A border firewall discards the UDP datagrams that Linux traceroute sends by default, so every hop beyond it is shown as asterisks. Which TWO options make traceroute send its probes with a protocol other than UDP? (Choose two.)

LPIC-1 Exam 102-500, objective 109. Networking fundamentals medium

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The options

Choose 2.

Correct -I

Correct. -I sends ICMP echo requests, the same probe type ping uses, which many firewalls permit even where UDP to high ports is blocked.

Correct -T

Correct. -T sends TCP SYN segments, by default to port 80, and combined with -p it can be aimed at a port the firewall is known to allow.

Not correct -U

Wrong. -U selects UDP probes to a fixed destination port, which is the family of probe already being blocked rather than an alternative to it.

Not correct -n

Wrong. -n only suppresses reverse DNS lookups of the responding hops; it changes presentation, not what is transmitted.

Not correct -q

Wrong. -q sets how many probes are sent per hop. More or fewer probes of a blocked protocol are still blocked.

Not correct -m

Wrong. -m raises the maximum TTL, extending the trace's reach in hops. It has no effect on which protocol the probes use.

Why

Linux traceroute defaults to UDP datagrams aimed at unlikely high ports, relying on the destination to answer with an ICMP port-unreachable message; that pattern is exactly what many perimeter filters drop. -I switches to ICMP echo and -T to TCP SYN, and because both require crafting raw packets they need root privileges or the appropriate capability, unlike the default UDP mode. When a trace dies at a firewall, changing the probe protocol is the first diagnostic step, and comparing the three often reveals precisely what the filter permits.

Where this comes from

Cited
manual page traceroute(8)

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