The Gardyan of Greyskull wrote:
>
> I have a very weird problem.
>
> I can connect to FTP servers but I can't "get", "dir" or "ls", although I
> can "cd".
>
> I also just noticed that I cannot receive e-mail from any outside account,
> although I can receive e-mail from an account inside prognet.com (my
> domain)
>
> Any ideas what could be going on? My system had been working just fine.
Sounds like you're dealing with some kind of firewall. get, dir, and
ls all require the establishment of an ftp data connection, which is
actually initiated (in the TCP sense) from the server, which cd doesn't
require this.
Many sites firewall by filtering out any TCP SYN packets which come,
un-solicited, from the outside. Here's an excerpt from Internet
Standard 7, which describes/defines the TCP protocol:
A connection progresses through a series of states during its
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional
because it represents the state when there is no TCB, and therefore,
no connection. Briefly the meanings of the states are:
LISTEN - represents waiting for a connection request from any remote
TCP and port.
SYN-SENT - represents waiting for a matching connection request
after having sent a connection request.
SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request.
ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer phase
of the connection.
FIN-WAIT-1 - represents waiting for a connection termination request
from the remote TCP, or an acknowledgment of the connection
termination request previously sent.
FIN-WAIT-2 - represents waiting for a connection termination request
from the remote TCP.
CLOSE-WAIT - represents waiting for a connection termination request
from the local user.
CLOSING - represents waiting for a connection termination request
acknowledgment from the remote TCP.
LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP
(which includes an acknowledgment of its connection termination
request).
TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request.
CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in figure 6 illustrates only state changes, together
with the causing events and resulting actions, but addresses neither
error conditions nor actions which are not connected with state
changes. In a later section, more detail is offered with respect to
the reaction of the TCP to events.
NOTE BENE: this diagram is only a summary and must not be taken as
the total specification.
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
Well this won't explain it all but it will get you started down
the road! The gist of it is that you can keep track of packets and
make sure that TCP connections are only initiated form the inside.
Bruce is incorrect in this case the you could be missing a route since
if that were the case, you wouldn't be able to connect to ftp servers.
You can do it because the ftp control connection is initiated by the
client. If you ask your administrators and they say: "yes, we're
preventing TCP connections initiated from outside", then there *is*
a solution. Just use the '-p' parameter to request "passive mode".
Using 'passive mode' (which netscape always uses, it looks like)
the data connection is initiated locally. For more details on FTP
protocol see Internet Standard 9.
Curious? Here are the URLs:
ftp://venera.isi.edu/in-notes/std/std7.txt
ftp://venera.isi.edu/in-notes/std/std9.txt
--
Jens B. Jorgensen
[EMAIL PROTECTED]
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