The svnserve program is a lightweight
server, capable of speaking to clients over TCP/IP using a
custom, stateful protocol. Clients contact an
svnserve server by using URLs that begin with
the svn:// or svn+ssh://
scheme. This section will explain the different ways of running
svnserve, how clients authenticate themselves
to the server, and how to configure appropriate access control
to your repositories.
There are a few different ways to run the svnserve program:
Run svnserve as a standalone daemon, listening for requests.
Have the Unix inetd daemon temporarily spawn svnserve whenever a request comes in on a certain port.
Have SSH invoke a temporary svnserve over an encrypted tunnel.
Run svnserve as a Microsoft Windows service.
The easiest option is to run svnserve
as a standalone “daemon” process. Use the
-d option for this:
$ svnserve -d $ # svnserve is now running, listening on port 3690
When running svnserve in daemon mode,
you can use the --listen-port and
--listen-host options to customize the
exact port and hostname to “bind” to.
Once we successfully start svnserve as
explained previously, it makes every repository on your system
available to the network. A client needs to specify an
absolute path in the repository URL. For
example, if a repository is located at
/var/svn/project1, a client would
reach it via
svn://host.example.com/var/svn/project1. To
increase security, you can pass the -r option
to svnserve, which restricts it to
exporting only repositories below that path. For
example:
$ svnserve -d -r /var/svn …
Using the -r option effectively
modifies the location that the program treats as the root of
the remote filesystem space. Clients then use URLs that
have that path portion removed from them, leaving much
shorter (and much less revealing) URLs:
$ svn checkout svn://host.example.com/project1 …
If you want inetd to launch the
process, you need to pass the -i
(--inetd) option. In the following
example, we've shown the output from running
svnserve -i at the command line, but note
that this isn't how you actually start the daemon; see the
paragraphs following the example for how to configure
inetd to start
svnserve.
$ svnserve -i ( success ( 1 2 ( ANONYMOUS ) ( edit-pipeline ) ) )
When invoked with the --inetd option,
svnserve attempts to speak with a
Subversion client via stdin and
stdout using a custom protocol. This is
the standard behavior for a program being run via
inetd. The IANA has reserved port 3690 for
the Subversion protocol, so on a Unix-like system you can add
lines to /etc/services such as these (if
they don't already exist):
svn 3690/tcp # Subversion svn 3690/udp # Subversion
If your system is using a classic Unix-like
inetd daemon, you can add this line to
/etc/inetd.conf:
svn stream tcp nowait svnowner /usr/bin/svnserve svnserve -i
Make sure “svnowner” is a user that has
appropriate permissions to access your repositories. Now,
when a client connection comes into your server on port 3690,
inetd will spawn an
svnserve process to service it. Of course,
you may also want to add -r to the
configuration line as well, to restrict which repositories are
exported.
A third way to invoke svnserve is in
tunnel mode, using the -t option. This
mode assumes that a remote-service program such as
rsh or ssh has
successfully authenticated a user and is now invoking a
private svnserve process as
that user. (Note that you, the user, will
rarely, if ever, have reason to invoke
svnserve with the -t at
the command line; instead, the SSH daemon
does so for you.) The svnserve program
behaves normally (communicating via
stdin and stdout)
and assumes that the traffic is being automatically
redirected over some sort of tunnel back to the client.
When svnserve is invoked by a tunnel
agent like this, be sure that the authenticated user has
full read and write access to the repository database files.
It's essentially the same as a local user accessing the
repository via file:// URLs.
This option is described in much more detail later in this chapter in the section called “Tunneling over SSH”.
If your Windows system is a descendant of Windows NT
(2000, 2003, XP, or Vista), you can
run svnserve as a standard Windows
service. This is typically a much nicer experience than
running it as a standalone daemon with
the --daemon (-d) option.
Using daemon mode requires launching a console, typing a
command, and then leaving the console window running
indefinitely. A Windows service, however, runs in the
background, can start at boot time automatically, and can be
started and stopped using the same consistent administration
interface as other Windows services.
You'll need to define the new service using the command-line tool SC.EXE. Much like the inetd configuration line, you must specify an exact invocation of svnserve for Windows to run at startup time:
C:\> sc create svn
binpath= "C:\svn\bin\svnserve.exe --service -r C:\repos"
displayname= "Subversion Server"
depend= Tcpip
start= auto
This defines a new Windows service named
“svn,” which executes a particular
svnserve.exe command when started (in
this case, rooted at C:\repos). There
are a number of caveats in the prior example,
however.
