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Set MTU to 1500 in windows and in wsl (common default)
In wsl: ping -4 -s1472 somehostname
Observe successful replies
In wsl: ping -4 -s1473 somehostname
Observe no successful replies
In windows: ping -4 -l1473 somehostname
Observe successful replies
Set MTU to 9000 in windows and in wsl (jumbo frames)
Repeat 3-8 above
In windows: ping -4 -l8000 somehostname
Observe successful replies
Run packet sniffer on somehostname to observe packets coming from wsl. You will observe that large packets, eg 8000 from step 11, arrive at somehostname intact but the replies to not make it into wsl.
Expected Behavior
The WSL instance should behave identically to the windows host with regard to MTU. Whatever packet size the host can send/receive, the WSL guest should be able to send/receive.
Actual Behavior
The WSL guest, when in mirrored mode, has an MTU that is capped at something less than 1500, even when the host MTU is very much larger (jumbo frames). When observing using a packet sniffer, it can be seen that wsl correctly sends packets up to the configured MTU size but the replies are dropped unless they are less than X (some value less than 1500), regardless of the configured MTU.
When not in mirrored mode, wsl is able to send/receive large packets, up to the MTU size, just fine. The problem is only observed when running in mirrored mode.
Diagnostic Logs
No response
The text was updated successfully, but these errors were encountered:
If this a feature request, please reply with '/feature'. If this is a question, reply with '/question'. Otherwise please attach logs by following the instructions below, your issue will not be reviewed unless they are added. These logs will help us understand what is going on in your machine.
How to collect WSL logs
Download and execute collect-wsl-logs.ps1 in an administrative powershell prompt:
The scipt will output the path of the log file once done.
Once completed please upload the output files to this Github issue.
Click here for more info on logging
If you choose to email these logs instead of attaching to the bug, please send them to wsl-gh-logs@microsoft.com with the number of the github issue in the subject, and in the message a link to your comment in the github issue and reply with '/emailed-logs'.
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Windows Version
Microsoft Windows [Version 10.0.22631.3593]
WSL Version
2.2.1.0
Are you using WSL 1 or WSL 2?
Kernel Version
5.15.150.1-microsoft-standard-WSL2
Distro Version
Fedora 40
Other Software
No response
Repro Steps
Expected Behavior
The WSL instance should behave identically to the windows host with regard to MTU. Whatever packet size the host can send/receive, the WSL guest should be able to send/receive.
Actual Behavior
The WSL guest, when in mirrored mode, has an MTU that is capped at something less than 1500, even when the host MTU is very much larger (jumbo frames). When observing using a packet sniffer, it can be seen that wsl correctly sends packets up to the configured MTU size but the replies are dropped unless they are less than X (some value less than 1500), regardless of the configured MTU.
When not in mirrored mode, wsl is able to send/receive large packets, up to the MTU size, just fine. The problem is only observed when running in mirrored mode.
Diagnostic Logs
No response
The text was updated successfully, but these errors were encountered: