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Linux list the outgoing interface for packets to a specific IP destination
https://forum.ivorde.com/linux-list-the-outgoing-interface-for-packets-to-a-specific-ip-destination-t14821.html
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Author:  mandrei99 [ Mon Jul 22, 2013 10:17 am ]
Post subject:  Linux list the outgoing interface for packets to a specific IP destination

To be able to get the outgoing interface for traffic to a specific destination, the iproute package needs to be installed:
Code:
# dpkg -S `which ip`
iproute: /sbin/ip


Use "apt-get install iproute" or "yum install iproute" command for Debian/Ubuntu and Centos/RHEL/Fedora (assuming correct repositories are enabled).

Code:
# ip ro get help
Usage: ip route { list | flush } SELECTOR
       ip route get ADDRESS [ from ADDRESS iif STRING ]
                            [ oif STRING ]  [ tos TOS ]
       ip route { add | del | change | append | replace | monitor } ROUTE
SELECTOR := [ root PREFIX ] [ match PREFIX ] [ exact PREFIX ]
            [ table TABLE_ID ] [ proto RTPROTO ]
            [ type TYPE ] [ scope SCOPE ]
ROUTE := NODE_SPEC [ INFO_SPEC ]
NODE_SPEC := [ TYPE ] PREFIX [ tos TOS ]
             [ table TABLE_ID ] [ proto RTPROTO ]
             [ scope SCOPE ] [ metric METRIC ]
             [ mpath MP_ALGO ]
INFO_SPEC := NH OPTIONS FLAGS [ nexthop NH ]...
NH := [ via ADDRESS ] [ dev STRING ] [ weight NUMBER ] NHFLAGS
OPTIONS := FLAGS [ mtu NUMBER ] [ advmss NUMBER ]
           [ rtt NUMBER ] [ rttvar NUMBER ]
           [ window NUMBER] [ cwnd NUMBER ] [ initcwnd NUMBER ]
           [ ssthresh NUMBER ] [ realms REALM ]
TYPE := [ unicast | local | broadcast | multicast | throw |
          unreachable | prohibit | blackhole | nat ]
TABLE_ID := [ local | main | default | all | NUMBER ]
SCOPE := [ host | link | global | NUMBER ]
FLAGS := [ equalize ]
MP_ALGO := { rr | drr | random | wrandom }
NHFLAGS := [ onlink | pervasive ]
RTPROTO := [ kernel | boot | static | NUMBER ]

# ip ro get 10.1.1.1
10.1.1.1 via 172.30.10.1 dev eth0  src 172.30.10.230
    cache  mtu 1500 advmss 1460 hoplimit 64


The above output shows that traffic for destination IP 10.1.1.1 will go out the box via eth0 interface using source 172.30.10.230

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