SOCKS Protocol Version 6
draft-olteanu-intarea-socks-6-05
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
|
|
|---|---|---|---|
| Authors | Vladimir Olteanu , Dragos Niculescu | ||
| Last updated | 2018-10-23 | ||
| Replaces | draft-intarea-olteanu-socks-6, draft-olteanu-socks-6 | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-olteanu-intarea-socks-6-05
Internet Area Working Group V. Olteanu
Internet-Draft D. Niculescu
Intended status: Experimental University Politehnica of Bucharest
Expires: April 26, 2019 October 23, 2018
SOCKS Protocol Version 6
draft-olteanu-intarea-socks-6-05
Abstract
The SOCKS protocol is used primarily to proxy TCP connections to
arbitrary destinations via the use of a proxy server. Under the
latest version of the protocol (version 5), it takes 2 RTTs (or 3, if
authentication is used) before data can flow between the client and
the server.
This memo proposes SOCKS version 6, which reduces the number of RTTs
used, takes full advantage of TCP Fast Open, and adds support for
0-RTT authentication.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on April 26, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Revision log . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements language . . . . . . . . . . . . . . . . . . . . 7
3. Mode of operation . . . . . . . . . . . . . . . . . . . . . . 7
4. Requests . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Version Mismatch Replies . . . . . . . . . . . . . . . . . . 10
6. Authentication Replies . . . . . . . . . . . . . . . . . . . 11
7. Operation Replies . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Handling CONNECT . . . . . . . . . . . . . . . . . . . . 13
7.2. Handling BIND . . . . . . . . . . . . . . . . . . . . . . 14
7.3. Handling UDP ASSOCIATE . . . . . . . . . . . . . . . . . 14
7.3.1. Proxying UDP servers . . . . . . . . . . . . . . . . 16
8. SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1. Stack options . . . . . . . . . . . . . . . . . . . . . . 16
8.1.1. IP TOS options . . . . . . . . . . . . . . . . . . . 18
8.1.2. TFO options . . . . . . . . . . . . . . . . . . . . . 18
8.1.3. Multipath TCP options . . . . . . . . . . . . . . . . 19
8.1.4. MPTCP Scheduler options . . . . . . . . . . . . . . . 19
8.1.5. Listen Backlog options . . . . . . . . . . . . . . . 20
8.2. Authentication Method options . . . . . . . . . . . . . . 21
8.3. Authentication Data options . . . . . . . . . . . . . . . 22
8.4. Idempotence options . . . . . . . . . . . . . . . . . . . 22
8.4.1. Requesting a fresh token window . . . . . . . . . . . 23
8.4.2. Spending a token . . . . . . . . . . . . . . . . . . 24
8.4.3. Handling Token Window Advertisements . . . . . . . . 26
9. Username/Password Authentication . . . . . . . . . . . . . . 26
10. TCP Fast Open on the Client-Proxy Leg . . . . . . . . . . . . 26
11. False Starts . . . . . . . . . . . . . . . . . . . . . . . . 27
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
12.1. Large requests . . . . . . . . . . . . . . . . . . . . . 27
12.2. Replay attacks . . . . . . . . . . . . . . . . . . . . . 28
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
15.1. Normative References . . . . . . . . . . . . . . . . . . 29
15.2. Informative References . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
Versions 4 and 5 [RFC1928] of the SOCKS protocol were developed two
decades ago and are in widespread use for circuit level gateways or
as circumvention tools, and enjoy wide support and usage from various
software, such as web browsers, SSH clients, and proxifiers.
However, their design needs an update in order to take advantage of
the new features of transport protocols, such as TCP Fast Open
[RFC7413], or to better assist newer transport protocols, such as
MPTCP [RFC6824].
One of the main issues faced by SOCKS version 5 is that, when taking
into account the TCP handshake, method negotiation, authentication,
connection request and grant, it may take up to 5 RTTs for a data
exchange to take place at the application layer. This is especially
costly in networks with a large delay at the access layer, such as
3G, 4G, or satelite.
The desire to reduce the number of RTTs manifests itself in the
design of newer security protocols. TLS version 1.3 [RFC8446]
defines a zero round trip (0-RTT) handshake mode for connections if
the client and server had previously communicated.
TCP Fast Open [RFC7413] is a TCP option that allows TCP to send data
in the SYN and receive a response in the first ACK, and aims at
obtaining a data response in one RTT. The SOCKS protocol needs to
concern itself with at least two TFO deployment scenarios: First,
when TFO is available end-to-end (at the client, at the proxy, and at
the server); second, when TFO is active between the client and the
proxy, but not at the server.
