Securing FTP with TLS
draft-murray-auth-ftp-ssl-16
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4217.
|
|
|---|---|---|---|
| Author | Paul Ford-Hutchinson | ||
| Last updated | 2020-01-21 (Latest revision 2005-02-10) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4217 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Ted Hardie | ||
| Send notices to | <mjc@uk.ibm.com>, <tjh@cryptsoft.com>, <ericm@lne.com> |
draft-murray-auth-ftp-ssl-16
Paul Ford-Hutchinson
<draft-murray-auth-ftp-ssl-16.txt> IBM UK Ltd
INTERNET-DRAFT (draft)
13th February, 2005
This document expires on 13th August, 2005
Securing FTP with TLS
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
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IPR Statement
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
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Index
Section 1. .......... Abstract
Section 2. .......... Introduction
Section 3. .......... Audience
Section 4. .......... Overview
Section 5. .......... Session negotiation on the control port
Section 6. .......... Clearing the control port
Section 7. .......... Response to FEAT command
Section 8. .......... Data Connection Behaviour
Section 9. .......... Mechanisms for the AUTH Command
Section 10. ......... Data Connection Security
Section 11. ......... A discussion of negotiation behaviour
Section 12. ......... Who negotiates what, where and how
Section 13. ......... Timing Diagrams
Section 14. ......... Discussion of the REIN command
Section 15. ......... Discussion of the STAT and ABOR commands
Section 16. ......... Security Considerations
Section 17. ......... IANA Considerations
Section 18. ......... Other Parameters
Section 19. ......... Network Management
Section 20. ......... Internationalization
Section 21. ......... Scalability & Limits
Section 22. ......... Applicability
Section 23. ......... Acknowledgements
Section 24. ......... References
Section 25. ......... Authors' Contact Addresses
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1. Abstract
This document describes a mechanism that can be used by FTP clients
and servers to implement security and authentication using the TLS
protocol defined by [RFC-2246] and the extensions to the FTP protocol
defined by [RFC-2228]. It describes the subset of the extensions
which are required and the parameters to be used; discusses some of
the policy issues that clients and servers will need to take;
considers some of the implications of those policies and discusses
some expected behaviours of implementations to allow interoperation.
This document is intended to provide TLS support for FTP in a similar
way to that provided for SMTP in [RFC-2487] and HTTP in [RFC-2817].
That this specification is in accordance with the FTP RFC [RFC-959]
and relies on the TLS protocol [RFC-2246] and the FTP security
extensions [RFC-2228].
2. Introduction
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and
"OPTIONAL" that appear in this document are to be interpreted as
described in [RFC-2119].
This document describes how three other documents should be combined
to provide a useful, interoperable, secure file transfer protocol.
Those documents are:-
RFC 959 [RFC-959]
The description of the Internet File Transfer Protocol
RFC 2246 [RFC-2246]
The description of the Transport Layer Security protocol
(developed from the Netscape Secure Sockets Layer (SSL)
protocol version 3.0).
RFC 2228 [RFC-2228]
Extensions to the FTP protocol to allow negotiation of security
mechanisms to allow authentication, confidentiality and message
integrity.
The security extensions to FTP in [RFC-2228] offer a comprehensive
set of commands and responses that can be used to add authentication,
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integrity and confidentiality to the FTP protocol. The TLS protocol
is a popular (due to its wholesale adoption in the HTTP environment)
mechanism for generally securing a socket connection.
TLS is not the only mechanism for securing file transfer, however it
does offer some of the following positive attributes:-
- Flexible security levels. TLS can support confidentiality,
integrity, authentication or some combination of all of these.
This allows clients and servers to dynamically, during a session,
decide on the level of security required for a particular data
transfer,
- Ability to provide strong authentication of the FTP server.
- It is possible to use TLS identities to authenticate client
users and not just client hosts.
- Formalised public key management. By use of well established
client identity mechanisms (supported by TLS) during the
authentication phase, certificate management may be built into a
central function. Whilst this may not be desirable for all uses
of secured file transfer, it offers advantages in certain
structured environments.
- Co-existence and interoperation with authentication mechanisms
that are already in place for the HTTPS protocol. This allows web
browsers to incorporate secure file transfer using the same
infrastructure that has been set up to allow secure web browsing.
The TLS protocol is a development of the Netscape Communication
Corporation's SSL protocol and this document can be used to allow the
FTP protocol to be used with either SSL or TLS. The actual protocol
used will be decided by the negotiation of the protected session by
the TLS/SSL layer. This document will only refer to the TLS
protocol, however, it is understood that the Client and Server MAY
actually be using SSL if they are so configured.
There are many ways in which these three protocols can be combined.
