Bug 612857

Summary: Default dhclient config ignores interface-mtu option
Product: [Fedora] Fedora Reporter: Steve Hill <steve>
Component: dhcpAssignee: Jiri Popelka <jpopelka>
Status: CLOSED NOTABUG QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: medium Docs Contact:
Priority: low    
Version: 13CC: jpopelka
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Hardware: All   
OS: Linux   
Fixed In Version: Doc Type: Bug Fix
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Last Closed: 2010-07-09 06:25:15 EDT Type: ---
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Description Steve Hill 2010-07-09 05:09:41 EDT
Description of problem:
In environments that use PPPoE links, an MTU of 1492 (at most) must be used on the PPPoE link instead of the usual 1500 used over Ethernet.  In theory this should require no further configuration elsewhere on the network since all endpoints should be able to do PMTU discovery.  Unfortunately, many firewalls/servers on the internet are administered by idiots and block the "ICMP frag needed" packets which are required for PMTU to work.  This results in intermittently broken TCP connections (a TCP connection will freeze indefinitely as soon as a packet greater than the MTU size is sent).

The work-around for this problem is to set the MTU on at least one of the endpoints to match the path MTU.  i.e. set the MTU on all of the machines on the local side of the PPPoE link to 1492.  This ensures that the TCP stacks of both endpoints will not attempt to exceed this packet size.

The DHCP protocol provides an "interface-mtu" option, which can be used to automatically set the MTU appropriately as client machines connect to the network.  Unfortunately, the default dhclient config ignores this option.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:
1. Add a "option interface-mtu 1492;" line to the dhcp server's config file and restart the DHCP server.
2. Restart the NIC on the client Fedora 13 machine.
3. Use "ip li" to observe that the MTU remains unchanged.

Actual results:
dhclient ignores the option and leaves the MTU unchanged.

Expected results:
dhclient should change the MTU to match what the DHCP server is advising.

Additional info:
Adding "also request interface-mtu;" to the client machine's /etc/dhcp/dhclient.conf resolves this issue.  This should be added to the dhclient package's default config.
Comment 1 Jiri Popelka 2010-07-09 05:29:11 EDT
(In reply to comment #0)
> Adding "also request interface-mtu;" to the client machine's
> /etc/dhcp/dhclient.conf resolves this issue.  This should be added to the
> dhclient package's default config.    

This is strange. I'm sure the interface-mtu option has been requested by default since dhcp-4.1.1-9.fc12 (see bug #566873 and bug #574629).
I'll check this later.
Comment 2 Jiri Popelka 2010-07-09 06:19:22 EDT
Just tested with dhclient-4.1.1-22.P1.fc13.i686 and it works for me.

Would it be possible to capture network traffic
on client machine with wireshark ?

Configure server to send interface-mtu option and
on client machine:
1) delete /etc/dhcp/dhclient.conf
2) install wireshark-gnome, start wireshark
3) Capture->Options select the Interface, press Start
4) restart the interface or run 'dhclient -d interface'
5) stop the capture
6) File -> Save, save the packet dump and attach it to this bug report

Comment 3 Steve Hill 2010-07-09 06:25:15 EDT
Hmm, strange.  I've just retested and it works, so I'm a bit confused.  Looks like I was wrong - apologies for raising this issue, I'll close it - many thanks.