tinc/doc/tinc.conf.5.in

.Dd 2010-01-16
.Dt TINC.CONF 5
.\" Manual page created by:
.\" Ivo Timmermans
.\" Guus Sliepen <guus@tinc-vpn.org>

.Sh NAME
.Nm tinc.conf
.Nd tinc daemon configuration

.Sh DESCRIPTION
The files in the
.Pa @sysconfdir@/tinc/
directory contain runtime and security information for the tinc daemon.

.Sh NETWORKS
It is perfectly ok for you to run more than one tinc daemon.
However, in its default form,
you will soon notice that you can't use two different configuration files without the
.Fl c
option.

.Pp
We have thought of another way of dealing with this: network names.
This means that you call
.Nm
with the
.Fl n
option, which will assign a name to this daemon.

.Pp
The effect of this is that the daemon will set its configuration root to
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa / ,
where 
.Ar NETNAME
is your argument to the
.Fl n
option.
You'll notice that messages appear in syslog as coming from
.Nm tincd. Ns Ar NETNAME .

.Pp
However, it is not strictly necessary that you call tinc with the
.Fl n
option.
In this case, the network name would just be empty,
and it will be used as such.
.Nm tinc
now looks for files in
.Pa @sysconfdir@/tinc/ ,
instead of 
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa / ;
the configuration file should be
.Pa @sysconfdir@/tinc/tinc.conf ,
and the host configuration files are now expected to be in
.Pa @sysconfdir@/tinc/hosts/ .

.Pp
But it is highly recommended that you use this feature of
.Nm tinc ,
because it will be so much clearer whom your daemon talks to.
Hence, we will assume that you use it.

.Sh NAMES
Each tinc daemon should have a name that is unique in the network which it will be part of.
The name will be used by other tinc daemons for identification.
The name has to be declared in the
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc.conf
file.

.Pp
To make things easy,
choose something that will give unique and easy to remember names to your tinc daemon(s).
You could try things like hostnames, owner surnames or location names.

.Sh PUBLIC/PRIVATE KEYS
You should use 
.Ic tincd -K
to generate public/private keypairs.
It will generate two keys.
The private key should be stored in a separate file
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /rsa_key.priv
\-\- where 
.Ar NETNAME
stands for the network (see 
.Sx NETWORKS )
above.
The public key should be stored in the host configuration file
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/ Ns Va NAME
\-\- where
.Va NAME
stands for the name of the local tinc daemon (see
.Sx NAMES ) .

.Sh SERVER CONFIGURATION
The server configuration of the daemon is done in the file
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc.conf .
This file consists of comments (lines started with a
.Li # )
or assignments in the form of:

.Pp
.Va Variable Li = Ar Value .

.Pp
The variable names are case insensitive, and any spaces, tabs,
newlines and carriage returns are ignored.
Note: it is not required that you put in the 
.Li =
sign, but doing so improves readability.
If you leave it out, remember to replace it with at least one space character.

.Pp
The server configuration is complemented with host specific configuration (see the next section).
Although all configuration options for the local host listed in this document can also be put in
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc.conf ,
it is recommended to put host specific configuration options in the host configuration file,
as this makes it easy to exchange with other nodes.

.Pp
Here are all valid variables, listed in alphabetical order.
The default value is given between parentheses.
.Bl -tag -width indent

.It Va AddressFamily Li = ipv4 | ipv6 | any Pq any
This option affects the address family of listening and outgoing sockets.
If
.Qq any
is selected, then depending on the operating system both IPv4 and IPv6 or just
IPv6 listening sockets will be created.

.It Va BindToAddress Li = Ar address Oo Ar port Oc Bq experimental
If your computer has more than one IPv4 or IPv6 address,
.Nm tinc
will by default listen on all of them for incoming connections.
Multiple
.Va BindToAddress
variables may be specified,
in which case listening sockets for each specified address are made.
.Pp
If no
.Ar port
is specified, the socket will be bound to the port specified by the
.Va Port
option, or to port 655 if neither is given.
To only bind to a specific port but not to a specific address, use
.Li *
for the
.Ar address .
.Pp
This option may not work on all platforms.

