getdomainname / setdomainname - glibc

Split from the chapter glibc: System Management into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <unistd.h>

/* unistd.h:928 gate: __USE_MISC */
int getdomainname(char *name, size_t len);        /* unistd.h:930 */
int setdomainname(const char *name, size_t len);  /* unistd.h:933 */

getdomainname.2’s gate history: _DEFAULT_SOURCE since glibc 2.21; _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500) before that – matching the header’s __USE_MISC block today.

Semantics

These get or set the NIS (YP) domain name of the host – more precisely, of the calling process’s UTS namespace (getdomainname.2). This is the yellow-pages/NIS domain, not the DNS domain part of the FQDN; the manual is emphatic that people confuse the two (sysinfo.texi, Host Identification). setdomainname writes the len bytes at name into the kernel’s domainname field and needs CAP_SYS_ADMIN; the 64-byte limit (with NUL) has held since Linux 1.0 (getdomainname.2 NOTES). getdomainname copies out the NUL-terminated name, failing with ENAMETOOLONG when it needs more than len bytes. On most architectures – x86 included – there is no getdomainname syscall: glibc implements it as a library function returning a copy of the domainname field of uname(2) (getdomainname.2 NOTES; sysinfo.texi line 298 says the same); setdomainname is a real syscall wrapper (uname covers the same kernel storage).

Gotchas and portability

• With NIS dead on modern systems the field is usually the empty string; treat it as “whatever the admin wrote”, not as DNS identity.
• The name lives in the same kernel slots that uname’s domainname member reports, and glibc’s struct utsname exposes that member only under __USE_GNU (__domainname otherwise, sys/utsname.h lines 64-70).
• setdomainname succeeds even when NIS is not in use – it is just kernel string storage.
• musl 1.2.5: both present (include/unistd.h lines 171-172).

Errors

EFAULT (bad name), EINVAL (negative len; setdomainname over the 64-byte limit), ENAMETOOLONG (glibc getdomainname truncation), EPERM (setdomainname without CAP_SYS_ADMIN) (getdomainname.2 ERRORS, same shape as gethostname.2).

References

• man2: getdomainname.2 (covers setdomainname), setdomainname.2 (.so link)
• glibc manual: sysinfo.texi, node Host Identification
• headers: /usr/include/unistd.h (lines 928-934)
• symbols: nm -D --with-symbol-versions libc.so.6