Why does traceroute not report complete route information when there is no ARP table entry for the gateway?
Issue
- traceroute can not report complete route information when there is no ARP table entry for the gateway. If "traceroute" command is run while the ARP table entry for a gateway is not yet created after the network interface is activated, the reply line from the gateway is missing in the output.
# traceroute 192.168.100.111
traceroute to 192.168.100.111 (192.168.100.111), 30 hops max, 40 byte
packets
1 (192.168.100.111) 3.156 ms 3.158 ms 3.159 ms
<This result shows as if the node communicated with 192.168.100.111
directly.>
- After the ARP entry for the gateway is created, "traceroute" reports complete information as follows.
# traceroute 192.168.100.111
traceroute to 192.168.100.111 (192.168.100.111), 30 hops max, 40 byte
packets
1 (192.168.10.129) 0.340 ms 0.388 ms 0.498 ms
2 (192.168.100.111) 1.991 ms 3.582 ms 4.278 ms
<The result shows 192.168.10.129 is on the path to 192.168.100.111 and
is relaying the packets.>
- Before and after the creation of the ARP entry for the gateway, there is the difference of behavior. That's found by monitoring tcpdump's ethernet packets. traceroute sends out a probe packet;
- with TTL = 5 from the first time as follows *before* the creation.
16:09:21.219427 IP (tos 0x0, ttl 5, id 19641, offset 0, flags [none],
proto 17, length: 68) 10.186.174.44.60114 > 10.185.132.18.33447: UDP,
length 40- with TTL = 1 at first as follows *after* the creation.
16:09:31.250217 IP (tos 0x0, ttl 1, id 19645, offset 0, flags [none],
proto 17, length: 68) 10.186.174.44.46578 > 10.185.132.18.33435: UDP,
length 40
- Is there something wrong with traceroute package?
Environment
- Red Hat Enterprise Linux 5 Update 3 to 5
- Architecture:x86_64
- Kernel Version:kernel-2.6.18-128.1.6.el5-x86_64
- Related Package Version:traceroute-2.0.1-5.el5-x86_64
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