.It Va BindToInterface Li = Ar interface Bq experimental
If your computer has more than one network interface,
.Nm tinc
will by default listen on all of them for incoming connections.
It is possible to bind only to a single interface with this variable.

.Pp
This option may not work on all platforms.

.It Va Broadcast Li = yes | no Po yes Pc Bq experimental
When disabled, tinc will drop all broadcast and multicast packets, in both router and switch mode.

.It Va ConnectTo Li = Ar name
Specifies which other tinc daemon to connect to on startup.
Multiple
.Va ConnectTo
variables may be specified,
in which case outgoing connections to each specified tinc daemon are made.
The names should be known to this tinc daemon
(i.e., there should be a host configuration file for the name on the
.Va ConnectTo
line).

.Pp
If you don't specify a host with
.Va ConnectTo ,
.Nm tinc
won't try to connect to other daemons at all,
and will instead just listen for incoming connections.

.It Va DecrementTTL Li = yes | no Po no Pc Bq experimental
When enabled,
.Nm tinc
will decrement the Time To Live field in IPv4 packets, or the Hop Limit field in IPv6 packets,
before forwarding a received packet to the virtual network device or to another node,
and will drop packets that have a TTL value of zero,
in which case it will send an ICMP Time Exceeded packet back.
.Pp
Do not use this option if you use switch mode and want to use IPv6.

.It Va Device Li = Ar device Po Pa /dev/tap0 , Pa /dev/net/tun No or other depending on platform Pc
The virtual network device to use.
.Nm tinc
will automatically detect what kind of device it is.
Note that you can only use one device per daemon.
Under Windows, use
.Va Interface
instead of
.Va Device .
The info pages of the tinc package contain more information
about configuring the virtual network device.

.It Va DeviceType Li = Ar type Pq platform dependent
The type of the virtual network device.
Tinc will normally automatically select the right type of tun/tap interface, and this option should not be used.
However, this option can be used to select one of the special interface types, if support for them is compiled in.
.Bl -tag -width indent

.It dummy
Use a dummy interface.
No packets are ever read or written to a virtual network device.
Useful for testing, or when setting up a node that only forwards packets for other nodes.

.It raw_socket
Open a raw socket, and bind it to a pre-existing
.Va Interface
(eth0 by default).
All packets are read from this interface.
Packets received for the local node are written to the raw socket.
However, at least on Linux, the operating system does not process IP packets destined for the local host.

.It multicast
Open a multicast UDP socket and bind it to the address and port (separated by spaces) and optionally a TTL value specified using
.Va Device .
Packets are read from and written to this multicast socket.
This can be used to connect to UML, QEMU or KVM instances listening on the same multicast address.
Do NOT connect multiple
.Nm tinc 
daemons to the same multicast address, this will very likely cause routing loops.
Also note that this can cause decrypted VPN packets to be sent out on a real network if misconfigured.

.It uml Pq not compiled in by default
Create a UNIX socket with the filename specified by
.Va Device ,
or
.Pa @localstatedir@/run/ Ns Ar NETNAME Ns Pa .umlsocket
if not specified.
.Nm tinc
will wait for a User Mode Linux instance to connect to this socket.

.It vde Pq not compiled in by default
Uses the libvdeplug library to connect to a Virtual Distributed Ethernet switch,
using the UNIX socket specified by
.Va Device ,
or
.Pa @localstatedir@/run/vde.ctl
if not specified.
.El

Also, in case tinc does not seem to correctly interpret packets received from the virtual network device,
it can be used to change the way packets are interpreted:

.Bl -tag -width indent

.It tun Pq BSD and Linux
Set type to tun.
Depending on the platform, this can either be with or without an address family header (see below).

.It tunnohead Pq BSD
Set type to tun without an address family header.
Tinc will expect packets read from the virtual network device to start with an IP header.
On some platforms IPv6 packets cannot be read from or written to the device in this mode.