This document describes the SOCKS protocol version 6. The key
improvements over SOCKS version 5 are:
o The client sends as much information upfront as possible, and does
not wait for the authentication process to conclude before
requesting the creation of a socket.
o The connection request also mimics the semantics of TCP Fast Open
[RFC7413]. As part of the connection request, the client can
supply the potential payload for the initial SYN that is sent out
to the server.
o The protocol can be extended via options without breaking
backward-compatibility.
o The protocol can leverage the aforementioned options to support
0-RTT authentication schemes.
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1.1. Revision log
Typos and minor clarifications are not listed.
draft-05
o Limited the "slow" authentication negociations to one (and
Authentication Replies to 2)
o Revamped the handling of the first bytes in the application data
stream
* False starts are now recommended. (Added the "False Start"
section.)
* Initial data is only available to clients willing to do "slow"
authentication. Moved the "Initial data size" field from
Requests to Authentication Method options.
* Initial data size capped at 2^13. Initial data can no longer
be dropped by the proxy.
* The TFO option can hint at the desired SYN payload size.
o Request: clarified the meaning of the Address and Port fields.
o Better reverse TCP proxy support: optional listen backlog for TCP
BIND
o TFO options can no longer be placed inside Operation Replies.
o IP TOS stack option
o Suggested a range for vendor-specific options.
o Revamped UDP functionality
* Now using fixed UDP ports
* DTLS support
o Stack options: renamed Proxy-Server leg to Proxy-Remote leg
draft-04
o Moved Token Expenditure Replies to the Authentication Reply.
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o Shifted the Initial Data Size field in the Request, in order to
make it easier to parse.
draft-03
o Shifted some fields in the Operation Reply to make it easier to
parse.
o Added connection attempt timeout response code to Operation
Replies.
o Proxies send an additional Authentication Reply after the
authentication phase. (Useful for token window advertisements.)
o Renamed the section "Connection Requests" to "Requests"
o Clarified the fact that proxies don't need to support any command
in particular.
o Added the section "TCP Fast Open on the Client-Proxy Leg"
o Options:
* Added constants for option kinds
* Salt options removed, along with the relevant section from
Security Considerations. (TLS 1.3 Makes AEAD mandatory.)
* Limited Authentication Data options to one per method.
* Relaxed proxy requirements with regard to handling multiple
Authentication Data options. (When the client violates the
above bullet point.)
* Removed interdependence between Authentication Method and
Authentication Data options.
* Clients SHOULD omit advertising the "No authentication
required" option. (Was MAY.)
* Idempotence options:
+ Token Window Advertisements are now part of successful
Authentication Replies (so that the proxy-server RTT has no
impact on their timeliness).
+ Proxies can't advetise token windows of size 0.
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+ Tweaked token expenditure response codes.
+ Support no longer mandatory on the proxy side.
* Revamped Socket options
+ Renamed Socket options to Stack options.
+ Banned contradictory socket options.
+ Added socket level for generic IP. Removed the "socket"
socket level.
+ Stack options no longer use option codes from setsockopt().
+ Changed MPTCP Scheduler constants.
draft-02
o Made support for Idempotence options mandatory for proxies.
o Clarified what happens when proxies can not or will not issue
tokens.
o Limited token windows to 2^31 - 1.
o Fixed definition of "less than" for tokens.
o NOOP commands now trigger Operation Replies.
o Renamed Authentication options to Authentication Data options.
o Authentication Data options are no longer mandatory.
o Authentication methods are now advertised via options.
o Shifted some Request fields.
o Option range for vendor-specific options.
o Socket options.
o Password authentication.
o Salt options.
draft-01
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o Added this section.
o Support for idempotent commands.
o Removed version numbers from operation replies.
o Request port number for SOCKS over TLS. Deprecate encryption/
encapsulation within SOCKS.
o Added Version Mismatch Replies.
o Renamed the AUTH command to NOOP.
o Shifted some fields to make requests and operation replies easier
to parse.
