Internet Area Working Group V. Olteanu Internet-Draft D. Niculescu Intended status: Experimental University Politehnica of Bucharest Expires: February 18, 2019 August 17, 2018 SOCKS Protocol Version 6 draft-olteanu-intarea-socks-6-04 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 February 18, 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 Olteanu & Niculescu Expires February 18, 2019 [Page 1] Internet-Draft SOCKS 6 August 2018 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 . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Revision log . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements language . . . . . . . . . . . . . . . . . . . . 6 3. Mode of operation . . . . . . . . . . . . . . . . . . . . . . 6 4. Requests . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Version Mismatch Replies . . . . . . . . . . . . . . . . . . 9 6. Authentication Replies . . . . . . . . . . . . . . . . . . . 10 7. Operation Replies . . . . . . . . . . . . . . . . . . . . . . 11 7.1. Handling CONNECT . . . . . . . . . . . . . . . . . . . . 13 7.2. Handling BIND . . . . . . . . . . . . . . . . . . . . . . 13 7.3. Handling UDP ASSOCIATE . . . . . . . . . . . . . . . . . 13 8. SOCKS Options . . . . . . . . . . . . . . . . . . . . . . . . 13 8.1. Stack options . . . . . . . . . . . . . . . . . . . . . . 13 8.1.1. TFO options . . . . . . . . . . . . . . . . . . . . . 15 8.1.2. Multipath TCP options . . . . . . . . . . . . . . . . 15 8.1.3. MPTCP Scheduler options . . . . . . . . . . . . . . . 16 8.2. Authentication Method options . . . . . . . . . . . . . . 17 8.3. Authentication Data options . . . . . . . . . . . . . . . 17 8.4. Idempotence options . . . . . . . . . . . . . . . . . . . 18 8.4.1. Requesting a fresh token window . . . . . . . . . . . 19 8.4.2. Spending a token . . . . . . . . . . . . . . . . . . 20 8.4.3. Handling Token Window Advertisements . . . . . . . . 21 9. Username/Password Authentication . . . . . . . . . . . . . . 21 10. TCP Fast Open on the Client-Proxy Leg . . . . . . . . . . . . 22 11. Security Considerations . . . . . . . . . . . . . . . . . . . 22 11.1. Large requests . . . . . . . . . . . . . . . . . . . . . 22 11.2. Replay attacks . . . . . . . . . . . . . . . . . . . . . 23 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 14.1. Normative References . . . . . . . . . . . . . . . . . . 24 14.2. Informative References . . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24 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 Olteanu & Niculescu Expires February 18, 2019 [Page 2] Internet-Draft SOCKS 6 August 2018 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 [I-D.ietf-tls-tls13] 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. 1.1. Revision log Typos and minor clarifications are not listed. draft-04 Olteanu & Niculescu Expires February 18, 2019 [Page 3] Internet-Draft SOCKS 6 August 2018 o Moved Token Expenditure Replies to the Authentication Reply. 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). Olteanu & Niculescu Expires February 18, 2019 [Page 4] Internet-Draft SOCKS 6 August 2018 + Proxies can't advetise token windows of size 0. + 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. Olteanu & Niculescu Expires February 18, 2019 [Page 5] Internet-Draft SOCKS 6 August 2018 draft-01 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 Olteanu & Niculescu Expires February 18, 2019 [Page 6] Internet-Draft SOCKS 6 August 2018 CLIENT PROXY +------------------------+ | Authentication methods | Request --------> Command code +------------------------------> | Address | | Port | | Options | | Initial data | +------------------------+ +-----------------------+ Authentication reply | Type | <----------------------------------+ Method <----- | Options | +-----------------------+ <-------------------(Authentication protocol)------------------> +-----------------------+ Operation reply | Reply code | <--------------------+ Bind address <------------------ | Bind port | | Options | | Initial data offset | +-----------------------+ 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 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. Olteanu & Niculescu Expires February 18, 2019 [Page 7] Internet-Draft SOCKS 6 August 2018 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 | Initial Data | Port | Address | Address | | Major | Minor | Code | Size | | Type | | +-------+-------+---------+--------------+------+---------+----------+ | 1 | 1 | 1 | 2 | 2 | 1 | Variable | +-------+-------+---------+--------------+------+---------+----------+ +-----------+----------+--------------+ | Number of | Options | Initial Data | | Options | | | +-----------+----------+--------------+ | 1 | Variable | 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. * 0x03 UDP ASSOCIATE: requests a UDP port association. o