.It tunifhead Pq BSD
Set type to tun with an address family header.
Tinc will expect packets read from the virtual network device
to start with a four byte header containing the address family,
followed by an IP header.
This mode should support both IPv4 and IPv6 packets.

.It tap Pq BSD and Linux
Set type to tap.
Tinc will expect packets read from the virtual network device
to start with an Ethernet header.
.El

.It Va DirectOnly Li = yes | no Po no Pc Bq experimental
When this option is enabled, packets that cannot be sent directly to the destination node,
but which would have to be forwarded by an intermediate node, are dropped instead.
When combined with the IndirectData option,
packets for nodes for which we do not have a meta connection with are also dropped.

.It Va Forwarding Li = off | internal | kernel Po internal Pc Bq experimental
This option selects the way indirect packets are forwarded.
.Bl -tag -width indent

.It off
Incoming packets that are not meant for the local node,
but which should be forwarded to another node, are dropped.

.It internal
Incoming packets that are meant for another node are forwarded by tinc internally.

.Pp
This is the default mode, and unless you really know you need another forwarding mode, don't change it.

.It kernel
Incoming packets are always sent to the TUN/TAP device, even if the packets are not for the local node.
This is less efficient, but allows the kernel to apply its routing and firewall rules on them,
and can also help debugging.
.El

.It Va GraphDumpFile Li = Ar filename Bq experimental
If this option is present,
.Nm tinc
will dump the current network graph to the file
.Ar filename
every minute, unless there were no changes to the graph.
The file is in a format that can be read by graphviz tools.
If
.Ar filename
starts with a pipe symbol |,
then the rest of the filename is interpreted as a shell command
that is executed, the graph is then sent to stdin.

.It Va Hostnames Li = yes | no Pq no
This option selects whether IP addresses (both real and on the VPN) should
be resolved. Since DNS lookups are blocking, it might affect tinc's
efficiency, even stopping the daemon for a few seconds every time it does
a lookup if your DNS server is not responding.

.Pp
This does not affect resolving hostnames to IP addresses from the
host configuration files.

.It Va IffOneQueue Li = yes | no Po no Pc Bq experimental
(Linux only) Set IFF_ONE_QUEUE flag on TUN/TAP devices.

.It Va Interface Li = Ar interface
Defines the name of the interface corresponding to the virtual network device.
Depending on the operating system and the type of device this may or may not actually set the name of the interface.
Under Windows, this variable is used to select which network interface will be used.
If you specified a
.Va Device ,
this variable is almost always already correctly set.

.It Va KeyExpire Li = Ar seconds Pq 3600
This option controls the period the encryption keys used to encrypt the data are valid.
It is common practice to change keys at regular intervals to make it even harder for crackers,
even though it is thought to be nearly impossible to crack a single key.

.It Va LocalDiscovery Li = yes | no Po no Pc Bq experimental
When enabled,
.Nm tinc
will try to detect peers that are on the same local network.
This will allow direct communication using LAN addresses, even if both peers are behind a NAT
and they only ConnectTo a third node outside the NAT,
which normally would prevent the peers from learning each other's LAN address.

.Pp
Currently, local discovery is implemented by sending broadcast packets to the LAN during path MTU discovery.
This feature may not work in all possible situations.

.It Va MACExpire Li = Ar seconds Pq 600
This option controls the amount of time MAC addresses are kept before they are removed.
This only has effect when
.Va Mode
is set to
.Qq switch .

.It Va MaxTimeout Li = Ar seconds Pq 900
This is the maximum delay before trying to reconnect to other tinc daemons.

.It Va Mode Li = router | switch | hub Pq router
This option selects the way packets are routed to other daemons.
.Bl -tag -width indent

.It router
In this mode
.Va Subnet
variables in the host configuration files will be used to form a routing table.
Only unicast packets of routable protocols (IPv4 and IPv6) are supported in this mode.