2. Requirements language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Mode of operation
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CLIENT PROXY
+------------------------+
| Authentication methods | Request
--------> Command code +------------------------------>
| Address |
| Port |
| Options |
+------------------------+
+------------------------+
--------> Initial data +------------------------------>
+------------------------+
+-----------------------+
Authentication Reply | Type |
<----------------------------------+ Method <-----
| Options |
+-----------------------+
<-------------------(Authentication protocol)------------------>
+-----------------------+
Authentication Reply | Type = Success |
<----------------------------------+ Method <-----
| Options |
+-----------------------+
+-----------------------+
Operation Reply | Reply code |
<--------------------+ Bind address <------------------
| Bind port |
| Options |
+-----------------------+
Figure 1: The SOCKS version 6 protocol message exchange
When a TCP-based client wishes to establish a connection to a server,
it must open a TCP connection to the appropriate SOCKS port on the
SOCKS proxy. The client then enters a negotiation phase, by sending
the request in figure Figure 1, that contains, in addition to fields
present in SOCKS 5 [RFC1928], fields that facilitate low RTT usage
and faster authentication negotiation.
Next, the server sends an authentication reply. If the request did
not contain the necessary authentication information, the proxy
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indicates an authentication method that must proceed. This may
trigger a longer authentication sequence that could include tokens
for ulterior faster authentications. The part labeled
"Authentication protocol" is specific to the authentication method
employed and is not expected to be employed for every connection
between a client and its proxy server. The authentication protocol
typically takes up 1 RTT or more.
If the authentication is successful, an operation reply is generated
by the proxy. It indicates whether the proxy was successful in
creating the requested socket or not.
In the fast case, when authentication is properly set up, the proxy
attempts to create the socket immediately after the receipt of the
request, thus achieving an operational conection in one RTT (provided
TFO functionality is available at the client, proxy, and server).
4. Requests
The client starts by sending a request to the proxy.
+---------------+---------+------+---------+----------+
| Version | Command | Port | Address | Address |
| Major | Minor | Code | | Type | |
+-------+-------+---------+------+---------+----------+
| 1 | 1 | 1 | 2 | 1 | Variable |
+-------+-------+---------+------+---------+----------+
+-----------+----------+
| Number of | Options |
| Options | |
+-----------+----------+
| 1 | Variable |
+-----------+----------+
Figure 2: SOCKS 6 Request
o Version: The major byte MUST be set to 0x06, and the minor byte
MUST be set to 0x00.
o Command Code:
* 0x00 NOOP: authenticate the client and do nothing.
* 0x01 CONNECT: requests the establishment of a TCP connection.
* 0x02 BIND: requests the establishment of a TCP port binding.
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* 0x03 UDP ASSOCIATE: requests a UDP port association.
o Address Type:
* 0x01: IPv4
* 0x03: Domain Name
* 0x04: IPv6
o Address: this field's format depends on the address type:
* IPv4: a 4-byte IPv4 address
* Domain Name: one byte that contains the length of the FQDN,
followed by the FQDN itself. The string is not NUL-terminated.
* IPv6: a 16-byte IPv6 address
o Port: the port in network byte order.
o Number of Options: the number of SOCKS options that appear in the
Options field.
o Options: see Section 8.
The Address and Port fields have different meanings based on the
Command Code: * NOOP: The fields have no meaning. The Address Type
field MUST be either 0x01 (IPv4) or 0x04 (IPv6). The Address and
Port fields MUST be 0. * CONNECT: The fields signify the address and
port to which the client wishes to connect. * BIND, UDP ASSOCIATE:
The fields indicate the desired bind address and port. If the client
does not require a certain address, it can set the Address Type field
to 0x01 (IPv4) or 0x04 (IPv6), and the Address field to 0. Likewise,
if the client does not require a certain port, it can set the Port
field to 0.
Clients can advertise their supported authentication methods by
including an Authentication Method option (see Section 8.2).
5. Version Mismatch Replies
Upon receipt of a request starting with a version number other than
6.0, the proxy sends the following response:
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+---------------+
| Version |
| Major | Minor |
+-------+-------+
| 1 | 1 |
+-------+-------+
Figure 3: SOCKS 6 Version Mismatch Reply
o Version: The major byte MUST be set to 0x06, and the minor byte
MUST be set to 0x00.
A client MUST close the connection after receiving such a reply.
6. Authentication Replies
Upon receipt of a valid request, the proxy sends an Authentication
Reply:
+---------------+------+--------+-----------+----------+
| Version | Type | Method | Number of | Options |
| Major | Minor | | | Options | |
+-------+-------+------+--------+-----------+----------+
| 1 | 1 | 1 | 1 | 1 | Variable |
+-------+-------+------+--------+-----------+----------+
Figure 4: SOCKS 6 Authentication Reply
o Version: The major byte MUST be set to 0x06, and the minor byte
MUST be set to 0x00.
o Type:
* 0x00: authentication successful.
