dhcp reduce packet size

Discussion in 'Tomato Firmware' started by jsmiddleton4, Apr 8, 2007.

  1. jsmiddleton4

    jsmiddleton4 Network Guru Member

    What does this do? I've read through FAQ's, etc., and don't see it listed.
  2. der_Kief

    der_Kief Super Moderator Staff Member Member

    there is also a readme :biggrin:

    this is part of the readme from version 1.02:

    Added an option in Advanced/DHCP for reduced DHCP packet size (WAN side). Toggling this may help fix problems with DHCP failing to aquire an address. The previous behavior was the same as enabled in 1.00 and 1.01, the default is now disabled, the same behavior as the original udhcpc.

  3. jsmiddleton4

    jsmiddleton4 Network Guru Member

    Thanks for taking time to post. You didn't answer the question but thanks for posting. WHAT does it actually do is the question.
  4. der_Kief

    der_Kief Super Moderator Staff Member Member


    i don't read your question well enough :wink:
    I think only Jon know what this exactly do ! So if you want to know that you have to ask him directly.

  5. jeradc

    jeradc LI Guru Member

    I bet you could install Wireshark and just compare the dhcp packets with and without that setting.
  6. turbo53

    turbo53 Network Guru Member

    OK...this thread got me curious and I started looking around the net. Here's what I found. This following link is related to DD-WRT, not Tomato. But I bet the issue is the same.


    It seems that udhcpc (the client that both DD-WRT and Tomato use to get the WAN IP via DHCP) has an issue. The DHCP discovery packet from udhcpc is 590 bytes long, but DHCP relay servers only handle up to 576 bytes. This can cause a problem if DHCP relay servers are sitting between you and the DHCP server used by your ISP.

    From the message thread above, it appears that the extra bytes are all padding...completely useless. One of the developers discussing the situation in the thread eliminated the padding and reduced the DHCP discovery packet to only 331 bytes.

    Tomato is probably using the updated udhcpc code and the "Reduce Packet Size" option just eliminates the padding to allow DHCP relay servers to work properly.
  7. jeradc

    jeradc LI Guru Member

    I wonder why the option is even there to have the padded dhcp packets then.
  8. jsmiddleton4

    jsmiddleton4 Network Guru Member

    This is why I am asking. If the "Reduce Packet Size" is the better option, why isn't it the default? To answer that question knowing what it does is essential to understanding the option.

    So I agree, if its better being reduced, why the "padding"?
  9. rcordorica

    rcordorica Network Guru Member

    With my SB4200 modem on RoadRunner cable, I have to use the "reduce packet size" option otherwise my modem often refuses to give out a DHCP IP address.

    I believe the option is in there because some modems are just badly coded. They expect a certain size packet and formatting otherwise they just wont work.
  10. turbo53

    turbo53 Network Guru Member

    Yeah...I was wondering the same thing when I wrote the previous message. But perhaps a clue comes from Jon's README entry in Tomato 1.02.

    I read this to mean that the default in Tomato 1.00 and 1.01 was to eliminate the padding. If Jon turned this off, there were probably some complaints from users that they were having DHCP issues. Maybe some DHCP servers NEED the padding since they expect the packet to be of a certain size, while other ones need the padding eliminated to facilitate DHCP relay.

    This is just speculation on my part :) Jon may be able to give a more definitive answer.

    Anyway, I haven't touched the option since my DHCP works fine and I'm of the "if it ain't broke, don't fix it" school.
  11. jsmiddleton4

    jsmiddleton4 Network Guru Member

    So we get to play with it and see if there are any problems one way or the other? Which is ok with me. I love to play with things....
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