Analysis of the File Transfer Protocol

7/30/2019 Analysis of the File Transfer Protocol

1/31

Analysis of the File Transfer Protocol (FTP)

byPriscilla Oppenheimer

There are many myths about how FTP works. The goal of this white paper is to briefly describeFTP protocol processes to dispel some of the myths. More detailed information about FTP

behavior is available in Chapter 9 of theTroubleshooting Campus Networksbook by Priscilla

Oppenheimer and Joseph Bardwell.

FTP was one of the first Internet protocols. It was designed for use on the Internet when theInternet was still a closed system that connected universities, government agencies, and a few

commercial companies involved in the development of the system. FTP's behavior on modern

networks, especially networks where security is a big concern, is problematic. This paper

provides the theoretical background you will need to get FTP working properly on a modernnetwork that uses firewalls and Network Address Translation (NAT).

FTP was designed long before the term client/servercame in vogue, but it does behave like aclient/server protocol nonetheless. FTP uses two TCP connections, one for control information

and one for the actual data. Analysis of an FTP session involves an examination of the FTP

commands sent on the control connection and an assessment of the TCP segments sent on thedata connection. With normaloractive FTP, the control connection is initiated by the client sideand the data connection is initiated by the server side. (Active mode is also sometimes called

port mode). The other option ispassive FTP, in which case the client side establishes the data

connection.

FTP and TCP Port Numbers

FTP uses different TCP port numbers depending on whether active or passive FTP is in use.

Before we cover FTP in more detail, we'll briefly discuss some basic concepts regarding TCP

port numbers. TCP uses port numbers to identify the sending and receiving application. A port

number helps TCP demultiplex byte streams and deliver bytes to the correct application.

TCP ports can be semi-permanent or ephemeral (short-lived). Servers listen on the semi-permanent ports for clients wishing to access services. Clients use ephemeral ports to identify

their end of a conversation. The client side only lasts while the client is using a service, whereas

a server port is usually open the entire time that a server is running.

TCP port numbers also fall into these three categories:

Well-known port numbers are used to identify standard services that run above TCP,

including FTP, HTTP, Telnet, SMTP, and so on. Well-known port numbers are 0 to

1,023.

Registered port numbers identify an application that has been registered with the InternetAssigned Numbers Assigned Numbers Authority (IANA). Registered port numbers are

1,024 to 49,151.

7/30/2019 Analysis of the File Transfer Protocol

2/31

Private port numbers are unregistered and can be dynamically assigned to any

application. Private port numbers are 49,152 to 65,535.

A registered port number is intended for use by only the registered application. However, you

will see port numbers that are supposedly "registered" get used as an ephemeral port by

applications that are not related to the registered application. You can get an up-to-date list ofport numbers from IANAhere.

FTP Active Versus Passive Mode

When troubleshooting FTP problems, one of the first questions you should ask is whether active

or passive mode is in use. Because their behaviors are quite different, the two modes experience

different problems. In the past, client implementations defaulted to active mode. These days,many client implementations default to passive mode due to security concerns with active mode.

FTP Active Mode

The steps for active FTP are as follows:

1. The client sends a TCP SYN to the well-known FTP control port (port 21) on the server.The client uses an ephemeral port as its source port.

2. The server sends the client a SYN ACK from port 21 to the ephemeral port on the client.3. The client sends an ACK. The client uses this connection to send FTP commands and the

server uses this connection to send FTP replies.

4. When the user requests a directory listing or initiates the sending or receiving of a file,the client software sends a PORT command that includes an ephemeral port number that

the client wishes the server to use when opening the data connection. The PORT

command also includes an IP address, which is usually the client's own IP address,although FTP also supports a third-party mode where a client can tell a server to open a

connection with a different host.

5. The server sends a SYN from port 20 to the client's ephemeral port number, which wasprovided to the server in the client's PORT command.

6. The client sends a SYN ACK from its ephemeral port to port 20.7. The server sends an ACK.8. The host that is sending data uses this new connection to send the data in TCP segments,

which the other host ACKs. (With some commands, such as STOR, the client sends data.

With other commands, such as RETR, the server sends data.)

9. After the data transfer is complete, the host sending data closes the data connection with a

FIN, which the other host ACKs. The other host also sends its own FIN, which thesending host ACKs.

10.The client can send more commands on the control connection, which may causeadditional data connections to be opened and then closed. At some point, when the user isfinished, the client closes the control connection with a FIN. The server ACKs the client's

FIN. The server also sends its own FIN, which the client ACKs.

Figure 1 shows a graphical representation of the first few steps of FTP active mode.

7/30/2019 Analysis of the File Transfer Protocol

3/31

Figure 1. FTP active mode.

The FTP PORT Command

The FTP PORT command causes problems for network support engineers in many ways. For

one thing, the encoding of the IP address and port number in a PORT message is not

straightforward. In addition, an application-layer protocol command theoretically shouldn't

include network-layer information (an IP address). This breaks the principles of protocollayering and can cause both interoperability and security problems. Figure 2 shows an example

of an FTP PORT command.

7/30/2019 Analysis of the File Transfer Protocol

4/31

Figure 2. An FTP PORT command from a client to a server.

