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diff --git a/DEVELOPING.md b/DEVELOPING.md new file mode 100644 index 0000000..1846b15 --- /dev/null +++ b/DEVELOPING.md @@ -0,0 +1,75 @@ +# 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. + |