* 0x01: further authentication needed.
o Method: The chosen authentication method.
o Number of Options: the number of SOCKS options that appear in the
Options field.
o Options: see Section 8.
Multihomed clients SHOULD cache the chosen method on a per-interface
basis and SHOULD NOT include Authentication Data options related to
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any other methods in further requests originating from the same
interface.
If the server signals that further authentication is needed and
selects "No Acceptable Methods", the client MUST close the
connection.
The client and proxy begin a method-specific negotiation. During
such negotiations, the proxy MAY supply information that allows the
client to authenticate a future request using an Authentication Data
option. The client and proxy SHOULD NOT negotiate the encryption of
the application data. Descriptions of such negotiations are beyond
the scope of this memo.
When the negotiation is complete (either successfully or
unsuccessfully), the proxy sends a second Authentication Reply. The
second Authentication Reply MUST either signal success or that there
are no more acceptable authentication methods.
7. Operation Replies
After the authentication negotiations are complete, the proxy sends
an Operation Reply:
+-------+------+---------+----------+-----------+----------+
| Reply | Bind | Address | Bind | Number of | Options |
| Code | Port | Type | Address | Options | |
+-------+------+---------+----------+-----------+----------+
| 1 | 2 | 1 | Variable | 1 | Variable |
+-------+------+---------+----------+-----------+----------+
Figure 5: SOCKS 6 Operation Reply
o Reply Code:
* 0x00: Succes
* 0x01: General SOCKS server failure
* 0x02: Connection not allowed by ruleset
* 0x03: Network unreachable
* 0x04: Host unreachable
* 0x05: Connection refused
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* 0x06: TTL expired
* 0x07: Command not supported
* 0x08: Address type not supported
* 0x09: Connection attempt timed out
o Bind Port: the proxy bound port in network byte order.
o Address Type:
* 0x01: IPv4
* 0x03: Domain Name
* 0x04: IPv6
o Bind Address: the proxy bound address in the following format:
* IPv4: a 4-byte IPv4 address
* Domain Name: one byte that contains the length of the FQDN,
followed by the FQDN itself. The string is not NUL-terminated.
* IPv6: a 16-byte IPv6 address
o Number of Options: the number of SOCKS options that appear in the
Options field.
o Options: see Section 8.
Proxy implementations MAY support any subset of the client commands
listed in Section 4.
If the proxy returns a reply code other than "Success", the client
MUST close the connection.
If the client issued an NOOP command, the client MUST close the
connection after receiving the Operation Reply.
7.1. Handling CONNECT
In case the client has issued a CONNECT request, data can now pass.
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7.2. Handling BIND
In case the client has issued a BIND request, it must wait for a
second Operation reply from the proxy, which signifies that a host
has connected to the bound port. The Bind Address and Bind Port
fields contain the address and port of the connecting host.
Afterwards, application data may pass.
7.3. Handling UDP ASSOCIATE
Proxies offering UDP functionality must be configured with a UDP port
used for relaying UDP datagrams to and from the client, and/or a port
used for relaying datagrams over DTLS.
Following a successful Operation Reply, the proxy sends a UDP
Association Initialization message:
+----------------+
| Association ID |
+----------------+
| 4 |
+----------------+
Figure 6: UDP Association Initialization
o Association ID: the identifier of the UDP association
Proxy implementations SHOULD generate Association IDs randomly or
pseudo-randomly.
Clients may start sending UDP datagrams to the proxy either in
plaintext, or over an established DTLS session, using the proxy's
configured UDP ports. A client's datagrams are prefixed by a SOCKS
Datagram Header, indicating the remote host's address and port:
+---------------+-------------+------+---------+----------+
| Version | Association | Port | Address | Address |
| Major | Minor | ID | | Type | |
+-------+-------+-------------+------+---------+----------+
| 1 | 1 | 4 | 2 | 1 | Variable |
+-------+-------+-------------+------+---------+----------+
Figure 7: SOCKS 6 Datagram Header
o Version: The major byte MUST be set to 0x06, and the minor byte
MUST be set to 0x00.
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o Association ID: the identifier of the UDP association
o Address Type:
* 0x01: IPv4
* 0x03: Domain Name
* 0x04: IPv6
o Address: this field's format depends on the address type:
* IPv4: a 4-byte IPv4 address
* Domain Name: one byte that contains the length of the FQDN,
followed by the FQDN itself. The string is not NUL-terminated.
* IPv6: a 16-byte IPv6 address
o Port: the port in network byte order.
