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.
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.