Can IP MF and DF flags be set simulataneously

Patrick Schaaf

2002-09-21 08:59:12 UTC

Vijay,

please take your question to a more general forum related to IP
networking, this mailing list is too specific in scope.

If you find that something is to be illegal here (which I doubt),
and iptables/netfilter does a wrong thing (that the rest of the
IP stack does right), then please come back.

Sorry if I seem harsh, but we can't cover all quirks of the TCP/IP
protocol suite here. This is no educational facility.

Finally, one hint: fragments of packets can and will be further
fragmented, when you have more than two MTUs in the path of
the packet. That's why I doubt that your suspicion is correct.

best regards
Patrick

Post by Vijay Gullapalli
Hi,
I came across this list when searching for TCP SYN
Proxy links on google. And I have a very basic
question about IP fragment flags DF (don't fragment)
and MF (more fragment) usage. If this is not the right
forum, please excuse me.
While monitoring NFS/UDP traffic between two Linux
machines, I noticed that when Path MTU is turned on,
the IP header has both the DF and MF bits set at the
time. Is it legal to have have both flags set at the
same time? Isn't it enough to have just the DF bit set
to do Path MTU discovery?
Having both on at the same time goes against these
flags usage described in Internet Protocol RFC 791.
Are there any other RFCs which supercede RFC 791 and
describe how/why these flags should be allowed to be
set at the same time.
Thank you,
Vijay Gullapalli
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