我得到这个：
atul@ubuntu:~$ sudo apt-get install traceroute
[sudo] password for atul:
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package traceroute is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'traceroute' has no installation candidate
谁能告诉我如何解决这个问题？