See INSTALL for build instructions. SMALL has hints for building smaller binaries, also see comments in default_options.h.
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.
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.
Default algorithm lists are specified in 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, see default_options.h.
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 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.
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.
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.
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.