This just means that there is no UPnP router (expected) and that one of the interfaces syncthing tried to announce itself on doesn’t support multicast / doesn’t have a multicast route.
This is nothing to worry about. If something doesn’t work, there is another cause for it though.
I installed 0.8.15 (32) over SSH on a remote linux VPS (digitalocean). I received a similar error message as the OP:
“No UPnP IGD device found, no port mapping created (read udp4 0.0.0.0:44121: i/o timeout)”
Syncthing made keys, created a node ID, and says it’s “ready to sync default”. However, the WebGUI at xxx.xxx.xxx:8080 will not open remotely. I tried both http:// and https:// but neither worked. What am I missing?
Edit: You can also use an ssh port forward. The easiest way to do this (at least with a Unix ssh client) is when you are already ssh:d to the box. In the ssh connection, hit <enter>~C (yes a tilde then C) and you’ll get a special ssh> prompt. Enter the command -L 8085:127.0.0.1:8080 and then you can reach syncthing on http://localhost:8085/ on your own computer.
If you’re going to make it remotely accessible you should probably set a username and password though… you can do that in the web interface immediately after making it world accessible.
I use a .htaccess file for accessing the GUI over the apache webserver
(also rewriting from http to https)
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteCond %{ENV:HTTPS} !=on
RewriteRule .* https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]
RewriteRule ^(.*) http://localhost:8080/$1 [P] #replace 8080 if you have another port
if you place this in www-root/syncthing you can just use https://example.com/syncthing to get the webinterface and it also uses the certificate of the webserver and no additional port needs to be opened if you have a firewall (for GUI, sync port has nothing to do with this)
I just stumbled across an excellent Syncthing / VPS tutorial that digitalocean published earlier this week. Their technical writers talk in plain English to a largely linuxphobic audience, and they use lots of cut n’ paste terminal commands and graphics to make it all easy.