Paul Ford-Hutchinson
<draft-murray-auth-ftp-ssl-05.txt> IBM UK Ltd
Martin Carpenter
Verisign Ltd
Tim Hudson
INTERNET-DRAFT (draft) RSA Australia Ltd
Eric Murray
Wave Systems Inc
Volker Wiegand
SuSE Linux
26th January, 2000
This document expires on 26th July, 2000
Securing FTP with TLS
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
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Index
1. .......... Abstract
2. .......... Introduction
3. .......... Audience
4. .......... Session negotiation on the control port
5. .......... Response to FEAT command
6. .......... Data Connection Behaviour
7. .......... Mechanisms for the AUTH Command
8. .......... SASL Considerations
9. .......... Data Connection Security
10. ......... A discussion of negotiation behaviour
11. ......... Who negotiates what, where and how
12. ......... Timing Diagrams
13. ......... Implications of [FTP-EXT]
14. ......... Discussion of the 'REIN' command
15. ......... Security Considerations
16. ......... IANA Considerations
17. ......... Network Management
18. ......... Internationalization
19. ......... Scalability & Limits
20. ......... Applicability
21. ......... Acknowledgements
22. ......... References
23. ......... Authors' Contact Addresses
Appendices
A. .......... Summary of [RFC-2246]
B. .......... Summary of [RFC-2228]
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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
that 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].
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 privacy, 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,
- It is possible to use X.509 certificates to authenticate client
users and not just client hosts.
- Formalised public key management. By use of X.509 public
certificates during the authentication phase, certificate
management can 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 such as access to
corporate data sources.
- 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.
Note 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].
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2. Introduction
This document is an attempt to describe how three other documents
should 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, privacy and message
integrity.
The File Transfer Protocol (FTP) currently defined in [RFC-959] and
in place on the Internet is an excellent mechanism for exchanging
files. The security extensions to FTP in [RFC-2228] offer a
comprehensive set of commands and responses that can be used to add
authentication, integrity and privacy 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.
There are many ways in which these three protocols can be combined
which would ensure that interoperation is impossible. This document
describes one method by which FTP can operate securely in such a way
as to provide both flexibility and interoperation. This necessitates
a brief description of the actual negotiation mechanism (if used); a
much more detailed description of the policies and practices that
would be required 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
This document is aimed at developers who wish to use TLS as a
security mechanism to secure FTP clients and/or servers.
4. Session negotiation on the control port
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4.1 Negotiated Session Security
In this scenario, 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.
4.1.1 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 or SSL {TLS-PARM}.
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.
4.1.2 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.
4.2 Implicit Session Security
In this scenario, the server listens on a distinct port {FTP-
TLSPORT} to the normal unsecured FTP server. Upon connection, the
client is expected to start the TLS negotiation. If the
negotiation fails or succeeds at an unacceptable level of security
then it will be a client and/or server policy decision to
disconnect the session.
5. 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 'SSL', 'TLS-P' or 'TLS-C' parameters separately.
Example reply (in same style is [RFC-2389])
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C> FEAT
S> 211-Extensions supported
S> AUTH TLS
S> PBSZ
S> PROT
S> 211 END
6. 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 will be the TLS client and the FTP
server will be the TLS server.
That is to say, it does not matter which side initiates the
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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],
and in this document.
7. 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 proposed mechanism name for negotiating
SSL/TLS will be the character string 'TLS'. This will allow the
client and server to negotiate SSL or TLS on the control connection
without altering the protection of the data channel. To protect the
data channel as well, the PBSZ, PROT command sequence should be used.
We call this "Explicit Data Channel Protection".
However, there are clients and servers that exist today which use the
string 'SSL' to indicate that negotiation should take place on the
control connection and that the data connection should be implicitly
protected (i.e. the PBSZ 0, PROT P command sequence is not required
but the client and server will protect the data channel as if it
had). This is "Implicit Data Channel Protection" and is included
primarily for backward compatibility.
To allow for streamlining of the negotiation, whilst allowing the
'SSL' string to sink peacefully into disuse, the strings 'TLS-P' and
'TLS-C' will also be defined. 'TLS-C' will be a synonym for 'TLS'
and 'TLS-P' a synonym for 'SSL'. Thus we allow for strict compliance
with [RFC-2228] by use of 'TLS' or 'TLS-C' and a quicker (2 less
commands) and perhaps more sensible option 'TLS-P' which will
implicitly secure the data connection at the same time as securing
the control connection.
