Mercurial > dropbear
view DEVELOPING.md @ 1930:299f4f19ba19
Add /usr/sbin and /sbin to default root PATH
When dropbear is used in a very restricted environment (such as in a
initrd), the default user shell is often also very restricted
and doesn't take care of setting the PATH so the user ends up
with the PATH set by dropbear. Unfortunately, dropbear always
sets "/usr/bin:/bin" as default PATH even for the root user
which should have /usr/sbin and /sbin too.
For a concrete instance of this problem, see the "Remote Unlocking"
section in this tutorial: https://paxswill.com/blog/2013/11/04/encrypted-raspberry-pi/
It speaks of a bug in the initramfs script because it's written "blkid"
instead of "/sbin/blkid"... this is just because the scripts from the
initramfs do not expect to have a PATH without the sbin directories and
because dropbear is not setting the PATH appropriately for the root user.
I'm thus suggesting to use the attached patch to fix this misbehaviour (I
did not test it, but it's easy enough). It might seem anecdotic but
multiple Kali users have been bitten by this.
From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903403
author | Raphael Hertzog <hertzog@debian.org> |
---|---|
date | Mon, 09 Jul 2018 16:27:53 +0200 |
parents | 57226fc75cb5 |
children |
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# Developer Notes ## Building See [INSTALL](INSTALL) for build instructions. [SMALL](SMALL) has hints for building smaller binaries, also see comments in default_options.h. ## Debug printing Set `#define DEBUG_TRACE 1` in localoptions.h to enable a `-v` option for dropbear and dbclient. That prints various details of the session. For development running `dropbear -F -E` is useful to run in the foreground. You can set `#define DEBUG_NOFORK 1` to make dropbear a one-shot server, easy to run under a debugger. ## Random sources Most cryptography requires a good random entropy source, both to generate secret keys and in the course of a session. Dropbear uses the Linux kernel's `getrandom()` syscall to ensure that the system RNG has been initialised before using it. On some systems there is insufficient entropy gathered during early boot - generating hostkeys then will block for some amount of time. Dropbear has a `-R` option to generate hostkeys upon the first connection as required - that will allow the system more time to gather entropy. ## Algorithms Default algorithm lists are specified in [common-algo.c](common-algo.c). They are in priority order, the client's first matching choice is used (see rfc4253). Dropbear client has `-c` and `-m` arguments to choose which are enabled at runtime (doesn't work for server as of June 2020). Enabling/disabling algorithms is done in [localoptions.h](localoptions.h), see [default_options.h](default_options.h). ## Style Source code is indented with tabs, width set to 4 (though width shouldn't matter much). Braces are on the same line as functions/loops/if - try to keep consistency with existing code. All `if` statements should have braces, no exceptions. Avoid using pointer arithmetic, instead the functions in [buffer.h](buffer.h) should be used. Some Dropbear platforms have old compilers. Variable declarations must be at the top of a scope and comments must be `/* */` rather than `//`. Pointer variables should be initialised to NULL - it can reduce the severity of bugs. ## Third party code Libtomcrypt and libtommath are periodically synced from upstream, so avoid making changes to that code which will need to be maintained. Improvements can be sent upstream to the libtom project. ## Non-root user Dropbear server will run fine as a non-root user, allowing logins only for that user. Password authentication probably won't work (can't read shadow passwords). You will need to create hostkeys that are readable. ## Connection setup Dropbear implements first_kex_packet_follows to reduce handshake latency (rfc 4253 7.1). Some less common implementations don't handle that, it can be a cause of problems connecting. Note also that Dropbear may send several ssh packets within a single TCP packet - it's just a stream.