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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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14.1.6 Reply codes<br />

FTP<br />

client<br />

Firewall<br />

FTP<br />

Server A<br />

Figure 14-5 An FTP proxy transfer through a firewall<br />

1<br />

In Figure 14-5:<br />

1. The FTP client opens a connection <strong>and</strong> logs on to the FTP server A.<br />

2. The FTP client issues a proxy open comm<strong>and</strong>, <strong>and</strong> a new control connection<br />

is established with FTP server B.<br />

3. The FTP client then issues a proxy get comm<strong>and</strong> (though this can also be a<br />

proxy put).<br />

4. A data connection is established between server A <strong>and</strong> server B. Following<br />

data connection establishment, the data flows from server B to server A.<br />

In order to manage these operations, the client <strong>and</strong> server conduct a dialog using<br />

the Telnet convention. The client issues comm<strong>and</strong>s, <strong>and</strong> the server responds<br />

with reply codes. The responses also include comments for the benefit of the<br />

user, but the client application uses only the codes. Reply codes are three digits<br />

2<br />

3<br />

<br />

4<br />

FTP<br />

Server B<br />

Chapter 14. File-related protocols 523

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