This document selects one method by which FTP can operate securely,
while providing both flexibility and interoperation. This
necessitates a brief description of the actual negotiation mechanism;
a detailed description of the required policies and practices and a
discussion of the expected behaviours of clients and servers to allow
either party to impose their security requirements on the FTP
session.
3. Audience
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This document is aimed at developers who wish to implement TLS as a
security mechanism to secure FTP clients and/or servers.
Systems administrators and architects should be fully aware of the
security implications discussed in [RFC-2228] which need to be
considered when choosing an implementation of this protocol and
configuring it to provide their required security.
4. Overview
A full description of the FTP security protocol enhancements is
contained in [RFC-2228]. This document describes how the AUTH, PROT,
PBSZ and CCC commands, defined therein, should be implemented with
the TLS protocol.
In summary; an FTP session is established on the normal control port.
A client requests TLS with the AUTH command and then decides if it
wishes to secure the data connections by use of the PBSZ and PROT
commands. Should a client wish to make the control connection revert
back into plaintext (once the authentication phase is completed, for
example) then the CCC command can be used.
Implementation of this protocol extension does not ensure that each
and every session and data transfer is secure, it merely provides the
tools to allow a client and/or server to negotiate an acceptable or
required level of security for that given session or data transfer.
However, it is possible to have a server implementation that is
capable of refusing to operate in an insecure fashion.
5. Session negotiation on the control port
The server listens on the normal FTP control port {FTP-PORT} and the
session initiation is not secured at all. Once the client wishes to
secure the session, the AUTH command is sent and the server MAY then
allow TLS negotiation to take place.
5.1 Client wants a secured session
If a client wishes to attempt to secure a session then it SHOULD,
in accordance with [RFC-2228] send the AUTH command with the
parameter requesting TLS {TLS-PARM} ('TLS').
The client then needs to behave according to its policies depending
on the response received from the server and also the result of the
TLS negotiation. i.e. A client which receives an AUTH rejection
MAY choose to continue with the session unprotected if it so
desires.
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5.2 Server wants a secured session
The FTP protocol does not allow a server to directly dictate client
behaviour, however the same effect can be achieved by refusing to
accept certain FTP commands until the session is secured to an
acceptable level to the server.
in either case, the server response to an 'AUTH TLS' command which it
will honour, is '234'.
The '334' response as defined in [RFC-2228] implies that an ADAT
exchange will follow. This document does not use the ADAT command
and so the '334' reply is incorrect.
The FTP protocol insists that a USER command be used to identify the
entity attempting to use the ftp server. Although the TLS
negotiation may be providing authentication information the USER
command MUST still be issued by the client. However, it will be a
server implementation issue to decide which credentials to accept and
what consistency checks to make between any client cert used and the
parameter on the USER command.
[RFC-2228] states that the user must reauthorize (that is, reissue
some or all of the USER, PASS, and ACCT commands) following an AUTH
command. Additionally, this document specifies that all other
transfer parameters (other than the AUTH parameter) must be reset,
almost as if a REIN command had been issued.
Reset transfer parameters after the AUTH command include (but are
not limited to): user identity, default data ports, TYPE, STRU,
MODE and current working directory.
6. Clearing the control port
There are circumstances where it may be desirable to protect the
control connection only during part of the session and then revert
back to a plaintext connection. (This is often due to the
limitations of boundary devices such as NAT and firewalls which
expect to be able to examine the content of the control connection in
order to modify their behaviour.)
Typically the AUTH, USER, PASS, PBSZ and PROT commands would be
protected within the TLS protocol and then the CCC command would be
issued to return to a plaintext socket state. This does have
important Security Issues (which are discussed in the Security
Considerations section) but this document does describe how the
command should be used, should the client and server, having
considered the issues, still wish to use it.
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When a server receives the CCC command, it should behave as follows:
If the server does not accept CCC commands (or does not understand
them) then a 500 reply should be sent.
Otherwise, if the control connection is not protected with TLS,
then a 533 reply should be sent.
Otherwise, if the server does not wish to allow the control
connection to be cleared at this time, then a 534 reply should be
sent.
Otherwise, the server is accepting the CCC command and should do
the following:
o send a 200 reply
o Shutdown the TLS session on the socket and leave it open
o Continue the control connection in plaintext, expecting the
next command from the client to be in plaintext.
o Not accept any more PBSZ or PROT commands. All subsequent
data transfers must be protected with the current PROT
settings.
