Linux系统双网卡（PPP+以太网）流量路由配置问题求助

系统环境 #

root@fl-imx6ull:~# uname -a
Linux fl-imx6ull 4.1.15 #1 SMP PREEMPT Tue Apr 8 08:26:30 MSK 2025 armv7l armv7l armv7l GNU/Linux

网络环境与需求 #

• 双网卡：SIMCOM A7682E调制解调器（PPP连接，设备名ppp0）、以太网（设备名eth0）
• 核心需求：
  • eth0仅用于访问192.168.3.0/24本地子网
  • ppp0作为默认路由，承载所有外网流量

当前问题现象 #

PPP单独连接时可正常访问外网；连接eth0后，路由表出现default dev eth0 scope link条目，导致外网流量被强制走eth0，最终访问失败：

root@fl-imx6ull:~# ping google.com
PING google.com (173.194.221.100) 56(84) bytes of data.
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=1 ttl=104 time=94.4 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=2 ttl=104 time=86.5 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=3 ttl=104 time=84.8 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=4 ttl=104 time=83.7 ms
fec 2188000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx
IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=21 ttl=104 time=78.6 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=22 ttl=104 time=84.8 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=23 ttl=104 time=75.7 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=24 ttl=104 time=83.6 ms
64 bytes from lm-in-f100.1e100.net (173.194.221.100): icmp_seq=25 ttl=104 time=73.4 ms
From fl-imx6ull.local (192.168.3.200) icmp_seq=61 Destination Host Unreachable
From fl-imx6ull.local (192.168.3.200) icmp_seq=62 Destination Host Unreachable

当前路由表状态：

root@fl-imx6ull:~# ip route
default dev eth0  scope link
default via 192.168.3.1 dev eth0
default dev ppp0  scope link
10.64.64.64 dev ppp0  proto kernel  scope link  src 10.184.51.81
169.254.0.0/16 dev eth0  proto kernel  scope link  src 169.254.220.182
192.168.3.0/24 dev eth0  proto kernel  scope link  src 192.168.3.200

现有配置 #

PPP配置文件（/etc/ppp/peers/simcom-pppd） #

# named simcom-pppd and place in /etc/ppp/peers
/dev/ttymxc2 115200
#Insert the username and password for authentication, default user and password are test
user "megafon" password "megafon"
# The chat script, customize your APN in this file
connect 'chat -s -v -f /etc/ppp/peers/simcom-connect-chat'
# The close script
disconnect 'chat -s -v -f /etc/ppp/peers/simcom-disconnect-chat'
# Hide password in debug messages
hide-password
# The phone is not required to authenticate
noauth
# Debug info from pppd
#debug
# If you want to use the HSDPA link as your gateway
defaultroute
# pppd must not propose any IP address to the peer
noipdefault
# No ppp compression
novj
novjccomp
noccp
ipcp-accept-local
ipcp-accept-remote
local
# For sanity, keep a lock on the serial line
lock
modem
dump
nodetach
# Hardware flow control
nocrtscts
remotename 3gppp
ipparam 3gppp
ipcp-max-failure 30
# Ask the peer for up to 2 DNS server addresses
usepeerdns

启动命令：pppd call simcom-pppd &

以太网配置文件（/etc/network/interfaces） #

# Wired or wireless interfaces
allow-hotplug eth0
auto eth0
iface eth0 inet static
    address 192.168.3.200
        netmask 255.255.255.0
    up route add default gw 192.168.3.1 ppp0
    down route del default gw 192.168.3.1 ppp0
        gateway 192.168.3.1

启动命令：ifup eth0

修复步骤 #

1. 修正以太网配置 #

编辑/etc/network/interfaces，删除多余的gateway配置和错误的up/down路由命令，仅保留静态地址与子网配置：

# Wired or wireless interfaces
allow-hotplug eth0
auto eth0
iface eth0 inet static
    address 192.168.3.200
    netmask 255.255.255.0

说明：去掉gateway后，eth0只会生成192.168.3.0/24的子网路由，不会添加默认路由，避免抢占ppp0的外网流量。

2. 修正PPP配置 #

编辑/etc/ppp/peers/simcom-pppd，将defaultroute替换为replacedefaultroute：

# 替换前
#defaultroute
# 替换后
replacedefaultroute

说明：replacedefaultroute会让pppd在连接时替换所有现有默认路由，确保ppp0的默认路由优先级最高。

3. 清理当前错误路由 #

执行命令删除eth0相关的默认路由条目：

ip route del default dev eth0
ip route del default via 192.168.3.1 dev eth0

4. 重启网络服务 #

# 重启PPP连接
pkill pppd
pppd call simcom-pppd &

# 重启以太网
ifdown eth0 && ifup eth0

验证修复 #

查看路由表，确认仅保留ppp0的默认路由和eth0的子网路由：

ip route

预期输出示例：

default dev ppp0  scope link
10.64.64.64 dev ppp0  proto kernel  scope link  src 10.184.51.81
192.168.3.0/24 dev eth0  proto kernel  scope link  src 192.168.3.200

此时分别测试ping外网（如google.com）和本地子网（如192.168.3.1），确认流量走向符合需求。

内容的提问来源于stack exchange，提问作者Дмитрий Стеняев