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/ttyUSB4
root@ms:~#
root@ms:~# cat /dev/ttyUSB2 &
[1] 1292
root@ms:~# echo -en "AT\r\n" > /dev/ttyUSB2
root@ms:~# AT
OK
AT
OK
root@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 down
root@ms:~#
root@ms:~# ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:4 errors:0 dropped:0 overruns:0 frame:0
TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX 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 10.64.64.64
route: resolving defatult
root@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] 1288
root@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 seconds
send (AT^M)
expect (OK)
AT^M^M
OK
-- got it
send (^MATZ^M)
expect (OK)
^M
ATZ^M^M
OK
-- got it
send (^MAT+CGDCONT=1,"IP","3gnet",,0,0^M)
expect (OK)
^M
AT+CGDCONT=1,"IP","3gnet",,0,0^M^M
OK
-- got it
send (ATDT*99#^M)
expect (CONNECT)
^M
ATDT*99#^M^M
CONNECT
-- got it
send (\d)
Script /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-connect finished (pid 1289), status = 0x0
Serial connection established.
using channel 1
Using interface ppp0
Connect: ppp0 <--> /dev/ttyUSB3
Warning - secret file /etc/ppp/pap-secrets has world and/or group access
sent [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 possible
sent [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 10.64.64.64
local IP address 10.221.137.224
remote IP address 10.64.64.64
primary DNS address 61.156.60.66
secondary DNS address 202.102.128.68
Script /etc/ppp/ip-up started (pid 1295)
Script /etc/ppp/ip-up finished (pid 1295), status = 0x0
root@ms:~#
4、检查IP/DNS/Route。
root@ms:~# ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:10.221.137.224 P-t-P:10.64.64.64 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:4 errors:0 dropped:0 overruns:0 frame:0
TX packets:4 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:52 (52.0 B) TX bytes:58 (58.0 B)
root@ms:~#
root@ms:~# cat /etc/resolv.conf
meserver 61.132.163.68
nameserver 202.102.213.68
root@ms:~#
root@ms:~# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 0.0.0.0 0.0.0.0 U 0 0 0 ppp0
10.64.64.64 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
root@ms:~#
root@ms:~# ping www.baidu.com
PING www.baidu.com (14.215.177.38): 56 data bytes
64 bytes from 14.215.177.38: seq=0 ttl=50 time=55.281 ms
64 bytes from 14.215.177.38: seq=1 ttl=50 time=58.692 ms
64 bytes from 14.215.177.38: seq=2 ttl=50 time=60.334 ms
^C
--- www.baidu.com ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 55.281/58.102/60.334 ms
root@ms:~#
5、关闭PPP call。
root@ms:~# killall pppd
Terminating on signal 15
Connect 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 bay
Script /usr/sbin/chat -s -v -f /etc/ppp/peers/wcdma-chat-disconnect finished (pid 1307), status = 0x0
Serial link disconnected.
Script /etc/ppp/ip-down finished (pid 1305), status = 0x0
[1]+ Done(5) pppd call wcdma
root@ms:~#
6、查看网络节点。
root@ms:~# ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX 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 www.baidu.com。
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-up
dns_file="/etc/resolv.conf"
rm "$dns_file"
ln /etc/ppp/resolv.conf "$dns_file"
ip-down脚本
#!/bin/bash
#ip-down
#set -vx
dns_file="/etc/resolv.conf"
rm $dns_file
cat > "$dns_file" <<EOF
# auto create by ip-down
nameserver 114.114.114.114
nameserver 8.8.8.8
EOF
chmod 755 "$dns_file"
/etc/init.d/networking restart
echo "Set dns for eth0"
5、设置了断线重拨（lcp-echo-failure、lcp-echo-interval）。
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