In the detail view of the packet shown in Figure 2, theWildPackets EtherPeek protocol analyzerdecodes the IP address parameter for the PORT command, followed by the port number. (See

PORT 192,168,10,232,6,127.) The 6, 127 portion becomes a port number by multiplying the firstdigit by 256 and adding the second digit. So the client specified a port number that is (6 x 256) +

127, which equals 1663. Figure 3 verifies that the server did indeed open a TCP connection from

port 20 to port 1663.

7/30/2019 Analysis of the File Transfer Protocol

5/31

Figure 3. A server in active mode opens a TCP connection for data transfer.

When FTP is used, network firewalls must be "stateful." The firewall must track FTP sessions

and be on the lookout for FTP PORT commands. The firewall needs to anticipate the connectionestablishment coming from the FTP server into the client's port specified in the PORT command.

If the network uses NAT, the NAT gateway must be stateful also. The gateway needs to translate

the IP address in the FTP PORT command to the address assigned to the client, and then

recalculate the TCP checksum. If the gateway doesn't perform these operations correctly, FTPcan fail.

Hackers can exploit the third-party feature of FTP by setting the IP address and port numberparameters in the PORT command to a target host's address and port numbers (sometimes called

anFTP bounce attack). For example, a hacker can cause an FTP server to continually send TCP

SYN packets from port 20 to a set of target ports, making it look like the server is initiating a

port scan. The target won't know that the attack is coming from the hacker's machine. It willappear that it's coming from the FTP server.

Some commonly-used FTP implementations set the IP address in the PORT command to 0.0.0.0,

with the intention being that the server should simply open the data connection on the same

client that opened the control connection. Setting the IP address to 0.0.0.0 can confuse firewalls.

7/30/2019 Analysis of the File Transfer Protocol

6/31

For exmple, with Cisco IOS release 6.x(x), the PIX fixup protocol for FTP does not allow the IP

address for the data connection to be different from the one already in use for the control

connection. The reason for this is to thwart a hacker who could use the PORT command tolaunch an attack on another host. Although the FTP implementations that set the IP address to

0.0.0.0 are not intentional hacks, they do trigger a problem with the fixup protocol and other

firewalls that were designed to disallow FTP third-party mode and to avoid FTP bounce attacks.

FTP Passive Mode

The steps for passive FTP are described in the following list. Steps 1 through 3 are the same as

the first three steps for active mode. Also, steps 9 through 11 are the same as the last three steps

for active mode.

1. The client sends a TCP SYN to the well-known FTP control port (port 21) on the server.The client uses an ephemeral port as the source port.

2. The server sends the client a SYN ACK from port 21 to the ephemeral port on the client.

3. The client sends an ACK. The client uses this connection to send FTP commands and theserver uses the connection to send FTP replies.4. When the user requests a directory listing or initiates the sending or receiving of a file,

the client software sends a PASV command to the server indicating the desire to enter

passive mode.

5. The server replies. The reply includes the IP address of the server and an ephemeral portnumber that the client should use when opening the connection for data transfer.

6. The client sends a SYN from a client-selected ephemeral port to the server's ephemeralport number, which was provided to the client in the reply to the client's PASV

command.7. The server sends a SYN ACK from its ephemeral port to the client's ephemeral port.

8. The client sends an ACK.9. The host that is sending data uses this new connection to send the data in TCP segments,which the other host ACKs. (With some commands, such as STOR, the client sends data.

With other commands, such as RETR, the server sends data.)

10.After the data transfer is complete, the host sending data closes the data connection with aFIN, which the other host ACKs. The other host also sends its own FIN, which the

sending host ACKs.

11.The client can send more commands on the control session, which may cause additionaldata connections to be opened and then closed. At some point, when the user is finished,the client closes the control connection with a FIN. The server ACKs the client's FIN.

The server also sends its own FIN, which the client ACKs.

Figure 4 shows a graphical representation of the first few steps of FTP passive mode.

7/30/2019 Analysis of the File Transfer Protocol

7/31

Figure 4. FTP passive mode.

The FTP PASV Command

A PASV request asks the server to accept a data connection on a new TCP port selected by the

server. There are no parameters to the PASV command. The server's response is a single line

showing the IP address of the server and the TCP port number where the server is acceptingconnections.

Figure 5 shows a server's reply to a client's PASV command. The server tells the client that the

server is listening on port 5365. (See 192,168,179,100,20,245). To calculate the port number,

multiply 20 times 256 and add 245. (20 x 256 +245 = 5365.)

7/30/2019 Analysis of the File Transfer Protocol

8/31

Figure 5. A server's reply to a client's PASV command.

After receiving a reply to its PASV command, the client opens a TCP connection from an

ephemeral port to the port number supplied by the server. Figure 6 shows the TCP connectionestablishment from the client. Notice that the destination port is 5365, as expected.

7/30/2019 Analysis of the File Transfer Protocol

9/31

Figure 6. A client in passive mode opens a TCP connection for data transfer.

Most people assume that passive mode causes fewer problems for firewalls than active mode, butnote that the client opens a connection to an ephemeral (not well-known) port when using

passive mode. Some firewalls and Cisco access lists block this. Also, the response from the

server comes from an ephemeral port and goes to an ephemeral port. Firewalls and Cisco access

lists might block this also. On a Cisco router you can use the established keyword with an accesslist to avoid this second problem. The established keyword tells the router to allow packets with

the ACK bit set. The SYN ACK from the server has the ACK bit set.

