DEVELOPING.md · xfan1024/oh-dropbear

# 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.

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