7. Response to the FEAT command
The FEAT command (introduced in [RFC-2389]) allows servers with
additional features to advertise these to a client by responding to
the FEAT command. If a server supports the FEAT command then it MUST
advertise supported AUTH, PBSZ and PROT commands in the reply as
described in section 3.2 of [RFC-2389]. Additionally, the AUTH
command should have a reply that identifies 'TLS' as one of the
possible parameters to AUTH. It is not necessary to identify the
'TLS-C' synonym separately.
Example reply (in same style as [RFC-2389])
C> FEAT
S> 211-Extensions supported
S> AUTH TLS
S> PBSZ
S> PROT
S> 211 END
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8. Data Connection Behaviour
The Data Connection in the FTP model can be used in one of three
ways. (Note: these descriptions are not necessarily placed in exact
chronological order, but do describe the steps required. - See
diagrams later for clarification.)
i) Classic FTP client/server data exchange
- The client obtains a port; sends the port number to the
server; the server connects to the client. The client issues a
send or receive request to the server on the control connection
and the data transfer commences on the data connection.
ii) Firewall-Friendly client/server data exchange (as discussed
in [RFC-1579]) using the PASV command to reverse the direction
of the data connection.
- The client requests that the server open a port; the server
obtains a port and returns the address and port number to the
client; the client connects to the server on this port. The
client issues a send or receive request on the control
connection and the data transfer commences on the data
connection.
iii) Client initiated server/server data exchange (proxy or
PASV connections)
- The client requests that server A opens a port; server A
obtains a port and returns it to the client; the client sends
this port number to server B. Server B connects to server A.
The client sends a send or receive request to server A and the
complement to server B and the data transfer commences. In
this model server A is the proxy or PASV host and is a client
for the Data Connection to server B.
For i) and ii) the FTP client MUST be the TLS client and the FTP
server MUST be the TLS server.
That is to say, it does not matter which side initiates the
connection with a connect() call or which side reacts to the
connection via the accept() call; the FTP client as defined in
[RFC-959] is always the TLS client as defined in [RFC-2246].
In scenario iii) there is a problem in that neither server A nor
server B is the TLS client given the fact that an FTP server must act
as a TLS server for Firewall-Friendly FTP [RFC-1579]. Thus this is
explicitly excluded in the security extensions document [RFC-2228],
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and in this document.
9. Mechanisms for the AUTH Command
The AUTH command takes a single parameter to define the security
mechanism to be negotiated. As the SSL/TLS protocols self-negotiate
their levels there is no need to distinguish SSL vs TLS in the
application layer. The mechanism name for negotiating TLS is the
character string identified in {TLS-PARM}. This allows the client
and server to negotiate TLS on the control connection without
altering the protection of the data channel. To protect the data
channel as well, the PBSZ command followed by the PROT command
sequence MUST be used.
Note: The data connection state MAY be modified by the client issuing
the PROT command with the new desired level of data channel
protection and the server replying in the affirmative. This data
channel protection negotiation can happen at any point in the session
(even straight after a PORT or PASV command) and as often as is
required.
See also Section 15, "IANA Considerations".
10. Data Connection Security
The Data Connection security level is determined by the PROT command
The PROT command, as specified in [RFC-2228] allows client/server
negotiation of the security level of the data connection. Once a
PROT command has been issued by the client and accepted by the
server returning the '200' reply, the security of subsequent data
connections MUST be at that level until another PROT command is
issued and accepted; the session ends; a REIN command is issued;
or the security of the session (via an AUTH command) is re-
negotiated.
Data Connection Security Negotiation (the PROT command)
Note: In line with [RFC-2228], there is no facility for securing
the Data connection with an insecure Control connection.
Specifically, the PROT command MUST be preceded by a PBSZ command
and a PBSZ command MUST be preceded by a successful security data
exchange (the TLS negotiation in this case)
The command defined in [RFC-2228] to negotiate data connection
security is the PROT command. As defined there are four values
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that the PROT command parameter can take.
'C' - Clear - neither Integrity nor Privacy
'S' - Safe - Integrity without Privacy
'E' - Confidential - Privacy without Integrity
'P' - Private - Integrity and Privacy
As TLS negotiation encompasses (and exceeds) the Safe /
Confidential / Private distinction, only Private (use TLS) and
Clear (don't use TLS) are used.
For TLS, the data connection can have one of two security levels.
1)Clear (requested by 'PROT C')
2)Private (requested by 'PROT P')
With 'Clear' protection level, the data connection is made without
TLS at all. Thus the connection is unauthenticated and has no
confidentiality or integrity. This might be the desired behaviour
for servers sending file lists, pre-encrypted data or non-
sensitive data (e.g. for anonymous FTP servers).
