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Text of the page (random words):
er application layer ports 21 for control 20 for data transfer rfc 959 internet protocol suite application layer bgp dhcp v6 dns ftp http http 3 https imap ipp irc ldap mgcp mqtt nntp ntp ospf pop ptp onc rpc rtp rtsp rip sip smtp snmp ssh telnet tls ssl xmpp more transport layer tcp udp dccp sctp rsvp quic more internet layer ip v4 v6 icmp v6 ndp ecn l4s igmp ipsec more link layer arp tunnels ppp mac more v t e the file transfer protocol ftp is a standard communication protocol used for the transfer of computer files from a server to a client over a computer network ftp is built on a client server model architecture using separate control and data connections between the client and the server 1 ftp users may authenticate themselves with a plain text sign in protocol normally in the form of a username and password but can connect anonymously if the server is configured to allow it for secure transmission that protects the username and password and encrypts the content ftp is often secured with ssl tls ftps or replaced with ssh file transfer protocol sftp the first ftp client applications were command line programs developed before operating systems had graphical user interfaces and are still shipped with most windows and unix like operating systems 2 3 many dedicated ftp clients and automation utilities have since been developed for desktops servers mobile devices and hardware and ftp has been incorporated into productivity applications such as html editors and file managers google chrome browser accessing an ftp server showing the ftp scheme in the url bar an ftp client used to be commonly integrated in web browsers where file servers are browsed with the uri scheme ftp in 2021 ftp support was dropped by google chrome and firefox 4 5 two major web browser vendors due to it being superseded by the more secure sftp and ftps although neither browser has implemented the newer protocols 6 7 history of ftp servers edit the original specification for the file transfer protocol was written by abhay bhushan and published as rfc 114 on 16 april 1971 until 1980 ftp ran on ncp the predecessor of tcp ip 2 the protocol was later replaced by a tcp ip version rfc 765 june 1980 and rfc 959 october 1985 the current specification several proposed standards amend rfc 959 for example rfc 1579 february 1994 enables firewall friendly ftp passive mode rfc 2228 june 1997 proposes security extensions rfc 2428 september 1998 adds support for ipv6 and defines a new type of passive mode 8 protocol overview edit communication and data transfer edit illustration of starting a passive connection using port 21 ftp may run in active or passive mode which determines how the data connection is established 9 this sense of mode is different from that of the mode command in the ftp protocol in active mode the client starts listening for incoming data connections from the server on port m it sends the ftp command port m to inform the server on which port it is listening the server then initiates a data channel to the client from its port 20 the ftp server data port in situations where the client is behind a firewall and unable to accept incoming tcp connections passive mode may be used in this mode the client uses the control connection to send a pasv command to the server and then receives a server ip address and server port number from the server 9 which the client then uses to open a data connection from an arbitrary client port to the server ip address and server port number received 10 both modes were updated in september 1998 to support ipv6 further changes were introduced to the passive mode at that time updating it to extended passive mode 11 the server responds over 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 for the response and the optional text represents a human readable explanation or request e g need account for storing file 1 an ongoing transfer of file data over the data connection can be aborted using an interrupt message sent over the control connection ftp needs two ports one for sending and one for receiving because it was originally designed to operate on top of network control protocol ncp which was a simplex protocol that utilized two port addresses establishing two connections for two way communications an odd and an even port were reserved for each application layer application or protocol the standardization of tcp and udp reduced the need for the use of two simplex ports for each application down to one duplex port 12 15 but the ftp protocol was never altered to only use one port and continued using two for backwards compatibility nat and firewall traversal edit 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 13 for nats an additional complication is that 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 solving 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 13 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 level gateway for this purpose 13 a model chart of how ftp works data types edit while transferring data over the network five data types are defined 2 3 8 ascii type a used for text data is converted if needed from the sending host s character representation to 8 bit ascii before transmission and again if necessary to the receiving host s character representation including newlines as a consequence this mode is inappropriate for files that contain data other than ascii image type i commonly called binary mode the sending machine sends each file byte by byte and the recipient stores the bytestream as it receives it image mode support has been recommended for all implementations of ftp ebcdic type e used for plain text between hosts using the ebcdic character set local type l n designed to support file transfer between machines which do not use 8 bit bytes e g 36 bit systems such as dec pdp 10s for example type l 9 would be used to transfer data in 9 bit bytes or type l 36 to transfer 36 bit words most contemporary ftp clients servers only support l 8 which is equivalent to i unicode text files using utf 8 type u defined in an expired internet draft 14 which never became an rfc though it has been implemented by several ftp clients servers note these data types are commonly called modes although ambiguously that word is also used to refer to active vs passive communication mode see above and the modes set by the ftp protocol mode command see below for text files type a and type e three different format control options are provided to control how the file will be printed non print type a n and type e n the file does not contain any carriage control characters intended for a printer telnet type a t and type e t the file contains telnet or in other words ascii c0 carriage control characters cr lf etc asa type a a and type e a the file contains asa carriage control characters these formats were mainly relevant to line printers most contemporary ftp clients servers only support the default format control of n file structures edit file organization is specified using the stru command the following file structures are defined in section 3 1 1 of rfc959 f or file structure stream oriented files are