---
name: "curvecpclient"
section: "1"
platform: "linux"
package: "nacl-tools"
version: "20110221-15"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/curvecpclient"
---

# curvecpclient(1)

Message-handling programs

## NAME

CurveCP — Message-handling programs

## SYNOPSIS

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

## DESCRIPTION

*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   **curvecpclient**.

**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 case.

## 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.

## OPTIONS

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

## SEE ALSO

curvecpserver (1), curvecpmessage (1), inetd (8), tcpserver (1).

## AUTHOR

This manual page was written by Sergiusz Pawlowicz debian@pawlowicz.name for  the **Debian** system (and may be used by others). The source  of this page is a webpage https://curvecp.org/messageapi.html .  Permission is granted to copy, distribute and/or modify this  document under public domain.

This manual page was rewritten for the **Debian** distribution  because the original program does not have a manual page.