In glibc's libresolv, this is caused by res_hnok in resolv/res_name.c returning 0: /* * Verify that a domain name uses an acceptable character set.

In glibc's libresolv, this is caused by res_hnok in resolv/res_name.c returning 0: /* * Verify that a domain name uses an acceptable character set. */ /* * Note the conspicuous absence of ctype macros in these definitions. On * non-ASCII hosts, we can't depend on string literals or ctype macros to * tell us anything about network-format data. The rest of the BIND system * is not careful about this, but for some reason, we're doing it right here. */ #define PERIOD 0x2e #define hyphenchar(c) ((c) == 0x2d) #define underscorechar(c) ((c) == 0x5f) #define bslashchar(c) ((c) == 0x5c) #define periodchar(c) ((c) == PERIOD) #define asterchar(c) ((c) == 0x2a) #define alphachar(c) (((c) >= 0x41 && (c) <= 0x5a) \ || ((c) >= 0x61 && (c) <= 0x7a)) #define digitchar(c) ((c) >= 0x30 && (c) <= 0x39) #define borderchar(c) (alphachar(c) || digitchar(c)) #define middlechar(c) (borderchar(c) || hyphenchar(c) || underscorechar(c)) #define domainchar(c) ((c) > 0x20 && (c) < 0x7f) int res_hnok(const char *dn) { int pch = PERIOD, ch = *dn++; while (ch != '\0') { int nch = *dn++; if (periodchar(ch)) { (void)NULL; } else if (periodchar(pch)) { if (!borderchar(ch)) return (0); } else if (periodchar(nch) || nch == '\0') { if (!borderchar(ch)) return (0); } else { if (!middlechar(ch)) return (0); } pch = ch, ch = nch; } return (1); }