First, notice that the svnserve.exe
program must always be invoked with the
--service option. Any other options to
svnserve must then be specified on the
same line, but you cannot add conflicting options such as
--daemon
(-d), --tunnel,
or --inetd (-i). Options
such as -r
or --listen-port are fine, though. Second,
be careful about spaces when invoking
the SC.EXE command: the key=
value patterns must have no spaces between
key= and must have exactly one space
before the value. Lastly, be careful
about spaces in your command line to be invoked. If a
directory name contains spaces (or other characters that
need escaping), place the entire inner value of
binpath in double quotes, by escaping
them:
C:\> sc create svn
binpath= "\"C:\program files\svn\bin\svnserve.exe\" --service -r C:\repos"
displayname= "Subversion Server"
depend= Tcpip
start= auto
Also note that the word binpath is
misleading—its value is a command
line, not the path to an executable. That's why
you need to surround it with quotes if it contains
embedded spaces.
Once the service is defined, it can be stopped, started, or queried using standard GUI tools (the Services administrative control panel), or at the command line:
C:\> net stop svn C:\> net start svn
The service can also be uninstalled (i.e., undefined) by
deleting its definition: sc delete svn.
Just be sure to stop the service first!
The SC.EXE program has many other
subcommands and options; run sc /? to
learn more about it.
When a client connects to an svnserve process, the following things happen:
The client selects a specific repository.
The server processes the repository's
conf/svnserve.conf file and begins to
enforce any authentication and authorization policies it
describes.
Depending on the defined policies, one of the following may occur:
The client may be allowed to make requests anonymously, without ever receiving an authentication challenge.
The client may be challenged for authentication at any time.
If operating in tunnel mode, the client will declare itself to be already externally authenticated (typically by SSH).
The svnserve server, by default, knows only how to send a CRAM-MD5 [42] authentication challenge. In essence, the server sends a small amount of data to the client. The client uses the MD5 hash algorithm to create a fingerprint of the data and password combined, and then sends the fingerprint as a response. The server performs the same computation with the stored password to verify that the result is identical. At no point does the actual password travel over the network.
If your svnserve server was built with SASL support, it not only knows how to send CRAM-MD5 challenges, but also likely knows a whole host of other authentication mechanisms. See the section called “Using svnserve with SASL” later in this chapter to learn how to configure SASL authentication and encryption.
It's also possible, of course, for the client to be externally authenticated via a tunnel agent, such as ssh. In that case, the server simply examines the user it's running as, and uses this name as the authenticated username. For more on this, see the later section, the section called “Tunneling over SSH”.
As you've already guessed, a repository's
svnserve.conf file is the central
mechanism for controlling authentication and authorization
policies. The file has the same format as other configuration
files (see the section called “Runtime Configuration Area”):
section names are marked by square brackets
([ and ]), comments
begin with hashes (#), and each section
contains specific variables that can be set (variable
= value). Let's walk through these files and learn
how to use them.
For now, the [general] section of
svnserve.conf has all the variables you
need. Begin by changing the values of those variables:
choose a name for a file that will contain your usernames
and passwords and choose an authentication realm:
[general] password-db = userfile realm = example realm
The realm is a name that you define.
It tells clients which sort of “authentication
namespace” they're connecting to; the Subversion
client displays it in the authentication prompt and uses it
as a key (along with the server's hostname and port) for
caching credentials on disk (see the section called “Client Credentials Caching”). The
password-db variable points to a separate
file that contains a list of usernames and passwords, using
the same familiar format. For example:
[users] harry = foopassword sally = barpassword
The value of password-db can be an
absolute or relative path to the users file. For many
admins, it's easy to keep the file right in the
conf/ area of the repository, alongside
svnserve.conf. On the other hand, it's
possible you may want to have two or more repositories share
the same users file; in that case, the file should probably
live in a more public place. The repositories sharing the
users file should also be configured to have the same realm,
since the list of users essentially defines an
authentication realm. Wherever the file lives, be sure to
set the file's read and write permissions appropriately. If
you know which user(s) svnserve will run
as, restrict read access to the users file as necessary.
