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I'm running Centos 6.4, and I got pptpd working, for the most part. I am connecting from a Windows 7 machine, and once connected I can access all the intranet sites as well as sites that are already open, or have been opened recently, like google.com. What I can't access is new sites (like when I tried connecting to serverfault), so I think it is related to DNS. I have been able to access a few sites by their IP, but most seem to redirect the IP to the domain name, which then fails. From cmd, I can ping new sites, so it is resolving DNS there. Is the command line resolving DNS through the LAN while Chrome tries to do it through the VPN? I've searched many forums and blogs looking for an answer, but still haven't found anything that fixed it for me. I have tried disabling selinux and also iptables (both non-persistently), just to reduce the number of variables.

Possibly related: these lines keep showing up in /var/log/messages

GRE: read(fd=7,buffer=6097c0,len=8260) from network failed: status = -1 error = Protocol not available
CTRL: GRE read or PTY write failed (gre,pty)=(7,6)

Here are some config files that might be relevant in determining what is wrong:

pptpd.conf

ppp /usr/sbin/pppd
option /etc/ppp/options.pptpd
logwtmp
remoteip 192.168.16.36-49 #these are reserved, not used by dhcp
localip 192.168.16.35

options.pptpd

name pptpd
refuse-pap
refuse-chap
refuse-mschap
require-mschap-v2
require-mppe-128
ms-dns 192.168.16.2 #domain controller, dns server
ms-wins 192.168.16.2
proxyarp
lock
nobsdcomp 
novj
novjccomp
nologfd
auth
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I discovered this problem after changing to a router with a lower default mtu, having problems you describe, but being baffled when small 404 pages and ssh made it through.

I solved it by setting the ppp server mtu lower in /etc/ppp/options by adding:

mtu 1300

Also, apparently windows can be a bit funny about mtu sizes over ppp. Good luck.

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