AT PPP拨号失败，linux环境下在如何调试？

首先PPP在options文件中可以放开debug & kedebug调试选项

其次Linux内核日记可以通过cat /var/log/syslog.1查看

下面是linux的相关操作步骤

1. cd /etc/ppp
2. ls
3. vi options
4. change #debug to debug

   # Increase debugging level (same as -d).  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. This information can be directed to a file by
   # setting up /etc/syslog.conf appropriately (see syslog.conf(5)).  (If
   # pppd is compiled with extra debugging enabled, it will log messages
   # using facility local2 instead of daemon).
   #debug

   debug

5. change #kdebug  n to kdebug 1

   # Enable debugging code in the kernel-level PPP driver.  The argument n
   # is a number which is the sum of the following values: 1 to enable
   # general debug messages, 2 to request that the contents of received
   # packets be printed, and 4 to request that the contents of transmitted
   # packets be printed.
   #kdebug n

   kdebug

6. 设备拨号
7. cat /var/log/syslog.1 > ppp_debug_history.log

总结：

这次遇到的问题是MRU大小不匹配问题，终端的MRU设置是默认的576（556+20），

但是服务器配置的MRU大小是1500，结果终端发起第一次拨号，MRU不匹配，导致拨号失败。