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I use NetworkManager to connect to VPN servers. My VPN has multiple servers, so I configured a dozen or so profiles. NetworkManager allows you to connect to multiple VPNs at once; if I do this, the routing table can end up with multiple default routes.
What does Linux do in this situation?
Is there any way to configure it so that it'll pick faster gateways?
Example (partial table):
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 10.8.8.1 0.0.0.0 UG 50 0 0 tun0
0.0.0.0 10.7.7.1 0.0.0.0 UG 51 0 0 tun1
0.0.0.0 10.8.8.1 0.0.0.0 UG 52 0 0 tun2
0.0.0.0 10.7.7.1 0.0.0.0 UG 53 0 0 tun3
0.0.0.0 10.8.8.1 0.0.0.0 UG 54 0 0 tun4
0.0.0.0 192.168.50.1 0.0.0.0 UG 100 0 0 enp0s31f6
Your extra gateways have different metric (and possibly have different tables, post ip rule show and ip route list table TTT where TTT are all tables mentioned by ip rule show). Metric is documented in man 8 route http://man7.org/linux/man-pages/man8/route.8.html
metric M
set the metric field in the routing table (used by routing
daemons) to M. If this option is not specified the metric for
inet6 (IPv6) address family defaults to '1', for inet (IPv4)
it defaults to '0'. You should always specify an explicit
metric value to not rely on those defaults - they also differ
from iproute2.
Metric The 'distance' to the target (usually counted in hops).
Rules are https://serverfault.com/questions/648276/routing-selection-specificity-vs-metric:
Single routing table... In this case, the kernel chooses:
the most specific route;
if there are multiple equally specific routes, the one with the smallest kernel metric.
So, when there are no multiple routing tables, linux will use only one reachable gateway with lowest metric (it probably will not use gateway from offline network interface).
Is there any way to configure it so that it'll pick faster gateways?
No, when the gateway is not used, you can't check how fast it is. Also you can't afford personal computer to have own AS to legally have several gateways which can connect your single IP to the internet. (So, picking different gateway will break all your TCP and UDP connections, as you will use different IP address in the internet).
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hello family first i show you my topology
topology
I have 4 frame relay switchs.
Frame-relay-switch1switch1
Frame-relay-switch2switch2
Frame-relay-switch3switch3
Frame-relay-switch4switch4
here is the configuration of the routers
router1
no ip address
encapsulation frame-relay
serial restart-delay 0!
interface Serial1/0.1 multipoint
ip address 192.168.1.1 255.255.255.0
snmp trap link-status
frame-relay map ip 192.168.1.3 102 broadcast
frame-relay map ip 192.168.1.2 101 broadcast
frame-relay interface-dlci 101
router2
interface Serial1/0
no ip address
encapsulation frame-relay
serial restart-delay 0!
interface Serial1/0.1 point-to-point
ip address 192.168.1.2 255.255.255.0
snmp trap link-status
frame-relay interface-dlci 103
router 3
interface Serial1/0
no ip address
encapsulation frame-relay
serial restart-delay 0!
interface Serial1/0.1 point-to-point
ip address 192.168.1.3 255.255.255.0
snmp trap link-status
frame-relay interface-dlci 101
the link between router 1 and router 2 is working but between router 1 and router 3 is not working
and i don't know why !!! help me please
hello Tang Li try this topology I hope it will help you
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In Lab we have two hosts say A and B.
host A has two NIC and host B has single NIC (see below diagram for network topology)
NIC-1 of host A is connected to lab network and
NIC-2 of host A is connected directly (end-to-end) to host B
----------
| | -> NIC1 ------> Lab Network (10.203.194.0/24)
| | (eth0 - 10.203.194.247/24)
| host A | ----------
| | -> NIC2 -------> NIC1 | host B |
----------- (eth1 - 10.203.209.230/24) -----------
(eth0 - 10.203.194.163/24)
From host A, I was able to ping 'lab network' whereas I couldn't connect/ping host B (10.203.194.163). I find a routing table of host A has default gw to lab network.
