Why Your Linux Router Won’t Forward Packets and How to Fix It
This article walks through a Linux routing experiment where misconfigured routes caused ARP failures, explains the root cause, and provides the correct routing commands and packet‑capture analysis that enable successful ping between two hosts through a Linux router.
Recently I performed a network experiment that required Linux packet forwarding; after encountering several issues and consulting Linux networking experts, I resolved them and documented the process for future reference.
Network Topology
Three machines are used: host ①, router ④, and host ⑦. Host ① pings host ⑦ with host ④ acting as the router.
Incorrect Routing Configuration
Initially the following commands were applied to configure the routing tables on the three machines: route add -net 10.0.4.0/24 dev eth1 On host ① the routing table displayed:
On host ⑦ the routing table displayed:
After enabling IP forwarding on the router (setting net.ipv4.ip_forward=1 in /etc/sysctl.conf) the ping from host ① to host ④ fails:
PING 10.0.4.3 (10.0.4.3) 56(84) bytes of data.
From 10.0.1.3 icmp_seq=2 Destination Host Unreachable
...Packet capture on router ④ shows host ① repeatedly sending ARP requests for 10.0.4.3, but the router does not forward ARP packets, so host ① never learns the MAC address of host ⑦.
Correct Configuration
Applying the proper route on host ① resolves the issue: route add -net 10.0.4.0/24 gw 10.0.1.2 The updated routing table on host ① becomes:
[root@h1 ~]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.0.4.0 10.0.1.2 255.255.255.0 UG 0 0 0 eth1
...Similarly, host ⑦’s routing table is configured to use the router as gateway.
After these changes, ping from host ① to host ⑦ succeeds. MAC addresses are:
① eth1: 00:16:EC:AF:CB:CB
④ eth1: 40:61:86:32:8F:0B
④ eth4: 40:61:86:32:8F:0E
⑦ eth1: 00:25:90:93:40:79Packet captures on router interfaces (eth1, eth4) show ICMP echo requests and replies traversing correctly.
Conclusion
Linux routers do not forward ARP packets by default; an incorrect static route causes the source host to remain stuck in ARP resolution, preventing communication. Using the correct gateway configuration or enabling ARP proxy on the router resolves the issue.
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