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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | USAGE EXAMPLE | CRYPTOGRAPHY | SETUP HOWTO | NOTE | BUGS | LEGAL | HISTORY | SEE ALSO | AUTHOR | COLOPHON | COLOPHON |
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CURVETUN(8) netsniff-ng toolkit CURVETUN(8)
curvetun - a lightweight curve25519 ip4/6 tunnel
curvetun [options]
curvetun is a lightweight, high-speed ECDH multiuser IP tunnel
for Linux that is based on epoll(2). curvetun uses the Linux
TUN/TAP interface and supports {IPv4, IPv6} over {IPv4, IPv6}
with UDP or TCP as carrier protocols.
It has an integrated packet forwarding tree, thus multiple users
with different IPs can be handled via a single tunnel device on
the server side, and flows are scheduled for processing in a CPU
efficient way, at least in the case of TCP as the carrier
protocol.
For key management, public-key cryptography based on elliptic
curves are used and packets are encrypted end-to-end by the
symmetric stream cipher Salsa20 and authenticated by the MAC
Poly1305, where keys have previously been computed with the ECDH
key agreement protocol Curve25519.
Cryptography is based on Daniel J. Bernstein's networking and
cryptography library “NaCl”. By design, curvetun does not provide
any particular pattern or default port numbers that gives
certainty that the connection from a particular flow is actually
running curvetun.
However, if you have a further need to bypass censorship, you can
try using curvetun in combination with Tor's obfsproxy or Telex.
Furthermore, curvetun also protects you against replay attacks
and DH man-in-the-middle attacks. Additionally, server-side
syslog event logging can also be disabled to avoid revealing
critical user connection data.
1. obfsproxy from the TOR project
https://www.torproject.org/projects/obfsproxy.html.en
2. Telex, anti-censorship in the network infrastructure
https://telex.cc/
-d <tundev>, --dev <tundev>
Defines the name of the tunnel device that is being
created. If this option is not set, then the default
names, curves{0,1,2,..} for a curvetun server, and
curvec{0,1,2,...} for a curvetun client are used.
-p <num>, --port <num>
Defines the port the curvetun server should listen on.
There is no default port for curvetun, so setting this
option for server bootstrap is mandatory. This option is
for servers only.
-t <server>, --stun <server>
If needed, this options enables an STUN lookup in order to
show public IP to port mapping and to punch a hole into
the firewall. In case you are unsure what STUN server to
use, simply use ''--stun stunserver.org''.
-c[=alias], --client[=alias]
Starts curvetun in client mode and connects to the given
connection alias that is defined in the configuration
file.
-k, --keygen
Generate private and public keypair. This must be done
initially.
-x, --export
Export user and key combination to stdout as a one-liner.
-C, --dumpc
Dump all known clients that may connect to the local
curvetun server and exit.
-S, --dumps
Dump all known servers curvetun as a client can connect
to, and exit.
-D, --nofork
Do not fork off as a client or server on startup.
-s, --server
Start curvetun in server mode. Additional parameters are
needed, at least the definition of the port that clients
can connect to is required.
-N, --no-logging
Disable all curvetun logging of user information. This
option can be used to enable curvetun users to connect
more anonymously. This option is for servers only.
-u, --udp
Use UDP as a carrier protocol instead of TCP. By default,
TCP is the carrier protocol. This option is for servers
only.
-4, --ipv4
Defines IPv4 as the underlying network protocol to be used
on the tunnel device. IPv4 is the default. This option is
for servers only.
-6, --ipv6
Defines IPv6 as the underlying network protocol to be used
on the tunnel device. This option is for servers only.
-v, --version
Show version information and exit.
-h, --help
Show user help and exit.
curvetun --server -4 -u -N --port 6666 --stun stunserver.org
Starts curvetun in server mode with IPv4 as network
protocol and UDP as a transport carrier protocol. The
curvetun server listens for incoming connections on port
6666 and performs an STUN lookup on startup to
stunserver.org.
curvetun --client=ethz
Starts curvetun in client mode and connects to the defined
connection alias ''ethz'' that is defined in the curvetun
~/.curvetun/servers configuration file.
curvetun --keygen
Generates initial keypairs and stores them in the
~/.curvetun/ directory.
curvetun --export
Export user data to stdout for configuration of a curvetun
server.
Encrypted IP tunnels are often used to create virtual private
networks (VPN), where parts of the network can only be reached
via an insecure or untrusted medium such as the Internet. Only a
few software utilities exist to create such tunnels, or, VPNs.
Two popular representatives of such software are OpenVPN and
VTUN.
The latter also introduced the TUN/TAP interfaces into the Linux
kernel. VTUN only has a rather basic encryption module, that does
not fit today's cryptographic needs. By default, MD5 is used to
create 128-Bit wide keys for the symmetric BlowFish cipher in ECB
mode [1].