Note: Regardless of the manner in which the data connection is
secured (either implicitly by use of 'TLS-P', 'SSL' or connection to
a well-known port for FTP protocol over TLS, or explicitly by use of
the PBSZ/PROT sequence) 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
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the session (even straight after a PORT or PASV command) and as often
as is required.
See also Section 16, "IANA Considerations".
8. SASL Considerations
SASL is the Simple Authentication Security Layer. Currently, its
definition can be found in the internet draft [RFC-2222]. This
document attempts to define the means by which a connection-based
protocol may identify and authenticate a client user to a server,
with additional optional negotiation of protection for the remainder
of that session.
Unfortunately, the SASL paradigm does not fit in neatly with the
FTP-TLS protocol, mainly due to the fact that FTP uses two
(independent) connections, and under FTP-TLS these may be at
different (and possibly renegotiable) protection levels.
Consequently, it is envisaged that SASL will sit underneath TLS on
the control connection, and TLS (on both, either or neither
connection) will be used for privacy and integrity (with optional
authentication from TLS on either connection).
9. Data Connection Security
The Data Connection security level is determined by two factors.
1) The mechanism used to negotiate security on the control
connection will dictate the default (i.e. un-negotiated) security
level of the data port.
2) 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 by returning the '200' reply, the security
of subsequent data connections should be at that level until
another PROT command is issued and accepted; the session ends; 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.
The command defined in [RFC-2228] to negotiate data connection
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security is the PROT command. As defined there are four values
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
2)Private
With 'Clear' protection level, the data connection is made without
TLS at all. Thus the connection is unauthenticated and has no
privacy 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, 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
However it should be noted that using a value of '0' to mean a
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streaming protocol is a reasonable use of '0' for that parameter
and is not ambiguous.
Initial Data Connection Security
For backward compatibility and ease of implementation the
following rules govern the initial expected protection setting of
the data connection.
Connections accepted on the 'secure FTP' port (see
{FTP-TLSPORT}).
The initial state of the data connection will be 'Private'
(Although this does not follow [RFC-2228], this is how such
clients tend to work today).
Connections accepted on the normal FTP port {FTP-PORT} with
TLS/SSL negotiated via an 'AUTH SSL' command.
The initial state of the data connection will be 'Private'
(Although this does not follow [RFC-2228], this is how such
clients tend to work today).
Connections accepted on the normal FTP port {FTP-PORT} with
TLS/SSL negotiated via an 'AUTH TLS' command.
The initial state of the data connection will be 'Clear'
(this is the correct behaviour as indicated by [RFC-2228].)
Note: Connections made on other ports may be still behave in one
of these ways, but that will be a local configuration issue.
10. A Discussion of Negotiation Behaviour
All these discussions assume that the negotiation has taken place by
issuing the AUTH command with a mechanism that does not implicitly
protect the data channel. Using a mechanism which does implicitly
secure the data channel or connecting to a port which is implicitly
protected will have similar issues.
10.1. 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
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ftp/anonymous)
- Deny secure USER commands for certain users (e.g.
ftp/anonymous)
- Define the level(s) of TLS/SSL 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)
Note: 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
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 can either continue to refuse to
service commands or issue a '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 that are against policy.
10.2. 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
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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 cause data connections to be opened and thus
may be rejected (before any 1xx) message due to an incorrect PROT
setting.
STOR
RETR
NLST
LIST
STOU
APPE
MLST (if [FTP-EXT] is implemented)
MLSD (if [FTP-EXT] is implemented)
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).
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 the control connection. If this is
the reason for the 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.
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10.3. 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 must 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.
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 must 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
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and the client should simply drop the control connection.
10.4. The client's view of the data connection
Client security policies
Clients do not typically have 'policies' as such, instead they
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 in toto, 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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11. Who negotiates what, where and how
11.1. Do we protect at all ?
Client issues AUTH <Mechanism>, server accepts or rejects.
If server needs AUTH, then it refuses to accept certain commands
until it gets a successfully protected session.
11.2. What level of protection do we use on the Control connection ?
Decided entirely by the TLS CipherSuite negotiation.
11.3. Do we protect data connections in general ?
Client issues PROT command, server accepts or rejects.
11.4. 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.
11.5. 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 that are to
be/have been negotiated and make decisions accordingly. However it
should be entirely possible, using the mechanisms described in this
document, to have a TLS client or server sitting on top of a generic
'TLS socket layer'. In this case, interoperability for a client with
a smart TLS-aware server may not be possible due to server policies.
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12. Timing Diagrams
12.1. Establishing a protected session
Client Server
control data data control
====================================================================
socket()
bind()
socket()
connect() ----------------------------------------------> accept()
AUTH TLS ---------------------------------------------->
<---------------------------------------------- 234
TLSneg() <----------------------------------------------> TLSneg()
PBSZ 0 ---------------------------------------------->
<---------------------------------------------- 200
PROT P ---------------------------------------------->
<---------------------------------------------- 200
USER fred ---------------------------------------------->
<---------------------------------------------- 331
PASS pass ---------------------------------------------->
<---------------------------------------------- 230
Note: 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).
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12.2. 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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12.3. 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: Implementors 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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12.4. 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()
close() ----------> close()
<-------------------------------------------------------- 226
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12.5. 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()
close() ---------> close()
<-------------------------------------------------------- 226
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13. Implications of [FTP-EXT]
13.1. Virtual hosts (HOST)
The proposed 'HOST' command allows a user, upon connection or
indeed at any time during the FTP session, to identify the host
which they wish to have a session. This extension allows a single
FTP server to serve requests that are, as far as the client is
concerned, sitting on different DNS names.
When an 'AUTH' command is received, the server should take into
consideration any previously issued 'HOST' or 'REIN' command to
determine which authentication tokens it should present to the
client.
13.2. MLST and MLSD
MLST and MLSD are directory listing commands and should be treated
in the same manner as NLST and LIST for the purposes of this
document.
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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.
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.
The defined behaviour for TLS protected FTP sessons will depend on
the manner of session initialisation.
If the session has been explicity protected (see section 4.1) then
the TLS session(s) will be cleared and the control and data
connections revert to unprotected, clear communications. It will be
acceptable to use cached TLS sessions for subsequent connections,
however a server should not mandate this.
If the session is implicitly protected (see section 4.2) then the
control connection will continue to be protected using the exisiting
negotiated TLS session and the data connection will revert to being
implicitly protected, irrespective of any 'PROT' commands preceding
the 'REIN'.
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15. Security Considerations
This entire document deals with security considerations related to
the File Transfer Protocol.
15.1. Verification of Authentication tokens
15.1.1. 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. (i.e.
SubjectCommonName or SubjectAltName of type dNSName).
For servers that support the 'HOST' extension defined in [FTP-
EXT], this may mean that the server has to refuse the 'AUTH'
command until a 'HOST' command has been issued, or else present
the credentials for a default DNS name (which may be flagged as
an exception by the connecting client to the user).
15.1.2. Client Certificates
- Deciding which client certificates to allow and defining
which fields define what authentication information is entirely
a server implementation issue.
- It is also server implementation issue to decide if the
authentication token presented for the data connection must
match the one used for the corresponding control connection.
15.2. Addressing FTP Security Considerations [RFC-2577]
15.2.1. 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.
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- 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...)
15.2.2. Restricting Access
This document presents a strong mechanism for solving the issue
raised in this section.
15.2.3. 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.
15.2.4. 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)
15.2.5. Protecting Usernames
This is not an issue when TLS is used as the primary
authentication mechanism.
15.2.6. Port Stealing
This proposal 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.
15.2.7. Software-Base Security Problems
Nothing in this proposal will affect the discussion in this
section.
16. IANA Considerations
{FTP-PORT} - The port assigned to the FTP control connection is 21.
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{FTP-TLSPORT} - A port to be assigned by the IANA for native TLS FTP
connections on the control socket. This has been provisionally
reserved as port 990.
{TLS-PARM} - A parameter for the AUTH command to indicate that TLS is
required. It is recommended that 'TLS', 'TLS-C', 'SSL' and 'TLS-P'
are acceptable, and mean the following :-
'TLS' or 'TLS-C' - the TLS protocol or the SSL protocol will be
negotiated on the control connection. The default protection
setting for the Data connection is 'Clear'.
'SSL' or 'TLS-P' - the TLS protocol or the SSL protocol will be
negotiated on the control connection. The default protection
setting for the Data connection is 'Private'. This is primarily
for backward compatibility.
Note - [RFC-2228] states that these parameters are case-
insensitive.
17. Network Management
NONE
18. Internationalization
NONE
19. 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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20. 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 privacy).
21. 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.
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22. References
[RFC-959] J. Postel, "File Transfer Protocol"
RFC 959, October 1985.
[RFC-1579] Bellovin, S., "Firewall-Friendly FTP"
RFC 1579, February 1994.