Clear Text Username and Password

Another infamous problem with FTP is that it sends the username and password as clear text,

that is, unencrypted. Anyone with a strategically-placed protocol analyzer can see the name andpassword. FTP sends the data in the files being transfered as clear text also. This might not seem

like a huge problem because it's difficult to place a protocol analyzer in the correct place to glean

the unencrypted information, but it's not impossible to do so. The problem is made worse by thefact that users tend to use the same password for many different applications. If hackers glean an

FTP password, they may be getting a password that is also used to access online checking

accounts or other confidential data.

7/30/2019 Analysis of the File Transfer Protocol

10/31

Alternatives to FTP

In this day and age, there's probably no excuse for using FTP any more, considering that there

are other more secure options for file transfer. Both SCP and SFTP, for example, are similar infunction to FTP but use Secure Shell (SSH) authentication and encryption. If you use a Unix-

based server, you should be able to invokescp orsftp from the command line. For moreinformation about SecureShell, see theOpenSSHWeb site.

If your use for FTP is limited to updating Web pages, there is another alternative called Web-

based Distributed Authoring and Versioning (WebDAV). WebDAV is a set of extensions to theHypertext Transfer Protocol (HTTP) to allow users to collaboratively edit and manage files on

remote Web servers. SeeRFC 2518for more information.

Summary

FTP was designed in the 1970s. At that time, the Internet was a closed network and security was

not a big concern. When FTP is used in modern environments with NAT gateways, firewalls,and Cisco access lists, problems can arise, whether you use active or passive mode. FTP is often

used for mission-critical applications on the public Internet, which is probably a mistake. There

have been many attempts to make FTP more secure. These attempts cause troubleshooting issuesand also fail to fix the most glaring security problem with FTP, which is that it sends the

username and password as clear text. Many alternatives to FTP are available, including SCP,

SFTP, and WebDAV.

Other FTP Resources

Bernstein, D. J.,FTP: File Transfer Protocol.

CERT Coordination Center,Problems with the FTP PORT Command or Why You Don't

Want Just any PORT in a Storm.

Postel, J. and J. Reynolds,RFC 959(the official specification for FTP).

7/30/2019 Analysis of the File Transfer Protocol

11/31

7/30/2019 Analysis of the File Transfer Protocol

12/31

7/30/2019 Analysis of the File Transfer Protocol

13/31

7/30/2019 Analysis of the File Transfer Protocol

14/31

7/30/2019 Analysis of the File Transfer Protocol

15/31

7/30/2019 Analysis of the File Transfer Protocol

16/31

What is FTP?

FTP is an acronym for File Transfer Protocol. It represents a widely-used, method for transferring filesbetween two computers. One of the computers must be set up as a host or server. The other, known asthe client, is the computer that you are using.

There are two basic types of operations:

When you retrieve a file from the server, you are performing a "get" operation. This isalso known as "downloading."

When you post a file to the server, your are performing a "put" operation. This is alsoknown as "uploading."

When you connect to an FTP server, you must send a username and password. In many cases theusername "anonymous" may be used to mean "I am a visitor." Even if you are visiting and FTP site andare not prompted for a username, the FTP server will assign the anonymous username to you. Most FTPservers ask which username you would like to use.

Types of FTP "Programs"

There are a number of programs to help transfer files using FTP. Technical people tend to use acommand line program built into most operating systems, and it has a simple name: 'ftp'. But in order tokeep network sniffers from reading your password when you connect to our ftp server, we use a secureftp server. This means that you must use a secure ftp client that uses "SFTP", or secure file transferprotocol.

There are a number of popular freeware FTP programs such as WinSCP, CuteFTP, FileZilla, Fugu, andSmartFTP. In addition there are commercially available programs for those with too much money. Forclass purposes, FileZilla is a great program to use because it is free to use and runs on Windows, Macs,and Linux environments.

Side note: another way to access files is to use your web browser. However, there are lots of traces ofyour username and password that will be left behind, so refrain from using this method.

I want to watch the ftp traffic and find which ftp urls are being accessed with tshark. Wireshark's DisplayFilters contains the fields http.request.uri and http.host, but these options are not available for ftp traffic.

The problem is that FTP is not a stateless transaction protocol like HTTP - with HTTP the client does asingle request which details all the parameters required to deliver the file, and the server responds with asingle message that contains all the metadata and the file contents.

By comparison, FTP is a chat-style protocol: to get something done you open a connection to the server

and it starts chatting with the server - login, change to some directory, list files, get me this file, etc.

You can listen into this conversation using wireshark like this:

tshark -i lo -f 'port 21' -l -t ad -n -R ftp.request.command -T fields -eftp.request.command -e ftp.request.arg

The output received when a user tries to retrieve a file from the FTP server (in this example using theclient software curl) might look like this:

7/30/2019 Analysis of the File Transfer Protocol

17/31

USER usernamePASS passwordPWDCWD DocumentsEPSVTYPE I

SIZE somefile.extRETR somefile.extQUIT

A bit of processing over that might give you a URL like log of file retrievals. For example, I came up withthis thing using perl:

tshark -i lo -f 'port 21' -l -t ad -n -R ftp.request.command \-T fields -e ftp.request.command -e ftp.request.arg | \perl -nle '

m|CWD\s*(\S+)| and do {$dir=$1;if ($dir =~ m,^/,) { $cwd=$dir } else { $cwd .= "/$dir"; }

};m|RETR\s*(\S+)| and print "$cwd/$1";'

For the same FTP session above, this script will yield a single line of output:

/Documents/somefile.ext

A classic example that shows people why using the FTP protocol is a bad idea:

You can begin to capture traffic by going to the left panel of the Wireshark window and clicking

on Capture/Interfaces. Choose the interface that goes "out" to the network (for example eth1) and

click on Start, and Wireshark will start examining all the packets in transit on the network.

