATE. It MUST drop any datagrams
arriving from any source IP address other than the one recorded for
the particular association.
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The FRAG field indicates whether or not this datagram is one of a
number of fragments. If implemented, the high-order bit indicates
end-of-fragment sequence, while a value of X'00' indicates that this
datagram is standalone. Values between 1 and 127 indicate the
fragment position within a fragment sequence. Each receiver will
have a REASSEMBLY QUEUE and a REASSEMBLY TIMER associated with these
fragments. The reassembly queue must be reinitialized and the
associated fragments abandoned whenever the REASSEMBLY TIMER expires,
or a new datagram arrives carrying a FRAG field whose value is less
than the highest FRAG value processed for this fragment sequence.
The reassembly timer MUST be no less than 5 seconds. It is
recommended that fragmentation be avoided by applications wherever
possible.
Implementation of fragmentation is optional; an implementation that
does not support fragmentation MUST drop any datagram whose FRAG
field is other than X'00'.
The programming interface for a SOCKS-aware UDP MUST report an
available buffer space for UDP datagrams that is smaller than the
actual space provided by the operating system:
o if ATYP is X'01' - 10+method_dependent octets smaller
o if ATYP is X'03' - 262+method_dependent octets smaller
o if ATYP is X'04' - 20+method_dependent octets smaller
8. Security Considerations
This document describes a protocol for the application-layer
traversal of IP network firewalls. The security of such traversal is
highly dependent on the particular authentication and encapsulation
methods provided in a particular implementation, and selected during
negotiation between SOCKS client and SOCKS server.
Careful consideration should be given by the administrator to the
selection of authentication methods.
9. References
[1] Koblas, D., "SOCKS", Proceedings: 1992 Usenix Security Symposium.
Author's Address
Marcus Leech
Bell-Northern Research Ltd
P.O. Box 3511, Stn. C,
Ottawa, ON
CANADA K1Y 4H7
Phone: (613) 763-9145
EMail: mleech@bnr.ca
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