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<a name="Interoperability"></a>
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Next: <a href="Compatibility-with-the-OpenSSL-library.html#Compatibility-with-the-OpenSSL-library" accesskey="n" rel="next">Compatibility with the OpenSSL library</a>, Previous: <a href="Channel-Bindings.html#Channel-Bindings" accesskey="p" rel="prev">Channel Bindings</a>, Up: <a href="Advanced-topics.html#Advanced-topics" accesskey="u" rel="up">Advanced topics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Interoperability-1"></a>
<h4 class="subsection">6.12.7 Interoperability</h4>
<p>The <acronym>TLS</acronym> protocols support many ciphersuites, extensions and version
numbers. As a result, few implementations are
not able to properly interoperate once faced with extensions or version protocols
they do not support and understand. The <acronym>TLS</acronym> protocol allows for a
graceful downgrade to the commonly supported options, but practice shows
it is not always implemented correctly.
</p>
<p>Because there is no way to achieve maximum interoperability with broken peers
without sacrificing security, <acronym>GnuTLS</acronym> ignores such peers by default.
This might not be acceptable in cases where maximum compatibility
is required. Thus we allow enabling compatibility with broken peers using
priority strings (see <a href="Priority-Strings.html#Priority-Strings">Priority Strings</a>). A conservative priority
string that would disable certain <acronym>TLS</acronym> protocol
options that are known to cause compatibility problems, is shown below.
</p><pre class="verbatim">NORMAL:%COMPAT
</pre>
<p>For very old broken peers that do not tolerate TLS version numbers over TLS 1.0
another priority string is:
</p><pre class="verbatim">NORMAL:-VERS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:%COMPAT
</pre><p>This priority string will in addition to above, only enable SSL 3.0 and
TLS 1.0 as protocols.
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