Now open a second terminal window and invoke a normal FTP session. Enter the login name

and password, run some FTP commands, then close the session. Return to the main Wiresharkwindow, and you should see that many packets have passed over the network since the moment

you started capturing. Click Stop Capture (or press Ctrl+E); then you can examine the traffic

you've got.

7/30/2019 Analysis of the File Transfer Protocol

18/31

Figuring out something from what is probably a large amount of traffic is not simple, until you

use a BPF filter. You want one that shows only packets that are part of an FTP connection, so in

the Filter field type "ftp." Immediately the traffic of your session should be highlighted, and in astunning display of poor security, you'll see clearly your username and password. It will look

something like this:

356 101.676753 10.100.1.1 192.168.0.4 FTP 86Response: 220 (vsFTPd 1.1.3)

360 104.546659 192.168.0.4 10.100.1.1 FTP 77

Request: USER wazi

362 104.594520 10.100.1.1 192.168.0.4 FTP 100

Response: 331 Please specify the password.

366 106.530150 192.168.0.4 10.100.1.1 FTP 77

Request: PASS mytest

371 108.922240 10.100.1.1 192.168.0.4 FTP 88

Response: 530 Login incorrect.

If this result doesn't convince your colleagues to quit using FTP and turn to OpenSSH, there's no

hope for them.

The Most Common Filters for Wireshark

On top of all of the filters we've look at so far, here are a few more useful ones.

ip.addr==192.168.0.1 --- Show all traffic from and to 192.168.0.1

tcp.port==80 --- Show all the traffic with 80 as a source or destination port

ip.src==192.168.0.1 and ip.dst==10.100.1.1 --- Show all the traffic that starts from192.168.0.1 and has as target 10.100.1.1

ftp --- Show only the traffic for the ftp protocol

http --- Show only the traffic for the http protocol

dns --- Show only the traffic for the dns protocol

http.request.uri contains string--- Show all http traffic where the url contains the word"string."

7/30/2019 Analysis of the File Transfer Protocol

19/31

As I had said thatFTPandHTTPare two of the some application protocols that pass the

username and password across the network in clear text. So here, we will do some sniffing of

FTP server details of some victim.

Install and run Wireshark

Go to Capture and select Interfaces from the drop-down.

I select the VMWare network adapter since I am using VMWare for demonstrating this. Go to

Options

7/30/2019 Analysis of the File Transfer Protocol

20/31

Check that the Promiscuous mode checkbox is selected and I also opted to use the Capturefilter to narrow the sniffed packets. That is, it targets the packet activity of thespecific IPused as

the Capture filter. Done with the setting, press the Start button. Now, wait for the victim to

login to the FTP server. I am using Ubuntu OS at the victim end on the same network.

I have setup the FTP server at some IP 192.168.48.128. The sooner the victim logs in. The login

details get sniffed and get listed out as:

7/30/2019 Analysis of the File Transfer Protocol

21/31

Check the rows highlighted in blue, in the above snapshot. One shows the user-name and the

other shows the password of the victim.

The best alternative is FTPS (FTP and SSL) - from the wikipedia page:

FTPS (also known as FTP Secure and FTP-SSL) is an extension to the commonly used File Transfer

Protocol (FTP) that adds support for the Transport Layer Security (TLS) and the Secure Sockets Layer

(SSL) cryptographic protocols.

FTPS should not be confused with the SSH File Transfer Protocol (SFTP), an incompatible secure file

transfer subsystem for the Secure Shell (SSH) protocol. It is also different from Secure FTP, the practice

of tunneling FTP through an SSH connection.

for FTP, there is always two sessions between client and server. The control session is opened by the

client on TCP port 21. Then for every subsequent data transfer another data session is required. The

control session is always long-lived.

, , , , ,

there is support for creating your own authentication and authorization providers. You can alsocreate providers for custom FTP logging and for determining the home directory information for

your FTP users.

FTP Binding Component is bound to either a service consumer or service provider, the interfacesexposed is defined by a WSDL. FTP Binding Component implements a set of specific

extensibility elements so that a service can be defined and bound to a FTP protocol.

7/30/2019 Analysis of the File Transfer Protocol

22/31

The FTP Binding Component supports the following extensibility elements:

1. Address: Specifies the FTP connectivity element information such as, FTP URL (host,port, login, password), directory listing style, user defined heuristics for directory listing

parsing.

2. Binding: FTP binding element, a marker element indicating a FTP binding. This elementdoes not have attributes.3. Operation: FTP operation element is a marker element indicating a FTP operation. This

element does not have attributes.4. Transfer: FTP transfer element, specifies a message transfer from a sender and receiver

perspective. For example, to specify a message transfer for a service request, there is a

sender and a receiver involved, the WSDL author specifies:

A. The target sender sends to: Represented by attribute ftp:sendTo, the target receiverreceives fromrepresented by attribute ftp:receiveFrom. Also, the additional

operations performed before a message is sent (PUT) to target or after a message is

received (GET) from the target and is called Pre/Post operations.