Although OpenSSL is used in both VTUN and OpenVPN, OpenVPN is
much more feature rich regarding ciphers and user authentication.
Nevertheless, letting people choose ciphers or authentication
methods is not necessarily a good thing: administrators could
either prefer speed over security and therefore choose weak
ciphers, so that the communication system will be as good as
without any cipher; they could choose weak passwords for
symmetric encryption or they could misconfigure the communication
system by having too much choice of ciphers and too little
experience for picking the right one.
Next to the administration issues, there are also software
development issues. Cryptographic libraries like OpenSSL are a
huge mess and too low-level and complex to fully understand or
correctly apply, so that they form further ground for
vulnerabilities of such software.
In 2010, the cryptographers Tanja Lange and Daniel J. Bernstein
have therefore created and published a cryptographic library for
networking, which is named NaCl (pronounced ''salt''). NaCl
addresses such problems as mentioned in OpenSSL and, in contrast
to the rather generic use of OpenSSL, was created with a strong
focus on public-key authenticated encryption based on elliptic
curve cryptography, which is used in curvetun. Partially quoting
Daniel J. Bernstein:
"RSA is somewhat older than elliptic-curve cryptography: RSA was
introduced in 1977, while elliptic-curve cryptography was
introduced in 1985. However, RSA has shown many more weaknesses
than elliptic-curve cryptography. RSA's effective security level
was dramatically reduced by the linear sieve in the late 1970s,
by the quadratic sieve and ECM in the 1980s, and by the number-
field sieve in the 1990s. For comparison, a few attacks have been
developed against some rare elliptic curves having special
algebraic structures, and the amount of computer power available
to attackers has predictably increased, but typical elliptic
curves require just as much computer power to break today as they
required twenty years ago.
IEEE P1363 standardized elliptic-curve cryptography in the late
1990s, including a stringent list of security criteria for
elliptic curves. NIST used the IEEE P1363 criteria to select
fifteen specific elliptic curves at five different security
levels. In 2005, NSA issued a new ''Suite B'' standard,
recommending the NIST elliptic curves (at two specific security
levels) for all public-key cryptography and withdrawing previous
recommendations of RSA."
curvetun uses a particular elliptic curve, Curve25519, introduced
in the following paper: Daniel J. Bernstein, ''Curve25519: new
Diffie-Hellman speed records,'' pages 207-228 in Proceedings of
PKC 2006, edited by Moti Yung, Yevgeniy Dodis, Aggelos Kiayias,
and Tal Malkin, Lecture Notes in Computer Science 3958, Springer,
2006, ISBN 3-540-33851-9.
This elliptic curve follows all of the standard IEEE P1363
security criteria. It also follows new recommendations that
achieve ''side-channel immunity'' and ''twist security'' while
improving speed. What this means is that secure implementations
of Curve25519 are considerably simpler and faster than secure
implementations of, for example, NIST P-256; there are fewer
opportunities for implementors to make mistakes that compromise
security, and mistakes are more easily caught by reviewers.
An attacker who spends a billion dollars on special-purpose chips
to attack Curve25519, using the best attacks available today, has
about 1 chance in 1000000000000000000000000000 of breaking
Curve25519 after a year of computation. One could achieve
similar levels of security with 3000-bit RSA, but encryption and
authentication with 3000-bit RSA are not nearly fast enough to
handle tunnel traffic and would require much more space in
network packets.
1. Security analysis of VTun
http://www.off.net/~jme/vtun_secu.html
2. NaCl: Networking and Cryptography library
http://nacl.cr.yp.to/
If you have not run curvetun before, you need to do an initial
setup once.
First, make sure that the servers and clients clocks are
periodically synced, for example, by running an NTP daemon. This
is necessary to protect against replay attacks. Also, make sure
you have read and write access to /dev/net/tun. You should not
run curvetun as root! Then, after you have assured this, the
first step is to generate keys and config files. On both the
client and server do:
curvetun -k
You are asked for a user name. You can use an email address or
whatever suits you. Here, we assume you have entered 'mysrv1' on
the server and 'myclient1' on the client side.
Now, all necessary files have been created under ~/.curvetun.
Files include “priv.key”, “pub.key”, “username”, “clients” and
“servers”.
“clients” and “servers” are empty at the beginning and need to be
filled. The “clients” file is meant for the server, so that it
knows what clients are allowed to connect. The “servers” file is
for the client, where it can select curvetun servers to connect
to. Both files are kept very simple, so that a single
configuration line per client or server is sufficient.
