CODEWITHSUNDEEP
pythontwistedtelnetnetcat
Fixee
Fixee

Reputation: 1639

Why does telnet work and netcat fail with this Twisted example?

I'm following a Twisted tutorialand tried the following code from that article:

# Read username, output from non-empty factory, drop connections

from twisted.internet import protocol, reactor
from twisted.protocols import basic

class FingerProtocol(basic.LineReceiver):
    def lineReceived(self, user):
        self.transport.write(self.factory.getUser(user)+"\r\n")
        self.transport.loseConnection()

class FingerFactory(protocol.ServerFactory):
    protocol = FingerProtocol

    def __init__(self, **kwargs):
        self.users = kwargs

    def getUser(self, user):
        return self.users.get(user, "No such user")

reactor.listenTCP(1079, FingerFactory(moshez='Happy and well'))
reactor.run()

I tried nc localhost 1079 and it just hangs: no replies. But then telnet localhost 1079 works just fine. Why?

Upvotes: 1

Views: 1333

Answers (2)

jaymz
jaymz

Reputation: 53

On linux, you need to use CRLF as EOL

From the nc man page:

-C, --crlf (Use CRLF as EOL) . This option tells Ncat to convert LF. line endings to CRLF. when taking input from standard input.. This is useful for talking to some stringent servers directly from a terminal in one of the many common plain-text protocols that use CRLF for end-of-line.

Upvotes: 2

Fixee
Fixee

Reputation: 1639

Using wireshark, I figured out the following: telnet was sending 0x0d and then 0x0a (ie, "\r\n") for the line terminator. However netcat was sending only 0x0a. This was on Ubuntu (and also on OS X).

Apparently the LineReceiver protocol in Twisted requires the \r in order to raise the "line received" event.

There are at least 6 versions of netcat out there, but the one on OS X (which is close to FreeBSD) has a -c option that appends \r\n to the end of each line. Using this option fixes the problem.

$ nc -c localhost 1079 
moshez
Happy and well

Note: LineReceiver has a class variable called delimiter that allows any EOL character to be used. Setting delimiter = '\n' obviates the need for the -c option to netcat.

Upvotes: 6

Related Questions