Following the receipt of the first datagram from the client, the
proxy makes a one-way mapping between the Association ID and:
o the 5-tuple of the UDP conversation, if the datagram was received
over plain UDP, or
o the DTLS connection, if the datagram was received over DTLS. The
DTLS connection is identified either by its 5-tuple, or some other
mechanism, like [I-D.ietf-tls-dtls-connection-id].
Further datagrams carrying the same Association ID, but not matching
the established mapping, are silently dropped.
The proxy then sends an UDP Association Confirmation message over the
TCP connection with the client:
+--------+
| Status |
+--------+
| 1 |
+--------+
Figure 8: UDP Association Confirmation
o Status: MUST be 0x00
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Following the confirmation message, UDP packets bound for the proxy's
bind address and port are relayed to the client, also prefixed by a
Datagram Header.
The UDP association remains active for as long as the TCP connection
between the client and the proxy is kept open.
7.3.1. Proxying UDP servers
Under some circumstances (e.g. when hosting a server), the SOCKS
client expects the remote host to send UDP datagrams first. As such,
the SOCKS client must trigger a UDP Association Confirmation without
having the proxy relay any datagrams on its behalf.
To that end, it sends an empty datagram prefixed by a Datagram Header
with an IP address and port consisting of zeroes. The client SHOULD
resend the empty datagram if an UDP Association Confirmation is not
received after a timeout.
8. SOCKS Options
SOCKS options have the following format:
+------+--------+-------------+
| Kind | Length | Option Data |
+------+--------+-------------+
| 1 | 1 | Variable |
+------+--------+-------------+
Figure 9: SOCKS 6 Option
o Kind: MUST be allocated by IANA. (See Section 13.)
o Length: The length of the option.
o Option Data: The contents are specific to each option kind.
Unless otherwise noted, client and proxy implementations MAY omit
supporting any of the options described in this document. Upon
encountering an unsupported option, a SOCKS endpoint MUST silently
ignore it.
8.1. Stack options
Stack options can be used by clients to alter the behavior of the
protocols on top of which SOCKS is running, as well the protcols used
by the proxy to communicate with the remote host (i.e. IP, TCP,
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UDP). A Stack option can affect either the proxy's protocol on the
client-proxy leg or on the proxy-remote leg. Clients can only place
Stack options inside SOCKS Requests.
Proxies MAY include Stack options in their Operation Replies to
signal their behavior. Said options MAY be unsolicited, i. e. the
proxy MAY send them to signal behaviour that was not explicitly
requested by the client.
In case of UDP ASSOCIATE, the stack options refer to the UDP traffic
relayed by the proxy.
Stack options that are part of the same message MUST NOT contradict
one another.
+------+--------+--------+--------+------+----------+
| Kind | Length | Leg | Level | Code | Data |
+------+--------+--------+--------+------+----------+
| 1 | 1 | 2 bits | 6 bits | 1 | Variable |
+------+--------+--------+--------+------+----------+
Figure 10: Stack Option
o Kind: 0x01 (Stack option)
o Length: The length of the option.
o Leg:
* 0x1: Client-Proxy Leg
* 0x2: Proxy-Remote Leg
* 0x3: Both Legs
o Level:
* 0x01: IP
* 0x02: IPv4
* 0x03: IPv6
* 0x04: TCP
* 0x05: UDP
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o Code: Option code
o Data: Option-specific data
8.1.1. IP TOS options
+------+--------+--------+--------+-----+
| Kind | Length | Leg | Level | TOS |
+------+--------+--------+--------+-----+
| 1 | 1 | 2 bits | 6 bits | 1 |
+------+--------+--------+--------+-----+
Figure 11: IP TOS Option
o Kind: 0x01 (Stack option)
o Length: 4
o Leg: Either 0x01, 0x02, or 0x03 (Client-Proxy, Proxy-Remote or
Both legs)
o Level: 0x04 (TCP).
o Code: 0x01
The client can use IP TOS options to request that the proxy use a
certain value for the IP TOS field. Likewise, the proxy can use IP
TOS options to advertise the TOS values being used.
8.1.2. TFO options
+------+--------+--------+--------+------+--------------+
| Kind | Length | Leg | Level | Code | Payload Size |
+------+--------+--------+--------+------+--------------+
| 1 | 1 | 2 bits | 6 bits | 1 | 2 |
+------+--------+--------+--------+------+--------------+
Figure 12: TFO Option
o Kind: 0x01 (Stack option)
o Length: 4
o Leg: 0x2 (Proxy-Remote leg).
o Level: 0x04 (TCP).
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o Code: 0x01
o Payload Size: The desired payload size of the TFO SYN. MUST be 0
in case of a BIND command.
