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>17.6. Preventing Server Spoofing</A
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>   While the server is running, it is not possible for a malicious user
   to interfere with client/server communications.  However, when the
   server is down it is possible for a local user to spoof the normal
   server by starting their own server.  The spoof server could read
   passwords and queries sent by clients, but could not return any data
   because the <TT
CLASS="VARNAME"
>PGDATA</TT
> directory would still be secure because
   of directory permissions. Spoofing is possible because any user can
   start a database server; a client cannot identify an invalid server
   unless it is specially configured.
  </P
><P
>   The simplest way to prevent invalid servers for <TT
CLASS="LITERAL"
>local</TT
>
   connections is to use a Unix domain socket directory (<A
HREF="runtime-config-connection.html#GUC-UNIX-SOCKET-DIRECTORY"
>unix_socket_directory</A
>) that has write permission only
   for a trusted local user.  This prevents a malicious user from creating
   their own socket file in that directory.  If you are concerned that
   some applications might still reference <TT
CLASS="FILENAME"
>/tmp</TT
> for the
   socket file and hence be vulnerable to spoofing, during operating system
   startup create symbolic link <TT
CLASS="FILENAME"
>/tmp/.s.PGSQL.5432</TT
> that points
   to the relocated socket file.  You also might need to modify your
   <TT
CLASS="FILENAME"
>/tmp</TT
> cleanup script to preserve the symbolic link.
  </P
><P
>   For TCP connections the server
   must accept only <TT
CLASS="LITERAL"
>hostssl</TT
> connections (<A
HREF="auth-pg-hba-conf.html"
>Section 21.1</A
>) and have SSL
   <TT
CLASS="FILENAME"
>server.key</TT
> (key) and
   <TT
CLASS="FILENAME"
>server.crt</TT
> (certificate) files (<A
HREF="ssl-tcp.html"
>Section 17.8</A
>). The TCP client must connect using
   <TT
CLASS="LITERAL"
>sslmode='require'</TT
> (<A
HREF="libpq-connect.html"
>Section 30.1</A
>) and have
   a <TT
CLASS="FILENAME"
>~/.postgresql/root.crt</TT
> SSL certificate (<A
HREF="libpq-ssl.html"
>Section 30.16</A
>).
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