[HELP] flags [DF] always set

Discussion in 'Linux Networking' started by lollolo, Nov 25, 2011.

  1. lollolo

    lollolo Guest

    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
     
    lollolo, Nov 25, 2011
    #1
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  2. lollolo

    Tauno Voipio Guest


    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, Nov 25, 2011
    #2
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  3. lollolo

    Jorgen Grahn Guest

    Yes, nowadays it is.
    Yes, in a situation which is normal nowadays.
    # sysctl -a | grep pmtu_disc
    net.ipv4.ip_no_pmtu_disc = 0

    You can google for the details.
    Yes, but that would probably stop the traffic completely, not slow it
    down.

    /Jorgen
     
    Jorgen Grahn, Nov 25, 2011
    #3
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