6.2 Network Model | Version Control with Subversion

This section is a general discussion of how a Subversion client and server interact with one another, regardless of the network implementation you're using. After reading, you'll have a good understanding of how a server can behave and the different ways in which a client can be configured to respond.

6.2.1 Requests and Responses

The Subversion client spends most of its time managing working copies. When it needs information from a repository, however, it makes a network request, and the server responds with an appropriate answer. The details of the network protocol are hidden from the user; the client attempts to access a URL, and depending on the URL schema, a particular protocol is used to contact the server (see Chapter 2). Users can run svn version to see which URL schemas and protocols the client knows how to use.

When the server process receives a client request, it typically demands that the client identify itself. It issues an authentication challenge to the client, and the client responds by providing credentials back to the server. Once authentication is complete, the server responds with the original information the client asked for. Notice that this system is different from systems such as CVS, where the client preemptively offers credentials (logs in) to the server before ever making a request. In Subversion, the server pulls credentials by challenging the client at the appropriate moment, rather than the client pushing them. This makes certain operations more elegant. For example, if a server is configured to allow anyone in the world to read a repository, then the server will never issue an authentication challenge when a client attempts to svn checkout.

If the client's network request writes new data to the repository (e.g., svn commit), then a new revision tree is created. If the client's request was authenticated, then the authenticated user's name is stored as the value of the svn:author property on the new revision (see Section 5.1.2). If the client was not authenticated (in other words, the server never issued an authentication challenge), then the revision's svn:author property is empty.^[1]

^[1] This problem is actually a FAQ, resulting from a misconfigured server setup.

6.2.2 Client Credentials Caching

Many servers are configured to require authentication on every request. This can become a big annoyance to users, who are forced to type their passwords over and over again.

Happily, the Subversion client has a remedy for this: a built-in system for caching authentication credentials on disk. By default, whenever the commandline client successfully authenticates itself to a server, it saves the credentials in the user's private runtime configuration area in ~/.subversion/auth/ on Unix-like systems or %APPDATA%/Subversion/auth/ on Windows. (The runtime area is covered further in Section 7.1 in Chapter 7.) Successful credentials are cached on disk, keyed on a combination of hostname, port, and authentication realm.

When the client receives an authentication challenge, it first looks for the appropriate credentials in the disk cache; if not present, or if the cached credentials fail to authenticate, then the client simply prompts the user for the information.

The security-paranoid people may be thinking to themselves: "Caching passwords on disk? That's terrible! You should never do that!" But, please, remain calm. First, the auth/ caching area is permission-protected, so only the user (owner) can read data from it, not the world at large. If that's still not safe enough for you, you can disable credential caching. To disable caching for a single command, pass the --no-auth-cache option:

$ svn commit -F log_msg.txt --no-auth-cache Authentication realm: <svn://host.example.com:3690> example realm Username:  joe Password for 'joe': Adding         newfile Transmitting file data . Committed revision 2324. # password was not cached, so a second commit still prompts us $ svn rm newfile $ svn commit -F new_msg.txt Authentication realm: <svn://host.example.com:3690> example realm Username:  joe [...]

Or, if you want to disable credential caching permanently, you can edit your runtime config file (located next to the auth/ directory). Simply set store-auth-creds to no, and no credentials will be cached on disk, ever.

[auth] store-auth-creds = no

Sometimes users will want to remove specific credentials from the disk cache. To do this, you need to navigate into the auth/ area and manually delete the appropriate cache file. Credentials are cached in individual files; if you look inside each file, you will see keys and values. The svn:realmstring key describes the particular server realm that the file is associated with:

$ ls ~/.subversion/auth/svn.simple/ 5671adf2865e267db74f09ba6f872c28         3893ed123b39500bca8a0b382839198e 5c3c22968347b390f349ff340196ed39 $ cat ~/.subversion/auth/svn.simple/5671adf2865e267db74f09ba6f872c28 K 8 username V 3 joe K 8 password V 4 blah K 15 svn:realmstring V 45 <https://svn.domain.com:443> Joe's repository END

Once you have located the proper cache file, just delete it.

One last word about client authentication behavior: a bit of explanation about the --username and --password options is needed. Many client subcommands accept these options; however, it is important to understand using these options does not automatically send credentials to the server. As discussed earlier, the server pulls credentials from the client when it deems necessary; the client cannot push them at will. If a username and/or password are passed as options, they will only be presented to the server if the server requests them.^[2] Typically, these options are used when:

^[2] Again, a common mistake is to misconfigure a server so that it never issues an authentication challenge. When users pass --username and --password options to the client, they're surprised to see that they're never used, i.e., new revisions still appear to have been committed anonymously!

The user wants to authenticate as a different user than her system login name.
A script wants to authenticate without using cached credentials.

Here is a final summary that describes how a Subversion client behaves when it receives an authentication challenge:

Check whether the user specified any credentials as command-line options, via --username and/or --password. If not, or if these options fail to authenticate successfully, then
Look up the server's realm in the runtime auth/ area, to see if the user already has the appropriate credentials cached. If not, or if the cached credentials fail to authenticate, then
Resort to prompting the user.

If the client successfully authenticates by any of the methods listed previously, it will attempt to cache the credentials on disk (unless the user has disabled this behavior, as mentioned earlier).