Open Radar

 
Summary:
Internet Sharing of a PPPoE connection via Wi-Fi does not work. The clients can connect without problems but any data transmissions (such as web browsing) fail. Reducing the MTU on the clients to the PPPoE MTU improves the situation but some websites still refuse to load.

Steps to Reproduce:
2 Macs running Mac OS X 10.7.3. Mac A has a PPPoE connection. (Typically, the server-assigned MTU is 1492 for PPPoE connections.) Mac A has Internet Sharing over WiFi configured (Channel 11, WEP 128 password setup but that doesn't seem to matter in any way).
Mac B connects via Wi-Fi to the network of Mac A.

Expected Results:
Network access on Mac B should just work. Configuring manual MTU settings shouldn't ever be needed for the average user. (Not even speaking of iOS devices that don't even offer adjusting the MTU size).

Actual Results:
ping to a remote server succeeds but any normal HTTP or HTTPS traffic times out. In the next step, I reduced the MTU for the Wi-Fi interface on Mac B to 1492 (or experimentally to even lower values). Then, most web sites could be opened without a problem. However, some websites (for example bugreport.apple.com) still failed to load at some time.
All iOS devices connecting to Mac B can't successfully load any websites.

Regression:
Internet Sharing on a Mac in the same network setup running Mac OS X 10.6.8 worked flawlessly.

Notes:
I verified using tcpdump that HTTP packets went in both directions on Mac A as well as on Mac B. So a firewalling issue (also Firewall was disabled in the system settings) seemed unlikely.

The symptom that reducing the MTU somewhat helps but doesn't completely solve the problem, does make sense: servers send data packets with a default packet size of 1500. If the ICMP packets to reduce the MTU size aren't routed back to the server, the server continues to send packets with a size of 1500. Those large packets aren't refragmented by Internet Sharing and thus aren’t forwarded correctly to the client. Other servers that do correctly receive and honor those ICMP packets work just fine.


See also:
https://discussions.apple.com/thread/3194185?start=0&tstart=0 (comment by RandomMarius at 18.12.2011 00:57: "The problem is that the Lion ICS server does not do packet refragmentation anymore... ")