Hello Benjamin, Neil, On Mon, Sep 19, 2011 at 3:03 PM, Neil Horman <nhorman@xxxxxxxxxxxxx> 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 page >> and the kernel code with regards to the IP DF flag for UDP sockets. >> >> The man page says that "The don't-fragment flag is set on all outgoing >> datagrams" and that the ip_no_pmtu_disc sysctl affects only SOCK_STREAM >> 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 wireshark >> (they don't have the DF flag set). 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? Thanks, Michael >> 1) in the words of `man 7 ip`: >> IP_MTU_DISCOVER (since Linux 2.2) >> Set or receive the Path MTU Discovery setting for a socket. >> When enabled, Linux will perform Path MTU Discovery as defined >> in RFC 1191 on this socket. The don't-fragment flag is set on >> all outgoing datagrams. The system-wide default is controlled >> by the /proc/sys/net/ipv4/ip_no_pmtu_disc file for SOCK_STREAM >> sockets, and disabled on all others. >> >> This is the text present in the latest version of the online manpages, >> http://webcache.googleusercontent.com/search?q=cache:http://www.kernel.org/doc/man-pages/reporting_bugs.html&ie=UTF-8 >> >> 2) in net/ipv4/af_inet.c:inet_create(): >> if (ipv4_config.no_pmtu_disc) >> inet->pmtudisc = IP_PMTUDISC_DONT; >> else >> inet->pmtudisc = 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_disc, which > means the stack should be free to fragment a frame as it sees fit according to > the MTU metric of the route its traversing, hence the cleared DF bit in the > fraem. > > RFC 1191 can apply equally well to udp, as tcp, and is evident in that you can > set the per-socket option IP_MTU_DISCOVER to any of the 4 acceptible values > offered (DONT/WANT/DO/PROBE), so theres no reason the sysctl governing the > default value at creation shouldn't apply as well. > Neil > > -- 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@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
