嵌入式Linux开发中，移远4G模组通过ppp拨号上网，命令操作过程如下。
chat：调制解调器的自动对话脚本，Chat程序定义了一个计算机和调制解调器之间对话交流，其主要目的是用来在本地PPPD和远端PPPD程序之间建立连接（简单说就是与4G模块进行AT命令交互的流程表，最终实现与ISP运营商的连接）。
pppd：点对点协议守护进程，其功能为实现ppp策略性的内容，包括所有鉴权、压缩/解压和加密/解密等扩展功能的控制协议。
pppdump：将使用pppd记录选项编写的文件转换为人类可读的格式。
pppstats：显示PPP连线状态。pppstats(point to point protocol status)。
一、Test AT Function。
方法一：
root@ms:~# ls /dev/ttyU*/dev/ttyUSB0 /dev/ttyUSB1 /dev/ttyUSB2 /dev/ttyUSB3 /dev/ttyUSB4root@ms:~#root@ms:~# cat /dev/ttyUSB2 &[1] 1292root@ms:~# echo -en "AT\r\n" > /dev/ttyUSB2root@ms:~# ATOKATOKroot@ms:~#
方法二（本人测试没反应）：
root@ms:~# busybox microcom /dev/ttyUSB2
二、Test PPP Function。
1、关闭其它网络，eth*和wlan*。不关闭，需手动更改默认路由。
root@ms:~# ifconfig eth0 downroot@ms:~#root@ms:~# ifconfiglo Link encap:Local Loopbackinet addr:127.0.0.1 Mask:255.0.0.0inet6 addr: ::1/128 Scope:HostUP LOOPBACK RUNNING MTU:16436 Metric:1RX packets:4 errors:0 dropped:0 overruns:0 frame:0TX packets:4 errors:0 dropped:0 overruns:0 carrier:0collisions:0 txqueuelen:0RX bytes:312 (312.0 B) TX bytes:312 (312.0 B)root@ms:~#
root@ms:~# route add defatult 0.0.0.0 mask 255.255.255.255 10.64.64.64route: resolving defatultroot@ms:~#
2、所需文件。移远EC20、EC25的Modem path为/dev/ttyUSB3。
必须文件：wcdma、wcdma-chat-connect、wcdma-chat-disconnect，目录为/etc/ppp/peers/。
不必须文件：ip-up、ip-down，目录为/etc/ppp。可从Ubuntu系统中拷贝。
/etc/ppp/ip-up、/etc/ppp/ip-down这两个文件分别在ppp链路成功、链接断开时执行一次。可以利用这个去实现pppoe断线自动重连。
3、启动ppp服务。
root@ms:~# pppd call wcdma &[1] 1288root@ms:~# pppd options in effect:debug # (from /etc/ppp/peers/wcdma)nodetach# (from /etc/ppp/peers/wcdma)dump # (from /etc/ppp/peers/wcdma)noauth# (from /etc/ppp/peers/wcdma)user password # (from /etc/ppp/peers/wcdma)remotename 3gppp# (from /etc/ppp/peers/wcdma)/dev/ttyUSB3 # (from /etc/ppp/peers/wcdma)115200# (from /etc/ppp/peers/wcdma)lock # (from /etc/ppp/peers/wcdma)connect /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-connect# (from /etc/ppp/peers/wcdma)disconnect /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-disconnect # (from /etc/ppp/peers/wcdma)crtscts # (from /etc/ppp/peers/wcdma)local # (from /etc/ppp/peers/wcdma)hide-password # (from /etc/ppp/peers/wcdma)novj # (from /etc/ppp/peers/wcdma)novjccomp# (from /etc/ppp/peers/wcdma)ipcp-accept-local# (from /etc/ppp/peers/wcdma)ipcp-accept-remote # (from /etc/ppp/peers/wcdma)ipparam 3gppp # (from /etc/ppp/peers/wcdma)noipdefault # (from /etc/ppp/peers/wcdma)defaultroute # (from /etc/ppp/peers/wcdma)usepeerdns # (from /etc/ppp/peers/wcdma)noccp # (from /etc/ppp/peers/wcdma)abort on (BUSY)abort on (NO CARRIER)abort on (NO DIALTONE)abort on (ERROR)abort on (NO ANSWER)timeout set to 120 secondssend (AT^M)expect (OK)AT^M^MOK-- got itsend (^MATZ^M)expect (OK)^MATZ^M^MOK-- got itsend (^MAT+CGDCONT=1,"IP","3gnet",,0,0^M)expect (OK)^MAT+CGDCONT=1,"IP","3gnet",,0,0^M^MOK-- got itsend (ATDT*99#^M)expect (CONNECT)^MATDT*99#^M^MCONNECT-- got itsend (\d)Script /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-connect finished (pid 1289), status = 0x0Serial connection established.using channel 1Using interface ppp0Connect: ppp0 <--> /dev/ttyUSB3Warning - secret file /etc/ppp/pap-secrets has world and/or group accesssent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x4794f369> <pcomp> <accomp>]rcvd [LCP ConfReq id=0x0 <asyncmap 0x0> <auth chap MD5> <magic 0xac3f292c> <pcomp> <accomp>]No auth is possiblesent [LCP ConfRej id=0x0 <auth chap MD5>]rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x4794f369> <pcomp> <accomp>]rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xac3f292c> <pcomp> <accomp>]sent [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0xac3f292c> <pcomp> <accomp>]sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns2 