From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Poirier Subject: Re: discrepancy in ip(7) wrt. IP DF flag for UDP sockets Date: Tue, 20 Sep 2011 09:29:54 -0400 Message-ID: <20110920132954.GA23041@synalogic.ca> References: <20110919121940.GA19942@synalogic.ca> <20110919130313.GA27819@hmsreliant.think-freely.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Neil Horman , linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Michael Kerrisk Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-man-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: netdev.vger.kernel.org On 11-09-20 08:14, Michael Kerrisk wrote: > Hello Benjamin, Neil, >=20 > On Mon, Sep 19, 2011 at 3:03 PM, Neil Horman = wrote: > > On Mon, Sep 19, 2011 at 08:19:40AM -0400, Benjamin Poirier wrote: > >> Hi, > >> > >> I noticed what appears to be a discrepancy between the ip(7) man p= age > >> and the kernel code with regards to the IP DF flag for UDP sockets= =2E > >> > >> The man page says that "The don't-fragment flag is set on all outg= oing > >> datagrams" and that the ip_no_pmtu_disc sysctl affects only SOCK_S= TREAM > >> sockets. This is quickly disproved by doing: > >> echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc > >> firing up netcat and looking at a few outgoing udp packets in wire= shark > >> (they don't have the DF flag set). >=20 > Could you describe the required change in terms of how the man page > text should look--i.e., rewrite the passage as you think it should > look? How about changing it to: IP_MTU_DISCOVER (since Linux 2.2) Set or receive the Path MTU Discovery setting for a socket. When enabled, the don't-fragment flag is set on all outgoing packets. Linux will perform Path MTU Discovery as defined in RFC 1191 on SOCK_STREAM sockets. For non-SOCK_STREAM sockets, it is the user's responsibility to packetize the data in MTU sized chunks and to do the retransmits if necessary. The kernel will reject (with EMSGSIZE) datagrams that are bigger than the known path MTU. The system-wide default is controlled by the /proc/sys/net/ipv4/ip_no_pmtu_disc file.=20 Path MTU discovery flags Meaning [...] There are some differences between _DO and _WANT that are glossed over in this description, but I suppose there's only so much detail you can put in... Thanks, -Ben >=20 > Thanks, >=20 > Michael >=20 >=20 > >> 1) in the words of `man 7 ip`: > >> IP_MTU_DISCOVER (since Linux 2.2) > >> =A0 =A0 =A0 Set or receive the Path MTU Discovery =A0setting =A0fo= r =A0a =A0socket. > >> =A0 =A0 =A0 When =A0enabled, Linux will perform Path MTU Discovery= as defined > >> =A0 =A0 =A0 in RFC 1191 on this socket. =A0The don't-fragment flag= is set =A0on > >> =A0 =A0 =A0 all =A0outgoing datagrams. =A0The system-wide default = is controlled > >> =A0 =A0 =A0 by the /proc/sys/net/ipv4/ip_no_pmtu_disc file for =A0= SOCK_STREAM > >> =A0 =A0 =A0 sockets, and disabled on all others. > >> > >> This is the text present in the latest version of the online manpa= ges, > >> http://webcache.googleusercontent.com/search?q=3Dcache:http://www.= kernel.org/doc/man-pages/reporting_bugs.html&ie=3DUTF-8 > >> > >> 2) in net/ipv4/af_inet.c:inet_create(): > >> =A0 =A0 =A0 if (ipv4_config.no_pmtu_disc) > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 inet->pmtudisc =3D IP_PMTUDISC_DONT; > >> =A0 =A0 =A0 else > >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 inet->pmtudisc =3D IP_PMTUDISC_WANT; > >> > >> and pmtudisc is left alone from there on for UDP sockets. > >> > >> What should be adjusted, the man page or the code? > >> > > The man page is wrong I think > > > > By my read, the code: > > 1) Affects UDP and TCP the same way (which makes sense to me) > > > > 2) Is doing exactly what you asked it to, since you set no_pmtu_dis= c, which > > means the stack should be free to fragment a frame as it sees fit a= ccording to > > the MTU metric of the route its traversing, hence the cleared DF bi= t in the > > fraem. > > > > RFC 1191 can apply equally well to udp, as tcp, and is evident in t= hat you can > > set the per-socket option IP_MTU_DISCOVER to any of the 4 acceptibl= e values > > offered (DONT/WANT/DO/PROBE), so theres no reason the sysctl govern= ing the > > default value at creation shouldn't apply as well. > > Neil > > > > >=20 >=20 >=20 > --=20 > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Author of "The Linux Programming Interface"; http://man7.org/tlpi/ -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html