viewed as an arbitrary sequence of bytes characters or words this is the usual file structure on unix systems and other systems such as cp m ms dos and microsoft windows section 3 1 1 1 r or record structure record oriented files are viewed as divided into records which may be fixed or variable length this file organization is common on mainframe and midrange systems such as mvs vm cms os 400 and vms which support record oriented filesystems p or page structure page oriented files are divided into pages which may either contain data or metadata each page may also have a header giving various attributes this file structure was specifically designed for tenex systems and is generally not supported on other platforms rfc1123 section 4 1 2 3 recommends that this structure not be implemented most contemporary ftp clients and servers only support stru f stru r is still in use in mainframe and minicomputer file transfer applications data transfer modes edit data transfer can be done in any of three modes 1 2 stream mode mode s data is sent as a continuous stream relieving ftp from doing any processing rather all processing is left up to tcp no end of file indicator is needed unless the data is divided into records block mode mode b designed primarily for transferring record oriented files stru r although it can also be used to transfer stream oriented stru f text files ftp puts each record or line of data into several blocks block header byte count and data field and then passes it on to tcp 8 compressed mode mode c extends mode b with data compression using run length encoding most contemporary ftp clients and servers do not implement mode b or mode c ftp clients and servers for mainframe and minicomputer operating systems are the exception to that some ftp software also implements a deflate based compressed mode sometimes called mode z after the command that enables it this mode was described in an internet draft but not standardized 15 gridftp defines additional modes mode e 16 and mode x 17 as extensions of mode b additional commands edit more recent implementations of ftp support the modify fact modification time mfmt command which allows a client to adjust that file attribute remotely enabling the preservation of that attribute when uploading files 18 19 to retrieve a remote file timestamp there s mdtm command some servers and clients support nonstandard syntax of the mdtm command with two arguments that works the same way as mfmt 20 login edit a computer at amundsen scott south pole station logging into an ftp server and transferring a file in 1994 ftp login uses normal username and password scheme for granting access 2 the username is sent to the server using the user command and the password is sent using the pass command 2 this sequence is unencrypted on the wire so may be vulnerable to a network sniffing attack 21 if the information provided by the client is accepted by the server the server will send a greeting to the client and the session will commence 2 if the server supports it users may log in without providing login credentials but the same server may authorize only limited access for such sessions 2 anonymous ftp edit a host that provides an ftp service may provide anonymous ftp access 2 users typically log into the service with an anonymous lower case and case sensitive in some ftp servers account when prompted for user name although users are commonly asked to send their email address instead of a password 3 no verification is actually performed on the supplied data 22 many ftp hosts whose purpose is to provide software updates will allow anonymous logins 3 software support edit filezilla and winscp two popular ftp client software running on windows file managers edit many file managers tend to have ftp access implemented such as file explorer formerly windows explorer on microsoft windows this client is only recommended for small file transfers from a server due to limitations compared to dedicated client software 23 it does not support sftp 24 both the native file managers for kde on linux dolphin and konqueror support ftp as well as sftp 25 26 primitive ftpd on android actively running an ftp and sftp server on android the my files file manager on samsung galaxy has a built in ftp and sftp client 27 web browser edit for a long time most common web browsers were able to retrieve files hosted on ftp servers although not all of them had support for protocol extensions such as ftps 3 28 when an ftp rather than an http url is supplied the accessible contents on the remote server are presented in a manner that is similar to that used for other web content google chrome removed ftp support entirely in chrome 88 also affecting other chromium based browsers such as microsoft edge 29 firefox 88 disabled ftp support by default with firefox 90 dropping support entirely 30 4 fireftp is a discontinued browser extension that was designed as a full featured ftp client to be run within firefox but when firefox dropped support for ftp the extension developer recommended using waterfox 31 some browsers such as the text based lynx still support ftp 32 uri syntax edit ftp uri syntax is described in rfc 1738 taking the form ftp user password host port path only the host is required more details on specifying a username and password may be found in the browsers documentation e g firefox 33 and internet explorer 34 by default most web browsers use passive pasv mode which more easily traverses end user firewalls some variation has existed in how different browsers treat path resolution in cases where there is a non root home directory for a user 35 download manager edit most common download managers can receive files hosted on ftp servers while some of them also give the interface to display the list of files hosted on ftp servers other edit libreoffice declared its ftp support deprecated from the 7 4 release this was later removed in the 24 2 release 36 37 apache openoffice another descent of openoffice org still supports ftp 38 39 40 security edit ftp was not designed to be a secure protocol and has many security weaknesses 41 in may 1999 the authors of rfc 2577 listed a vulnerability to the following problems brute force attack ftp bounce attack packet capture port stealing guessing the next open port and usurping a legitimate connection spoofing attack username enumeration dos or ddos ftp does not encrypt its traffic all transmissions are in clear text and usernames passwords commands and data can be read by anyone able to perform packet capture sniffing on the network 2 41 this problem is common to many of the internet protocol specifications such as smtp telnet pop and imap that were designed prior to the creation of encryption mechanisms such as tls or ssl 8 common solutions to this problem include using the secure versions of the insecure protocols e g ftps instead of ftp and telnets instead of telnet using a different more secure protocol that can handle the job e g ssh file transfer protocol or secure copy protocol using a secure tunnel such as secure shell ssh or virtual private network vpn ftp over ssh edit ftp over ssh is the practice of tunnel...
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