Why can I ping an IP address but not 'traceroute' it?

Question

I can ping an IP address, but I can't traceroute it. How could this be?

[USERNAME@HOSTNAME ~]$ ping CENSORED.CENSORED
PING CENSORED.CENSORED (CENSORED) 56(84) bytes of data.
64 bytes from CENSORED.CENSORED (CENSORED): icmp_req=1 ttl=49 time=52.8 ms
64 bytes from CENSORED.CENSORED (CENSORED): icmp_req=2 ttl=49 time=49.4 ms
64 bytes from CENSORED.CENSORED (CENSORED): icmp_req=3 ttl=49 time=49.2 ms
64 bytes from CENSORED.CENSORED (CENSORED): icmp_req=4 ttl=49 time=50.4 ms
^C
--- CENSORED.CENSORED ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3003ms
rtt min/avg/max/mdev = 49.276/50.494/52.804/1.401 ms
[USERNAME@HOSTNAME ~]$
[USERNAME@HOSTNAME ~]$ traceroute CENSORED.CENSORED
traceroute to CENSORED.CENSORED (CENSORED), 30 hops max, 60 byte packets
 1  CENSORED (CENSORED)  5.733 ms  6.000 ms  5.977 ms
 2  CENSORED (CENSORED)  0.428 ms  0.417 ms  0.393 ms
 3  CENSORED (CENSORED)  1.726 ms  1.718 ms  1.682 ms
 4  CENSORED (CENSORED)  26.699 ms  26.693 ms  26.670 ms
 5  CENSORED (CENSORED)  27.785 ms  27.769 ms  27.746 ms
 6  * * *
 7  * * *
 8  * * *
 9  * * *
10  * * *
11  * * *
12  * * *
13  * * *
14  * * *
15  * * *
16  * * *
17  * * *
18  * * *
19  * * *
20  * * *
21  * * *
22  * * *
23  * * *
24  * * *
25  * * *
26  * * *
27  * * *
28  * * *
29  * * *
30  * * *
[USERNAME@HOSTNAME ~]$

The fifth CENSORED IP address in the traceroute isn't the same as at the "ping CENSORED.CENSORED".

Answer 1

Try using a different method in your traceroute, for example TCP SYN or ICMP instead of the default UDP method.

For example note the difference between ICMP and TCP:

x@x:~$ ping -qc4 94.254.2.51
PING 94.254.2.51 (94.254.2.51) 56(84) bytes of data.
--- 94.254.3.90 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3009ms
rtt min/avg/max/mdev = 7.781/7.807/7.836/0.067 ms

x@x:~$ sudo traceroute -I 94.254.2.51
traceroute to 94.254.2.51 (94.254.2.51), 30 hops max, 40 byte packets
1  <REDACTED>
2  <REDACTED>
3  <REDACTED>
4  <REDACTED>
5  netnod-ix-ge-a-sth-1500.bahnhof.net (194.68.123.85)  1.307 ms  1.299 ms  1.432 ms
6  sto-cr1.sto-cr3.bahnhof.net (85.24.151.165)  7.166 ms  7.364 ms  7.336 ms
7  sto-cr3.gav-cr1.bahnhof.net (85.24.151.195)  7.251 ms  7.099 ms  7.220 ms
8  zitius-a322-gw-c.bahnhof.net (85.24.153.249)  7.059 ms  7.074 ms  7.145 ms
9  h-2-51.A322.priv.bahnhof.se (94.254.2.51)  7.619 ms  7.750 ms  8.070 ms

x@x:~$ sudo traceroute -T 94.254.2.51
traceroute to 94.254.2.51 (94.254.2.51), 30 hops max, 40 byte packets
1  <REDACTED>
2  <REDACTED>
3  <REDACTED>
4  <REDACTED>
5  netnod-ix-ge-a-sth-1500.bahnhof.net (194.68.123.85)  1.621 ms  1.683 ms  1.817 ms
6  sto-cr1.sto-cr3.bahnhof.net (85.24.151.165)  8.530 ms  7.861 ms  7.820 ms
7  sto-cr3.gav-cr1.bahnhof.net (85.24.151.195)  7.724 ms  7.539 ms  7.486 ms
8  zitius-a322-gw-c.bahnhof.net (85.24.153.249)  7.572 ms  7.537 ms  7.553 ms
9  * * *
10  * * *
11  * * *
12  * * *
13  * * *

Answer 2

Traceroute is based on ICMP or UDP packets. It effectively pings each router on the path between you and censored.censored. It increases the Time-To-Live (TTL) for each subsequent packet it sends (from 1-30 normally) expecting that as each packet is sent with an increased TTL from the last, the next router in the path will return an error code.

If hop 6 isn't responding, it's probably specifically blocking ICMP/UDP messages. Ping therefore works because the routers between you and it are just passing the ICMP/UDP packets through to it rather than responding to them, as they do with a traceroute.

Answer 3

Traceroute relies on ICMP messages, which some routers might be configured to not respond to.

Answer 4

Sometimes it's worth using ping to get traceroute-like information:

#!/bin/bash
for TTL in 1 2 3 4 5 6 7 8 9 10 11 12
do
    ping -c 1 -n -t $TTL a.b.c.d
done

By calling ping with a -t $TTL argument, you can sometimes elude the firewall, and find out IP addresses and so forth of routers behind firewalls.