What changes were required to connect Lab network and host B FROM host A.
I can go for a bridge connection connecting both eth0 and eth1 interface in host A; but I don't prefer that option because host B should not recieve any packet specific to lab network and that is the reason I did not connect host B directly to lab network.
I am looking for some routing option that is specific to the destination/source and the interface has to be decided based on that.
Finally, I was able to resolve the issue by simply adding the rule in the route table.
$>route add –host 10.203.194.163 dev eth1
and now my routing table looks like:
Destination Gateway Genmask Flags MSS Windows irtt Iface
10.203.194.163 0.0.0.0 255.255.255.255 UH 0 0 0 eth1
10.203.194.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 10.203.194.1 0.0.0.0 UG 0 0 0 eth0
and when I ping from host B to host A and running tcpdump on eth0, I won't get any ICMP packets and in eth1 I can see all ICMP packets from host B :)
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Could you please explain the following basic ip routing table:
Destination Gateway Genmask Flags Metric Ref Use Iface
10.80.0.0 192.176.20.1 255.255.255.0 UG 0 0 0 eth0
192.176.100.0 * 255.255.255.0 U 0 0 0 int1
10.1.1.0 192.176.20.1 255.255.255.0 UG 0 0 0 eth0
10.13.1.0 192.176.100.1 255.255.255.0 UG 0 0 0 int1
192.176.0.0 * 255.255.0.0 U 1 0 0 eth0
default 192.176.20.2 0.0.0.0 UG 0 0 0 eth0
Well you have 2 local interfaces on the router; eth0 and int1. I guess it's easiest to go through line by line of the routing table.
Destination Gateway Genmask Flags Metric Ref Use Iface
10.80.0.0 192.176.20.1 255.255.255.0 UG 0 0 0 eth0
This states that any packet with a destination of 10.80.0.0/24 will be sent out eth0 using a gateway of 192.176.20.1.
Destination Gateway Genmask Flags Metric Ref Use Iface
192.176.100.0 * 255.255.255.0 U 0 0 0 int1
This states that any packet with a destination of 192.176.100.0/24 will be sent out int1 without using a gateway.
Destination Gateway Genmask Flags Metric Ref Use Iface
10.1.1.0 192.176.20.1 255.255.255.0 UG 0 0 0 eth0
This states that any packet with a destination of 10.1.1.0/24 will be sent out eth0 using a gateway of 192.176.20.1.
Destination Gateway Genmask Flags Metric Ref Use Iface
10.13.1.0 192.176.100.1 255.255.255.0 UG 0 0 0 int1
This states that any packet with a destination of 10.13.1.0/24 will be sent out int1 using a gateway of 192.176.100.1.
Destination Gateway Genmask Flags Metric Ref Use Iface
192.176.0.0 * 255.255.0.0 U 1 0 0 eth0
This states that any packet with a destination of 192.176.0.0/16 will be sent out eth0 without using a gateway.
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.176.20.2 0.0.0.0 UG 0 0 0 eth0
This states that any packet that doesn't match any of the above routes will be forwarded out eth0 with a gateway of 192.176.20.2. Most likely a route to the internet (also called Gateway of Last Resort, if I'm not mistaken).
If you need more information, don't hesitate to ask.
Extra info on the specifics: http://www.cyberciti.biz/faq/what-is-a-routing-table/
Destination = the address of the network that the packet is headed to
The "default" means that if the destination is not found in any of the other rules, than use this rule.
Genmask = The subnet mask
If there is more than one address in the routing table that works for the outgoing packet, the rule with the higher subnet mask will be used. If those are tied, then metric comes into play but that will be different based on what protocol is being used.
