[HELP] flags [DF] always set

Hi,
via tcpdump i noticed the Don't Fragment flag in the ip packets
transmitted/received from my pc.

It seems it only sends ip packets with this flags set, indipendently from
the type of traffic (LAN or internet).

Is this normal?
I though DF flag was only used in some situation. Is there a way (maybe
via /proc or /sys ) to reset the situation to the original configuration?

Could this problem affect negatively the network speed while using
internet? (maybe some packets get lost 'cause of some router with too low
MTU , unable to fragment because of the DF flag set )


thanks, lollolo

On 25.11.11 1:23 , lollolo wrote:
> Hi,
> via tcpdump i noticed the Don't Fragment flag in the ip packets
> transmitted/received from my pc.
>
> It seems it only sends ip packets with this flags set, indipendently from
> the type of traffic (LAN or internet).
>
> Is this normal?
> I though DF flag was only used in some situation. Is there a way (maybe
> via /proc or /sys ) to reset the situation to the original configuration?
>
> Could this problem affect negatively the network speed while using
> internet? (maybe some packets get lost 'cause of some router with too low
> MTU , unable to fragment because of the DF flag set )
>
>
> thanks, lollolo


TCP uses the DF flags to detect the path MTU, the maximum segment size
that gets through without IP fragmentation. If the nodes in the path
behave correctly, they will return an ICMP message to the sender, if
a segment is too large for the path.

--

Tauno Voipio

On Fri, 2011-11-25, lollolo wrote:
> Hi,
> via tcpdump i noticed the Don't Fragment flag in the ip packets
> transmitted/received from my pc.
>
> It seems it only sends ip packets with this flags set, indipendently from
> the type of traffic (LAN or internet).
>
> Is this normal?

Yes, nowadays it is.

> I though DF flag was only used in some situation.

Yes, in a situation which is normal nowadays.

> Is there a way (maybe
> via /proc or /sys ) to reset the situation to the original configuration?

# sysctl -a | grep pmtu_disc
net.ipv4.ip_no_pmtu_disc = 0

You can google for the details.

> Could this problem affect negatively the network speed while using
> internet? (maybe some packets get lost 'cause of some router with too low
> MTU , unable to fragment because of the DF flag set )

Yes, but that would probably stop the traffic completely, not slow it
down.

/Jorgen

--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .