First of all --- I really didn't know where to post this question: askubuntu, unix, superuser... I am posting this one here because the only cases I have seen are on Ubuntu and I have a report for Arch... so trying to get the common denominator.(1)If I try out of the box to connect to portal.office.com (and enter a valid user/password), from my Ubuntu 14.04 workstation at work, the connection just sits there forever. But if I reduce the MTU of the active internet interface (for example sudo ifconfig eth0 MTU 1400) to anything less or equal 1400, then the connection automagically works. This does not happen when I connect from home with the same system. What can cause this behavior? An intermediate router? Notice that no other application/web site has any problem with the default MTU of 1500. (I think this could be related, and it was the hint to try lowering MTU, but I still have no idea about what's happening, and this peeves me). (1) moreover, I spent a lot of time finding this, so if I can help some desperate googler, the better.
Office 365 online from Unix working only with MTU <= 1400
networking
I would suggest that this is probably due to something blocking ICMP packets of types 3 and 4 from O365 back to your client PC. These packet types report on packet fragmentation across network links. I would have expected these to be classed as related packets for a connection and so permitted as part of a TCP conversation, but perhaps not.If you have a firewall on your client PC it might be worth disabling temporarily (if safe to do so) or accepting inbound ICMP type 3 and 4 packets:iptables -I INPUT -p icmp --icmp-type 4 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPTiptables -I INPUT -p icmp --icmp-type 3 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPTPotentially useful reading:Path MTU Discovery and Filtering ICMPRFC 1191Path MTU DiscoveryOf course, this may not help if your edge router is blocking them too.