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97 lines
3.6 KiB
97 lines
3.6 KiB
<chapter id="security">
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<title>&kppp; and security issues</title>
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<para>This section is mainly for superusers (<systemitem>root</systemitem>)
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people with high security demands, or simply technically interested people. It
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is not necessary to read this if you only use &Linux; at home for yourself,
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although you may learn a thing or two in any case.</para>
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<sect1 id="security-restricting-access">
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<title>Restricting access to &kppp;</title>
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<para>A system administrator might want to restrict access as to who is allowed
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to use &kppp;. There are two ways to accomplish this.</para>
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<sect2 id="security-group-permissions">
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<title>Restricting access with group permissions</title>
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<para>Create a new group (you might want to name it
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<systemitem>dialout</systemitem> or similar), and put every user that should be
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allowed to use &kppp; into that group. Then type at the prompt:</para>
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<screen><prompt>#</prompt> <userinput><command>chown</command> <option>root.dialout</option> <filename>/opt/kde/bin/kppp</filename></userinput>
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<prompt>#</prompt> <userinput><command>chmod</command> <option>4750</option> <filename>/opt/kde/bin/kppp</filename></userinput>
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</screen>
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<para>This assumes that &kde; was installed in <filename class="directory">
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/opt/kde/</filename> and that your new group is named
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<systemitem>dialout</systemitem>.</para>
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</sect2>
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<sect2 id="security-kppps-way">
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<title>Restricting access &kppp;'s way</title>
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<para>Before doing anything, &kppp; checks if there is a file named
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<filename>/etc/kppp.allow</filename>. If such a file exists, only users named in
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this file are allowed to dial out. This file must be readable by everyone (but
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of course <emphasis>NOT</emphasis> writable.) Only login names are recognized,
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so you cannot use <acronym>UID</acronym>'s in this file. Here is a short
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example:</para>
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<screen>
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# /etc/kppp.allow
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# comment lines like this are ignored
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# as well as empty lines
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fred
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karl
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daisy
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</screen>
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<para>In the example above, only the users <systemitem>fred</systemitem>,
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<systemitem>karl</systemitem> and <systemitem>daisy</systemitem> are allowed to
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dial out, as well as every user with a <acronym>UID</acronym> of 0 (so you don't
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have to explicitly list root in the file).</para>
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</sect2>
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</sect1>
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<sect1 id="security-why-suid">
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<title>&kppp; has the <acronym>SUID</acronym> bit on? What about
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security?</title>
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<para>It's virtually impossible to write a dialer without the
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<acronym>SUID</acronym> bit that is both safe and easy to use for inexperienced
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users. &kppp; addresses the security issues with the following strategy.</para>
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<itemizedlist>
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<listitem>
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<para>Immediately after the program starts, &kppp; forks.</para>
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</listitem>
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<listitem>
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<para>The master process, which handles all the <acronym>GUI</acronym> operations
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(such as user interaction), drops the <acronym>SUID</acronym> state after the
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fork, and runs with normal user privileges.</para>
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</listitem>
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<listitem>
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<para>The slave process keeps its privileges, and is responsible for all
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actions that need <systemitem>root</systemitem> privileges. To
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keep this part safe, no &kde; or &Qt; library calls are used here, just simple
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library calls. The source code for this process is short (around 500 lines) and
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well documented, so it's easy for you to check it for security holes.</para>
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</listitem>
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<listitem>
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<para>Master and slave processes communicate with standard &UNIX;
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<acronym>IPC</acronym>.</para>
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</listitem>
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</itemizedlist>
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<para>Special thanks to Harri Porten for writing this excellent piece of code.
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It was thought to be impossible, but he managed it within a week.</para>
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</sect1>
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</chapter>
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