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 <[email protected]>
> 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
>