If the data connection security level is 'Private' then a TLS
negotiation must take place on the data connection, to the
satisfaction of the Client and Server prior to any data being
transmitted over the connection. The TLS layers of the Client and
Server will be responsible for negotiating the exact TLS Cipher
Suites that will be used (and thus the eventual security of the
connection).
In addition, the PBSZ (protection buffer size) command, as
detailed in [RFC-2228], is compulsory prior to any PROT command.
This document also defines a data channel encapsulation mechanism
for protected data buffers. For FTP-TLS, which appears to the FTP
application as a streaming protection mechanism, this is not
required. Thus the PBSZ command MUST still be issued, but must
have a parameter of '0' to indicate that no buffering is taking
place and the data connection should not be encapsulated.
Note that PBSZ 0 is not in the grammar of [RFC-2228], section
8.1, where it is stated:
PBSZ <sp> <decimal-integer> <CRLF> <decimal-integer> ::= any
decimal integer from 1 to (2^32)-1
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However it should be noted that using a value of '0' to mean a
streaming protocol is a reasonable use of '0' for that parameter
and is not ambiguous.
Initial Data Connection Security
The initial state of the data connection MUST be 'Clear' (this is
the behaviour as indicated by [RFC-2228].)
11. A Discussion of Negotiation Behaviour
As [RFC-2228] allows security qualities to be negotiated, enabled and
disabled dynamically, this can make implementations seem quite complex.
However, in any given instance the behaviour should be quite
straightforward. Either the server will be enforcing the policy of the
server host or it will be providing security capabilities requested by
the client. Either the client will be conforming to the server's
policy or will be endeavouring to provide the capabilities which the
user desires.
11.1. The server's view of the control connection
A server MAY have a policy statement somewhere that might:
- Deny any command before TLS is negotiated (this might cause
problems if a SITE or some such command is required prior to
login)
- Deny certain commands before TLS is negotiated (such as USER,
PASS or ACCT)
- Deny insecure USER commands for certain users (e.g. not
ftp/anonymous)
- Deny secure USER commands for certain users (e.g.
ftp/anonymous)
- Define the level(s) of TLS to be allowed
- Define the CipherSuites allowed to be used (perhaps on a per
host/domain/... basis)
- Allow TLS authentication as a substitute for local
authentication.
- Define data connection policies (see next section)
It is possible that the TLS negotiation may not be completed
satisfactorily for the server, in which case it can be one of
these states.
The TLS negotiation failed completely
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In this case, the control connection should still be up
in unprotected mode and the server SHOULD issue an
unprotected '421' reply to end the session.
The TLS negotiation completed successfully, but the server
decides that the session parameters are not acceptable (e.g.
Distinguished Name in the client certificate is not
permitted to use the server)
In this case, the control connection should still be up
in a protected state, so the server MAY either continue
to refuse to service commands or issue a protected '421'
reply and close the connection.
The TLS negotiation failed during the TLS handshake
In this case, the control connection is in an unknown
state and the server SHOULD simply drop the control
connection.
Server code will be responsible for implementing the required
policies and ensuring that the client is prevented from
circumventing the chosen security by refusing to service those
commands which are against policy.
11.2. The server's view of the data connection
The server can take one of four basic views of the data connection
1 - Don't allow encryption at all (in which case the PROT
command should not allow any value other than 'C' - if it is
allowed at all)
2 - Allow the client to choose protection or not
3 - Insist on data protection (in which case the PROT command
must be issued prior to the first attempted data transfer)
4 - Decide on one of the above three for each and every data
connection
The server SHOULD only check the status of the data protection
level (for options 3 and 4 above) on the actual command that will
initiate the data transfer (and not on the PORT or PASV). The
following commands, defined in [RFC-959] cause data connections to
be opened and thus may be rejected (before any 1xx) message due to
an incorrect PROT setting.
STOR
RETR
NLST
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LIST
STOU
APPE
The reply to indicate that the PROT setting is incorrect is
'521 data connection cannot be opened with this PROT setting'
If the protection level indicates that TLS is required, then it
should be negotiated once the data connection is made. Thus, the
'150' reply only states that the command can be used given the
current PROT level. Should the server not like the TLS
negotiation then it will close the data port immediately and
follow the '150' command with a '522' reply indicating that the
TLS negotiation failed or was unacceptable. (Note: this means
that the application can pass a standard list of CipherSuites to
the TLS layer for negotiation and review the one negotiated for
applicability in each instance).
The Security Considerations section discusses the issue of cross-
checking any certificates used to authenticate the data connection
with the one(s) used to authenticate the control connection. This
is an important security step.
It is quite reasonable for the server to insist that the data
connection uses a TLS cached session. This might be a cache of a
previous data connection or of a cleared control connection. If
this is the reason for the refusal to allow the data transfer then
the '522' reply should indicate this.
