DF bit is needed to discover the MTU on the application level, exactly because MTU discovery can be broken on lower levels of the network stack. QUIC specifies this procedure here: https://tools.ietf.org/html/draft-ietf-quic-transport-05#page-53
On Saturday, April 24, 2021 at 1:37:26 PM UTC-7 ico....@gmail.com wrote: > I'm curious, why do you need to set DF bit? I'm asking, because as a > network engineer I'm often experiencing broken apps with this setting. It > usually ends up network pointing to app and vice versa. > New tunnel along the path, either GRE or IPSec is lowering MTU, new > technologies like Cisco SD-WAN are all tunnel inside tunnel inside MPLS and > MTU can go as low as 1260. > Different workarounds are configured on routers as dynamic mss size > adjustment. > In your case you are at least trying to discover MTU, but path could be in > many cases asymmetric. > > On Friday, April 23, 2021 at 8:20:15 AM UTC+2 Marten Seemann wrote: > >> For my QUIC implementation <https://github.com/lucas-clemente/quic-go/>, >> I'm trying to set the DF bit on outgoing packets. >> >> The following code works fine on Linux: >> addr, _ := net.ResolveUDPAddr("udp", ":0") >> conn, _ := net.ListenUDP("udp", addr) >> rawConn, _ := conn.SyscallConn() >> rawConn.Control(func(fd uintptr) { >> unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_MTU_DISCOVER, 1) >> }) >> >> On OSX, the sockopt doesn't have a named constant, the correct value to >> use here is 28. The code above only works if I'm listening on an address >> that's unambiguously IPv4, e.g. *127.0.0.1:0 <http://127.0.0.1:0>*. >> If I listen on an address that allows me to send and receive both IPv4 >> and IPv6 (like *:0*), it doesn't work anymore: *SetsockoptInt *now >> returns error 22 (invalid argument). That's unfortunate as users of the >> library often listen on *:0*. >> >> Is there any way to make this work analogously to how it works on Linux? >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/9407910f-0cd2-4a35-ba24-9b367c9fda50n%40googlegroups.com.
