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| Application Layer | |
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BGP · DHCP · DNS · FTP · HTTP · IMAP · IRC · LDAP · MGCP · NNTP · NTP · POP · RIP · RPC · RTP · SIP · SMTP · SNMP · SSH · Telnet · TLS/SSL · XMPP · (more) |
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| Transport Layer | |
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| Internet Layer | |
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| ARP/InARP · NDP · OSPF · Tunnels (L2TP) · PPP · Media Access Control (Ethernet, DSL, ISDN, FDDI) · (more) | |
File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP/IP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications, which solves the problem of different end host configurations (i.e., Operating Systems, file names).[1] FTP is used with user-based password authentication or with anonymous user access.
Applications were originally interactive command-line tools with a standardized command syntax, but graphical user interfaces have been developed for all desktop operating systems in use today.
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The original specification for the File Transfer Protocol was written by Abhay Bhushan and published as RFC 114 on 16 April 1971 and later replaced by RFC 765 (June 1980) and RFC 959 (October 1985), the current specification. Several proposed standards amend RFC 959, for example RFC 2228 (June 1997) proposes security extensions and RFC 2428 (September 1998) adds support for IPv6 and defines a new type of passive mode. [2]
The protocol is specified in RFC 959, which is summarized below.[3]
A client makes a TCP connection to the server's port 21. This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, opened by the server from its port 20 to a client port (specified in the negotiation dialog) as required to transfer file data. The control connection is used for session administration (i.e., commands, identification, passwords)[4] exchanged between the client and server using a telnet-like protocol. For example "RETR filename" would transfer the specified file from the server to the client. Due to this two-port structure, FTP is considered an out-of-band, as opposed to an in-band protocol such as HTTP[4].
The server responds on the control connection with three digit status codes in ASCII with an optional text message, for example "200" (or "200 OK.") means that the last command was successful. The numbers represent the code number and the optional text represent explanations (i.e., <OK>) or needed parameters (i.e., <Need account for storing file>)[1]. A file transfer in progress over the data connection can be aborted using an interrupt message sent over the control connection.
FTP can be run in active or passive mode, which determine how the data connection is established. In active mode, the client sends the server the IP address and port number on which the client will listen and the server initiates the TCP connection. In situations where the client is behind a firewall and unable to accept incoming TCP connections, passive mode may be used, where the client sends a PASV command to the server and receives an IP address and port number in return, which the client uses to open the data connection to the server.[3] Both modes were updated in September 1998 to add support for IPv6 and made some other changes to passive mode, making it extended passive mode[5].
While transferring data over the network, four data representations can be used[2]:
For text files, different format control and record structure options are provided. These features were designed to facilitate files containing Telnet or ASA formatting.
Data transfer can be done in any of three modes[1]:
The original FTP specification has many security concerns. In May 1999, the following flaws were addressed[6]:
FTP has no encryption tools meaning all transmissions are in clear text; user names, passwords, FTP commands and transferred files can be read by anyone sniffing on the network. This is a problem common to many Internet protocol specifications written prior to the creation of SSL, such as HTTP, SMTP and Telnet[2]. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP as specified in RFC 4217.
A host that provides an FTP service may additionally provide anonymous FTP access. Users typically log into the service with an 'anonymous' account when prompted for user name. Although users are commonly asked to send their email address in lieu of a password, no verification is actually performed on the supplied data[7]; examples of anonymous FTP servers can be found here.
Where FTP access is restricted, a remote FTP (or FTPmail) service can be used to circumvent the problem. An e-mail containing the FTP commands to be performed is sent to a remote FTP server, which is a mail server that parses the incoming e-mail, executes the FTP commands, and sends back an e-mail with any downloaded files as an attachment. Obviously this is less flexible than an FTP client, as it is not possible to view directories interactively or to modify commands, and there can also be problems with large file attachments in the response not getting through mail servers. As most internet users these days have ready access to FTP, this procedure is no longer in everyday use.
Most recent web browsers can retrieve files hosted on FTP servers, although they may not support protocol extensions such as FTPS[8]. When an FTP—rather than HTTP—URL is supplied, the accessible contents of the remote server is presented in a manner similar to that used for other Web content. Firefox has a full-featured FTP client in the form of an extension called FireFTP[1]
FTP URL syntax is described in RFC1738[9], taking the form:
ftp://[<user>[:<password>]@]<host>[:<port>]/<url-path>[9]
(The bracketed parts are optional.) For example:
ftp://public.ftp-servers.example.com/mydirectory/myfile.txt
or:
ftp://user001:secretpassword@private.ftp-servers.example.com/mydirectory/myfile.txt
More details on specifying a user name and password may be found in the browsers' documentation, such as, for example, Firefox and Internet Explorer.
By default, most web browsers use passive (PASV) mode, which more easily traverses end-user firewalls.
FTP normally transfers data by having the server connect back to the client, after the PORT command is sent by the client. This is problematic for both NATs and firewalls, which do not allow connections from the Internet towards internal hosts. For NATs, an additional complication is the representation of the IP addresses and port number in the PORT command refer to the internal host's IP address and port, rather than the public IP address and port of the NAT.
There are two approaches to this problem. One is that the FTP client and FTP server use the PASV command, which causes the data connection to be established from the FTP client to the server. This is widely used by modern FTP clients. Another approach is for the NAT to alter the values of the PORT command, using an application layer gateways for this purpose.
FTP over SSH (not SFTP) refers to the practice of tunneling a normal FTP session over an SSH connection.
Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will protect only that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels), which bypass the SSH connection, and thus have no confidentiality, integrity protection, etc.
Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security's software suite, the GPL licensed FONC, and Co:Z FTPSSH Proxy are three software packages that support this mode.
FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol.
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