Note: this has an important impact on client design, but allows
servers to minimise the cycles used during TLS negotiation by
refusing to perform a full negotiation with a previously
authenticated client.
It should be noted that the TLS authentication of the server will
be authentication of the server host itself and not a user on the
server host.
11.3. The client's view of the control connection
In most cases it is likely that the client will be using TLS
because the server would refuse to interact insecurely. To allow
for this, clients SHOULD be able to be flexible enough to manage
the securing of a session at the appropriate time and still allow
the user/server policies to dictate exactly when in the session
the security is negotiated.
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In the case where it is the client that is insisting on the
securing of the session, it will need to ensure that the
negotiations are all completed satisfactorily and will need to be
able to inform the user sensibly should the server not support, or
be prepared to use, the required security levels.
Clients SHOULD be coded in such a manner as to allow the timing of
the AUTH, PBSZ and PROT commands to be flexible and dictated by
the server. It is quite reasonable for a server to refuse certain
commands prior to these commands, similarly it is quite possible
that a SITE or quoted command might be needed by a server prior to
the AUTH. A client MUST allow a user to override the timing of
these commands to suit a specific server.
For example, a client SHOULD NOT insist on sending the AUTH as the
first command in a session, nor should it insist on issuing a
PBSZ, PROT pair directly after the AUTH. This may well be the
default behaviour, but must be overridable by a user.
Note: The TLS negotiation may not be completed satisfactorily for
the client, in which case it will be in one of these states:
The TLS negotiation failed completely
In this case, the control connection should still be up
in unprotected mode and the client should issue an
unprotected QUIT command to end the session.
The TLS negotiation completed successfully, but the client
decides that the session parameters are not acceptable (e.g.
Distinguished Name in certificate is not the actual server
expected)
In this case, the control connection should still be up
in a protected state, so the client should issue a
protected QUIT command to end the session.
The TLS negotiation failed during the TLS handshake
In this case, the control connection is in an unknown
state and the client should simply drop the control
connection.
11.4. The client's view of the data connection
Client security policies
Clients do not typically have 'policies' as such, instead they
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rely on the user defining their actions and, to a certain extent,
are reactive to the server policy. Thus a client will need to
have commands that will allow the user to switch the protection
level of the data connection dynamically, however, there may be a
general 'policy' that attempts all LIST and NLST commands on a
Clear connection first (and automatically switches to Private if
it fails). In this case there would need to be a user command
available to ensure that a given data transfer was not attempted
on an insecure data connection.
Clients also need to understand that the level of the PROT setting
is only checked for a particular data transfer after that transfer
has been requested. Thus a refusal by the server to accept a
particular data transfer should not be read by the client as a
refusal to accept that data protection level completely, as not
only may other data transfers be acceptable at that protection
level, but it is entirely possible that the same transfer may be
accepted at the same protection level at a later point in the
session.
It should be noted that the TLS authentication of the client
should be authentication of a user on the client host and not the
client host itself.
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12. Who negotiates what, where and how
12.1. Do we protect at all ?
Client issues 'AUTH TLS', server accepts or rejects.
If server needs AUTH, then it refuses to accept certain commands
until it gets a successfully protected session.
12.2. What level of protection do we use on the Control connection ?
Decided entirely by the TLS CipherSuite negotiation.
12.3. Do we protect data connections in general ?
Client issues PROT command, server accepts or rejects.
12.4. Is protection required for a particular data transfer ?
A client would already have issued a PROT command if it required
the connection to be protected.
If a server needs to have the connection protected then it will
reply to the STOR/RETR/NLST/... command with a '522' indicating
that the current state of the data connection protection level is
not sufficient for that data transfer at that time.
12.5. What level of protection is required for a particular data
transfer ?
Decided entirely by the TLS CipherSuite negotiation.
Thus it can be seen that, for flexibility, it is desirable for the
FTP application to be able to interact with the TLS layer upon which
it sits to define and discover the exact TLS CipherSuites which are
to be/have been negotiated and make decisions accordingly.
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13. Timing Diagrams
These timing diagrams aim to help explain exactly how the TLS handshake
and session protection fits into the existing logic of the FTP protocol.
Of course, the FTP protocol itself is not well described with respect to
timing of commands and responses in [RFC-959], so this is partly based
on empirical observation of existing widespread client and server
implementations.
