On 01Jun2021 20:43, Stuart Henderson <s...@spacehopper.org> wrote: >On 2021-06-01, Cameron Simpson <c...@cskk.id.au> wrote: >> If I had TCP keep alive turned on, both ends might tidy themselves up. >> I can't enable that on the clients (various mail readers) or, >> apparently, on the server configuration. I can't do it in PF because PF >> just copies packets. I can't seem to do it in relayd either, though that >> seems the obvious way to intercept the connection for this purpose. > >It looks like courier-imap does enable SO_KEEPALIVE if available.
Hmm. Ok. I wonder how recent that is? I have 5.0.6 IIRC, and current is 5.1.something. >By default, keepalive timers are long; on a random Linux I had handy: > >$ grep . /proc/sys/net/ipv4/tcp_keepalive_* >/proc/sys/net/ipv4/tcp_keepalive_intvl:75 >/proc/sys/net/ipv4/tcp_keepalive_probes:9 >/proc/sys/net/ipv4/tcp_keepalive_time:7200 > >7200s (2h) initially, then every 75 seconds. (OpenBSD default times are >long too; 14400 "slowhz" intervals = also 2h). Ah. A long time indeed. Yes, winding these down will help - the above times are in the same magnitude as the time required to hit the connection limits. >> Plan B is to build the latest courier-imap from source if I find the >> time, but there may be no build option for this. I guess a single >> setsockopt() call in the source would be enough, _if_ that can be done >> on the accept end, which I haven't checked. > >https://tldp.org/HOWTO/TCP-Keepalive-HOWTO/addsupport.html but I don't think >you'll need it. Ta. >So you probably just need to lower tcp_keepalive_time, and perhaps adjust >tcp_keepalive_intvl. Note there is a tradeoff especially with mobile >clients; they will need to wake and transmit more often, so faster >keepalives will result in more battery/data use. I can wind it down to a handful of minutes without any serious impact I'd expect. >> Plan B0 might be to disable IMAP IDLE support. Hmm. > >Depends on timings whether that will help; think it's a last ditch effort >though, I think it will make things noticably worse for clients. Courier lets me change the advertised capabilities (it is not clear if that affects the actual capabilities). Not joy; possibly some clinets will try IDLE even if it isn't advertised and just cope if not supported, so maybe some clients are using IDLE successfully anyway. At any rate, dropping IDLE from the advertised list didn't help, and my hourly "restart imapd" cron is live again :-( I'll look at the keepalive settings on the server, many thanks! Cheers, Cameron Simpson <c...@cskk.id.au>
