Which file on a Linux system maps service names such as ssh, smtp and http to their port numbers and transport protocols?
LPIC-1 Exam 102-500, objective 109. Networking fundamentals easy
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The options
Not correct /etc/protocols
Wrong. /etc/protocols maps IP protocol names to protocol numbers (icmp 1, tcp 6, udp 17). It has nothing to do with port numbers.
Not correct /etc/nsswitch.conf
Wrong. nsswitch.conf decides which databases are consulted for lookups (including the `services` database) and in what order, but it stores no name-to-port mappings itself.
Correct /etc/services
Correct. /etc/services holds lines such as `ssh 22/tcp` and `domain 53/udp`, which let tools print or accept service names instead of numbers.
Not correct /etc/hosts
Wrong. /etc/hosts is the static mapping of IP addresses to hostnames and aliases, consulted before or after DNS depending on nsswitch.conf.
Why
/etc/services is why `ss -t` prints `ssh` instead of `22` and why `nc host smtp` works. Suppress the lookup with the numeric flag most tools offer (`ss -n`, `netstat -n`). You can query the database portably with `getent services ssh`.
Where this comes from
- Cited
- LPI exam objective 109.1
- What it says
- Know the purpose of /etc/services.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 109 and space the ones you get wrong.