13.1. Establishing a protected session
Client Server
control data data control
====================================================================
socket()
bind()
socket()
connect() ----------------------------------------------> accept()
<---------------------------------------------- 220
AUTH TLS ---------------------------------------------->
<---------------------------------------------- 234
TLSneg() <----------------------------------------------> TLSneg()
PBSZ 0 ---------------------------------------------->
<---------------------------------------------- 200
PROT P ---------------------------------------------->
<---------------------------------------------- 200
USER fred ---------------------------------------------->
<---------------------------------------------- 331
PASS pass ---------------------------------------------->
<---------------------------------------------- 230
Note 1: the order of the PBSZ/PROT pair and the USER/PASS pair (with
respect to each other) is not important (i.e. the USER/PASS can happen
prior to the PBSZ/PROT - or indeed the server can refuse to allow a
PBSZ/PROT pair until the USER/PASS pair has happened).
Note 2: the PASS command might not be required at all (if the USER
parameter and any client identity presented provide sufficient
authentication). The server would indicate this by issuing a '232'
reply to the USER command instead of the '331' which requests a PASS
from the client. (see below)
Note 3: the AUTH command might not be the first command after the
receipt of the 220 welcome message.
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13.2. Establishing a protected session without a password request
(the TLS authentication is sufficient)
Client Server
control data data control
====================================================================
socket()
bind()
socket()
connect() ----------------------------------------------> accept()
<---------------------------------------------- 220
AUTH TLS ---------------------------------------------->
<---------------------------------------------- 234
TLSneg() <----------------------------------------------> TLSneg()
PBSZ 0 ---------------------------------------------->
<---------------------------------------------- 200
PROT P ---------------------------------------------->
<---------------------------------------------- 200
USER fred ---------------------------------------------->
<---------------------------------------------- 232
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13.3. Establishing a protected session and then clearing with the CCC
command
Client Server
control data data control
====================================================================
socket()
bind()
socket()
connect() ----------------------------------------------> accept()
<---------------------------------------------- 220
AUTH TLS ---------------------------------------------->
<---------------------------------------------- 234
TLSneg() <----------------------------------------------> TLSneg()
PBSZ 0 ---------------------------------------------->
<---------------------------------------------- 200
PROT P ---------------------------------------------->
<---------------------------------------------- 200
USER fred ---------------------------------------------->
<---------------------------------------------- 232
CCC ---------------------------------------------->
<---------------------------------------------- 200
TLSshutdown() <-------------------------------------> TLSshutdown()
- rest of control session continues in plaintext with protected data
transfers (due to PROT P)
Note: this has serious security issues (see Security Considerations
section) but may be useful in a firewall/NAT scenario.
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13.4. A standard data transfer without protection.
Client Server
control data data control
====================================================================
socket()
bind()
PORT w,x,y,z,a,b ----------------------------------------->
<----------------------------------------------------- 200
STOR file ------------------------------------------------>
socket()
bind()
<----------------------------------------------------- 150
accept() <----------- connect()
write() -----------> read()
close() -----------> close()
<----------------------------------------------------- 226
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13.5. A firewall-friendly data transfer without protection
Client Server
control data data control
====================================================================
PASV -------------------------------------------------------->
socket()
bind()
<------------------------------------------ 227 (w,x,y,z,a,b)
socket()
STOR file --------------------------------------------------->
connect() ----------> accept()
<-------------------------------------------------------- 150
write() ----------> read()
close() ----------> close()
<-------------------------------------------------------- 226
Note: Implementers should be aware that then connect()/accept()
function is performed prior to the receipt of the reply from the
STOR command. This contrasts with situation when (non-firewall-
friendly) PORT is used prior to the STOR, and the accept()/connect()
is performed after the reply from the aforementioned STOR has been
dealt with.
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13.6. A standard data transfer with protection
Client Server
control data data control
====================================================================
socket()
bind()
PORT w,x,y,z,a,b -------------------------------------------->
<-------------------------------------------------------- 200
STOR file --------------------------------------------------->
socket()
bind()
<-------------------------------------------------------- 150
accept() <---------- connect()
TLSneg() <----------> TLSneg()
TLSwrite() ----------> TLSread()
TLSshutdown() -------> TLSshutdown()
close() ----------> close()
<-------------------------------------------------------- 226
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13.7. A firewall-friendly data transfer with protection
Client Server
control data data control
====================================================================
PASV -------------------------------------------------------->
socket()
bind()
<------------------------------------------ 227 (w,x,y,z,a,b)
socket()
STOR file --------------------------------------------------->
connect() ----------> accept()
<-------------------------------------------------------- 150
TLSneg() <---------> TLSneg()
TLSwrite() ---------> TLSread()
TLSshutdown() -------> TLSshutdown()
close() ---------> close()
<-------------------------------------------------------- 226
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14. Discussion of the REIN command
The REIN command, defined in [RFC-959], allows the user to reset the
state of the FTP session. From [RFC-959]:
REINITIALIZE (REIN)
This command terminates a USER, flushing all I/O and account
information, except to allow any transfer in progress to be
completed. All parameters are reset to the default settings
and the control connection is left open. This is identical to
the state in which a user finds himself immediately after the
control connection is opened. A USER command may be expected
to follow.
