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CurveCP — Message-handling programs


curvecpclient [-q (optional)] [-Q (optional)] [-v (optional)] [-c keydir(optional)]
[sname] [pk] [ip] [port] [ext] [prog]


This manual page documents briefly the CurveCP commands.

A traditional UNIX-style server such as ftpd handles just one network connection, reading
input from stdin and writing output to stdout. A "superserver" such as inetd or tcpserver
listens for network connections and starts a separate server process for each connection.

The CurveCP command-line tools have an extra level of modularity. The curvecpserver
superserver listens for network connections. For each connection, curvecpserver starts the
curvecpmessage message handler; curvecpmessage then starts a server such as ftpd. Then
ftpd sends a stream of data to curvecpmessage, which in turn sends messages to
curvecpserver, which encrypts and authenticates the messages and sends them inside network
packets. At the same time curvecpclient receives network packets, verifies and decrypts
messages inside the packets, and passes the messages to curvecpmessage; curvecpmessage
sends a stream of data to ftpd. The same curvecpmessage tool is also used by

curvecpserver and curvecpclient can use programs other than curvecpmessage. Those programs
can directly generate messages in the CurveCP message format without talking to separate
tools such as ftpd; or they can support a completely different protocol that reuses
CurveCP's cryptographic layer but transmits different kinds of messages.

This page explains what programmers have to do to write curvecpmessage replacements that
talk to curvecpserver and curvecpclient.

Incoming messagess

File descriptor 8 is a pipe. Read from this pipe a length byte n, between 1 and 68, and a
16*n-byte message. Repeat. The pipe is set to non-blocking mode; be prepared for EAGAIN
and EWOULDBLOCK, even in the middle of a message.

This pipe reading must always be active. The curvecpclient and curvecpserver programs
assume that every message is read immediately. If you can't handle a message immediately,
read it and put it onto a queue. If you don't have queue space, throw the message away;
this shouldn't cause trouble, since you have to be able to handle missing messages in any

Outgoing messagess

File descriptor 9 is a pipe. Write to this pipe a length byte n, between 1 and 68, and a
16*n-byte message. Repeat. The pipe is set to non-blocking mode; be prepared for EAGAIN
and EWOULDBLOCK, even in the middle of a message.

As a client, do not use length bytes above 40 until a message has arrived from the server.
(The messages inside CurveCP Initiate packets are limited to 640 bytes.)

The CurveCP server does not start until it has received a message from the client.
Furthermore, the CurveCP server must receive this message within 60 seconds of the client
starting up. (The CurveCP Initiate packet is valid for only 60 seconds after the
corresponding CurveCP Cookie packet.) This does not mean that the client must start
sending messages immediately, but it does mean that waiting for more than a second to send
a message is a bad idea.


How to use curvecpclient:

-q optional
no error messages

-Q optional
print error messages (default)

-v optional
print extra information

-c keydir optional
use this public-key directory

sname server's name

pk server's public key

ip server's IP address

port server's UDP port

ext server's extension

prog run this client

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