B. messageCorrelate: If enabled, a UUID tagging based message correlation scheme willbe used to correlate requestresponse of a synchronous service invoking.5. Message: FTP message element, specifies a message transfer from a service consumer

and/or service provider perspective. The WSDL author can specify:A. The message repository represented as attribute ftp:messageRepository. A base

directory where all the working directories for a message transfer are created, such as,

A. inbox : Is used for posting request (by consumer) and polling request (by provider)B. instage : Is used for staging requestC. outbox : Is used for posting response (by provider) and polling response (by

consumer)

D. outstage : Is used for staging responseB. messageCorrelate : If enabled, a UUID tagging based message correlation scheme

will be used to correlate request-response of a synchronous service invoking.

Beginning with FTP 7.5, the FTP service supports extensibility for custom authentication and roles,

logging, and home directory lookups. When you create an extensibility provider, you must first

register it in the .NET Global Assembly Cache (GAC) for managed-code providers, or in your system's

registry for native-code (COM) providers.

The following table lists the managed-code interfaces that are exposed by the FTP service.

IFtpAuthenticationProvider Interface (Managed) [IIS 7]

Provides an interface for authenticating a FTP user.IFtpHomeDirectoryProvider Interface (Managed) [IIS 7]

Provides an interface for retrieving the path to a user's FTP home directory.IFtpLogProvider Interface (Managed) [IIS 7]

Provides an interface for logging FTP activity.IFtpRoleProvider Interface (Managed) [IIS 7]

Provides an interface for authorization and role checks.

7/30/2019 Analysis of the File Transfer Protocol

23/31

An Overview of the File Transfer Protocol

The File Transfer Protocol (FTP) was one of the first efforts to create a standard means of

exchanging files over a TCP/IP network, so the FTP has been around since the 1970's. The FTPwas designed with as much flexibility as possible, so it could be used over networks other than

TCP/IP, as well as being engineered to have the capability with exchanging files with a broad

variety of machines.

The base specification isRFC 959and is dated October 1985. There are some additional RFCsrelating to FTP, but it should be noted that even as of this writing (December 2001) that most of

the new additions are not in widespread use. The purpose of this document is to provide general

information about how the protocol works without getting into too many technical details. RFC959 should be consulted for details on the protocol.

Control Connection -- the conversation channel

The protocol can be thought of as interactive, because clients and servers actually have a

conversation where they authenticate themselves and negotiate file transfers. In addition, theprotocol specifies that the client and server do not exchange data on the conversation channel.

Instead, clients and servers negotiate how to send data files on separate connections, with one

connection for each data transfer. Note that a directory listing is considered a file transfer.

To illustrate, we'll just present (an admittedly contrived) example of how the FTP would workbetween human beings rather than computer systems. For our example, we'll assume we have a

client, Carl Clinton, who wishes to transfer files from Acme Mail Service that manages his post

office box. Below is a transcript of a phone call between Carl Clinton and Acme Mail Service.

Clinton: (Dials the phone number for the mail service)

Service:"Hello, this is the Acme Mail Service. How may I help you today?"

Clinton:"Hello, this is Carl Clinton. I would like to access mailboxnumber MB1234."

Service:"OK, Mr. Clinton, I need to verify that you may access mailboxMB1234. What is your password?"

Clinton:"My password is QXJ4Z2AF."

Service:"Thank you Mr. Clinton, you may proceed."

Clinton:"For now, I'm only interested in looking at the bills andinvoices, so look at the folder marked "bills" in my mailbox."

Service:"OK."

Clinton:"Please prepare to have your assistant call my secretary at +1 402555 1234."

Service:"OK."

Clinton:"Now call my secretary and tell him the names of all the items inthe bills folder of my mailbox. Tell me when you have finished."

Server:"My assistant is calling your secretary now."

Server:"My assistant has sent the names of the items."

Clinton: (Receives the list from his secretary and notices a bill from Yoyodyne Systems.)"Please prepare to have your assistant send to my fax machine +1

7/30/2019 Analysis of the File Transfer Protocol

24/31

402 555 7777."

Service:"OK."

Clinton:"Now fax a copy of the bill from Yoyodyne Systems."

Server:"My assistant is calling your fax machine now."

Server:"My assistant has finished faxing the item."

Clinton:"Thank you, that is all. Good bye."

Server:"Goodbye."

Now let's look at how this same conversation would appear between computer systems

communicating with the FTP protocol over a TCP/IP connection.

Client: Connects to the FTP service at port 21 on theIP address 172.16.62.36.

Server:220 Hello, this is the AcmeMail Service.

Client:USER MB1234

Server:331 Password required toaccess user account MB1234.

Client:PASS QXJ4Z2AF Note that this password is not encrypted. TheFTP is susceptible to eavesdropping!

Server:230 Logged in.

Client:CWD Bills Change directory to "Bills."

Server:250 "/home/MB1234/Bills" isnew working directory.

Client:PORT 192,168,1,2,7,138 The client wants the server to send to portnumber 1930 on IP address 192.168.1.2. In

this case, 192.168.1.2 is the IP address of theclient machine.

Server:200 PORT command successful.

Client:LIST Send the list of files in "Bills."

Server:150 Opening ASCII mode dataconnection for /bin/ls.

The server now connects out from its port 20on 172.16.62.36 to port 1930 on 192.168.1.2.

