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Can no longer access ProtonMail after switching to Zen?

andrewmoore

Member
Hi All,

I moved to a new ISP, Zen Internet, from Virgin Media yesterday and ever since I can no longer access any Proton services, including Mail and Calendar. This applies to all devices on my network.

Attempting to navigate to https://mail.proton.me in a web browser just times out eventually. Screenshot of Chrome (all browsers are the same). The same applies to all Proton owned domains, such as https://protonmail.com.

DNS (1.1.1.1) appears to be working fine:
$ host mail.proton.me
mail.proton.me has address 185.70.42.37
mail.proton.me mail is handled by 20 mailsec.protonmail.ch.
mail.proton.me mail is handled by 10 mail.protonmail.ch.

Traceroute:
$ traceroute mail.proton.me
traceroute to mail.proton.me (185.70.42.37), 64 hops max, 52 byte packets
1 172.16.1.1 (172.16.1.1) 0.765 ms 0.524 ms 0.386 ms
2 lo0-0.bng2.ixn-lon.zen.net.uk (51.148.77.129) 6.772 ms 11.199 ms 10.711 ms
3 lag-4.p1.ixn-lon.zen.net.uk (51.148.73.89) 4.451 ms 3.906 ms
lag-5.p2.ixn-lon.zen.net.uk (51.148.73.91) 4.471 ms
4 lag-2.br1.ixn-lon.zen.net.uk (51.148.73.195) 4.190 ms
lag-1.br1.ixn-lon.zen.net.uk (51.148.73.181) 3.798 ms 4.238 ms
5 lag51.fr5.lcy1.llnw.net (87.248.216.236) 4.619 ms 4.273 ms 4.591 ms
6 siteprotect.security.neustar (68.142.82.33) 4.403 ms 5.213 ms 4.744 ms
7 * * *
8 gre01.lcy.siteprotect.security.neustar (156.154.218.0) 4.626 ms 3.599 ms 4.609 ms
9 156.154.165.61 (156.154.165.61) 21.334 ms 22.204 ms
156.154.253.101 (156.154.253.101) 22.669 ms
10 * * *
11 185-70-42-37.protonmail.ch (185.70.42.37) 22.440 ms 22.325 ms 21.090 ms

curl fails:
$ curl -I https://mail.proton.me:443
curl: (35) error:02FFF036:system library:func(4095):Connection reset by peer

If I switch to alternate routing on the iOS app, use 4G or connect using a VPN, the problem goes away.

No other websites have this issue.

Anyone else on Zen who can share some insight? They are a well regarded ISP and I can't believe they would block Proton, so I'm wondering if the issue lies elsewhere.

Thanks in advance!
 
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You have done enough investigation to be 99% certain its Zen, just go to them direct and get them to remove the block.
Thanks. I've raised a ticket with Zen.
This could also be the other way around and for whatever reason, the client is being denied.

Maybe ask Proton too?
It could be Proton, so I've raised a ticket with them too. If it's not their fault, they tend to be quite good at reaching out to ISPs to remove blocks as well. The "Connection reset by peer" curl error could also mean Zen are terminating the TCP connection.

Thanks both.
 
  • Like
Reactions: Msh
Solved!

Turns out the issue was on my end. I'd missed the MSS clamping setting on my firewall, which should be set to 1452 bytes to prevent packet fragmentation when using PPPoE.

Proton must just be particularly sensitive to packet/TCP fragmentation, whilst every other site handled it fine. The fact that it was just Proton sent me on a wild goose chase, but it turned out to be a red herring in the end.
 
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