.Pp
This is the default mode, and unless you really know you need another mode, don't change it.

.It switch
In this mode the MAC addresses of the packets on the VPN will be used to
dynamically create a routing table just like an Ethernet switch does.
Unicast, multicast and broadcast packets of every protocol that runs over Ethernet are supported in this mode
at the cost of frequent broadcast ARP requests and routing table updates.

.Pp
This mode is primarily useful if you want to bridge Ethernet segments.

.It hub
This mode is almost the same as the switch mode, but instead
every packet will be broadcast to the other daemons
while no routing table is managed.
.El

.It Va Name Li = Ar name Bq required
This is the name which identifies this tinc daemon.
It must be unique for the virtual private network this daemon will connect to.

.It Va PingInterval Li = Ar seconds Pq 60
The number of seconds of inactivity that
.Nm tinc
will wait before sending a probe to the other end.

.It Va PingTimeout Li = Ar seconds Pq 5
The number of seconds to wait for a response to pings or to allow meta
connections to block. If the other end doesn't respond within this time,
the connection is terminated,
and the others will be notified of this.

.It Va PriorityInheritance Li = yes | no Po no Pc Bq experimental
When this option is enabled the value of the TOS field of tunneled IPv4 packets
will be inherited by the UDP packets that are sent out.

.It Va PrivateKey Li = Ar key Bq obsolete
The private RSA key of this tinc daemon.
It will allow this tinc daemon to authenticate itself to other daemons.

.It Va PrivateKeyFile Li = Ar filename Po Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /rsa_key.priv Pc
The file in which the private RSA key of this tinc daemon resides.
Note that there must be exactly one of
.Va PrivateKey
or
.Va PrivateKeyFile
specified in the configuration file.

.It Va ProcessPriority Li = low | normal | high
When this option is used the priority of the tincd process will be adjusted.
Increasing the priority may help to reduce latency and packet loss on the VPN.

.It Va ReplayWindow Li = Ar bytes Pq 16
This is the size of the replay tracking window for each remote node, in bytes.
The window is a bitfield which tracks 1 packet per bit, so for example
the default setting of 16 will track up to 128 packets in the window.  In high
bandwidth scenarios, setting this to a higher value can reduce packet loss from
the interaction of replay tracking with underlying real packet loss and/or
reordering.  Setting this to zero will disable replay tracking completely and
pass all traffic, but leaves tinc vulnerable to replay-based attacks on your
traffic.

.It Va StrictSubnets Li = yes | no Po no Pc Bq experimental
When this option is enabled tinc will only use Subnet statements which are
present in the host config files in the local
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/
directory.

.It Va TunnelServer Li = yes | no Po no Pc Bq experimental
When this option is enabled tinc will no longer forward information between other tinc daemons,
and will only allow connections with nodes for which host config files are present in the local
.Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/
directory.
Setting this options also implicitly sets StrictSubnets.

.It Va UDPRcvBuf Li = Ar bytes Pq OS default
Sets the socket receive buffer size for the UDP socket, in bytes.
If unset, the default buffer size will be used by the operating system.

.It Va UDPSndBuf Li = Ar bytes Pq OS default
Sets the socket send buffer size for the UDP socket, in bytes.
If unset, the default buffer size will be used by the operating system.
.El

.Sh HOST CONFIGURATION FILES
The host configuration files contain all information needed
to establish a connection to those hosts.
A host configuration file is also required for the local tinc daemon,
it will use it to read in it's listen port, public key and subnets.

.Pp
The idea is that these files are portable.
You can safely mail your own host configuration file to someone else.
That other person can then copy it to his own hosts directory,
and now his tinc daemon will be able to connect to your tinc daemon.
Since host configuration files only contain public keys,
no secrets are revealed by sending out this information.
.Bl -tag -width indent

.It Va Address Li = Ar address Oo Ar port Oc Bq recommended
The IP address or hostname of this tinc daemon on the real network.
This will only be used when trying to make an outgoing connection to this tinc daemon.
Optionally, a port can be specified to use for this address.
Multiple
.Va Address
variables can be specified, in which case each address will be tried until a working
connection has been established.

