Linux – Cannot ping through one of the network interfaces

icmplinuxpingroutetcp

Setup

ubuntu linux VM ( virtual-box) is configured with two interfaces, eth0 and eth1.

eth0 is on a bridged network and directly connected to external network.

eth1 is on "nat network", which is also connected to external network.

Problem

Unable to ping via eth0. Able to connect to the host via TCP.

ping -I eth0  -c2 google.com
PING google.com (172.217.1.238) from 10.254.185.16 eth0: 56(84) bytes of data.
From company.com (10.254.185.16) icmp_seq=1 Destination Host Unreachable
From company.com (10.254.185.16) icmp_seq=2 Destination Host Unreachable

--- google.com ping statistics ---
2 packets transmitted, 0 received, +2 errors, 100% packet loss, time 1008ms
pipe 2


telnet -b 10.254.185.16 google.com 80
Trying 172.217.1.238...
Connected to google.com.
Escape character is '^]'.

ping works fine via eth1, which is the default route.

ping -I eth1  -c2 google.com
PING google.com (172.217.1.238) from 10.0.2.4 eth1: 56(84) bytes of data.
64 bytes from lax17s02-in-f14.1e100.net (172.217.1.238): icmp_seq=1 ttl=49 time=11.5 ms
64 bytes from lax17s02-in-f14.1e100.net (172.217.1.238): icmp_seq=2 ttl=49 time=11.3 ms

--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1000ms
rtt min/avg/max/mdev = 11.310/11.446/11.582/0.136 ms

Details

default route is through eth1.

route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         10.0.2.1        0.0.0.0         UG    0      0        0 eth1
10.0.2.0        *               255.255.255.0   U     0      0        0 eth1
10.254.184.0    *               255.255.248.0   U     0      0        0 eth0
192.168.122.0   *               255.255.255.0   U     0      0        0 virbr0


ip route
default via 10.0.2.1 dev eth1
10.0.2.0/24 dev eth1  proto kernel  scope link  src 10.0.2.4
10.254.184.0/21 dev eth0  proto kernel  scope link  src 10.254.185.16
192.168.122.0/24 dev virbr0  proto kernel  scope link  src 192.168.122.1

eth0 is configured through another routing table

ip route show table eth0
default via 10.254.184.1 dev eth0
10.254.184.0/21 dev eth0  scope link  src 10.254.185.16


ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 08:00:27:6f:a1:e6
inet addr:10.254.185.16  Bcast:10.254.191.255     Mask:255.255.248.0
inet6 addr: fe80::a00:27ff:fe6f:a1e6/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:2123 errors:0 dropped:0 overruns:0 frame:0
TX packets:1280 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:237141 (237.1 KB)  TX bytes:225214 (225.2 KB)


iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     udp  --  anywhere             anywhere             udp dpt:domain
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:domain
ACCEPT     udp  --  anywhere             anywhere             udp dpt:bootps
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:bootps

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             192.168.122.0/24     ctstate RELATED,ESTABLISHED
ACCEPT     all  --  192.168.122.0/24     anywhere
ACCEPT     all  --  anywhere             anywhere
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable
REJECT     all  --  anywhere             anywhere             reject-with icmp-port-unreachable

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     udp  --  anywhere             anywhere             udp dpt:bootpc

IP Rule 

ip rule
0:  from all lookup local
32763:  from all to 10.246.240.0/20 lookup eth0
32764:  from 10.246.240.0/20 lookup eth0
32765:  from 10.246.242.68 lookup eth0
32766:  from all lookup main
32767:  from all lookup default

Traceroute output

traceroute -T r2d2.company.com
traceroute to r2d2.company.com (10.254.194.217), 30 hops max, 60 byte packets
1  nambi-ubuntu-dell-t5600.company.com (10.254.194.217)  13.181 ms  13.164 ms  13.142 ms


traceroute -I r2d2.company.com
traceroute to r2d2.company.com (10.246.20.141), 30 hops max, 60 byte packets
1  10.0.2.1 (10.0.2.1)  0.178 ms  0.139 ms  0.137 ms
2  * * *
3  te1-30-sjl1-2-cc01.company.com (10.246.100.81)  3.193 ms  3.174 ms  3.520 ms
4  te1-2-sjl2-2-cd02.companyy.com (10.246.100.62)  2.771 ms  2.853 ms  2.835 ms
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Best Answer

Instead of simple ping, use traceroute -I. You can also use iptables to log your pings and your telnet packets with outgoing interfaces.

My recollections of bizarre configurations like yours are a bit hazy, but you will very probably find that even though you specify a source IP, that does not mean that your packets will leave through that interface! I'd say your ping packets do but your telnet packets don't, and something is PATting and reverse-routing them correctly. You say that eth0 has a different routing table, and you show the table, but I don't see the rule that sends packets to that table. Execute ip rule list to see those rules.

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