VPN PPTP - GRE: Bad checksum from pppd
I just set up a PPTP server on Debian 7 and SOMETIMES I got a problem during the authentication. So I've checked the syslog and here's a short dump :
Mar 26 22:24:21 vps52953 pptpd[26709]: CTRL: Client 80.25.192.102 control connection started
Mar 26 22:24:21 vps52953 pptpd[26709]: CTRL: Starting call (launching pppd, opening GRE)
Mar 26 22:24:21 vps52953 pppd[26710]: Plugin /usr/lib/pptpd/pptpd-logwtmp.so loaded.
Mar 26 22:24:21 vps52953 pppd[26710]: pppd 2.4.5 started by root, uid 0
Mar 26 22:24:21 vps52953 pppd[26710]: Using interface ppp2
Mar 26 22:24:21 vps52953 pppd[26710]: Connect: ppp2 <--> /dev/pts/3
Mar 26 22:24:21 vps52953 pptpd[26709]: GRE: Bad checksum from pppd.
Mar 26 22:24:21 vps52953 pptpd[26709]: GRE: read(fd=7,buffer=7fe23141c460,len=8260) from network failed: status = -1 error = Protocol not available
Mar 26 22:24:21 vps52953 pptpd[26709]: CTRL: GRE read or PTY write failed (gre,pty)=(7,6)
Mar 26 22:24:21 vps52953 pptpd[26709]: CTRL: Reaping child PPP[26710]
Mar 26 22:24:21 vps52953 pppd[26710]: Hangup (SIGHUP)
Mar 26 22:24:21 vps52953 pppd[26710]: Modem hangup
Mar 26 22:24:21 vps52953 pppd[26710]: Connection terminated.
Mar 26 22:24:21 vps52953 pppd[26710]: Exit.
Mar 26 22:24:21 vps52953 pptpd[26709]: CTRL: Client 80.25.192.102 control connection finished
The problem seems appear only with my Android device (Nexus 5). If anyone could help me to solve this strange error, it should be great !
Thanks !
Answer
The problem may simply be that the GRE packets are not getting through.
To resolve this issue, make sure the network firewall/3G network permits GRE protocol 47 (You will want to find and enable a "PPTP passthrough" feature on your router that knows how to handle the GRE packets). Also, make sure that the network firewall permits TCP traffic on port 1723. Both of these conditions must be met to establish VPN connectivity by using PPTP. If you are using Arno's IPTables Firewall, enable the plugin for PPTP then.
Hope it could solve the problem.