Server:226 Listing completed. That succeeded, so the data is now sent overthe established data connection.

Client:PORT 192,168,1,2,7,139 The client wants the server to send to portnumber 1931 on the client machine.

Server:200 PORT command successful.Client:RETR Yoyodyne.TXT Download "Yoyodyne.TXT."

Server:150 Opening ASCII mode dataconnection for Yoyodyne.TXT.

The server now connects out from its port 20on 172.16.62.36 to port 1931 on 192.168.1.2.

Server:226 Transfer completed. That succeeded, so the data is now sent overthe established data connection.

Client:QUIT

7/30/2019 Analysis of the File Transfer Protocol

25/31

Server:221 Goodbye.

When using FTP, users frequently employ the FTP client programs rather than directly

communicating with the FTP server. Here's our same example using the stock "ftp" program

which is usually installed as /usr/bin/ftp on UNIX systems (and FTP.EXE on Windows). Theitems the user types are in bold.

ksh$ /usr/bin/ftp

ftp> open ftp.acmemail.example.com

Connected to ftp.acmemail.example.com (172.16.62.36).

220 Hello, this is the Acme Mail Service.

Name (ftp.acmemail.example.com:root): MB1234

331 Password required to access user account MB1234.

Password: QXJ4Z2AF

230 Logged in.

ftp> cd Bills

250 "/home/MB1234/Bills" is new working directory.

ftp> ls

200 PORT command successful.

150 Opening ASCII mode data connection for /bin/ls.

-rw-r--r-- 1 ftpuser ftpusers 14886 Dec 3 15:22 Acmemail.TXT

-rw-r--r-- 1 ftpuser ftpusers 317000 Dec 4 17:40 Yoyodyne.TXT

226 Listing completed.

ftp> get Yoyodyne.TXT

local: Yoyodyne.TXT remote: Yoyodyne.TXT

200 PORT command successful.

150 Opening ASCII mode data connection for Yoyodyne.TXT.

226 Transfer completed.

317000 bytes received in 0.0262 secs (1.2e+04 Kbytes/sec)

ftp> quit

221 Goodbye.

As you can see, FTP is designed to allow users to browse the file system much like you would

with a regular UNIX login shell or MS-DOS command prompt. This differs from other protocolsthat are transactional (i.e. HTTP), where a connection is established, clients issue a single

message to a server that replies with a single reply, and the connection is closed. On the other

hand, client programs can be constructed to simulate a transactional environment if they know inadvance what they need to do. In effect, FTP is a stateful sequence of one or more transactions.

Command primitives, result codes and textual responses

7/30/2019 Analysis of the File Transfer Protocol

26/31

The client is always responsible for initiating requests. These requests are issued with FTP

command primitives, which are typically 3 or 4 characters each. For example, the command

primitive to change the working directory is CWD.

The server replies are specially formatted to contain a 3-digit result code first, followed by a

space character, followed by descriptive text (there is also a format for multi-line responses).

The protocol specifies that clients must only rely upon the numeric result code, since thedescriptive text is allowed to vary (with a few exceptions). In practice, the result text is often

helpful for debugging, but is generally no longer useful for end users.

Authentication

Although it is not required by protocol, in effect clients must always login to the FTP server with

a username and password before the server will allow the client to access the service.

There is also a de facto standard for guest access, where "anonymous" (or "ftp") are used as theusername and an e-mail address is customarily used as the password in a way for a polite netizento let the server administrator know who is using the guest login. Because users do not want to

divulge their e-mail addresses to protect against unsolicited bulk e-mail, this has subsequently

evolved to the point where the password is just some arbitrary text.

Types of data connections

The protocol has built-in support for different types of data transfers. The two mandated types

are ASCII for text (specified by the client sending "TYPE A" to the server), and "image" for

binary data (specified by "TYPE I").

ASCII transfers are useful when the server machine and client machine have different standardsfor text. For example, MS-DOS and Microsoft Windows use a carriage return and linefeed

sequence to denote an end-of-line, but UNIX systems use just a linefeed. When ASCII transfersare specified, this enables a client to always be able to translate the data into its own native text

format.

Binary transfers can be used for any type of raw data that requires no translation. Client

programs should use binary transfers unless they know that the file in question is text.

The protocol does not have any advanced support for character sets for pathnames nor file

contents. There is no way to specify UNICODE, for example. For ASCII, it is 7-bit ASCII only.

Unfortunately, the burden of deciding what transfer type to use is left to the client, unlike HTTP,

which can inform the client what type of data is being sent. Clients often simply choose totransfer everything in binary, and perform any necessary translation after the file is downloaded.

Additionally, binary transfers are inherently more efficient to send over the network since theclient and server do not need to perform on-the-fly translation of the data.

It should be noted that ASCII transfers are mandated by the protocol as the default transfer typeunless the client requests otherwise!

The PORT and PASV conundrum -- Active and Passive data connections

7/30/2019 Analysis of the File Transfer Protocol

27/31

Although it was purposely designed into the protocol as a feature, FTP's use ofseparate data

connections cause numerous problems for things like firewalls, routers, proxieswhich want to

restrict or delegate TCP connections, as well as things like IP stacks which want to do dynamicstateful inspection of TCP connections.

