SOCKS5: SOCKS Protocol Version 5
A general framework that lets TCP and UDP applications pass through a firewall via a SOCKS server, with pluggable authentication.
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>.
@misc{rfc1928,
series = {Request for Comments},
number = 1928,
howpublished = {RFC 1928},
publisher = {RFC Editor},
doi = {10.17487/RFC1928},
url = {https://www.rfc-editor.org/info/rfc1928},
author = {M. Leech and M. Ganis and Y. Lee and R. Kuris and D. Koblas and L. Jones},
title = {{SOCKS Protocol Version 5}},
pagetotal = 9,
year = 1996,
month = mar,
} RFC text
Contents
Network Working Group M. Leech
Request for Comments: 1928 Bell-Northern Research Ltd
Category: Standards Track M. Ganis
International Business Machines
Y. Lee
NEC Systems Laboratory
R. Kuris
Unify Corporation
D. Koblas
Independent Consultant
L. Jones
Hewlett-Packard Company
March 1996
SOCKS Protocol Version 5
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. 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.
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.
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.
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.
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.
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 | RSV | ATYP | DST.ADDR | DST.PORT |
+----+-----+-------+------+----------+----------+
| 1 | 1 | X'00' | 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 RSV RESERVED
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'
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 | RSV | ATYP | BND.ADDR | BND.PORT |
+----+-----+-------+------+----------+----------+
| 1 | 1 | X'00' | 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 RSV RESERVED
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
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 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.
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.
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.
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
AI ELI5
On this page
§1Why SOCKS Exists
Network firewalls isolate an organization's internal network from an outside network such as the Internet. They typically act as application-layer gateways offering controlled TELNET, FTP and SMTP access. As more sophisticated application protocols appeared, a general framework was needed for them to traverse a firewall transparently and securely, with strong authentication as fine-grained as practical.
SOCKS version 5 is conceptually a shim layer between the application layer and the transport layer. For that reason it does not provide network-layer gateway services such as forwarding ICMP messages.
It evolves SOCKS version 4, which gave unsecured firewall traversal for TCP applications. Version 5 adds UDP, a framework for generalized strong authentication, and addressing by domain name and IPv6 address. Using SOCKS typically means recompiling or relinking TCP client applications with a SOCKS library.
A company keeps its computers behind a wall. SOCKS is the single gate in that wall, and it checks who you are before it lets your messages through.
§2Connecting and Choosing Authentication
A TCP client that needs to reach something only reachable through the firewall opens a TCP connection to the SOCKS server, conventionally on TCP port 1080. It then negotiates an authentication method, authenticates, and sends a relay request. The server either sets up the connection or denies it.
The client first sends a version identifier and method selection message: a VER octet (X'05'), an NMETHODS octet and one to 255 method octets. The server picks one and replies with VER and METHOD. If the chosen method is X'FF', none of the offered methods is acceptable and the client MUST close the connection.
Compliant implementations MUST support GSSAPI and SHOULD support USERNAME/PASSWORD. After selection, the client and server run a method-specific sub-negotiation, described in separate memos. If the method provides encapsulation for integrity or confidentiality, later requests and replies MUST use it.
First you knock and say which ways you can prove who you are, such as a password. The doorman picks one, or tells you none of them is good enough and you must leave.
- VER, NMETHODS, METHODSOffers authentication methods.
- VER, METHODX'FF' means none acceptable.
- Method sub-negotiationDefined in separate memos.
- Request (CMD, ATYP, DST.ADDR, DST.PORT)
- Reply (REP, ATYP, BND.ADDR, BND.PORT)
§3Requests and Addresses
Once authentication is done, the client sends a request: VER, CMD, a reserved octet X'00', ATYP, DST.ADDR and a two-octet DST.PORT in network octet order. There are three commands.
