Add DEVELOPING.md

DEVELOPING.md

@@ -0,0 +1,51 @@
+# 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).
+
+## 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.
+

INSTALL

@@ -28,6 +28,8 @@ Basic Dropbear build instructions:
 recompile after changing the PROGRAMS list, you *MUST* "make clean" before
 recompiling - bad things will happen otherwise)
 
+DEVELOPING.md has some notes on other developer topics, including debugging.
+
 See MULTI for instructions on making all-in-one binaries.
 
 If you want to compile statically use ./configure --enable-static