Configure Linux Server with Multiple NICs
For linux server with multiple nics, it is a common issue that only one of the nic IP is reachable. The issue is due to missing routes of other nics. This article will show how to fix the issue.
Problem description
At first I created a VM from Ubuntu 24.04 cloud image on two networks. The VM has two nics: ens3 and ens4. By default, only ens3 is enabled in netplan. After enabling ens4 in netplan config and applying netplan configuration, the VM has two assigned IPs.
Now ping the two IPs from another server which can access both networks.
Only ping to the first IP worked. The IP on ens4 is not pingable.
Root analysis
Use tcpdump to monitor the packets, it shows the VM has received the ICMP request and sent the ICMP reply. However, it does not reach the source host.
The test vm has the default route via ens3 but no default route for ens4.
Solve the problem
iproute2 provides ip-rule tool to manage complicated routing policies.
echo "100 Table_ens3" >> /etc/iproute2/rt_tables
echo "101 Table_ens4" >> /etc/iproute2/rt_tables
ip rule add from 192.168.4.0/24 table Table_ens3
ip rule add from 192.168.8.0/24 table Table_ens4
ip route add default via 192.168.4.1 dev ens3 table Table_ens3
ip route add default via 192.168.8.1 dev ens4 table Table_ens4
After applying the above changes, the source server is able to ping IPs of both nics.
References
- [1] http://linux-ip.net/html/tools-ip-rule.html