The quic-go maintainer said that 2 MB is not enough. [1]

Is there any chance we can change the OpenBSD kernel to allow larger buffer sizes?
Otherwise, I think I'll replace those instructions with a note explaining that
QUIC will not work.


[1] https://github.com/quic-go/quic-go/issues/3476#issuecomment-3831234358




On Sunday, February 1st, 2026 at 12:23, Douglas Silva <doug.hs@proton.me> wrote:

> Hi, I've applied the suggested changes. Diff's below.
> 
> > can the linux code be ported across? (we don't have
> > SO_RCVBUFFORCE which ignore the max limit, but
> > SO_RCVBUF will at least allow up to 2MB). if so,
> > doing that (and changing the amount it's trying to
> > raise to cap it to 2MB on OpenBSD, or at least
> > retry capped to 2MB if it fails) is a better
> > approach than telling the user to unilaterally
> > raise the socket buffer for all programs via sysctl.
> 
> I've also sent that suggestion to quic-go. I wish I
> could make a PR, but I don't feel confident with Go.
> 
> [1] https://github.com/quic-go/quic-go/issues/3476#issuecomment-3831146698
> 
> 
> Index: pkg/README
> ===================================================================
> RCS file: /cvs/ports/net/syncthing/pkg/README,v
> diff -u -p -u -p -r1.9 README
> --- pkg/README	1 Jan 2026 22:24:37 -0000	1.9
> +++ pkg/README	1 Feb 2026 15:13:22 -0000
> @@ -22,8 +22,8 @@ Starting via RC script
>  Syncthing can be started via the included RC script, thus running
>  Syncthing as a system service. This is fine for single user
>  configurations. Under this mode of operation, Syncthing runs as the
> -_syncthing user, and the default location for folders is
> -${LOCALSTATEDIR}/syncthing.
> +_syncthing user (or whatever `syncthing_user` is set to in RC), and
> +the default location for folders is ${LOCALSTATEDIR}/syncthing.
> 
>  Starting Manually
>  -----------------
> @@ -45,6 +45,17 @@ This will result in a new tmux(1) sessio
>  boot. The user can later use `tmux attach` to view and control their
>  Syncthing process.
> 
> +Logging
> +=======
> +
> +By default, logs are not saved anywhere; but you can read the logs
> +for the current session on the web GUI.
> +
> +To log to a file, set these flags:
> +
> +	# rcctl set syncthing flags --log-file /var/log/syncthing/syncthing.log \
> +		--log-max-old-files 5 --log-max-size 2000000
> +
>  File Descriptor Limits
>  ======================
> 
> @@ -87,3 +98,22 @@ Another option is to turn off the file w
>  This will result in much reduced file descriptor usage at the cost of a
>  (configurable) latency. See "watch for changes" and "full rescan interval" in
>  the "advanced" tab in a folder's settings (on the web UI).
> +
> +UDP Buffer Size
> +===============
> +
> +Syncthing uses quic-go for QUIC transport. On other platforms, it is
> +able to increase the UDP buffer size automatically on startup; but it does
> +not have an implementation for OpenBSD, so you get this warning instead:
> +
> +	2026-01-24 12:33:01 INF failed to increase receive buffer size
> +	(wanted: 7168 kiB, got 0 kiB). See
> +	https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.
> +
> +This can be manually raised system-wide by setting a sysctl. Note that the
> +maximum size is 2097152 (~2 MB), which is well below what quic-go expects.
> +Hopefully it should be enough for it to minimally function.
> +
> +       # sysctl net.inet.udp.recvspace=2097152
> +
> +This can be set at boot in sysctl.conf(5).
> 
> 
> 
> 
> 
> 
> 
> 
> On Monday, January 26th, 2026 at 12:34, Stuart Henderson <stu@spacehopper.org> wrote:
> 
> >
> >
> >
> >
> > On 2026/01/26 13:24, Douglas Silva wrote:
> >
> > > +By default, logs are not saved anywhere; but you can read the logs
> > > +for the current session on the web GUI.
> > > +
> > > +To log to a file, use `syncthing_flags`. Example:
> > > +
> > > + syncthing_flags="--log-file /var/log/syncthing/syncthing.log --log-max-old-files 5 --log-max-size 2000000"
> >
> >
> > the recommended interface is 'rcctl set' for this, also wrap the line
> > e.g.
> >
> > rcctl set syncthing flags --log-file /var/log/syncthing/syncthing.log \
> > --log-max-old-files 5 --log-max-size 2000000
> >
> > > +UDP Buffer Size
> > > +===============
> >
> >
