The first version of my work-in-progress NNTP server I tested with a real newsreader had enough code to handle connections, give the greeting and reject every command except QUIT (whose implementation was fully RFC 3977 conforming).
Here’s an excerpt from my server log when giving tin the Treatment:
2009-05-02T01:05:49 nntpd for 127.0.0.1:55432 terminating 2009-05-02T15:24:31 nntpd listening at 0.0.0.0:5555 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 starts 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 ==> 200 posting allowed 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 <== CAPABILITIES 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 ==> 500 unrecognized command 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 <== MODE READER 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 ==> 500 unrecognized command 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 <== CAPABILITIES 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 ==> 500 unrecognized command 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 <== MODE READER 2009-05-02T15:24:46 nntpd for 127.0.0.1:34844 ==> 500 unrecognized command
This went on and on and on for five seconds, until tin crashed (which I believe is a bug – a NNTP client ought to either give up after getting 500 to CAPABILITIES, or ignore the 500 from MODE READER – I’m sure there are functional NNTP servers predating CAPABILITIES that do not implement MODE READER either). My server promptly crashed as well – my logic for handling socket errors was faulty (fixed).
Next victim was slrn. It was much better behaved:
2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 starts 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 200 posting allowed 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== MODE READER 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== XOVER 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== XHDR Path 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== NEWGROUPS 071115 214035 GMT 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.comp.shells.dash 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.comp.version-control.packaging 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.linux.drivers.ath5k.user 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.linux.kernel 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.linux.kernel.announce 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.comp.compilers.cyclone.general 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.comp.version-control.git 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 <== GROUP gmane.comp.compilers.pcc 2009-05-02T15:37:07 nntpd for 127.0.0.1:40000 ==> 500 unrecognized command 2009-05-02T15:37:21 nntpd for 127.0.0.1:40000 <== QUIT 2009-05-02T15:37:21 nntpd for 127.0.0.1:40000 ==> 205 bye 2009-05-02T15:37:21 nntpd for 127.0.0.1:40000 terminating
Unfortunately, slrn went and deleted my subscriptions to those groups. The bastard. 
Debian bug #526646
201 is a better greeting response code in this case
True. I wonder why I didn’t use that. Git history shows I switched to 201 after implementing the main READER commands. Turned back to 200 after implementing POST.