The client needs to export its public key data for the server
curvetun -x
where it prints a string in the following format:
myclient1;11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11
\_______/
\_____________________________________________________________________________________________/
username 32 byte public key for 'myclient1'
This line is transferred to the server admin (yes, we assume a
manual on-site key exchange scenario where, for example, the
admin sets up server and clients), where the admin then adds this
entry into his ''clients'' file like:
server$ echo
"myclient1;11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:11:" \
"11:11:11:11:11:11:11:11:11:11:11:11:11:11:11" >>
~/.curvetun/clients
The server admin can check if the server has registered it
properly as follows:
server$ curvetun -C
which prints all parsed clients from ''~/.curvetun/clients''.
This process could easily be automated or scripted with, for
example, Perl and LDAP.
Now, the client ''myclient1'' is known to the server; that
completes the server configuration. The next step is to tell the
client where it needs to connect to the server.
We assume in this example that the tunnel server has a public IP
address, e.g. 1.2.3.4, runs on port 6666 and uses UDP as a
carrier protocol. In case you are behind NAT, you can use
curvetun's ''--stun'' option for starting the server, to obtain
your mapping. However, in this example we continue with 1.2.3.4
and 6666, UDP.
First, the server needs to export its key to the client, as
follows:
server$ curvetun -x
where it prints a string in the following format:
mysrv1;22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22
\____/
\_____________________________________________________________________________________________/
username 32 byte public key for 'mysrv1'
^-- you need this public key
Thus, you now have the server IP address, server port, server
transport protocol and the server's public key at hand. On the
client side it can be put all together in the config as follows:
client$ echo
"myfirstserver;1.2.3.4;6666;udp;22:22:22:22:22:22:22:22:22:22:" \
"22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:22:"
\
"22:22" >> ~/.curvetun/servers
The client can check its config using:
client$ curvetun -S
Then we start the server with:
server$ curvetun -s -p 6666 -u
server# ifconfig curves0 up
server# ifconfig curves0 10.0.0.1/24
Then, we start the client with:
client$ curvetun -c=myfirstserver
client# ifconfig curvec0 up
client# ifconfig curvec0 10.0.0.2/24
Also, client-side information, errors, or warnings will appear in
syslog! By now we should be able to ping the server:
client$ ping 10.0.0.1
That's it! Routing example:
Server side's public IP on eth0 is, for example, 1.2.3.4:
server$ ... start curvetun server ...
server# ifconfig curves0 up
server# ifconfig curves0 10.0.0.1/24
server# echo 1 > /proc/sys/net/ipv4/ip_forward
server# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
server# iptables -A FORWARD -i eth0 -o curves0 -m state --state
RELATED,ESTABLISHED -j ACCEPT
server# iptables -A FORWARD -i curves0 -o eth0 -j ACCEPT
Client side's IP on eth0 is, for example, 5.6.7.8:
client$ ... start curvetun client ...
client# ... lookup your default gateway (e.g. via route, here:
5.6.7.9) ...
client# ifconfig curvec0 up
client# ifconfig curvec0 10.0.0.2/24
client# route add -net 1.2.3.0 netmask 255.255.255.0 gw 5.6.7.9
dev eth0
client# route add default gw 10.0.0.1
client# route del default gw 5.6.7.9
That should be it, happy browsing and emailing via curvetun
tunnels!
This software is an experimental prototype intended for
researchers. It will most likely mature over time, but it is
currently not advised to use this software when life is put at
risk.
Blackhole tunneling is currently not supported.
curvetun is licensed under the GNU GPL version 2.0.
curvetun was originally written for the netsniff-ng toolkit by
Daniel Borkmann. It is currently maintained by Tobias Klauser
<tklauser@distanz.ch> and Daniel Borkmann
<dborkma@tik.ee.ethz.ch>.
netsniff-ng(8), trafgen(8), mausezahn(8), bpfc(8), ifpps(8),
flowtop(8), astraceroute(8)
Manpage was written by Daniel Borkmann.
This page is part of the Linux netsniff-ng toolkit project. A
description of the project, and information about reporting bugs,
can be found at http://netsniff-ng.org/.
This page is part of the netsniff-ng (a free Linux networking
toolkit) project. Information about the project can be found at
⟨http://netsniff-ng.org/⟩. If you have a bug report for this
manual page, send it to netsniff-ng@googlegroups.com. This page
was obtained from the project's upstream Git repository
⟨https://github.com/netsniff-ng/netsniff-ng⟩ on 2024-06-14. (At
that time, the date of the most recent commit that was found in
the repository was 2023-02-01.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Linux 03 March 2013 CURVETUN(8)
Pages that refer to this page: astraceroute(8), bpfc(8), flowtop(8), ifpps(8), mausezahn(8), netsniff-ng(8), trafgen(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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