Address Type: * 0x01: IPv4 * 0x03: Domain Name Olteanu & Niculescu Expires February 18, 2019 [Page 8] Internet-Draft SOCKS 6 August 2018 * 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. o Initial Data Size: A two-byte number in network byte order. In case of NOOP, BIND or UDP ASSOCIATE, this field MUST be set to 0. In case of CONNECT, this is the number of bytes of initial data that are supplied in the following field. o Initial Data: The first octets of the data stream. Clients can advertise their supported authentication methods by including an Authentication Method option (see Section 8.2). The server MAY truncate the initial data to an arbitrary size and disregard the rest. This is will be communicated later to the client, should the authentication process be successful (see Section 7). As such, server implementations do not have to buffer the initial data while waiting for the (potentially malicious) client to authenticate. 5. Version Mismatch Replies Upon receipt of a request starting with a version number other than 6.0, the proxy sends the following response: Olteanu & Niculescu Expires February 18, 2019 [Page 9] Internet-Draft SOCKS 6 August 2018 +---------------+ | 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 Olteanu & Niculescu Expires February 18, 2019 [Page 10] Internet-Draft SOCKS 6 August 2018 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 another Authentication Reply. 7. Operation Replies After the authentication negotiations are complete, the server sends an Operation Reply: +-------+--------------+------+---------+----------+ | Reply | Initial Data | Bind | Address | Bind | | Code | Offset | Port | Type | Address | +-------+--------------+------+---------+----------+ | 1 | 2 | 2 | 1 | Variable | +-------+--------------+------+---------+----------+ +-----------+----------+ | Number of | Options | | Options | | +-----------+----------+ | 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 Olteanu & Niculescu Expires February 18, 2019 [Page 11] Internet-Draft SOCKS 6 August 2018 * 0x05: Connection refused * 0x06: TTL expired * 0x07: Command not supported * 0x08: Address type not supported * 0x09: Connection attempt timed out o Initial Data Offset: A two-byte number in network byte order. In case of BIND or UDP ASSOCIATE, this field MUST be set to 0. In case of CONNECT, it represents the offset in the plain data stream from which the client is expected to continue sending data. 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. Olteanu & Niculescu Expires February 18, 2019 [Page 12] Internet-Draft SOCKS 6 August 2018 7.1. Handling CONNECT In case the client has issued a CONNECT request, data can now pass. The client MUST resume the data stream at the offset indicated by the Initial Data Offset field. 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 The relay of UDP packets is handled exactly as in SOCKS 5 [RFC1928]. 8. SOCKS Options SOCKS options have the following format: +---------------+-------------+ | Kind | Length | Option Data | +------+--------+-------------+ | 1 | 1 | Variable | +------+--------+-------------+ Figure 6: SOCKS 6 Option o Kind: MUST be allocated by IANA. (See Section 12.) 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 server (i.e. IP, TCP, UDP). A Stack option can affect either the proxy's protocol on the Olteanu & Niculescu Expires February 18, 2019 [Page 13] Internet-Draft SOCKS 6 August 2018 client-proxy leg or on the proxy-server 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. 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 7: Stack Option o Kind: 0x01 (Stack option) o Length: The length of the option. o Leg: * 0x1: Client-Proxy Leg * 0x2: Proxy-Server Leg * 0x3: Both Legs o Level: * 0x01: IP * 0x02: IPv4 * 0x03: IPv6 * 0x04: TCP * 0x05: UDP o Code: Option code o Data: Option-specific data Olteanu & Niculescu Expires February 18, 2019 [Page 14] Internet-Draft SOCKS 6 August 2018 8.1.1. TFO options +---------------+--------+--------+------+ | Kind | Length | Leg | Level | Code | +------+--------+--------+--------+------+ | 1 | 1 | 2 bits | 6 bits | 1 | +------+--------+--------+--------+------+ Figure 8: TFO Option o Kind: 0x01 (Stack option) o Length: MUST be 4. o Leg: MUST be 0x2 (Proxy-Server Leg). o Level: 0x04 (TCP). o Code: 0x01 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 proxy MAY include a TFO option in the Operation reply if TFO was attempted, the operation succeded and the remote server supports TFO. In case of a BIND command, the proxy MAY include a TFO option in the first Operation reply to signal that it will accept an incoming TFO connection. 8.1.2. 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 9: Multipath TCP Option o Kind: 0x01 (Stack option) Olteanu & Niculescu Expires February 18, 2019 [Page 15] Internet-Draft SOCKS 6 August 2018 o Length: 4. o Leg: MUST be 0x2 (Proxy-Server Leg). o Level: 0x04 (TCP). o Code: 0x02 8.1.3. 