title: PPTP connection error: GRE: Bad checksum from pppd
body: <p>I have setup a PPTP server on my own server. I follow this tutorial <a href="https://www.digitalocean.com/community/articles/how-to-setup-your-own-vpn-with-pptp">https://www.digitalocean.com/community/articles/how-to-setup-your-own-vpn-with-pptp</a></p>
  
  <p>But when I try to use my mobile phone or my PC to connect my PPTP server, it says PPTP server hang up,username or password is wrong. But I checked my username and password, they are ok. In my PPTP server log, something like this:</p>
  
  <pre><code>Dec 14 03:57:46 localhost pptpd[2071]: CTRL: Client 121.32.107.56 control connection started
  
  
  Dec 14 03:57:46 localhost pptpd[2071]: CTRL: Starting call (launching pppd, opening GRE)
  Dec 14 03:57:46 localhost pppd[2072]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
  Dec 14 03:57:46 localhost pppd[2072]: pppd 2.4.5 started by root, uid 0
  Dec 14 03:57:46 localhost pppd[2072]: Using interface ppp1
  Dec 14 03:57:46 localhost pppd[2072]: Connect: ppp1 &lt;--&gt; /dev/pts/3
  Dec 14 03:57:46 localhost pptpd[2071]: GRE: Bad checksum from pppd.
  Dec 14 03:58:16 localhost pppd[2072]: LCP: timeout sending Config-Requests
  Dec 14 03:58:16 localhost pppd[2072]: Connection terminated.
  Dec 14 03:58:16 localhost pppd[2072]: Modem hangup
  Dec 14 03:58:16 localhost pppd[2072]: Exit.
  Dec 14 03:58:16 localhost pptpd[2071]: GRE: read(fd=6,buffer=804f620,len=8196) from PTY             failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
  Dec 14 03:58:16 localhost pptpd[2071]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
  Dec 14 03:58:16 localhost pptpd[2071]: CTRL: Reaping child PPP[2072]
  Dec 14 03:58:16 localhost pptpd[2071]: CTRL: Client 121.32.107.56 control connection finished
  </code></pre>