replace problematic hostentdb with hostsreader

the hostentdb introduced between 4.2 and 4.3
(via af5c6f0c6a )
had several issues:
- it caused breakage on FreeBSD and was commented out there
- prevented usage of the hostdb when proxy_dns was turned off
  (issue #42)
- required dynamic memory allocation which was accessed from several
  threads
- wouldnt reflect changes to the hosts file made during program run

the only sensible solution is to remove the hostentdb and replace it
with a home-grown hosts parser (we can't use gethostent() since
that would mess up the gethostent()-state from different threads).

the new parser used here is deliberately held simple and only meant
to provide the user with means to reference hardcoded ipv4 addresses
via his hosts file.

fixes #42

src/hostsreader.c

@@ -0,0 +1,119 @@
+#include <stdio.h>
+#include <ctype.h>
+#include <string.h>
+
+/*
+   simple reader for /etc/hosts
+   it only supports comments, blank lines and lines consisting of an ipv4 hostname pair.
+   this is required so we can return entries from the host db without messing up the
+   non-thread-safe state of libc's gethostent().
+
+*/
+
+struct hostsreader {
+	FILE *f;
+	char* ip, *name;
+};
+
+int hostsreader_open(struct hostsreader *ctx) {
+	if(!(ctx->f = fopen("/etc/hosts", "r"))) return 0;
+	return 1;
+}
+
+void hostsreader_close(struct hostsreader *ctx) {
+	fclose(ctx->f);
+}
+
+static int isnumericipv4(const char* ipstring);
+int hostsreader_get(struct hostsreader *ctx, char* buf, size_t bufsize) {
+	while(1) {
+		if(!fgets(buf, bufsize, ctx->f)) return 0;
+		if(*buf == '#') continue;
+		ctx->ip = buf;
+		while(*buf && !isspace(*buf) && bufsize) {
+			buf++;
+			bufsize--;
+		}
+		if(!bufsize || !*buf || buf == ctx->ip) continue;
+		*buf = 0;
+		buf++;
+		while(*buf && isspace(*buf) && bufsize) {
+			buf++;
+			bufsize--;
+		}
+		if(!bufsize || !*buf) continue;
+		ctx->name = buf;
+		while(*buf && !isspace(*buf) && bufsize) {
+			buf++;
+			bufsize--;
+		}
+		if(!bufsize || !*buf) continue;
+		*buf = 0;
+		if(isnumericipv4(ctx->ip)) return 1;
+	}
+}
+
+char* hostsreader_get_ip_for_name(const char* name, char* buf, size_t bufsize) {
+	struct hostsreader ctx;
+	char *res = 0;
+	if(!hostsreader_open(&ctx)) return 0;
+	while(hostsreader_get(&ctx, buf, bufsize)) {
+		if(!strcmp(ctx.name, name)) {
+			res = ctx.ip;
+			break;
+		}
+	}
+	hostsreader_close(&ctx);
+	return res;
+}
+
+#include "ip_type.h"
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <arpa/inet.h>
+ip_type hostsreader_get_numeric_ip_for_name(const char* name) {
+	char *hres;
+	char buf[320];
+	if((hres = hostsreader_get_ip_for_name(name, buf, sizeof buf))) {
+		struct in_addr c;
+		inet_aton(hres, &c);
+		ip_type res;
+		memcpy(res.octet, &c.s_addr, 4);
+		return res;
+	} else return ip_type_invalid;
+}
+
+#ifdef HOSTSREADER_TEST
+int main() {
+	char buf[256];
+	char * ret = hostsreader_get_ip_for_name("goo", buf, sizeof buf);
+	printf("%s\n", ret ? ret : "null");
+}
+#endif
+
+/* isnumericipv4() taken from libulz */
+static int isnumericipv4(const char* ipstring) {
+	size_t x = 0, n = 0, d = 0;
+	int wasdot = 0;
+	while(1) {
+		switch(ipstring[x]) {
+			case 0: goto done;
+			case '.':
+				if(!n || wasdot) return 0;
+				d++;
+				wasdot = 1;
+				break;
+			case '0': case '1': case '2': case '3': case '4':
+			case '5': case '6': case '7': case '8': case '9':
+				n++;
+				wasdot = 0;
+				break;
+			default:
+				return 0;
+		}
+		x++;
+	}
+	done:
+	if(d == 3 && n >= 4 && n <= 12) return 1;
+	return 0;
+}