Debugging Linux pppd / PPTP VPN Problems

I've already written about setting up PPTP VPN client for proprietary Microsoft Point-to-Point vpn server using Linux pptp client. Learn how to troubleshoot and resolve common PPTP network connection problems.

You need to run pppd in debug mode by passing debug option. The debug option enables connection debugging facilities. If this option is given, pppd will log the contents of all control packets sent or received in a readable form. The packets are logged through syslog with facility daemon and level debug.

Step # 1: Find out location of your debug file, enter:
# grep debug /etc/syslog.conf
Sample output:

*.=debug;\
        news.none;mail.none     -/var/log/debug
#       *.=debug;*.=info;\
        *.=debug;*.=info;\

/var/log/debug is your debug file.
Step # 2: Start pppd with debug option, enter:
# pppd debug call office.vpn
To see debug log, enter:
# tail -f /var/log/debug
Sample output:

Jul 30 16:10:56 vivek-desktop pppd[30951]: using channel 28
Jul 30 16:10:57 vivek-desktop pppd[30951]: sent [LCP ConfReq id=0x1    ]
Jul 30 16:10:58 vivek-desktop pppd[30951]: rcvd [LCP ConfReq id=0x89     ]
Jul 30 16:10:58 vivek-desktop pppd[30951]: sent [LCP ConfAck id=0x89     ]
Jul 30 16:10:58 vivek-desktop pppd[30951]: rcvd [LCP ConfAck id=0x1    ]
Jul 30 16:10:58 vivek-desktop pppd[30951]: sent [LCP EchoReq id=0x0 magic=0xa3123563]
Jul 30 16:10:58 vivek-desktop pppd[30951]: rcvd [CHAP Challenge id=0x1 , name = ""]
Jul 30 16:10:58 vivek-desktop pppd[30951]: sent [CHAP Response id=0x1 <23962ce1340a0315661377969b543b870000000000000000f69db90c5ba79e5207ac61b073af5d732fbd6a605a7740d000>, name = "vivekgite"]
Jul 30 16:10:58 vivek-desktop pppd[30951]: rcvd [LCP EchoRep id=0x0 magic=0x61a31410]
Jul 30 16:11:07 vivek-desktop pppd[30951]: rcvd [CHAP Failure id=0x1 "E=691 R=0 M=Login incorrect"]
Jul 30 16:11:07 vivek-desktop pppd[30951]: sent [LCP TermReq id=0x2 "Failed to authenticate ourselves to peer"]
Jul 30 16:11:07 vivek-desktop pppd[30951]: rcvd [LCP TermReq id=0x8a]
Jul 30 16:11:07 vivek-desktop pppd[30951]: sent [LCP TermAck id=0x8a]
Jul 30 16:11:07 vivek-desktop pppd[30951]: rcvd [LCP TermAck id=0x8b]
Jul 30 16:11:07 vivek-desktop pppd[30951]: Script pptp offcie.vpn  --nolaunchpppd finished (pid 30952), status = 0x0

You can see my password or username failed (vpn authentication problem) to authenticate itself against the VPN server. Without debug message I won't able to solve any problem. You can fix chap authentication by editing /etc/ppp/chap-secrets file. See setting up PPTP VPN client tutorial for other debugging techniques.