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I am trying to set up a system where a single server has 2 VLANs and 2 corresponding network interfaces. I want to be able to bind services to a specific interface so it only receives traffic that comes from that IP. This is the current setup:

(ifconfig)
  enp28s0f0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9000
        inet A.B.1.171  netmask 255.255.255.0  broadcast A.B.1.255
        inet6 [.....]  prefixlen 64  scopeid 0x20<link>
        ether [.....]  txqueuelen 1000  (Ethernet)
        RX packets 37262  bytes 6954634 (6.6 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 28457  bytes 5529921 (5.2 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

  enp28s0f1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9000
        inet C.D.1.112  netmask 255.255.255.0  broadcast C.D.1.255
        inet6 [.....]  prefixlen 64  scopeid 0x20<link>
        ether [.....]  txqueuelen 1000  (Ethernet)
        RX packets 2314778  bytes 212155740 (202.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 5446  bytes 604308 (590.1 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

  lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 6600  bytes 420545 (410.6 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 6600  bytes 420545 (410.6 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

(netstat -rn)
  Kernel IP routing table
  Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
  0.0.0.0         A.B.1.1      0.0.0.0         UG        0 0          0 enp28s0f0
  0.0.0.0         C.D.1.1      0.0.0.0         UG        0 0          0 enp28s0f1
  A.B.1.0      0.0.0.0         255.255.255.0   U         0 0          0 enp28s0f0
  C.D.1.0      0.0.0.0         255.255.255.0   U         0 0          0 enp28s0f1

(ip route)
  default via A.B.1.1 dev enp28s0f0 proto static metric 100 
  default via C.D.1.1 dev enp28s0f1 proto static metric 101 
  A.B.1.0/24 dev enp28s0f0 proto kernel scope link src A.B.1.171 metric 100 
  C.D.1.0/24 dev enp28s0f1 proto kernel scope link src C.D.1.112 metric 101

(I have replaced sensitive info with A,B,C,D)

Currently all inbound traffic is being routed through enp28s0f0. I am able to bind service A to the enp28s0f0 interface IP A.B.1.171 and received traffic it normally. When making a request to service A via the enp28s0f1 interface IP C.D.1.112 it does not go to Service A as expected.

However when I bind service B to the enp28s0f1 interface IP I am not able to receive any traffic at all (I tried requests to both A.B.1.171 and C.D.1.112). My suspicion is that it has something to do with the same gateway 0.0.0.0 being used for both interface subnets, but I'm not completely sure.

Also possibly useful to note that I am currently only using 1 routing table - the default one - this may have an impact but I'm not sure.

The server is running RHEL 7.9. Any help/guidance is appreciated!

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  • 0.0.0.0 in "Gateway" column means that there is no gateway for this particular route, the subnet is directly connected to the interface. What bothers me more is both A.B.1.1 and C.D.1.1 used as default gateway. Having two different default routes is asking for trouble. Create routes to particular networks available via these interfaces, not two default routes.
    – raj
    May 13, 2021 at 16:51
  • Sorry, can you elaborate a bit on the last part "create routes to particular networks...". I'm new to linux networking so not really sure how to approach that. May 13, 2021 at 17:33
  • You can't have two interfaces that are intended to have traffic from anywhere (ie. default route, 0.0.0.0 in the "Destination" field). Only one interface can have a default route. The other interface should have route(s) to specific network(s), ie. for example if the interface A.B.1.171 is intended to connect to particular network X.Y.0.0/16, then you should remove a default route via A.B.1.1 and create a specific route to that network X.Y.0.0/16 instead.
    – raj
    May 13, 2021 at 18:18
  • @raj I think (and i could be wrong) your talking about traffic from A.B.1.171 -> outside / C.D.1.112 -> outside -- I'm talking about traffic outside -> A.B.1.171 / outside -> C.D.1.112 (and the outside -> C.D.1.112 is whats not working) May 13, 2021 at 22:56
  • Can you edit your question to add an example fitting your last comment to make it more clear?
    – A.B
    May 14, 2021 at 2:42

1 Answer 1

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From the routing table output that you shared, you have two default gateways and that is a problem. You just need one default gateway and the rest should be specific destination networks.

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