If a SOCKS Request contains a TFO option, the proxy SHOULD attempt to
use TFO in case of a CONNECT command, or accept TFO in case of a BIND
command. Otherwise, the proxy MUST NOT attempt to use TFO in case of
a CONNECT command, or accept TFO in case of a BIND command.
In case of a CONNECT command, the client can indicate the desired
payload size of the SYN. The proxy MAY use a different payload size
than the one indicated.
8.1.3. Multipath TCP options
In case of a CONNECT command, the proxy can inform the client that
the connection to the server is an MPTCP connection.
+------+--------+--------+--------+------+
| Kind | Length | Leg | Level | Code |
+------+--------+--------+--------+------+
| 1 | 1 | 2 bits | 6 bits | 1 |
+------+--------+--------+--------+------+
Figure 13: Multipath TCP Option
o Kind: 0x01 (Stack option)
o Length: 4
o Leg: 0x2 (Proxy-Remote leg)
o Level: 0x04 (TCP).
o Code: 0x02
8.1.4. MPTCP Scheduler options
In case of a CONNECT or BIND command, a client can use an MPTCP
Scheduler option to indicate its preferred scheduler for the
connection.
A proxy can use an MPTCP Scheduler option to inform the client about
what scheduler is in use.
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+------+--------+--------+--------+------+-----------+
| Kind | Length | Leg | Level | Code | Scheduler |
+------+--------+--------+--------+------+-----------+
| 1 | 1 | 2 bits | 6 bits | 1 | 1 |
+------+--------+--------+--------+------+-----------+
Figure 14: MPTCP Scheduler Option
o Kind: 0x01 (Stack option)
o Length: 5
o Leg: Either 0x01, 0x02, or 0x03 (Client-Proxy, Proxy-Remote or
Both legs).
o Level: 0x04 (TCP)
o Code: 0x03
o Scheduler:
* 0x01: Default
* 0x02: Round-Robin
* 0x03: Redundant
8.1.5. Listen Backlog options
+------+--------+--------+--------+---------+
| Kind | Length | Leg | Level | Backlog |
+------+--------+--------+--------+---------+
| 1 | 1 | 2 bits | 6 bits | 2 |
+------+--------+--------+--------+---------+
Figure 15: Listen Backlog Option
o Kind: 0x01 (Stack option)
o Length: 5
o Leg: 0x02 (Proxy-Remote leg)
o Level: 0x04 (TCP)
o Code: 0x04
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o Backlog: The length of the listen backlog. MUST be greater than
1.
The default behavior of the BIND does not allow a client to
simultaneously handle multiple connections to the same bind address.
An authenticated client can alter BIND's behavior by adding a TCP
Listen Backlog Option to a BIND Request.
In response, the proxy sends a TCP Listen Backlog Option as part of
the Operation Reply, with the Backlog field signalling the actual
backlog used. The proxy SHOULD NOT use a backlog longer than
requested.
Following the successful negotiation of a backlog, the proxy listens
for incoming connections for as long as the initial connection stays
open. The initial connection is not used to relay data between the
client and a remote host.
To accept connections, the client issues further BIND Requests using
the bind address and port supplied by the proxy in the initial
Operation Reply.
8.2. Authentication Method options
Authentication Method options are placed in SOCKS Requests to
advertise supported authentication methods. In case of a CONNECT
Request, they are also used to specify the amount of initial data
supplied before any method-specific authentication negotiations take
place.
+------+--------+---------------------+----------+
| Kind | Length | Initial Data Length | Methods |
+------+--------+---------------------+----------+
| 1 | 1 | 2 | Variable |
+------+--------+---------------------+----------+
Figure 16: Authentication Method Option
o Kind: 0x02 (Authentication Method option)
o Length: The length of the option.
o Initial Data Size: A two-byte number in network byte order. In
case of CONNECT, this is the number of bytes of initial data that
are supplied by the client immediately following the Request.
This number MUST NOT be larger than 2^13.
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o Methods: One byte per advertised method. Method numbers are
assigned by IANA.
Clients MUST support the "No authentication required" method.
Clients SHOULD omit advertising the "No authentication required"
option.
8.3. Authentication Data options
Authentication Data options carry method-specific authentication
data. They can be part of SOCKS Requests and Authentication Replies.
