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Mgetty+Sendfax with Vgetty Extensions (FAQ)
Section - Why the occasional "tcsetattr failed: I/O error" message?

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Q: Occasionally, mainly after "clean" user logouts (that is, the user
   typed "exit" instead of just hanging up), I get the message
	09/08 21:26:26 yS2  lowering DTR to reset Modem
	09/08 21:26:27 yS2  tcsetattr failed: I/O error
   in the mgetty log file, and a similar I/O error message in the syslog

A: Well, this is a Linux and SunOS specific problem: if the modem is still
   connected to the other end when mgetty starts, mgetty will force it to
   hangup by lowering DTR (and sending +++ATH, in case DTR drop won't
   suffice).  This will make the modem lower the DCD (carrier detect)
   line.  Unfortunately, this will trigger a security mechanism in the
   Linux kernel, which will prevent all further access via that file
   descriptor.  This is done to prevent one well-known password hack
   (I won't that explain in detail).

   Mgetty knows about that problem, and, upon noticing an error at this
   point during modem initialization, will simply reopen the port and
   redo all modem / port setup stuff.

   Because suppressing that error message would be messy, it keeps
   appearing, but it is harmless (... "trying again").

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