iCloud private relay issue in macOS 27 Beta 4

Question

Created Aug ’26

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Although I have reduced my Mac's MTU to solve the MTU blackhole problem, Safari is quite slow if I enabled iCloud private relay (scenario 1). At the same time, Chrome functions normally.

When I turns off iCloud private relay, the Safari's performance improves immediately.

Packets captured by tcpdump at a LAN gateway (FreeBSD) shows iCloud private relay uses UDP protocol and doesn't fallback to TCP protocol. If the MTU blackhole is a problem, the connection will switch to TCP protocol, given prior tcpdump captures.

I am waiting for macOS 27 Beta 5 but still no update released yet.

Answer 1

TomHsiung OP

Aug ’26

Update:

We have discovered that force iCloud private relay back to TCP mode (and setting back the MTU of a Mac to default/1500-byte) signifcantly improved the performance of Safari browsing, including Youtube videos.

We wrote a single PF rule on a FreeBSD LAN gateway to block UDP packets to port 443 and echo ICMP to the Mac.

Answer 2

TomHsiung OP

1w

We wish we can set the packet size of iCloud private relay. It currently send default initial packets of 1378-byte, which doesn't fit our need. We have to reduce its size to 1350-byte or less.