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7. Security

This section just summarizes security features of Epos documented elsewhere to assist the administrator in setting up a multi-user installation of Epos, especially in a UNIX-like environment.

Epos can be compiled in two ways; either as a simple monolithic binary or as a Text-to-Speech Control Protocol server. For technical reasons (especially the latency), it is strongly recommended to choose the latter method for most situations.

7.1 TTSCP-related Issues


TTSCP as such includes support for user name and plain text password based authentication. At the moment, Epos doesn't implement any name space for this authentication mechanism, except for a randomly-generated server password. Thus, all connections are either completely anonymous, or authenticated at the "server" level (which can still be restricted from setting or reading specific options.)

In the default configuration, the server password is stored in /var/run/epos.pwd. Any process with rights sufficient to read this file is able to authenticate as the server over TTSCP. For debugging purposes, it is also possible to force Epos to accept an additional server password with the debug_password option.

TTSCP Connection Handles

In TTSCP, a control connection can issue commands that affect any other connection (e.g. interrupt a command in progress). To prevent malicious use, the handle necessary to refer to a connection is only available to the connection originator, and is generated at random. Consequently, privacy in TTSCP relies on the properties of the underlying network connection and on the length of the data connection handle. Each byte of handle length carries a perplexity of six bits; reasonable handle lengths (as controlled with the handle_size option therefore fall within the range 10 to 200. Be sure to adjust max_line_len as well if using extremely long connection handles.

restr.ini File

Many TTSCP options are rarely set over TTSCP, and some of them could be used for malicious ends. In these cases, the administrator may restrict their use in the restr.ini as described in the respective section. Note especially that unlisted options can be set freely, and the default restr.ini file as distributed with Epos may occasionally omit a slightly dangerous option. Therefore, you may want to grep out a list of all options and to leave out only options you're going to treat as necessary for users and harmless for the server. On the other hand, almost no options affect the security directly, because they're only set within the context of the current TTSCP connection, and the few exceptions are unlikely to have been overlooked.

Access to the Local Sound Card

At the moment, any TTSCP client is capable of writing arbitrary waveform data to the local soundcard. If you consider this a security risk, disable the #localsound output module by turning the localsound option off.

The interfaces supported are Open Sound System and Microsoft Multimedia System.

Access to the Local File System

TTSCP in theory offers file system based input and output modules. For example, that allows the client to write some intermittent product to a server-side file and then the client may decide to use it repeatedly as an input to multiple streams. However, the name space for the files doesn't have to correspond to the file system root of the TTSCP server. With Epos, you can configure the location of the TTSCP name space using the --pseudo_root_dir option. Furthermore, writing to it is disabled by default; this restriction can be overridden by setting the --writefs option.

7.2 Privileged Executables

It is not recommended to run Epos as a setuid or setgid binary, not because of a known security weakness, but because of the possibility of yet unknown bugs. If necessary, Epos should run setgid to a group with minimal privileges.

Note also that the base_dir parameter can be given to Epos on the command-line, thus by-passing the restrictions specified in the restr.ini file. This issue may be solved in later versions of Epos; at the moment you have to adjust the source manually.

Instead of a privileged executable, run Epos with the desired privileges at system start-up and use a TTSCP client to control it from then on.

7.3 local_only Option

By default, the TTSCP server only binds to the loopback interface. That way, only local users can see and use Epos. Turn off the local_only option to allow remote users to communicate with Epos on your machine.

Theoretically, this security measure should not be needed, but it is very simple and thus likely to reduce the risk caused by bugs hidden in more complex corners of the Epos TTSCP implementation.

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