The tcpdump program (tcpdump -p -i ppp1 -s 1500 not port login) will
not report any packets after adding a default route to eth0 in the setup
below. The packet generating command is ping 192.168.2.42
It has been verified at the ppp1 peer that packets really arrive there via
ppp. Tcpdumping eth0 locally also confirms that packets don't go via eth0.
After adding/deleting the default route tcpdump needs to be restarted
to notice the difference. It uses the af_packet kernel module and maybe
its usage count must drop to zero temporarily to make a difference (I
don't know).
iapetus ~# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.2.42 * 255.255.255.255 UH 0 0 0 ppp1
192.168.0.39 * 255.255.255.255 UH 0 0 0 eth0
192.168.2.0 * 255.255.255.0 U 0 0 0 eth0
localnet * 255.255.255.0 U 0 0 0 eth0
loopback * 255.0.0.0 U 0 0 0 lo
iapetus ~# ifconfig
eth0 Link encap:Ethernet HWaddr 00:50:04:21:99:04
inet addr:192.168.0.39 Bcast:192.168.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:34216 errors:0 dropped:0 overruns:0 frame:0
TX packets:52668 errors:0 dropped:0 overruns:0 carrier:5
collisions:117 txqueuelen:100
Interrupt:10 Base address:0x300
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16144 Metric:1
RX packets:16037 errors:0 dropped:0 overruns:0 frame:0
TX packets:16037 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
ppp1 Link encap:Point-to-Point Protocol
inet addr:192.168.2.201 P-t-P:192.168.2.42 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:331 errors:0 dropped:0 overruns:0 frame:0
TX packets:330 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
Adding a default route:
iapetus ~# route add default dev eth0
iapetus ~# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.2.42 * 255.255.255.255 UH 0 0 0 ppp1
192.168.0.39 * 255.255.255.255 UH 0 0 0 eth0
192.168.2.0 * 255.255.255.0 U 0 0 0 eth0
localnet * 255.255.255.0 U 0 0 0 eth0
loopback * 255.0.0.0 U 0 0 0 lo
default * 0.0.0.0 U 0 0 0 eth0
--
Frank
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