readme.security

Linux下的TFTP源代码

$Id$

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Starting in version 0.27, tftp-hpa has the option of a "use Unix
permissions" mode.  In this mode, tftpd can access any file accessible
by the tftpd effective user, specified via the -u option.  This means
that files no longer need to be set to o+r or o+w.

If file creation is enabled (via the -c option), the -p option also
changes the default umask from 0 (anyone can read or write) to
"unchanged" (inherited from the calling process.)  The -U option can
be used to override the default umask; this is recommended.

The sanest setup, from a security standpoint, for tftpd to run in is
probably the following:

1. Create a separate "tftpd" user and group only used for tftpd;
2. Have all your boot files in a single directory tree (usually called 
   /tftpboot).
3. Specify "-p -u tftpd -s /tftpboot" on the tftpd command line; if
   you want clients to be able to create files use
   "-p -c -U 002 -u tftpd -s /tftpboot" (replace 002 with whatever
   umask is appropriate for your setup.)

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Starting in version 0.17, tftp-hpa operates in genuine "wait" mode,
which means that an in.tftpd process hangs around for some time after
the last service request has arrived.  This speeds up servicing a
subsequent request, which apparently has been a problem in the past,
resulting in "request storms" as the client keeps retrying, resulting
in multiple connections on the server which the client has already
abandoned.

This also means that spawning tftp via tcpd is useless (in fact, this
indirection seems to be part of the reason for these "request
storms.")  Instead, tftp-hpa supports calling the tcpwrapper library
directly.  Thus, if your /etc/inetd.conf looks like this (all on one
line):

tftp	dgram	udp	wait	root	/usr/sbin/tcpd
/usr/sbin/in.tftpd -s /tftpboot -r blksize

... it's better to change to ...

tftp	dgram	udp	wait	root	/usr/sbin/in.tftpd
in.tftpd -s /tftpboot -r blksize

You should make sure that you are using "wait" option in tftpd; you
also need to have tftpd spawned as root in order for chroot (-s) to
work.  tftpd automatically drops privilege and changes user ID to
"nobody" by default; the appropriate user ID for tftpd can be
specified with the -u option (e.g. "-u tftpuser").

If you are running a busy boot server, I would suggest to instead use
kernel-based firewalling rules, and to compile tftpd without
tcpwrapper support, in order to provide significantly better
performance.  To do so, specify the --without-tcpwrappers option to
configure when compiling; see the INSTALL.tftp file for more information.