FTP File Transfer Protocol Graeme Strachan

Agenda  An Overview  A Demonstration  An Activity

History  Created to allow transfer of data between servers - was published as an RFC in 1973  Became available for clients to access/upload/download files  1 st wave of the internet  It was the backbone of all file transactions on the internet  Had a number of major revisions E.g. the change from NCP to TCP/IP  HTTP has now passed it in volume of transactions  Still widely used – faster than HTTP

FTP Overview  FTP (File Transfer Protocol) is used to transfer data from one computer to another over the Internet, or through a network.  Client and Server  Uses TCP only. (no UDP)  Uses two ports, 'data' - port 20  the data port is not always on port 20 'command' - port 21 (also known as the control port)

Active session  Step 1, Client's command port contacts the server's command port and sends the command PORT  Step 2 - Server then sends an ACK back to the client's command port.  Step 3 - Server initiates a connection on its local data port to the data port the client specified earlier.  Step 4 - client sends an ACK to the client's data port.  ACK = Acknowledgement

 Client side.  The FTP client doesn't make the actual connection to the data port of the server.  It simply tells the server what port it is listening on.  The server connects back to the specified port on the client.  From the client side firewall this appears to be an outside system initiating a connection to an internal client--something that is usually blocked. Active session - Problem

Passive Session  Step 1 - Client contacts the server on the command port and issues the PASV command.  Step 2 - Server replies with PORT 2024, telling the client which port it is listening to for the data connection.  Step 3 - Client then initiates the data connection from its data port to the specified server data port.  Step 4 - Server sends back an ACK to the client's data port.

Passive Session - considerations  Passive mode FTP solves many client side problems.  Server side - needs to allow any remote connection to high numbered ports on the server.  Some FTP daemons allow the administrator to specify a range of ports which the FTP server will use.  Some clients do not support passive mode. (eg Solaris command line FTP utility).  Increased usage of web browsers as an FTP client. Most browsers only support passive mode when accessing ftp:// URLs. This can either be good or bad depending on what the servers and firewalls are configured to support.

TCP Protocol Map ( ) FTP runs exclusively over TCP. TCP

Criticisms of FTP  Passwords and file contents are sent in clear text Packet Sniffers  Multiple TCP/IP connections are used. Firewall software needs additional logic to account for these.  Hard to filter in active mode on the client side as the client must open a random port. Largely resolved by using passive mode FTP.  It is possible to abuse the protocol by telling a server to send data to a port of a third computer.  FTP is a high latency due to the number of commands needed to initiate a transfer.  No integrity check on the receiver side. integrity checks have to be managed externally.  No error detection. Relies on TCP layer for error control (comparatively weak checksum by modern standards).  No date/timestamp attribute transfer. Uploaded files are given a new current timestamp (SFTP does).

Variations  TFTP Trivial File Transfer Protocol Very simple Minimal memory Useful for:  booting computers and routers which did not have any data storage.  Transfer of small files between hosts on a network.  Remote X Window terminal or other thin client boots from a network host or server.  TFTP has been used by viruses/worms, (e.g. Blaster), as a way to spread and infect new hosts.

Variations  SSH – Secure Shell Uses public-key cryptography for authentication. Typically used to log into a remote machine and execute commands Supports tunnelling  Secure FTP - FTP over SSH Don’t confuse with  SSL/TLS – FTPS (SSL -Secure Sockets Layer)  SFTP - SSH File Transfer Protocol A common misconception is that SFTP is simply FTP run over SSH Uses SSH Rebuilt from the ground up  Secure Copy Protocol – SCP Uses SSH Older protocol

Demonstrations  2 Servers  Anonymous  User: test Password: testuser

Command Line

Client commands helphelp - Displays descriptions for ftp commands

Filezilla

Filezilla –

Filezilla – test, testuser