1 files changed, 91 insertions, 0 deletions
diff --git a/lib/canon-host.c b/lib/canon-host.c
new file mode 100644
index 0000000..dff0212
--- /dev/null
+++ b/lib/canon-host.c
@@ -0,0 +1,91 @@
+/* Host name canonicalization
+
+   Copyright (C) 2005-2023 Free Software Foundation, Inc.
+
+   Written by Derek Price <derek@ximbiot.com>.
+
+   This file is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation, either version 3 of the
+   License, or (at your option) any later version.
+
+   This file is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "canon-host.h"
+
+#include <string.h>
+#include <netdb.h>
+
+/* Store the last error for the single-threaded version of this function.  */
+static int last_cherror;
+
+/* Single-threaded of wrapper for canon_host_r.  After a NULL return, error
+   messages may be retrieved via ch_strerror().  */
+char *
+canon_host (const char *host)
+{
+  return canon_host_r (host, &last_cherror);
+}
+
+/* Return a malloc'd string containing the canonical hostname associated with
+   HOST, or NULL if a canonical name cannot be determined.  On NULL return,
+   if CHERROR is not NULL, set *CHERROR to an error code as returned by
+   getaddrinfo().  Use ch_strerror_r() or gai_strerror() to convert a *CHERROR
+   value to a string suitable for error messages.
+
+   WARNINGS
+     HOST must be a string representation of a resolvable name for this host.
+     Strings containing an IP address in dotted decimal notation will be
+     returned as-is, without further resolution.
+
+     The use of the word "canonical" in this context is unfortunate but
+     entrenched.  The value returned by this function will be the end result
+     of the resolution of any CNAME chains in the DNS.  There may only be one
+     such value for any given hostname, though the actual IP address
+     referenced by this value and the device using that IP address may each
+     actually have any number of such "canonical" hostnames.  See the POSIX
+     getaddrinfo spec
+     <https://pubs.opengroup.org/onlinepubs/9699919799/functions/getaddrinfo.html>,
+     RFC 1034 <https://www.ietf.org/rfc/rfc1034.txt>, & RFC 2181
+     <https://www.ietf.org/rfc/rfc2181.txt> for more on what this confusing
+     term really refers to. */
+char *
+canon_host_r (char const *host, int *cherror)
+{
+  char *retval = NULL;
+  static struct addrinfo hints;
+  struct addrinfo *res = NULL;
+  int status;
+
+  hints.ai_flags = AI_CANONNAME;
+  status = getaddrinfo (host, NULL, &hints, &res);
+  if (!status)
+    {
+      /* https://lists.gnu.org/r/bug-coreutils/2006-09/msg00300.html
+         says Darwin 7.9.0 getaddrinfo returns 0 but sets
+         res->ai_canonname to NULL.  */
+      retval = strdup (res->ai_canonname ? res->ai_canonname : host);
+      if (!retval && cherror)
+        *cherror = EAI_MEMORY;
+      freeaddrinfo (res);
+    }
+  else if (cherror)
+    *cherror = status;
+
+  return retval;
+}
+
+/* Return a string describing the last error encountered by canon_host.  */
+const char *
+ch_strerror (void)
+{
+  return gai_strerror (last_cherror);
+}