.It Va Cipher Li = Ar cipher Pq blowfish
The symmetric cipher algorithm used to encrypt UDP packets.
Any cipher supported by OpenSSL is recognised.
Furthermore, specifying
.Qq none
will turn off packet encryption.
It is best to use only those ciphers which support CBC mode.

.It Va ClampMSS Li = yes | no Pq yes
This option specifies whether tinc should clamp the maximum segment size (MSS)
of TCP packets to the path MTU. This helps in situations where ICMP
Fragmentation Needed or Packet too Big messages are dropped by firewalls.

.It Va Compression Li = Ar level Pq 0
This option sets the level of compression used for UDP packets.
Possible values are 0 (off), 1 (fast zlib) and any integer up to 9 (best zlib),
10 (fast lzo) and 11 (best lzo).

.It Va Digest Li = Ar digest Pq sha1
The digest algorithm used to authenticate UDP packets.
Any digest supported by OpenSSL is recognised.
Furthermore, specifying
.Qq none
will turn off packet authentication.

.It Va IndirectData Li = yes | no Pq no
This option specifies whether other tinc daemons besides the one you specified with
.Va ConnectTo
can make a direct connection to you.
This is especially useful if you are behind a firewall
and it is impossible to make a connection from the outside to your tinc daemon.
Otherwise, it is best to leave this option out or set it to no.

.It Va MACLength Li = Ar length Pq 4
The length of the message authentication code used to authenticate UDP packets.
Can be anything from
.Qq 0
up to the length of the digest produced by the digest algorithm.

.It Va PMTU Li = Ar mtu Po 1514 Pc
This option controls the initial path MTU to this node.

.It Va PMTUDiscovery Li = yes | no Po yes Pc
When this option is enabled, tinc will try to discover the path MTU to this node.
After the path MTU has been discovered, it will be enforced on the VPN.

.It Va Port Li = Ar port Pq 655
The port number on which this tinc daemon is listening for incoming connections,
which is used if no port number is specified in an
.Va Address
statement.

.It Va PublicKey Li = Ar key Bq obsolete
The public RSA key of this tinc daemon.
It will be used to cryptographically verify it's identity and to set up a secure connection.

.It Va PublicKeyFile Li = Ar filename Bq obsolete
The file in which the public RSA key of this tinc daemon resides.

.Pp
From version 1.0pre4 on
.Nm tinc
will store the public key directly into the host configuration file in PEM format,
the above two options then are not necessary.
Either the PEM format is used, or exactly one of the above two options must be specified
in each host configuration file,
if you want to be able to establish a connection with that host.

.It Va Subnet Li = Ar address Ns Op Li / Ns Ar prefixlength Ns Op Li # Ns Ar weight
The subnet which this tinc daemon will serve.
.Nm tinc
tries to look up which other daemon it should send a packet to by searching the appropriate subnet.
If the packet matches a subnet,
it will be sent to the daemon who has this subnet in his host configuration file.
Multiple
.Va Subnet
variables can be specified.

.Pp
Subnets can either be single MAC, IPv4 or IPv6 addresses,
in which case a subnet consisting of only that single address is assumed,
or they can be a IPv4 or IPv6 network address with a prefixlength.
Shorthand notations are not supported.
For example, IPv4 subnets must be in a form like 192.168.1.0/24,
where 192.168.1.0 is the network address and 24 is the number of bits set in the netmask.
Note that subnets like 192.168.1.1/24 are invalid!
Read a networking HOWTO/FAQ/guide if you don't understand this.
IPv6 subnets are notated like fec0:0:0:1:0:0:0:0/64.
MAC addresses are notated like 0:1a:2b:3c:4d:5e.

.Pp
A Subnet can be given a weight to indicate its priority over identical Subnets
owned by different nodes.  The default weight is 10. Lower values indicate
higher priority. Packets will be sent to the node with the highest priority,
unless that node is not reachable, in which case the node with the next highest
priority will be tried, and so on.