Authentication Data options have the following format:
+------+--------+--------+---------------------+
| Kind | Length | Method | Authentication Data |
+------+--------+--------+---------------------+
| 1 | 1 | 1 | Variable |
+------+--------+--------+---------------------+
Figure 17: Authentication Data Option
o Kind: 0x03 (Authentication Data option)
o Length: The length of the option.
o Method: The number of the authentication method. These numbers
are assigned by IANA.
o Authentication Data: The contents are specific to each method.
Clients SHOULD only place one Authentication Data option per
authentication method. Server implementations MAY silently ignore
all Authentication Data options for the same method aside from an
arbitrarily chosen one.
8.4. Idempotence options
To protect against duplicate SOCKS Requests, authenticated clients
can request, and then spend, idempotence tokens. A token can only be
spent on a single SOCKS request.
Tokens are 4-byte unsigned integers in a modular 4-byte space.
Therefore, if x and y are tokens, x is less than y if 0 < (y - x) <
2^31 in unsigned 32-bit arithmetic.
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Proxies grant contiguous ranges of tokens called token windows.
Token windows are defined by their base (the first token in the
range) and size. Windows can be shifted (i. e. have their base
increased, while retaining their size) unilaterally by the proxy.
Requesting and spending tokens is done via Idempotence options:
+------+--------+------+-------------+
| Kind | Length | Type | Option Data |
+------+--------+------+-------------+
| 1 | 1 | 1 | Variable |
+------+--------+------+-------------+
Figure 18: Idempotence Option
o Kind: 0x04 (Idempotence option)
o Length: The length of the option.
o Type:
* 0x00: Token Request
* 0x01: Token Window Advertisement
* 0x02: Token Expenditure
* 0x03: Token Expenditure Reply
o Option Data: The contents are specific to each type.
8.4.1. Requesting a fresh token window
A client can obtain a fresh window of tokens by sending a Token
Request option as part of a SOCKS Request:
+------+--------+------+-------------+
| Kind | Length | Type | Window Size |
+------+--------+------+-------------+
| 1 | 1 | 1 | 4 |
+------+--------+------+-------------+
Figure 19: Token Request
o Kind: MUST be allocated by IANA. (See Section 13.)
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o Length: 7
o Type: 0x00 (Token Request)
o Window Size: The requested window size.
If a token window is issued, the proxy then includes a Token Window
Advertisement option in the corresponding successful Authentication
Reply:
+------+--------+------+-------------+-------------+
| Kind | Length | Type | Window Base | Window Size |
+------+--------+------+-------------+-------------+
| 1 | 1 | 1 | 4 | 4 |
+------+--------+------+-------------+-------------+
Figure 20: Token Window Advertisement
o Kind: 0x04 (Idempotence option)
o Length: 11
o Type: 0x01 (Token Grant)
o Window Base: The first token in the window.
o Window Size: The window size. This value SHOULD be lower or equal
to the requested window size. Window sizes MUST be less than
2^31. Window sizes MUST NOT be 0.
If no token window is issued, the proxy MUST silently ignore the
Token Request.
8.4.2. Spending a token
The client can attempt to spend a token by including a Token
Expenditure option in its SOCKS request:
+------+--------+------+-------+
| Kind | Length | Type | Token |
+------+--------+------+-------+
| 1 | 1 | 1 | 4 |
+------+--------+------+-------+
Figure 21: Token Expenditure
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o Kind: 0x04 (Idempotence option)
o Length: 7
o Type: 0x02 (Token Expenditure)
o Token: The token being spent.
Clients SHOULD prioritize spending the smaller tokens.
The proxy responds by sending a Token Expenditure Reply option as
part of the successful Authentication Reply:
+------+--------+------+---------------+
| Kind | Length | Type | Response Code |
+------+--------+------+---------------+
| 1 | 1 | 1 | 1 |
+------+--------+------+---------------+
Figure 22: Token Expenditure Response
o Kind: 0x04 (Idempotence option)
o Length: 4
o Type: 0x03 (Token Expenditure Response)
o Response Code:
* 0x01: Success: The token was spent successfully.
* 0x02: No Window: The proxy does not have a token window
associated with the client.
* 0x03: Out of Window: The token is not within the window.
* 0x04: Duplicate: The token has already been spent.
If eligible, the token is spent as soon as the client authenticates.
If the token is not eligible for spending, the proxy MUST NOT attempt
to honor the client's SOCKS Request; further, it MUST indicate a
General SOCKS server failure in the Operation Reply.
Proxy implementations SHOULD also send a Token Window Advertisement
if:
o the token is out of window, or
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o by the proxy's internal logic, successfully spending the token
caused the window to shift.
Proxy implementations SHOULD NOT shift the window's base beyond the
highest unspent token.
