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).
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?
Thanks,
-Ben
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