The protocol does not mandate a particular port number or a direction that a data connection

uses. For example, the easy way out would have been for the protocol's designers to mandatethat alldata connections must originate from the client machine and terminate at port 20 on theserver machine.

Instead, for maximum flexibility, the protocol allows the client to choose one of two methods.

The first method, which we'll call "Active", is where the client requests that the server originate a

data connection and terminate at an IP address and port number of the client's choosing. Theimportant thing to note here is that the server connects out to the client.

Client:"Please connect to me at port 1931 on IP address192.168.1.2, then send the data."

Server:"OK"

Or, the client can request that the server to assign an IP address and port number on the serverside and have the client originate a connection to the server address. We call this method"Passive" and note that the client connects out to the server.

Client:"Please tell me where I can get the data."

Server:"Connect to me at port 4023 on 172.16.62.36."

The active method uses the FTP command primitive PORT, so the first example using the actualFTP protocol would resemble this:

Client:PORT 192,168,1,2,7,139

Server:200 PORT command successful.

The passive method uses the FTP command primitive PASV, so the second example using theactual FTP protocol would resemble this:

Client:PASV

Server:Entering Passive Mode (172,16,62,36,133,111)

It should be noted that FTP servers are required to implement PORT, but are not required to

implement PASV. The default has traditionally been PORT for this reason, but in practice it is now

preferred to use PASV whenever possible because firewalls may be present on the client sidewhichoften cause problems.

Partial data connections -- resuming downloads

The protocol provides a means to only transfer a portion of a file, by having a client specify a

starting offset into the file (using the REST primitive, i.e. "restart point"). If an FTP session failswhile a data transfer is in progress and has to be reestablished, a client can request that the serverrestart the transfer at the offset the client specifies. Note that not all FTP servers support this

feature.

7/30/2019 Analysis of the File Transfer Protocol

28/31

Directory listings

The base standard of the FTP protocol provides two types of listings, a simple name list (NLST)

and a human-readable extended listing (LIST). The name list consists of lines of text, whereeach line contains exactly one file name and nothing else.

The extended listing is not intended to be machine-readable and the protocol does not mandateany particular format. The de facto standard is for it to be in UNIX "/bin/ls -l" format, but

although most servers try to emulate that format even on non-UNIX FTP servers, it is still

common for servers to provide their own proprietary format. The important thing to note here isthat this listing can contain any type of data and cannot be relied upon. Additionally, even those

that appear in "/bin/ls -l" format cannot be relied upon for the validity of the fields. Forexample the date and time could be in local time or GMT.

Newer FTP server implementations support a machine-readable listing primitive (MLSD) which is

suitable for client programs to get reliable metadata information about files, but this feature is

still relatively rare. That leaves the simple name list as the only reliable way to get filenames,

but it doesn't tell a client program anything else (such as if the item is a file or a directory!).

Functional concerns

Despite a rich feature set, there are some glaring omissions. For example, the base specificationdoesn't even provide for clients to query a file's size or modification date. However, most FTP

servers in use now support a de facto extension to the specification which provides the SIZE and

MDTM primitives, and even newer servers support the extremely useful MLSD and MSLT primitives

which can provide a wealth of information in a standardized format.

There is also no 100% accurate way for a client to determine if a particular pathname refers to a

file or directory, unless MLSD orMLST is available. Since the protocol also does not provide a

way to transfer an entire directory of items at once, the consequence is that there is no 100%accurate way to download an entire directory tree.

The end result is that FTP is not particularly suited to "mirroring" files and directories, although

FTP client programs use heuristics to make calculated guesses when possible.

Despite the guesswork that clients can use for determining metadata for files to download, there's

little they can do for files that they upload. There is no standard way to preserve an uploadedfile's modification time. FTP is platform agnostic, so there aren't standard ways to preserve

platform-specific metadata such as UNIX permissions and user IDs or Mac OS file type and

creator codes.

Separate connections for data transfers are also a mixed blessing. For high performance, it

would be best to use a single connection and perform multiple data transfers before closing it.Even better would be for a method to use a single connection for both the control connection

conversation and data transfers. Since each data connection uses an ephemeral (random) port

number, it is possible to "run out" of connections. For details on this phenomenon, a separatearticleis available.

Security concerns

7/30/2019 Analysis of the File Transfer Protocol

29/31

It is important to note that the base specification, as implemented by the vast majority of the

world's FTP servers, does not have any special handling for encrypted communication of any

kind. When clients login to FTP servers, they are sending clear text usernames and passwords!This means that anyone with a packet sniffer between the client and server could surreptitiously

steal passwords.

Besides passwords, potential attackers could not only monitor the entire conversation on the FTPcontrol connection, they could also monitor the contents of the data transfers themselves.

There have been proposals to make the FTP protocol more secure, but these proposals have notseen widespread adoption.

Therefore, unless the IP protocol layer itself is secure (for example, encrypted using IPsec), FTP

should not be used if sensitive login information is to be exchanged over an insecure network, or

if the files containing sensitive material are being transferred over an insecure network.

A TCP/IPv4 connection consists of two endpoints, and each endpoint consists of an IP address

and a port number. Therefore, when a client user connects to a server computer, an establishedconnection can be thought of as the 4-tuple of (server IP, server port, client IP, client port).

Usually three of the four are readily known -- client machine uses its own IP address and when

connecting to a remote service, the server machine's IP address and service port number arerequired.