0.0.0.0>]rcvd [LCP DiscReq id=0x2 magic=0xac3f292c]rcvd [IPCP ConfReq id=0x0]sent [IPCP ConfNak id=0x0 <addr 0.0.0.0>]rcvd [IPCP ConfNak id=0x1 <addr 10.221.137.224> <ms-dns1 61.156.60.66> <ms-dns2 202.102.128.68>]sent [IPCP ConfReq id=0x2 <addr 10.221.137.224> <ms-dns1 61.156.60.66> <ms-dns2 202.102.128.68>]rcvd [IPCP ConfReq id=0x1]sent [IPCP ConfAck id=0x1]rcvd [IPCP ConfAck id=0x2 <addr 10.221.137.224> <ms-dns1 61.156.60.66> <ms-dns2 202.102.128.68>]Could not determine remote IP address: defaulting to 10.64.64.64local IP address 10.221.137.224remote IP address 10.64.64.64primary DNS address 61.156.60.66secondary DNS address 202.102.128.68Script /etc/ppp/ip-up started (pid 1295)Script /etc/ppp/ip-up finished (pid 1295), status = 0x0root@ms:~#
4、检查IP/DNS/Route。
root@ms:~# ifconfig ppp0ppp0Link encap:Point-to-Point Protocolinet addr:10.221.137.224 P-t-P:10.64.64.64 Mask:255.255.255.255UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1RX packets:4 errors:0 dropped:0 overruns:0 frame:0TX packets:4 errors:0 dropped:0 overruns:0 carrier:0collisions:0 txqueuelen:3RX bytes:52 (52.0 B) TX bytes:58 (58.0 B)root@ms:~#root@ms:~# cat /etc/resolv.confmeserver 61.132.163.68nameserver 202.102.213.68root@ms:~#root@ms:~# route -nKernel IP routing tableDestinationGateway Genmask Flags Metric Ref Use Iface0.0.0.0 0.0.0.0 0.0.0.0 U00 0 ppp010.64.64.640.0.0.0 255.255.255.255 UH 00 0 ppp0root@ms:~#root@ms:~# ping PING (14.215.177.38): 56 data bytes64 bytes from 14.215.177.38: seq=0 ttl=50 time=55.281 ms64 bytes from 14.215.177.38: seq=1 ttl=50 time=58.692 ms64 bytes from 14.215.177.38: seq=2 ttl=50 time=60.334 ms^C--- ping statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max = 55.281/58.102/60.334 msroot@ms:~#
5、关闭PPP call。
root@ms:~# killall pppdTerminating on signal 15Connect time 3.0 minutes.Sent 488 bytes, received 971 bytes.Script /etc/ppp/ip-down started (pid 1305)sent [LCP TermReq id=0x2 "User request"]rcvd [LCP TermAck id=0x2]Connection terminated.root@ms:~# abort on (ERROR)abort on (NO DIALTONE)\send (\k"^M)send (+++ATH"^M)Good bayScript /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-disconnect finished (pid 1307), status = 0x0Serial link disconnected.Script /etc/ppp/ip-down finished (pid 1305), status = 0x0[1]+ Done(5) pppd call wcdmaroot@ms:~#
6、查看网络节点。
root@ms:~# ifconfiglo Link encap:Local Loopbackinet addr:127.0.0.1 Mask:255.0.0.0inet6 addr: ::1/128 Scope:HostUP LOOPBACK RUNNING MTU:16436 Metric:1RX packets:0 errors:0 dropped:0 overruns:0 frame:0TX packets:0 errors:0 dropped:0 overruns:0 carrier:0collisions:0 txqueuelen:0RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)root@ms:~#
附：
1、指定网络节点ping命令：ping -I ppp0 。
2、在拨号之前已经启用了以太网eth0，ping广域网失败，原因有两个，一个是路由表中的默认值为eth0生成的，另一个是默认dn服务器地址不能被ppp0识别。
3、默认网关解决如下。
在pppd脚本中使能了defaultroute后，pppd会在拨号成功后向路由表中添加一条默认网关信息，但是因为之前已经有一条默认网关了，于是添加失败，默认网关依然是之前eth设置的，因此在pppd call gprs之前我们应该先route del default来删除默认路由（最好写个脚本来实现删除默认路由–拨号等一系列操作）。
或者将ppp0设置为默认路由。
先删除原来路由，route del default
再将ppp0设置为默认路由，route add default dev ppp0
4、dns问题解决如下。
在pppd脚本中使能了usepeerdns后，pppd会在拨号成功后，在/etc/ppp下生成resolv.conf，这是ISP运营商提供的dns，我们应该将拷贝或者连接到/etc目录下，当然这件事就可以交给ip-up来做。
按照ppp拨号过程中打印信息可以发现，在退出pppd进程时，会调用/etc/ppp/ip-down。因此删除默认网关，恢复dns就交由ip-down脚本做了。
ip-up脚本：
#!/bin/bash#ip-updns_file="/etc/resolv.conf"rm "$dns_file"ln /etc/ppp/resolv.conf "$dns_file"
ip-down脚本
#!/bin/bash#ip-down#set -vxdns_file="/etc/resolv.conf"rm $dns_filecat > "$dns_file" <<EOF# auto create by ip-downnameserver 114.114.114.114nameserver 8.8.8.8EOFchmod 755 "$dns_file"/etc/init.d/networking restartecho "Set dns for eth0"
