Hello!
I have a similar problem on Debian Linux kernel 4.9.0-4-amd64
Summary
$ letsencrypt certonly --webroot -w /home/www-data/ip/ -d api.pessom.ru --email=***@pessom.ru --agree-tos --dry-run
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Obtaining a new certificate
Performing the following challenges:
http-01 challenge for api.pessom.ru
Using the webroot path /home/www-data/ip for all unmatched domains.
Waiting for verification...
Cleaning up challenges
Failed authorization procedure. api.pessom.ru (http-01): urn:acme:error:connection :: The server could not connect to the client to verify the domain :: Fetching http://api.pessom.ru/.well-known/acme-challenge/7DFxVLj7oCnJGLbiCpTlI_UHA2fxfyCQFy-tmRzY7gc: Timeout
IMPORTANT NOTES:
- The following errors were reported by the server:
Domain: api.pessom.ru
Type: connection
Detail: Fetching
http://api.pessom.ru/.well-known/acme-challenge/7DFxVLj7oCnJGLbiCpTlI_UHA2fxfyCQFy-tmRzY7gc:
Timeout
To fix these errors, please make sure that your domain name was
entered correctly and the DNS A record(s) for that domain
contain(s) the right IP address. Additionally, please check that
your computer has a publicly routable IP address and that no
firewalls are preventing the server from communicating with the
client. If you're using the webroot plugin, you should also verify
that you are serving files from the webroot path you provided.
80 port is open.
The configuration was not changed, except for updating the system
this is output tcpdump
$ tcpdump -ieth0 -e port 80 -w -
Summary
���yu�|��Y;�JJRTb-p�E<w�@8/"4�H�����P?��i{u�
0)w���Y��JJ��RTb-E<@@����4�HP��M�L�?��q ���
w�0)w��Y�9BBRTb-p�E4w�@8/)4�H�����P?�M�L���_8
0)x
w��Yi:BBRTb-p�E4w�@8/(4�H�����P?�M�L���_7
0)x
w��Y�<����RTb-E�[Z@@B���4�HP��M�L�?�㜡
w�0)x
HTTP/1.0 400 Bad request
Cache-Control: no-cache
Connection: close
Content-Type: text/html
<html><body><h1>400 Bad request</h1>
Your browser sent an invalid request.
</body></html>
)