What is not immediately evident is that when a connection is established that the client side of

the connection uses a port number. Unless a client program explicitly requests a specific port

number, the port number used is an ephemeral port number. Ephemeral ports are temporaryports assigned by a machine's IP stack, and are assigned from a designated range of ports for this

purpose. When the connection terminates, the ephemeral port is available for reuse, although

most IP stacks won't reuse that port number until the entire pool of ephemeral ports have been

used. So, if the client program reconnects, it will be assigned a different ephemeral port numberfor its side of the new connection.

Similarly, for UDP/IP, when a datagram is sent by a client from an unbound port number, an

ephemeral port number is assigned automatically so the receiving end can reply to the sender.

Limits Implied by the Ephemeral Port Range

Another important ramification of the ephemeral port range is that it limits the maximum numberof connections from one machine to a specific service on a remote machine! The TCP/IP

protocol uses the connection's 4-tuple to distinguish between connections, so if the ephemeral

port range is only 4000 ports wide, it means that there can only be 4000 unique connections froma client machine to a remote service at one time.

A port range of 4000 may seem large, but it is actually small for 21st century computingdemands when you consider that a TCP connection must expire through the TIME_WAIT state

before it is really completed. For example, even if both sides of a connection properly close their

ends of the connection, due to TCP's error control, each side must wait until the TIME_WAITstate is expired before the connection's resources can really be disposed. The TIME_WAIT state

is twice the MSL (maximum segment lifetime) which, depending on the IP stack, is usually

7/30/2019 Analysis of the File Transfer Protocol

30/31

configured to be 240 seconds total. That means that you could have only 4000 connections per

240 second window, and in practice this can be exhausted.

Problems Exacerbated by the File Transfer Protocol

For example, the File Transfer Protocol (FTP) uses a separate data connection for each datatransfer, so this phenomenon can be experienced when FTP'ing hundreds of small files over a

fast local network. When all ephemeral ports are in use, a compatible BSD Sockets TCP/IPstack will return "Address already in use" errors.

The second problem with the FTP is that with the default mode of operation, the separate data

transfer connections originate from the server machine back to the client machine to an

ephemeral port number requested by the client program. Since many firewalls restrict incomingconnections, when the FTP data connection tries to connect to an ephemeral port number on the

client machine, the connection may be denied by the firewall since the port number isn't a well-

known service port.

It may be best to illustrate this by showing a sample FTP session:Client:USER anonymous

Server:331 Guest login ok, send youre-mail address as password.

Client:PASS NcFTP@

Server:230 Logged in anonymously.

Client:PORT 192,168,1,2,7,138 The client wants the server to send to portnumber 1930 on IP address 192.168.1.2.

Server:200 PORT command successful.

Client:LIST

Server:150 Opening ASCII mode dataconnection for /bin/ls.

The server now connects out from port 20to port 1930 on 192.168.1.2.

Server:226 Listing completed. That succeeded, so the data is now sent overthe established data connection.

Client:QUIT

Server:221 Goodbye.

Now, had there been a restrictive firewall on the client machine, when the connection from theserver to port 1930 on the client would have failed. Note that the client program did not

explicitly ask for port 1930 -- it just asked for an unused port number to use for this temporary

data connection.

Traditional Configuration of the Ephemeral Port Range

The BSD Sockets TCP/IP stack used ports 1024 through 4999 as ephemeral ports. Additionally,

ports 1 through 1023 were intended for systems services running as the superuser, so those ports

are called reserved ports.

As discussed earlier, BSD's choice of the ephemeral port range is unfortunate because of itsrelatively small size (3975 ports) and its low numbered position. We feel that the default range

7/30/2019 Analysis of the File Transfer Protocol

31/31

should be 49152 through 65535, which is both much larger (16383 ports) and is at the very top

of the full port range.

Firewalling the Ephemeral Port Range

For firewalls, often administrators choose to restrict access to as many port numbers as possible.For cases where inbound connections to the ephemeral ports is required, an entire range of ports

must be opened. It is imperative that when opening a range of ports on the firewall that nosystem services are listening on ports in the open range. Administrators will often want to open

a specific range on the firewall, and then for each machine on the internal network, make sure

that the ephemeral port range on the machine coincides with the open range on the firewall.

It must be made clear that the ephemeral port range on machines on the internal network often donot coincide with each other since different operating systems may use different ranges. That is

why it can be time consuming to manually configure each machine's ephemeral port range so it

coincides with the open range on the firewall. As a result, administrators often end up changing

to a policy of allowing all incoming ports and deny access to specific ports when needed.

Luckily, it may not be necessary to open the ephemeral port range. It is usually only necessary

when FTP is being served to the outside world (Passive "PASV" data connections use inbound

ephemeral ports), or when FTP client access must work in non-passive mode ("PORT"

connections from the server are inbound to clients using ephemeral ports).

Changing the Ephemeral Port Range

It is desirable to change which port numbers are used for the ephemeral port range for any of the

following:

To use a larger range so that more simultaneous connections are possible

To shift the range to the higher numbered ports. The higher numbered ports should beused as ephemeral ports because they are less likely to be used as port numbers for

system services. Well-known service ports have traditionally been assigned to lowerport numbers.

To change the range to coincide with other systems for purposes of firewalls andautomatic network address translation.

When changing the range, we suggest you change it to 49152 through 65535. If you need a

larger range, continue downward from 49152, but leave 65535 as your upper bound.