Mac OS X / El Capitan connectivity issues

A user with a Macbook running OS X El Capitan connected to the Turris Omnia router over wifi.

They could browse the web but when they try to use Thunderbird (v45) to connect to their IMAP server (TLS on port 993), the TLS handshake doesn’t complete.

I manually set the MTU on the Macbook down to 1280 in case it was an MTU issue.

I ran a packet capture on the router and observed retransmissions and Dup ACK messages.

A Linux laptop running Thunderbird is fine on the same wifi.

Has anybody seen similar problems with Mac OS X and Turris Omnia / OpenWRT?

Hmm. Never had any problem with my Mac running Serria, but I don’t use Thunderbird, I use AirMail. No problem with my iPad or iPhone either.

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I used to have similar weird problems with Ubiquity EdgeRouter. TLS handshake was broken for some sites and not for others. It was definitely a MTU issue.

If I remember well in the end it was related to something called TCP MSS Clamping and it was triggerred by specific fragmentation of packets going through my ISP. A terrible thing to diagnose and resolve. Good luck.

The mtu_fix parameter is supposed to resolve that and I already have it enabled for all interfaces, it hasn’t helped.