NetDiag+
How to find your Path MTU on iPhone
The usual advice is to run ping with a fixed payload size and keep lowering it until packets stop coming back. That works, but it is slow, error-prone, and on iOS there is no terminal to do it in. Here is the 30-second version.
The 30-second method
- Open NetDiag+ and go to Diagnostics → Path MTU.
- Leave the default target (or type any host you care about) and tap Discover. The app runs a binary search with the don't-fragment flag set and reads the ICMP responses.
- You get one recommended value in bytes, plus the detected connection type (PPPoE, fiber, 5G, tunnel). Copy it and enter it in your router's MTU field — or in your VPN client.
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Common MTU values for reference
| MTU | Typical case |
|---|---|
1500 | Ethernet, cable, fiber, most Wi-Fi |
1492 | PPPoE (DSL) |
1480 | GRE / IPIP tunnel |
1472 | largest ping payload that fits in 1500 (1500 − 28) |
1420 | WireGuard over IPv4 |
1400 | OpenVPN (typical) |
1280 | IPv6 minimum — most routers refuse anything lower |
Measuring from a desktop instead
The same binary search by hand. Start at a 1472-byte payload and lower it until the ping stops failing, then add 28 bytes (20 IP + 8 ICMP) to get the MTU.
macOS / Linux
ping -D -s 1472 1.1.1.1Windows
ping -f -l 1472 1.1.1.1Questions people actually ask
My router doesn't let me change MTU
Set the MTU on the device instead (network settings on macOS, Windows or Linux), or in your VPN client — WireGuard uses MTU, OpenVPN uses tun-mtu. You can also ask your provider to change it, or switch the provider's box to bridge mode and use your own router.
Why does the test use a public host and not my DNS server?
Path MTU measures the path to the internet, not the path to your DNS resolver — changing DNS servers does not change the result. If you want to test a specific route, enter that host in the field before running Discover.
What do the individual probe rows mean?
Each row is one probe packet. Green means it fit; orange means a router along the path said it was too big. The recommended value is the largest packet that fit — that is the only number you need.
The result is below 1280 — is that normal?
Rarely. Most routers only accept 1280–1500 because 1280 is the minimum MTU required for IPv6. A very low reading usually means a mobile carrier is filtering the probe packets, or a tunnel is adding a lot of overhead. Run it again against a different host before trusting the number.
Cloud gaming says my MTU is too low
Services like Boosteroid or GeForce NOW require roughly 1452 bytes or more. Measure the real path value first: if you are below the requirement, the limit is usually in the carrier's tunnel or a PPPoE overhead, and no router setting can go below what the operator gives you.
Once you know the number, the next question is where to type it. Where the MTU field hides on popular routers →