[RFC-2222] J. Myers, "Simple Authentication and Security Layer"
RFC 2222, October 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.
[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.
[FTP-EXT] R Elz, P Hethmon "Extensions to FTP"
draft-ietf-ftpext-mlst-07.txt, June 1999.
[SRA-FTP] "SRA - Secure RPC Authentication for TELNET and FTP Version
1.1"
file://ftp.funet.fi/security/login/telnet/doc/sra/sra.README
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23. Authors' Contact Addresses
Please send comments to Paul Ford-Hutchinson at the address below
Tim Hudson Paul Ford-Hutchinson
RSA Data Security IBM UK Ltd
Australia Pty Ltd PO Box 31
Birmingham Road
Warwick
United Kingdom
tel - +61 7 3227 4444 +44 1926 462005
fax - +61 7 3227 4400 +44 1926 496482
email - tjh@rsasecurity.com.au paulfordh@uk.ibm.com
Martin Carpenter Eric Murray
Verisign Ltd Wave Systems Inc.
email - mcarpenter@verisign.com ericm@lne.com
Volker Wiegand
SuSE Linux
email - wiegand@suse.de
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Appendices
A. Summary of [RFC-2246]
The TLS protocol was developed by the IETF TLS working group. It is
based on the SSL protocol proposed by Netscape Communications
Corporation. The structure of the start of a TLS session allows
negotiation of the level of the protocol to be used - in this way, a
client or server can simultaneously support TLS and SSL and negotiate
the most appropriate for the connection.
The TLS protocol defines three security mechanisms that may be used
(almost) independently. They are Authentication, Integrity and
Privacy. It is possible to have an Authenticated session with no
Privacy and with or without Integrity (useful for anonymous FTP
sites, or sites with pre-encrypted data). For example, sessions with
Authentication, Privacy and Integrity would be useful for control
connections over an insecure network and data connections
transferring confidential material.
The TLS protocol allows unauthenticated sessions; server
authentication or client and server authentication. There is no
mechanism for authenticating a client without first authenticating
the server.
The basic mechanism of the TLS protocol is that (for an
Authenticated, Private session) asymmetric encryption is used to
authenticate clients and servers and exchange a session key for
symmetric encryption which is to be used for the rest of the session.
The structure of the TLS session initialisation is that the client
initiates the session with a 'ClientHello' message. The server will
respond with a 'ServerHello' and the session negotiation will
continue.
The TLS protocol allows session caching which is achieved by the
client requesting that the server re-use a session context (Cipher
Suite and symmetric key) in the ClientHello message. There is no
reason why a second connection could not request a 'cached' session
with the same context as an existing session.
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B. Summary of [RFC-2228]
Extensions to FTP
The FTP Security Extensions document has 8 new commands to enhance
the FTP protocol to allow negotiation of security and exchange of
security data. Three of these commands (the AUTH, PBSZ and PROT
commands) are used by this document to allow an FTP client to
negotiate TLS with the server. The other commands are not required.
i) AUTH
This command is a request by the client to use an authentication
and/or security mechanism.
The client will issue an 'AUTH <Mechanism>' command
which will be a request to the server to secure the control
connection using the TLS (or SSL) protocol. It also governs the
initial protection setting of the data channel (which may be
changed by a subsequent PROT command).
ii) ADAT
This command is used to transmit security data required by the
security mechanism agreed in a preceding AUTH command.
This document does not use the ADAT command.
iii) PROT
This command is used by the client to instruct the type of
security that is required on the Data connection.
The 'PROT C' command will mean that TLS should not be used to
secure the data connection; 'PROT P' means that TLS should be
used. 'PROT E' and 'PROT S' are not defined and generate
a '536' reply from the server.
iv) PBSZ
This command is used to negotiate the size of the buffer to be
used during secured data transfer.
The PBSZ command must be issued prior to the PROT command. The
PBSZ command cannot be sent on an insecure control connection.
For FTP and TLS the only valid value for the parameter is '0', all
other values should receive a '200' reply with the text 'PBSZ=0'
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included.
v) CCC
This command is used to specify that the control channel no longer
requires protection.
This document does not use the CCC command.
vi) MIC
This command is used to send a normal FTP command with integrity
protection.
This document does not use the MIC command.
vii) CONF
This command is used to send a normal FTP command with
confidentiality protection (encrypted).
This document does not use the CONF command.
viii) ENC
This command is used to send a normal FTP command with
confidentiality and integrity protection.
This document does not use the ENC command.
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Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
This document expires on 26th July, 2000
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