.It Va TCPOnly Li = yes | no Pq no Bq obsolete
If this variable is set to yes,
then the packets are tunnelled over the TCP connection instead of a UDP connection.
This is especially useful for those who want to run a tinc daemon
from behind a masquerading firewall,
or if UDP packet routing is disabled somehow.
Setting this options also implicitly sets IndirectData.

.Pp
Since version 1.0.10, tinc will automatically detect whether communication via
UDP is possible or not.
.El

.Sh SCRIPTS
Apart from reading the server and host configuration files,
tinc can also run scripts at certain moments.
Under Windows (not Cygwin), the scripts should have the extension
.Pa .bat .
.Bl -tag -width indent

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-up
This is the most important script.
If it is present it will be executed right after the tinc daemon has been started and has connected to the virtual network device.
It should be used to set up the corresponding network interface,
but can also be used to start other things.
Under Windows you can use the Network Connections control panel instead of creating this script.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-down
This script is started right before the tinc daemon quits.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/ Ns Ar HOST Ns Pa -up
This script is started when the tinc daemon with name
.Ar HOST
becomes reachable.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/ Ns Ar HOST Ns Pa -down
This script is started when the tinc daemon with name
.Ar HOST
becomes unreachable.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /host-up
This script is started when any host becomes reachable.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /host-down
This script is started when any host becomes unreachable.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /subnet-up
This script is started when a Subnet becomes reachable.
The Subnet and the node it belongs to are passed in environment variables.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /subnet-down
This script is started when a Subnet becomes unreachable.
.El

.Pp
The scripts are started without command line arguments, but can make use of certain environment variables.
Under UNIX like operating systems the names of environment variables must be preceded by a
.Li $ 
in scripts.
Under Windows, in
.Pa .bat
files, they have to be put between
.Li %
signs.
.Bl -tag -width indent

.It Ev NETNAME
If a netname was specified, this environment variable contains it.

.It Ev NAME
Contains the name of this tinc daemon.

.It Ev DEVICE
Contains the name of the virtual network device that tinc uses.

.It Ev INTERFACE
Contains the name of the virtual network interface that tinc uses.
This should be used for commands like
.Pa ifconfig .

.It Ev NODE
When a host becomes (un)reachable, this is set to its name.
If a subnet becomes (un)reachable, this is set to the owner of that subnet.

.It Ev REMOTEADDRESS
When a host becomes (un)reachable, this is set to its real address.

.It Ev REMOTEPORT
When a host becomes (un)reachable, this is set to the port number it uses for communication with other tinc daemons.

.It Ev SUBNET
When a subnet becomes (un)reachable, this is set to the subnet.

.It Ev WEIGHT
When a subnet becomes (un)reachable, this is set to the subnet weight.
.El

.Sh FILES
The most important files are:
.Bl -tag -width indent

.It Pa @sysconfdir@/tinc/
The top directory for configuration files.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc.conf
The default name of the server configuration file for net
.Ar NETNAME .

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /hosts/
Host configuration files are kept in this directory.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-up
If an executable file with this name exists,
it will be executed right after the tinc daemon has connected to the virtual network device.
It can be used to set up the corresponding network interface.

.It Pa @sysconfdir@/tinc/ Ns Ar NETNAME Ns Pa /tinc-down
If an executable file with this name exists,
it will be executed right before the tinc daemon is going to close
its connection to the virtual network device.
.El

.Sh SEE ALSO
.Xr tincd 8 ,
.Pa http://www.tinc-vpn.org/ ,
.Pa http://www.linuxdoc.org/LDP/nag2/ .

.Pp
The full documentation for
.Nm tinc
is maintained as a Texinfo manual.
If the info and tinc programs are properly installed at your site, the command
.Ic info tinc
should give you access to the complete manual.

.Pp
.Nm tinc
comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it under certain conditions;
see the file COPYING for details.