CONNECT(X'01') asks the server to connect to the destination.BIND(X'02') is for protocols where the client must accept a connection from the server. FTP is the example, with data transfers coming back from server to client. It is expected to be used only for secondary connections after a primary one is set up with CONNECT.UDP ASSOCIATE(X'03') sets up a UDP relay association.
The ATYP field says what kind of address follows: X'01' is an IPv4 address of 4 octets, X'04' is an IPv6 address of 16 octets, and X'03' is a fully-qualified domain name whose first octet gives the number of name octets that follow, with no terminating NUL.
The server typically evaluates the request using the source and destination addresses.
After you are let in, you tell the doorman what you want: call this address, wait here for someone to call me, or pass along some quick postcards. You can name the place by number or by its name.
§4Replies and Command Behaviour
The server answers with VER, REP, a reserved octet, ATYP, BND.ADDR and BND.PORT. REP of X'00' means success; other values name the failure. Reserved fields are set to X'00'.
For CONNECT, the bound address and port are what the server assigned to connect to the target. This address often differs from the one the client used to reach the server, since such servers are often multi-homed.
For BIND, the server sends two replies. The first comes after it creates and binds a socket and gives the address and port it is listening on, which the client typically passes to the application server over the primary connection. The second comes after the incoming connection succeeds or fails and carries the address and port of the connecting host.
For UDP ASSOCIATE, DST.ADDR and DST.PORT say where the client expects to send UDP from; if it does not know yet it MUST use all zeros. The reply's bound address and port say where the client MUST send UDP messages to be relayed. The association ends when the TCP connection that carried the request ends.
When a reply signals failure, the server MUST terminate the TCP connection shortly after, no more than 10 seconds after detecting the cause. After a successful CONNECT or BIND the client may pass data, using method-dependent encapsulation if the method has any.
The doorman tells you whether it worked, and if not, why: the line was busy, the place does not exist, or the rules say no. If he says no, he hangs up soon after.
- X'00'succeeded — The request worked.
- X'01'general SOCKS server failure — Unspecified server problem.
- X'02'connection not allowed by ruleset — Refused by the server's rules.
- X'03'Network unreachable — No route to the network.
- X'04'Host unreachable — The host could not be reached.
- X'05'Connection refused — The target refused the connection.
- X'06'TTL expired — Time to live ran out.
- X'07'Command not supported — The CMD is not supported.
- X'08'Address type not supported — The ATYP is not supported.
§5Relaying UDP Datagrams
A UDP client MUST send its datagrams to the UDP port given in BND.PORT of the UDP ASSOCIATE reply, wrapped in any method-dependent encapsulation. Each datagram carries a UDP request header: a two-octet reserved field X'0000', a one-octet FRAG, ATYP, DST.ADDR, DST.PORT and then the data.
The relay forwards datagrams silently, with no notice to the client, and drops those it cannot or will not relay. Replies from remote hosts are wrapped in the same header. The relay MUST learn from the SOCKS server the expected client IP address for the association and MUST drop datagrams from any other source address.
Fragmentation is optional. In FRAG, the high-order bit marks the end of a fragment sequence, X'00' means a standalone datagram, and values 1 to 127 give the position. Receivers keep a reassembly queue and timer, and the timer MUST be no less than 5 seconds. An implementation without fragmentation MUST drop any datagram whose FRAG is not X'00'. Applications are advised to avoid fragmentation.
A SOCKS-aware UDP interface MUST report available buffer space smaller than the real space: by 10 octets plus method overhead for IPv4, 262 for domain names and 20 for IPv6.
For quick postcards, you hand them to a special mailbox the doorman names. He forwards them and brings back replies, but he only accepts mail from the person he expects.
§6Security Considerations
The security of firewall traversal with SOCKS depends heavily on the authentication and encapsulation methods an implementation offers and those chosen during negotiation. The RFC adds only that administrators should consider the choice of authentication methods carefully.
How safe the gate is depends on how good the ID check is, so the people running it should pick the check carefully.