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Re: svn+ssh long-lived daemon

From: Greg Stein <gstein_at_gmail.com>
Date: Sun, 22 Nov 2015 07:40:42 -0600

When svnserve first landed in svn, it was sold as a "simple" mechanism to
allow people to use their existing SSH setup, rather than moving to
Apache/TLS. I expressed my fear that svnserve was going to grow features,
until one day it looked just like httpd.

And yes... it has grown many, many features. Many reproducing what has
already been done in httpd. And this is Yet Another.

I don't like this kind of feature reproduction, as I consider it a waste of
energy, and am registering my -0 on it.


On Thu, Nov 19, 2015 at 12:22 PM, Philip Martin <philip.martin_at_wandisco.com>

> How do we get good performance when using public/private key pairs for
> authentication?
> One reason to use svn+ssh is a preference for key pairs over passwords
> for authentication. The svnbook describes how to setup a single system
> user ID that can be used by multiple repository users:
> http://svnbook.red-bean.com/nightly/en/svn.serverconfig.svnserve.html#svn.serverconfig.svnserve.sshtricks.fixedcmd
> The ssh authorized_keys file is used to map public keys to Subversion
> user IDs via the --tunnel-user option. Maintaining an authorized_keys
> file like this is not much different from maintaining a Subversion
> password file, the difference being it maps public keys to users rather
> than users to passwords and passwords do not have to be maintained.
> One big disadvantage of this setup is that the repository access pattern
> gives poor Subversion performance. The reason is that svn+ssh has no
> long-lived daemon but instead starts a new svnserve tunnel process for
> each connection. This process has a small FSFS cache and that cache
> starts empty. It's not possible to make the cache significantly bigger
> as there may be a large number of processes, and even if it were larger
> each operation starts with a new, empty, cache. To get good performance
> while using key pairs we need a long-lived daemon of some sort.
> I can simulate such a daemon by running a conventional svnserve daemon
> and then replacing the svnserve tunnel process in the authorized_keys
> file with a proxy process such as
> command="socat STDIO TCP:localhost:3690"
> In this case the client sets up an ssh tunnel, runs the socat proxy on
> the server, and the svn protocol stream from the client gets handled by
> the long-lived daemon. While this solves the performance problem it
> doesn't really work: after authenticating to setup the ssh tunnel a
> second authentication is required to pass a username to the svnserve
> daemon. Running the svnserve daemon in anonymous mode solves the second
> authentication problem but results in commits without svn:author and
> disallows authz. A proxy like this is mostly pointless if Subversion
> passwords still have to be maintained.
> What we need is a new daemon that accepts an equivalent of --tunnel-user
> for each connection. Here's a possible design:
> 1. A new svnserve "tunnel daemon" mode
> svnserve --tunnel-daemon -T -r path -M size
> This creates a named pipe and blocks reading from the pipe.
> 2. A new svnserve "tunnel proxy" started via the authorized_keys file:
> svnserve --tunnel-proxy --tunnel-user name
> This creates/binds/listens a localhost TCP socket and then writes
> the port and the tunnel username to the named pipe. It then blocks
> accepting a connection on the socket.
> 3. The tunnel daemon main thread receives the port and username and
> hands these to a worker thread. The main thread goes back to
> reading the named pipe. The worker thread connects back to the
> tunnel proxy using the given port. Then the worker thread runs as
> it would for our current svnserve tunnel mode.
> 4. The tunnel proxy accepts the connection from the tunnel daemon and
> starts to proxy. It reads from stdin and writes to the socket, and
> reads from the socket and writes to stdout.
> The tunnel daemon will be able to read/write the repository as any
> Subversion user. The named pipe can have the same OS permissions as the
> repository and this limits communication between the tunnel-proxy and
> the tunnel-daemon to system users that can already write to the
> repository. Reads and writes on the named pipe should be atomic
> provided they are less than PIPE_BUF.
> Does this sound like a good idea?
> APR has named pipe support but it is APR_ENOTIMPL on Windows so this
> feature may start out as Unix only.
> Are there alternative ways to get a long-lived daemon to do
> authentication with public/private key pairs? Can svn:// with SASL do
> it? It might be possible to extend SASL, but I think this would involve
> client and server changes.
> --
> Philip Martin
> WANdisco
Received on 2015-11-22 14:40:57 CET

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