Proxy implementations MAY include a Token Window Advertisement in any
Authentication Reply that indicates success.
8.4.3. Handling Token Window Advertisements
Even though the proxy increases the window's base monotonically,
there is no mechanism whereby a SOCKS client can receive the Token
Window Advertisements in order. As such, clients SHOULD disregard
unsolicited Token Window Advertisements with a Window Base less than
the previously known value.
9. Username/Password Authentication
Username/Password authentication is carried out as in [RFC1929].
Clients can also attempt to authenticate by placing the Username/
Password request in an Authentication Data Option, provided that it
is no longer than 252 bytes.
+------+--------+--------+---------------------------+
| Kind | Length | Method | Username/Password request |
+------+--------+--------+---------------------------+
| 1 | 1 | 1 | Variable |
+------+--------+--------+---------------------------+
Figure 23: Password authentication via a SOCKS Option
o Kind: MUST be allocated by IANA. (See Section 13.)
o Length: The length of the option.
o Method: 0x02 (Username/Password).
o Username/Password request: The Username/Password request, as
described in [RFC1929].
10. TCP Fast Open on the Client-Proxy Leg
TFO breaks TCP semantics, causing replays of the data in the SYN's
payload under certain rare circumstances [RFC7413]. A replayed SOCKS
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Request could itself result in a replayed connection on behalf of the
client.
As such, client implementations SHOULD NOT use TFO on the client-
proxy leg unless:
o The protocol running on top of SOCKS tolerates the risks of TFO,
or
o The SYN's payload does not contain any application data (so that
no data is replayed to the server, even though duplicate
connections are still possible), or
o The client uses Idempotence Options, making replays impossible, or
o SOCKS is running on top of TLS and Early Data is not used.
11. False Starts
In case of CONNECT Requests, the client MAY start sending application
data as soon as possible, as long as doing so does not incur the risk
of breaking the SOCKS protocol.
Clients must work around the authentication phase by doing any of the
following:
o If the Request does not contain an Authentication Method option,
the authentication phase is guaranteed not to happen. In this
case, application data MAY be sent immediately after the Request.
o Application data MAY be sent immediately after receiving an
Authentication Reply indicating success.
o When performing a method-specific authentication sequence,
application data MAY be sent immediately after the last client
message.
12. Security Considerations
12.1. Large requests
Given the format of the request message, a malicious client could
craft a request that is in excess of 80 KB and proxies could be prone
to DDoS attacks.
To mitigate such attacks, proxy implementations SHOULD be able to
incrementally parse the requests. Proxies MAY close the connection
to the client if:
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o the request is not fully received after a certain timeout, or
o the number of options exceeds an imposed hard cap, or
o the total size of the options exceeds an imposed hard cap.
Further, the server MAY choose not to buffer any initial data beyond
what would be expected to fit in a TFO SYN's payload.
12.2. Replay attacks
In TLS 1.3, early data (which is likely to contain a full SOCKS
request) is prone to replay attacks.
While Token Expenditure options can be used to mitigate replay
attacks, the initial Token Request is still vulnerable. As such,
client implementations SHOULD NOT make use of TLS early data when
sending a Token Request.
13. IANA Considerations
This document requests that IANA allocate 1-byte option kinds for
SOCKS 6 options. Further, this document requests the following
option kinds:
o Stack options: 0x01
o Authentication Method options: 0x02
o Authentication Data options: 0x03
o Idempotence options: 0x04
o A range for vendor-specific options: 0xC0-0xFF
This document also requests that IANA allocate a TCP and UDP port for
SOCKS over TLS and DTLS, respectively.
14. Acknowledgements
The protocol described in this draft builds upon and is a direct
continuation of SOCKS 5 [RFC1928].
15. References
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15.1. Normative References
[RFC1929] Leech, M., "Username/Password Authentication for SOCKS
V5", RFC 1929, DOI 10.17487/RFC1929, March 1996,
<https://www.rfc-editor.org/info/rfc1929>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
15.2. Informative References
[I-D.ietf-tls-dtls-connection-id]
Rescorla, E., Tschofenig, H., Fossati, T., and T. Gondrom,
"Connection Identifiers for DTLS 1.2", draft-ietf-tls-
dtls-connection-id-02 (work in progress), October 2018.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
Authors' Addresses
Vladimir Olteanu
University Politehnica of Bucharest
Email: vladimir.olteanu@cs.pub.ro
Dragos Niculescu
University Politehnica of Bucharest
Email: dragos.niculescu@cs.pub.ro
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