Outbound port on sockets

[Grant Edwards]

On 2006-09-15, Steve Holden <steve at holdenweb.com> wrote:

>> I don't know what "multi-homing problems are either".
>> Apparently there must be some ftp clients that require the
>> source port for the data connection to be port 20.
>> 
>> The RFC is pretty vague. It does say the server and clinet but
>> must "support the use of the default data port [port 20]" or
>> something like that. But, it's not all all clear to me what
>> that is supposed to mean.  My reading is that they must support
>> the default port as the destination port for a data connection
>> untill it's been changed by receipt of a PORT command.

> Standard (port-mode) FTP has the client send a PORT command to the 
> server when data transfer is required. The server then makes a 
> connection to the indicated port from its own port 20.

I agree that's certainly the way it seems to work.  When I read
the RFC, I couldn't tell if it intended to require that the
source port be 20.  It almost sounded like it meant that the
data connection destination was 20 unless a PORT command was
received, but that wouldn't make much sense.

> Passive mode was introduced so that the server is not required
> to make a connection inbound to the client, as more and more
> firewalls were interposed at the perimeter of networks,
> blocking the inbound requests to clients from servers.
>
> I suspect that the reason for the comment is simply that the
> connection out from the server is being bound to the same
> interface (*IP address*) that the inbound request arrived on.
> That way it's less likely that the data stream will be routed
> differently from the control (port 21) stream.

I think that's probably right.  It just dawned on me that
"multi-homed" refers to having more than one network interface.

-- 
Grant Edwards                   grante             Yow!  Yow!! That's a GOOD
                                  at               IDEA!! Eating a whole FIELD
                               visi.com            of COUGH MEDICINE should
                                                   make you feel MUCH BETTER!!