There are two more variables to set in the
svnserve.conf file: they determine what
unauthenticated (anonymous) and authenticated users are
allowed to do. The variables anon-access
and auth-access can be set to the value
none, read, or
write. Setting the value to
none prohibits both reading and writing;
read allows read-only access to the
repository, and write allows complete
read/write access to the repository. For example:
[general] password-db = userfile realm = example realm # anonymous users can only read the repository anon-access = read # authenticated users can both read and write auth-access = write
The example settings are, in fact, the default values of the variables, should you forget to define them. If you want to be even more conservative, you can block anonymous access completely:
[general] password-db = userfile realm = example realm # anonymous users aren't allowed anon-access = none # authenticated users can both read and write auth-access = write
The server process understands not only
these “blanket” access controls to the
repository, but also finer-grained access restrictions placed
on specific files and directories within the repository. To
make use of this feature, you need to define a file containing
more detailed rules, and then set
the authz-db variable to point to it:
[general] password-db = userfile realm = example realm # Specific access rules for specific locations authz-db = authzfile
We discuss the syntax of the authzfile file
in detail later in this chapter, in
the section called “Path-Based Authorization”. Note
that the authz-db variable isn't mutually
exclusive with the anon-access
and auth-access variables; if all the
variables are defined at once, all
of the rules must be satisfied before access is allowed.
For many teams, the built-in CRAM-MD5 authentication is all they need from svnserve. However, if your server (and your Subversion clients) were built with the Cyrus Simple Authentication and Security Layer (SASL) library, you have a number of authentication and encryption options available to you.
Normally, when a subversion client connects to svnserve, the server sends a greeting that advertises a list of the capabilities it supports, and the client responds with a similar list of capabilities. If the server is configured to require authentication, it then sends a challenge that lists the authentication mechanisms available; the client responds by choosing one of the mechanisms, and then authentication is carried out in some number of round-trip messages. Even when SASL capabilities aren't present, the client and server inherently know how to use the CRAM-MD5 and ANONYMOUS mechanisms (see the section called “Built-in Authentication and Authorization”). If server and client were linked against SASL, a number of other authentication mechanisms may also be available. However, you'll need to explicitly configure SASL on the server side to advertise them.
To activate specific SASL mechanisms on the server,
you'll need to do two things. First, create
a [sasl] section in your
repository's svnserve.conf file with an
initial key-value pair:
[sasl]
use-sasl = true
Second, create a main SASL configuration file
called svn.conf in a place where the
SASL library can find it—typically in the directory
where SASL plug-ins are located. You'll have to locate the
plug-in directory on your particular system, such
as /usr/lib/sasl2/
or /etc/sasl2/. (Note that this
is not
the svnserve.conf file that lives
within a repository!)
On a Windows server, you'll also have to edit the system
registry (using a tool such as regedit)
to tell SASL where to find things. Create a registry key
named [HKEY_LOCAL_MACHINE\SOFTWARE\Carnegie
Mellon\Project Cyrus\SASL Library], and place two
keys inside it: a key called SearchPath
(whose value is a path to the directory containing the SASL
sasl*.dll plug-in libraries), and a key
called
ConfFile (whose value is a path to the
parent directory containing
the svn.conf file you created).
Because SASL provides so many different kinds of
authentication mechanisms, it would be foolish (and far
beyond the scope of this book) to try to describe every
possible server-side configuration. Instead, we recommend
that you read the documentation supplied in the
doc/ subdirectory of the SASL source
code. It goes into great detail about every mechanism and
how to configure the server appropriately for each. For the
purposes of this discussion, we'll just demonstrate a simple
example of configuring the DIGEST-MD5 mechanism. For
example, if your subversion.conf
(or svn.conf) file contains the
following:
pwcheck_method: auxprop auxprop_plugin: sasldb sasldb_path: /etc/my_sasldb mech_list: DIGEST-MD5
you've told SASL to advertise the DIGEST-MD5
mechanism to clients and to check user passwords against a
private password database located
at /etc/my_sasldb. A system
administrator can then use
the saslpasswd2 program to add or modify
usernames and passwords in the database:
$ saslpasswd2 -c -f /etc/my_sasldb -u realm username
A few words of warning: first, make sure the
“realm” argument
to saslpasswd2 matches the same realm
you've defined in your
repository's svnserve.conf file; if
they don't match, authentication will fail. Also, due to a
shortcoming in SASL, the common realm must be a string with
no space characters. Finally, if you decide to go with the
standard SASL password database, make sure
the svnserve program has read access to
the file (and possibly write access as well, if you're using
a mechanism such as OTP).
