Suddenly, my 2 remote devices are dis-connected. I tried to add a new device(aspspc in the below screenshot), but it did not work. Before, I remember when we add new device, the remote device would get a notification. But now it seems the devices are not talking to each other.
Do you have any clues how to debug this? The console did not give much useful information.
Below is the screenshot.
Can you post a screenshot from the other side? Also can you click on the 3/5 number next to Discovery and post a screenshot from there?
Check what Discovery 3/5 shows when you click on the blue text.
Did not realize that I can click the discovery 
Seems that IPV6 does not work. Yes, indeed I only have link-local IPv6 address.
The first screenshot is from my laptop. And the 2nd is from a dis-connected remote linux. Both complaing about ipv6.
The weird part is that IPv4 global discovery is slso broken. Any weird firewall rules or security middleware?
@bt90 I am not sure. I use wifi in my institution. I do not know if they set up some new firewall rules. But yesterday syncthing was working properly. Is there any way I can check if the package is blocked by firewall?
Check if you can access those https urls. I suspect you cannot.
In the remote device (linux inside my institution), v6 says Unable to connect, the other two says Secure Connection Failed.
However, something is different when I am using my home network with my laptop. The discovery became 4/5 when at home. When I using institution wifi, this is 3/5. Besides, when I am at home, I can now connect to one of the remote devices(lab-pc, this PC is not in my institution network). Still, at home I cannot connect to the institution linux. I highly suspect that my institution blocks the outcoming and incoming traffic of syncthing.
Could you try the following from inside the institution?
openssl s_client discovery-v4.syncthing.net:443
This morning, when using my institution’s wifi, the discovery of both my laptop and the linux become 4/5. And it start to sync!!!
Thanks guys. Although I do not know what happened.
Below is the result of the openssl command.
CONNECTED(00000003)
depth=0 CN = syncthing
verify error:num=18:self signed certificate
verify return:1
depth=0 CN = syncthing
verify return:1
---
Certificate chain
0 s:CN = syncthing
i:CN = syncthing
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=CN = syncthing
issuer=CN = syncthing
---
No client certificate CA names sent
Requested Signature Algorithms: RSA-PSS+SHA256:ECDSA+SHA256:Ed25519:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA+SHA256:RSA+SHA384:RSA+SHA512:ECDSA+SHA384:ECDSA+SHA512:RSA+SHA1:ECDSA+SHA1
Shared Requested Signature Algorithms: RSA-PSS+SHA256:ECDSA+SHA256:Ed25519:RSA-PSS+SHA384:RSA-PSS+SHA512:RSA+SHA256:RSA+SHA384:RSA+SHA512:ECDSA+SHA384:ECDSA+SHA512
Peer signing digest: SHA384
Peer signature type: ECDSA
Server Temp Key: X25519, 253 bits
---
SSL handshake has read 870 bytes and written 430 bytes
Verification error: self signed certificate
---
New, TLSv1.3, Cipher is TLS_AES_128_GCM_SHA256
Server public key is 384 bit
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
Early data was not sent
Verify return code: 18 (self signed certificate)
---
---
Post-Handshake New Session Ticket arrived:
SSL-Session:
Protocol : TLSv1.3
Cipher : TLS_AES_128_GCM_SHA256
Session-ID: 8EB6ABE6E2B7DC89AD2AB2A6F858720B62431F28932221E8B4E68831EE66AAF8
Session-ID-ctx:
Resumption PSK: A08B231A6B3BEF473142B0100BD4072B265F52201412506DE1757CB3EDEB196C
PSK identity: None
PSK identity hint: None
SRP username: None
TLS session ticket lifetime hint: 604800 (seconds)
TLS session ticket:
0000 - 21 a1 71 ff 06 fe bd 52-d6 74 ed 4b fb 97 7a a2 !.q....R.t.K..z.
0010 - 4a 10 73 5f 7f ec 58 87-26 70 e7 07 dd 36 25 65 J.s_..X.&p...6%e
0020 - 47 4e fc 73 f5 d0 d7 b0-87 c7 aa 35 57 57 46 47 GN.s.......5WWFG
0030 - 97 b1 20 2f eb a5 cc a6-a4 07 4e 5d ce 86 28 b0 .. /......N]..(.
0040 - bc 9e c3 f0 0b 3c 0e 31-51 b4 fc 92 6f b1 e1 38 .....<.1Q...o..8
0050 - 09 94 72 2a 4c 7f cf c1-f0 b4 97 15 53 fd dd 7c ..r*L.......S..|
0060 - be b1 e3 96 57 4e 87 19-c7 ....WN...
Start Time: 1695820621
Timeout : 7200 (sec)
Verify return code: 18 (self signed certificate)
Extended master secret: no
Max Early Data: 0
---
read R BLOCK
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