Mention a pre-2.2.5 bug in the tun driver that gets tickled

by the "set mtu" command (now that we can see the IO error
in the log file).

FAQ/FAQ.sgml

@@ -1,12 +1,12 @@
 <!DOCTYPE linuxdoc PUBLIC "-//FreeBSD//DTD linuxdoc//EN">
-<!-- $Id: FAQ.sgml,v 1.79 1997-10-03 20:53:27 wosch Exp $ -->
+<!-- $Id: FAQ.sgml,v 1.80 1997-10-04 00:19:58 brian Exp $ -->
 
   <article>
 
 <title>Frequently Asked Questions for FreeBSD 2.X
 <author>Maintainer: Peter da Silva <tt><htmlurl url='mailto:peter@taronga.com'
   name='&lt;peter@taronga.com&gt;'></tt>
-<date>$Date: 1997-10-03 20:53:27 $</date>
+<date>$Date: 1997-10-04 00:19:58 $</date>
 
 <abstract>
 This is the FAQ for FreeBSD systems version 2.X  All entries are
@@ -3548,6 +3548,27 @@ MYADDR:
   disable pred1
 </verb>
 
+          <sect2>
+            <heading>Ppp locks up during file transfers with IO errors</heading>
+
+            <p>
+              Under FreeBSD 2.2.2 and before, there was a bug in the tun
+              driver that prevents incoming packets of a size larger than
+              the tun interface's MTU size.  Receipt of a packet greater than
+              the MTU size results in an IO error being logged via syslogd.
+
+            <p>
+              The ppp specification says that an MRU of 1500 should
+              <bf>always</bf> be accepted as a minimum, despite any LCP
+              negotiations, therefore it is possible that should you decrease
+              the MTU to less than 1500, your ISP will transmit packets of
+              1500 regardless, and you will tickle this non-feature - locking
+              up your link.
+
+            <p>
+              The problem can be circumvented by never setting an MTU of
+              less than 1500 under FreeBSD 2.2.2 or before.
+
           <sect2>
             <heading>None of this helps - I'm desperate !</heading>