REALHOSTNAME_SA(3)

✨ REALHOSTNAME_SA(3) — the libutil glow-up ✨

okay so bestie, this is a manpage from the LOCAL section of the manual, and it's about a function called realhostname_sa(). it lives in <libutil.h> and honestly? it's giving "i just want to know what host this socket address actually is" energy. let's get into it 💅

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🌸 NAME 🌸

realhostname_sa — converts a struct sockaddr into the real host name, fr fr.

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🎀 LIBRARY 🎀

System Utilities Library, aka libutil. link it with -lutil or it's not gonna slay. periodt.

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🦋 SYNOPSIS 🦋

#include <sys/types.h>
#include <netinet/in.h>
#include <libutil.h>

int
realhostname_sa(char *host, size_t hsize, struct sockaddr *addr,
    int addrlen);

like, that's the whole vibe. include the headers, call the function. easy.

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💖 DESCRIPTION 💖

so, realhostname_sa() takes addr and turns it into the corresponding host name. but it's not just gonna hand you any old name — it does a double check 😤. it resolves addr to a host name, and then it makes sure that host name resolves back to addr. no cap, it's verifying the relationship both ways. it's not gonna let a fake name slide through.

here's the tea on host and hsize: host must point to a buffer of at least hsize bytes. and heads up bestie — this function is always going to write to host, no exceptions. so don't pass in something const or you're gonna have a bad time.

now for the plot twist: if the name resolution doesn't work both ways, or if the host name is literally longer than hsize bytes, then it falls back to getnameinfo(3) with NI_NUMERICHOST specified, and converts addr into an ASCII form. basically, if it can't give you a cute name, it gives you the numeric address instead. it's giving "backup plan, but make it aesthetic."

⚠️ bestie, major caveat: if the string written to host is exactly hsize bytes long, then host will not be NUL terminated. like, it's not playing. watch your buffer length or you're lowkey on your own 😭.

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👑 RETURN VALUES 👑

realhostname_sa() returns one of these constants, all defined in <libutil.h>. here's the vibe check on each:

• HOSTNAME_FOUND — slay! a valid host name was found. iconic.
• HOSTNAME_INCORRECTNAME — a host name was found, but it didn't resolve back to the passed ip. rude. in this case, host now contains the numeric value of ip.
• HOSTNAME_INVALIDADDR — the ip could not be resolved at all. also, host now contains the numeric value of ip.
• HOSTNAME_INVALIDNAME — a host name was found, but it couldn't be resolved back to any ip number. so, host now contains the numeric value of ip. it's giving "we tried, but the math didn't math."

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🌈 SEE ALSO 🌈

getaddrinfo(3), getnameinfo(3), realhostname(3)

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and that's the whole page, bestie! 🫶 this one came from the Debian manual, dated January 11, 2000 — like, a whole 💫 vintage icon 💫 — and it's the REALHOSTNAME_SA(3) page from the LOCAL section. go forth and resolve those hostnames responsibly, okay? mwah 💋