When this command is processed by the server, the TLS session(s)
MUST be cleared and the control and data connections revert to
unprotected, clear communications. It MAY be acceptable to use
cached TLS sessions for subsequent connections, however a server MUST
NOT mandate this.
If the REIN command is being used to clear a TLS session then the
reply to the REIN command MUST be sent in a protected session prior
to the session(s) being cleared.
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15. Discussion of the STAT and ABOR commands
The ABOR and STAT commands and the use of TCP Urgent Pointers
[RFC-959] describes the use of Telnet commands (IP and DM) and the
TCP Urgent pointer to indicate the transmission of commands on the
control channel during the execution of a data transfer. FTP uses
the Telnet Interrupt Process and Data Mark commands in conjunction
with Urgent data to preface two commands: ABOR (Abort Transfer)
and STAT (Status request).
The Urgent Pointer was used because in a Unix implementation the
receipt of a TCP packet marked as Urgent would result in the
execution of the SIGURG interrupt handler. This reliance on
interrupt handlers was necessary on systems which did not
implement select() or did not support multiple threads. TLS does
not support the notion of Urgent data.
When TLS is implemented as a security method in FTP the server
SHOULD NOT rely on the use of SIGURG to process input on the
control channel during data transfers. The client MUST send all
data including Telnet commands across the TLS session.
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16. Security Considerations
This document discusses how TLS may be used in conjunction with
[RFC-2228] to provide mechanisms for securing FTP sessions.
Discussions about security rationale and security properties are
contained within the [RFC-2228] document and are not repeated here.
16.1. Verification of Authentication tokens
In this section, we assume that X.509 certificates will be used for
the TLS authentication. If some other identity token is used (e.g.
kerberos tickets - see [RFC-2712]) then similar, mechanism specific,
considerations will need to be made.
16.1.1. Server Certificates
- Although it is entirely an implementation decision, it is
recommended that certificates used for server authentication of
the TLS session contain the server identification information
in a similar manner to those used for http servers. (see
[RFC-2818])
- It is strongly recommended that the certificate used for
server authentication of Data connections is the same
certificate as that used for the corresponding Control
connection. If different certificates are to be used, there
should be some other mechanism which the client can use to
cross-check the data and control connection server identities.
- If Server Certificates are not used, then many of the
security benefits will not be realised. For Example, in an
anonymous Diffie-Hellman environment, there is no server
identity authentication and so there is little protection
against man-in-the-middle attacks.
16.1.2. Client Certificates
- Deciding which client certificates to allow and defining
which fields define what authentication information is entirely
a server implementation issue.
- However, it is strongly recommended that the certificate used
for client authentication of Data connections is the same
certificate as that used for the corresponding Control
connection. If different certificates are to be used, there
should be some other mechanism which the server can use to
cross-check the data and control connection client identities.
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- If Client Certificates are not used, then many of the
security benefits will not be realised. For Example, it would
still be possible for a malicious client to hijack a data
connection.
16.2. Addressing FTP Security Considerations [RFC-2577]
16.2.1. Bounce Attack
A bounce attack should be harder in a secured FTP environment
because:
- The FTP server that is being used to initiate a false
connection will always be a 'server' in the TLS context.
Therefore, only services that act as 'clients' in the TLS
context could be vulnerable. This would be a counter-
intuitive way to implement TLS on a service.
- The FTP server would detect that the authentication
credentials for the data connection are not the same as
those for the control connection, thus the server policies
could be set to drop the data connection.
- Genuine users are less likely to initiate such attacks
when the authentication is strong and malicious users are
less likely to gain access to the FTP server if the
authentication is not easily subverted (password guessing,
network tracing, etc...)
16.2.2. Restricting Access
This document presents a strong mechanism for solving the issue
raised in this section.
16.2.3. Protecting Passwords
The twin solutions of strong authentication and data
confidentiality ensure that this is not an issue when TLS is
used to protect the control session.
16.2.4. Privacy
The TLS protocol ensures data confidentiality by encryption.
Privacy (e.g. access to download logs, user profile
information, etc...) is outside the scope of this document (and
[RFC-2577] presumably)
16.2.5. Protecting Usernames
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This is not an issue when TLS is used as the primary
authentication mechanism.