This is just one simple way of configuring SASL. Many other authentication mechanisms are available, and passwords can be stored in other places such as in LDAP or a SQL database. Consult the full SASL documentation for details.
Remember that if you configure your server to only allow certain SASL authentication mechanisms, this forces all connecting clients to have SASL support as well. Any Subversion client built without SASL support (which includes all pre-1.5 clients) will be unable to authenticate. On the one hand, this sort of restriction may be exactly what you want (“My clients must all use Kerberos!”). However, if you still want non-SASL clients to be able to authenticate, be sure to advertise the CRAM-MD5 mechanism as an option. All clients are able to use CRAM-MD5, whether they have SASL capabilities or not.
SASL is also able to perform data encryption if a
particular mechanism supports it. The built-in CRAM-MD5
mechanism doesn't support encryption, but DIGEST-MD5 does,
and mechanisms such as SRP actually require use of the
OpenSSL library. To enable or disable different levels of
encryption, you can set two values in your repository's
svnserve.conf file:
[sasl] use-sasl = true min-encryption = 128 max-encryption = 256
The min-encryption and
max-encryption variables control the
level of encryption demanded by the server. To disable
encryption completely, set both values to 0. To enable
simple checksumming of data (i.e., prevent tampering and
guarantee data integrity without encryption), set both
values to 1. If you wish to allow—but not
require—encryption, set the minimum value to 0, and
the maximum value to some bit length. To require encryption
unconditionally, set both values to numbers greater than 1.
In our previous example, we require clients to do at least
128-bit encryption, but no more than 256-bit
encryption.
svnserve's built-in authentication (and SASL support) can be very handy, because it avoids the need to create real system accounts. On the other hand, some administrators already have well-established SSH authentication frameworks in place. In these situations, all of the project's users already have system accounts and the ability to “SSH into” the server machine.
It's easy to use SSH in conjunction with
svnserve. The client simply uses the
svn+ssh:// URL scheme to connect:
$ whoami harry $ svn list svn+ssh://host.example.com/repos/project harryssh@host.example.com's password: ***** foo bar baz …
In this example, the Subversion client is invoking a local
ssh process, connecting to
host.example.com, authenticating as the
user harryssh (according to SSH user
configuration), then spawning a private
svnserve process on the remote machine
running as the user harryssh. The
svnserve command is being invoked in tunnel
mode (-t), and its network protocol is being
“tunneled” over the encrypted connection by
ssh, the tunnel agent.
If the client performs a commit, the authenticated username
harryssh will be used as the author
of the new revision.
The important thing to understand here is that the Subversion client is not connecting to a running svnserve daemon. This method of access doesn't require a daemon, nor does it notice one if present. It relies wholly on the ability of ssh to spawn a temporary svnserve process, which then terminates when the network connection is closed.
When using svn+ssh:// URLs to access a
repository, remember that it's the ssh
program prompting for authentication, and
not the svn client
program. That means there's no automatic password-caching
going on (see the section called “Client Credentials Caching”). The
Subversion client often makes multiple connections to the
repository, though users don't normally notice this due to the
password caching feature. When using
svn+ssh:// URLs, however, users may be
annoyed by ssh repeatedly asking for a
password for every outbound connection. The solution is to
use a separate SSH password-caching tool such as
ssh-agent on a Unix-like system, or
pageant on Windows.
When running over a tunnel, authorization is primarily
controlled by operating system permissions to the repository's
database files; it's very much the same as if Harry were
accessing the repository directly via a
file:// URL. If multiple system users are
going to be accessing the repository directly, you may want to
place them into a common group, and you'll need to be careful
about umasks (be sure to read the section called “Supporting Multiple Repository Access Methods” later in this
chapter). But even in the case of tunneling, you can still use the
svnserve.conf file to block access, by
simply setting auth-access = read
or auth-access = none.
[43]
You'd think that the story of SSH tunneling would end
here, but it doesn't. Subversion allows you to create custom
tunnel behaviors in your runtime config
file (see the section called “Runtime Configuration Area”.) For
example, suppose you want to use RSH instead of SSH.