Interface = On-board connection
For example, the laptop I am on currently has three interfaces:
Ethernet card
Wireless card
Bluetooth
Routers generally have at least 2 interfaces for each side, 1 for each network they are a part of. For most home routers, 1 interface is part of your home network and the other is part of the external network (usually the internet) headed toward your ISP.
Gateway = Next hop
This is where the packet will be sent if the destination is not on the same network as the sender.
If the value is "*" or "On-link" or the address of the current device... these all mean the same thing. It means that the packet is addressed to a device that is directly reachable by the current host. In other words, they're on the same network so the gateway won't actually be used because the host will know the data link layer (MAC) address of the destination and be able to send it directly there. These values are just used for human readability in this case.
As for the process of sending a packet:
Destination and Genmask are used to figure out which rules are a
match for the destination address of the outgoing packet.
Interface is used to decide what communication device on the machine
to send it through.
Gateway is the address of the device on the same network that it will
send the packet to.
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I have a linux machine Iam using as a router with multible interfaces and multible internet connection
say eth0--isp1
eth1--isp2
eth2--isp3
the gateway is eth0 throw isp1
and eth3--local1 10.0.0.x
eth4--local2 192.168.1.x
i need local1 ip 192.168.1.10 to go throw isp3, eth2
thanks
You need to use policy routing for this. You create a new routing table and use it when the source IP is 192.168.1.10.
ip route add ... table $TABLENUM # your usual routes, for the new table
...
ip route add default via $ISP3 table $TABLENUM # gateway for the new table
ip rule add from 192.168.1.10/32 lookup $TABLENUM # use the new table for this IP
ip rule add to 192.168.1.10/32 lookup $TABLENUM # make it symmetric, for clarity
Try route add 192.168.1.0 netmask 255.255.255.0 dev eth4--local, on the assumption these are locally-connected networks. If not, you will need to specify a gateway machine to route the packets through also. Truthfully your question is malformed (and probably belongs on serverfault). If you already have addresses on those networks, these routes should already exist. If you do not, your problem is likely more complicated than you think it is.
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I have a system with 2 interfaces eth0, and eth1.
eth0 is 192.168.0.250 and connected to gateway 192.168.0.2.
eth1 is connected to 192.123.123.10 via a swtich.
I am trying to route packets from 192.123.123.10 to gateway 192.168.0.2, which means I need to route 192.123.123.x packets coming into eth1 interface out via eth0 interface.
I set ip_forward file to 1.
I ran this command:
route add -net 192.123.0.0 netmask 255.255.255.0 dev eth0
route add default gw 192.168.0.2
I can ping from 129.123.123.10 to 192.168.0.250, but I can't ping to 192.168.0.2
I think the packets are not being forwarded to eth0.
My routing table looks something like this:
gteway Genmask Flags Ref Iface
192.123.123.0 * 255.255.255.0 U eth1
192.168.0.0 * 255.255.255.0 U eth0
192.123.0.0 * 255.255.255.0 U eth0
default 192.168.0.2 0.0.0.0 UG eth0
Can anyone tell me what is missing?
Thank you in advance.
You are missing your back path route.
The host 192.168.0.2 see packet coming from 192.123.123.10 but he doesn't know how to route the reply packet back since it doesn't have the return route.
You can do two things:
1- create a route on 192.168.0.2 machine to handle traffic directed to 192.123.123.0/24
2- NAT on your 192.168.0.250 host with the command below:
iptables -t nat -A POSTROUTING -s 129.123.123.0/24 -j SNAT --to-source 192.168.0.250
It's not your routing table on this system that you need to be concerned about. It's the routing tables of the other systems. 192.168.0.2 knows nothing about the 192.123.X.X network being routed to 192.168.0.250. Similarly the hosts on 192.123.X.X need to route the 192.168.X.X network over to 192.123.123.10.
I'm fairly certain this can be achieved using iptables and port forwarding rules. There is some more information here http://www.revsys.com/writings/quicktips/nat.html about how to forward packets between interfaces.