16.2.6. Port Stealing
This specification will do little for the Denial of Service
element of this section, however, strong authentication on the
data connection will prevent unauthorised connections
retrieving or submitting files. Of course, this is only the
case where strong client authentication is being used. If
client certificates are not used then port stealing by a rogue
client is still a problem. If no strong authentication is in
use at all (e.g. anonymous Diffie-Hellman) then the port
stealing problem will still remain.
16.2.7. Software-Base Security Problems
Nothing in this specification will affect the discussion in
this section.
16.3. Issues with the CCC command
Using the CCC command can create security issues. For a full
description, see the "CLEAR COMMAND CHANNEL (CCC)" section of
[RFC-2228]. Clients should not assume that a server will allow
the CCC command to be processed.
Server implementations may wish to refuse to process the CCC
command on a session which has not passed through some form of
client authentication (e.g. TLS client auth or FTP USER/PASS).
This can prevent anonymous clients from repeatedly requesting AUTH
TLS followed by CCC to tie up resources on the server.
17. IANA Considerations
{FTP-PORT} - The port assigned to the FTP control connection is 21.
18. Other Parameters
{TLS-PARM} - The parameter for the AUTH command to indicate that TLS
is required. To request the TLS protocol in accordance with this
document, the client MUST use 'TLS'
To maintain backward compatibility with older versions of this
document, the server SHOULD accept 'TLS-C' as a synonym for 'TLS'
Note - [RFC-2228] states that these parameters are case-
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insensitive.
19. Network Management
NONE
20. Internationalization
NONE
21. Scalability & Limits
There are no issues other than those concerned with the ability of
the server to refuse to have a complete TLS negotiation for each and
every data connection, which will allow servers to retain throughput
whilst using cycles only when necessary.
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22. Applicability
This mechanism is generally applicable as a mechanism for securing
the FTP protocol. It is unlikely that anonymous FTP clients or
servers will require such security (although some might like the
authentication features without the confidentiality).
23. Acknowledgements
o Netscape Communications Corporation for the original SSL protocol.
o Eric Young for the SSLeay libraries.
o University of California, Berkley for the original implementations
of FTP and ftpd on which the initial implementation of these
extensions were layered.
o IETF CAT working group.
o IETF TLS working group.
o IETF FTPEXT working group.
o Jeff Altman for the ABOR and STAT discussion.
o The various people who have help author this document throughout
its protracted draft stages, namely Martin Carpenter, Eric Murray,
Tim Hudson and Volker Wiegand.
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24. References
24.1. Normative References
[RFC-959] J. Postel, "File Transfer Protocol"
RFC 959, October 1985.
[RFC-2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels"
RFC 2119, March 1997.
[RFC-2228] M. Horowitz, S. Lunt, "FTP Security Extensions"
RFC 2228, October 1997.
[RFC-2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0"
RFC 2246, January 1999.
[RFC-2389] P Hethmon, R.Elz, "Feature Negotiation Mechanism for
the File Transfer Protocol"
RFC 2389, August 1998.
24.2. Informative References
[RFC-1579] S. Bellovin, "Firewall-Friendly FTP"
RFC 1579, February 1994.
[RFC-2222] J. Myers, "Simple Authentication and Security Layer"
RFC 2222, October 1997.
[RFC-2487] P Hoffman, "SMTP Service Extension for Secure SMTP over
TLS"
RFC 2487, January 1999.
[RFC-2577] M Allman, S Ostermann, "FTP Security Considerations"
RFC 2577, May 1999.
[RFC-2712] A. Medvinsky, M. Hur, "Addition of Kerberos Cipher
Suites to Transport Layer Security (TLS)"
RFC 2712, October 1999.
[RFC-2817] R. Khare, S. Lawrence, "Upgrading to TLS Within
HTTP/1.1"
RFC 2817, May 2000.
[RFC-2818] E. Rescorla, "HTTP Over TLS"
RFC 2818, May 2000.
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25. Author's Contact Address
The FTP-TLS draft information site is at http://www.ford-
hutchinson.com/~fh-1-pfh/ftps-ext.html
Please send comments to Paul Ford-Hutchinson at the address below
Paul Ford-Hutchinson
IBM UK Ltd
PO Box 31
Birmingham Road
Warwick
United Kingdom
tel - +44 1926 462005
email - paulfordh@uk.ibm.com
26.1. Additional Authors' Contact Addresses
Tim Hudson Volker Wiegand
RSA Data Security SuSE Linux
Australia Pty Ltd
tel - +61 7 3227 4444
email - tjh@rsasecurity.com.au wiegand@suse.de
Martin Carpenter Eric Murray
Verisign Ltd Wave Systems Inc.
email - mcarpenter@verisign.com ericm@lne.com
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Full Copyright Statement
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This document expires on 13th August, 2005
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