Bug 681721

Summary: dhclient sets MTU to 576 for addresses obtained via DHCP
Product: Red Hat Enterprise Linux 6 Reporter: Stuart R. Kirk <Stuart.Kirk>
Component: dhcpAssignee: Jiri Popelka <jpopelka>
Status: CLOSED ERRATA QA Contact: Release Test Team <release-test-team>
Severity: high Docs Contact:
Priority: unspecified    
Version: 6.1CC: merlin, mganisin, ovasik, tcapek
Target Milestone: rcKeywords: Patch
Target Release: ---   
Hardware: x86_64   
OS: Linux   
Whiteboard:
Fixed In Version: dhcp-4.1.1-17.P1.el6 Doc Type: Bug Fix
Doc Text:
DHCP servers at some ISPs send to clients the "interface-mtu" option with the value of 576. Such a low MTU (Maximum Transmission Unit) can cause throughput problems with UDP traffic, among other things. With this update, the dhclient utility now sets the interface MTU only if the value obtained from the server is higher than 576.
 Story Points: ---
Clone Of: Environment:
Last Closed: 2011-05-19 14:05:20 UTC Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Attachments:
Description Flags
set interface MTU only if it's higher than 576 none

Description Stuart R. Kirk 2011-03-03 02:14:12 UTC 
Description of problem:

Similar to BZ# 574629, the same/similar problem seems to exist in RHEL 6.0.  I have a server configured as a NAT gateway and what I initially believed to be a rule set issue appears to be an issue with the interface itself.  The external (ISP in this case) facing interface obtains its address through DHCP and each time it does it the MTU is reset to the X.25 default of 576.  This causes significant throughput issues.

Version-Release number of selected component (if applicable):
dhclient-4.1.1-12.P1.el6_0.2.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Install RHEL 6.0 with above version of dhclient
2. Configure an interface to obtain an address through DHCP
3. Execute dhclient and observe the mtu which is set on the interface.
4. Execute "ip link set dev eth0 mtu 1500"
5. Problem fixed.
  
Actual results:
MTU is set to 576 on startup or service network restart.

Expected results:
MTU should be 1500 on startup or service network restart.

Additional info:
Placing MTU=1500 in the /etc/sysconfig/network-scripts/ifcfg-ethX file does nothing.  dhclient does not seem to read this file and the MTU must be changed as noted above after the inferface is brought up.
Comment 2 Jiri Popelka 2011-03-03 09:25:07 UTC 
Created attachment 482017 [details]
set interface MTU only if it's higher than 576

Yes, this is the same problem as bug #574629 (also discussed in bug #566873).

I guess your ISP is Comcast or Cablevision ;-)

Attached patch solves the problem.
Comment 7 Tomas Capek 2011-05-10 14:25:32 UTC 
    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
DHCP servers at some ISPs send to clients the "interface-mtu" option with the value of 576. Such a low MTU (Maximum Transmission Unit) can cause throughput problems with UDP traffic, among other things. With this update, the dhclient utility now sets the interface MTU only if the value obtained from the server is higher than 576.
Comment 8 errata-xmlrpc 2011-05-19 14:05:20 UTC 
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2011-0697.html