> > can the linux code be ported across? (we don't have SO_RCVBUFFORCE which
> > ignore the max limit, but SO_RCVBUF will at least allow up to 2MB).
> > if so, doing that (and changing the amount it's trying to raise to
> > cap it to 2MB on OpenBSD, or at least retry capped to 2MB if it fails)
> > is a better approach than telling the user to unilaterally raise the
> > socket buffer for all programs via sysctl.
> >
> > there's certainly no point warning above 2MB, the OS won't accept it,
> > so there's nothing the user can do.
> >
> > if the code can't be changed, ...
> >
> > > +quic-go requests about 8 MB of UDP buffer space. On some platforms, it is able to increase the limit automatically on startup. On OpenBSD this fails with the following warning:
> > > +
> > > + 2026-01-24 12:33:01 INF failed to increase receive buffer size (wanted: 7168 kiB, got 0 kiB). See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.
> > > +
> > > +We can manually increase it by changing `net.inet.udp.recvspace`. Unfortunately, this limit can only be raised to a maximum of 2097152 (approx. 2 MB). For context, see the discussion at https://github.com/quic-go/quic-go/issues/3476.
> > > +
> > > + # sysctl net.inet.udp.recvspace=2097152
> > > +
> > > +Add that to sysctl.conf(5) to permanently set it.
> >
> >
> > text needs word-wrapping.
> >
> > it doesn't attempt to change the limit at all on OpenBSD, there is
> > simply no code for it, so saying that it has "failed" doesn't quite
> > sit right...
> >
> > ---
> > syncthing uses quic-go for QUIC transport. This wants to increase UDP
> > socket buffers, however it does not has an OpenBSD implementation to
> > actually do this, and otherwise logs a warning, e.g.:
> >
> > INF failed to increase receive buffer size (wanted: 7168 kiB, got 0 kiB).
> > See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.
> >
> > This can be manually raised for the entire system by setting a sysctl.
> > The maximum available is 2097152 (2MB).
> >
> > # sysctl net.inet.udp.recvspace=2097152
> >
> > This can be set at boot in sysctl.conf(5).
> > ---
> >
> >
> > > On Sunday, January 25th, 2026 at 13:00, Douglas Silva doug.hs@proton.me wrote:
> > >
> > > > Hi,
> > > >
> > > > When you start Syncthing, one of the first log entries is this warning:
> > > >
> > > > 2026-01-24 12:33:01 INF failed to increase receive buffer size (wanted: 7168 kiB, got 0 kiB). See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes for details.
> > > >
> > > > The linked wiki page explains that quic-go tries to automatically increase the buffer size on some platforms (not BSDs), and it has instructions for manually doing it on others; but the section on BSDs doesn't apply to OpenBSD. There is an open issue [1] about it.
> > > >
> > > > Someone there has figured out that the sysctl variable to change here is "net.inet.udp.recvspace", but it can only be increased to about 2 MB (2097152) — well below the target 7.5 MB (8441037). I have no idea whether quic-go is able to work properly with this. Either way, the warning persists.
> > > >
> > > > So, my idea is to:
> > > >
> > > > 1. Modify our package README with instructions to increase the buffer size to maximum.
> > > > 2. Silence the warning by setting the variable "QUIC_GO_DISABLE_RECEIVE_BUFFER_WARNING" [2] [3]
> > > >
> > > > What do you think?
> > > >
> > > > [1] https://github.com/quic-go/quic-go/issues/3476
> > > > [2] https://github.com/quic-go/quic-go/blob/4a8af22606c2d237da39212a734b80026a9a0bb0/sys_conn.go#L59
> > > > [3] https://github.com/quic-go/quic-go/issues/3801#issuecomment-1882365505
> >
> >
> > On 2026/01/26 14:25, Edd Barrett wrote:
> >
> > > Hi,
> > >
> > > On Mon, Jan 26, 2026 at 01:24:45PM +0000, Douglas Silva wrote:
> > >
> > > > Below are my proposed additions to the README file, assuming we don't want to silence the warning.
> > > >
> > > > I've added a short section on logging too.
> > >
> > > Looks good, but please wrap long lines at column 80 (as mentioned in
> > > README.template).
> > >
> > > Also, from memory QUIC is optional, and syncthing will work without it. Is it
> > > worth mentioning this?
> > >
> > > --
> > > Best Regards
> > > Edd Barrett
> > >
> > > https://www.theunixzoo.co.uk