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. +---------------+--------+--------+------+-----------+ | Kind | Length | Leg | Level | Code | Scheduler | +------+--------+--------+--------+------+-----------+ | 1 | 1 | 2 bits | 6 bits | 1 | 1 | +------+--------+--------+--------+------+-----------+ Figure 10: MPTCP Scheduler Option o Kind: 0x01 (Stack option) o Length: MUST be 5. o Leg: Either 0x01, 0x02, or 0x03 (Client-Proxy, Proxy-Client or Both legs). o Level: 0x04 (TCP). o Code: 0x03 o Scheduler: * 0x01: Default * 0x02: Round-Robin * 0x03: Redundant Olteanu & Niculescu Expires February 18, 2019 [Page 16] Internet-Draft SOCKS 6 August 2018 8.2. Authentication Method options Authentication Method options are used by clients to advertise supported authentication methods. They can be part of SOCKS Requests. +---------------+----------+ | Kind | Length | Methods | +------+--------+----------+ | 1 | 1 | Variable | +------+--------+----------+ Figure 11: Authentication Method Option o Kind: 0x02 (Authentication Method option) o Length: The length of the option. 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 12: 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. Olteanu & Niculescu Expires February 18, 2019 [Page 17] Internet-Draft SOCKS 6 August 2018 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. 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 13: 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. Olteanu & Niculescu Expires February 18, 2019 [Page 18] Internet-Draft SOCKS 6 August 2018 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 14: Token Request o Kind: MUST be allocated by IANA. (See Section 12.) 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 15: 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. Olteanu & Niculescu Expires February 18, 2019 [Page 19] Internet-Draft SOCKS 6 August 2018 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 16: Token Expenditure 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 17: 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. Olteanu & Niculescu Expires February 18, 2019 [Page 20] Internet-Draft SOCKS 6 August 2018 * 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 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 unsollicited 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. Olteanu & Niculescu Expires February 18, 2019 [Page 21] Internet-Draft SOCKS 6 August 2018 +---------------+--------+---------------------------+ | Kind | Length | Method | Username/Password request | +------+--------+--------+---------------------------+ | 1 | 1 | 1 | Variable | +------+--------+--------+---------------------------+ Figure 18: Password authentication via a SOCKS Option o Kind: MUST be allocated by IANA. (See Section 12.) 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 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. Security Considerations 11.1. Large requests Given the format of the request message, a malicious client could craft a request that is in excess of 100 KB and proxies could be prone to DDoS attacks. Olteanu & Niculescu Expires February 18, 2019 [Page 22] Internet-Draft SOCKS 6 August 2018 To mitigate such attacks, proxy implementations SHOULD be able to incrementally parse the requests. Proxies MAY close the connection to the client if: 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, or o the size of the initial data excedes a 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. 11.2. 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. 12. 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 This document also requests that IANA allocate a port for SOCKS over TLS. Olteanu & Niculescu Expires February 18, 2019 [Page 23] Internet-Draft SOCKS 6 August 2018 13. Acknowledgements The protocol described in this draft builds upon and is a direct continuation of SOCKS 5 [RFC1928]. 14. References 14.1. Normative References [RFC1929] Leech, M., "Username/Password Authentication for SOCKS V5", RFC 1929, DOI 10.17487/RFC1929, March 1996, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . 14.2. Informative References [I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", draft-ietf-tls-tls13-28 (work in progress), March 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, . [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, . [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, . Authors' Addresses Vladimir Olteanu University Politehnica of Bucharest Email: vladimir.olteanu@cs.pub.ro Olteanu & Niculescu Expires February 18, 2019 [Page 24] Internet-Draft SOCKS 6 August 2018 Dragos Niculescu University Politehnica of Bucharest Email: dragos.niculescu@cs.pub.ro Olteanu & Niculescu Expires February 18, 2019 [Page 25]