[44]
In the [tunnels] section of your
config file, simply define it like
this:
[tunnels] rsh = rsh
And now, you can use this new tunnel definition by using a
URL scheme that matches the name of your new variable:
svn+rsh://host/path. When using the new
URL scheme, the Subversion client will actually be running the
command rsh host svnserve -t behind the
scenes. If you include a username in the URL (e.g.,
svn+rsh://username@host/path), the client
will also include that in its command (rsh
username@host svnserve -t). But you can define new
tunneling schemes to be much more clever than that:
[tunnels] joessh = $JOESSH /opt/alternate/ssh -p 29934
This example demonstrates a couple of things. First, it
shows how to make the Subversion client launch a very specific
tunneling binary (the one located at
/opt/alternate/ssh) with specific
options. In this case, accessing an
svn+joessh:// URL would invoke the
particular SSH binary with -p 29934 as
arguments—useful if you want the tunnel program to
connect to a nonstandard port.
Second, it shows how to define a custom environment
variable that can override the name of the tunneling program.
Setting the SVN_SSH environment variable is
a convenient way to override the default SSH tunnel agent.
But if you need to have several different overrides for
different servers, each perhaps contacting a different port or
passing a different set of options to SSH, you can use the
mechanism demonstrated in this example. Now if we were to set
the JOESSH environment variable, its value
would override the entire value of the tunnel
variable—$JOESSH would be executed
instead of /opt/alternate/ssh -p
29934.
It's possible to control not only the way in which the client invokes ssh, but also to control the behavior of sshd on your server machine. In this section, we'll show how to control the exact svnserve command executed by sshd, as well as how to have multiple users share a single system account.
To begin, locate the home directory of the account
you'll be using to launch svnserve. Make
sure the account has an SSH public/private keypair
installed, and that the user can log in via public-key
authentication. Password authentication will not work,
since all of the following SSH tricks revolve around using
the SSH authorized_keys file.
If it doesn't already exist, create the
authorized_keys file (on Unix,
typically ~/.ssh/authorized_keys).
Each line in this file describes a public key that is
allowed to connect. The lines are typically of the
form:
ssh-dsa AAAABtce9euch… user@example.com
The first field describes the type of key, the second
field is the base64-encoded key itself, and the third field
is a comment. However, it's a lesser known fact that the
entire line can be preceded by a command
field:
command="program" ssh-dsa AAAABtce9euch… user@example.com
When the command field is set, the
SSH daemon will run the named program instead of the
typical tunnel-mode svnserve invocation that the
Subversion client asks for. This opens the door to a number
of server-side tricks. In the following examples, we
abbreviate the lines of the file as:
command="program" TYPE KEY COMMENT
Because we can specify the executed server-side command, it's easy to name a specific svnserve binary to run and to pass it extra arguments:
command="/path/to/svnserve -t -r /virtual/root" TYPE KEY COMMENT
In this example, /path/to/svnserve
might be a custom wrapper script
around svnserve which sets the umask (see
the section called “Supporting Multiple Repository Access Methods”.) It also
shows how to anchor svnserve in a virtual
root directory, just as one often does when
running svnserve as a daemon process.
This might be done either to restrict access to parts of the
system, or simply to relieve the user of having to type an
absolute path in the svn+ssh://
URL.
It's also possible to have multiple users share a single
account. Instead of creating a separate system account for
each user, generate a public/private key pair for each
person. Then place each public key into
the authorized_users file, one per
line, and use the --tunnel-user
option:
command="svnserve -t --tunnel-user=harry" TYPE1 KEY1 harry@example.com command="svnserve -t --tunnel-user=sally" TYPE2 KEY2 sally@example.com
This example allows both Harry and Sally to connect to
the same account via public key authentication. Each of
them has a custom command that will be executed;
the --tunnel-user option
tells svnserve to assume that the named
argument is the authenticated user. Without
--tunnel-user, it would appear as though
all commits were coming from the one shared system
account.
A final word of caution: giving a user access to the
server via public-key in a shared account might still allow
other forms of SSH access, even if you've set
the command value
in authorized_keys. For example, the
user may still get shell access through SSH or be able to
perform X11 or general port forwarding through your server.
To give the user as little permission as possible, you may
want to specify a number of restrictive options immediately
after the command:
command="svnserve -t --tunnel-user=harry",no-port-forwarding,no-agent-forw arding,no-X11-forwarding,no-pty TYPE1 KEY1 harry@example.com
Note that this all must be on one line—truly on
one line—since SSH authorized_keys
files do not even allow the conventional backslash character
(\) for line continuation. The only
reason we've shown it with a line break is to fit it on
the physical page of a book.