Write webserver part in C

It is a probably working FastCGI program.
Other parts will be ready later.

clang -g doskast-trigger-connect.c -o t -l fcgi -l gnutls

webserver/doskast-trigger-connect.c

@@ -0,0 +1,130 @@
+#include <ctype.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcgios.h>
+#include <fcgi_stdio.h>
+#include <gnutls/gnutls.h>
+#include <gnutls/crypto.h>
+
+// allow to redefine as gcc -D...
+#ifndef DIR
+#define DIR "/tmp"
+#endif
+
+#define HTTP_OK 200
+#define HTTP_BAD_REQUEST 400
+#define HTTP_ERROR 500
+
+/* Here we could verify validity of a IPv4 or IPv6 address,
+ * but that is very complex. Just sanitize input. */
+int
+_verify_ip(char *ip){
+	int rc = 0;
+	for (int i = 0; i < strlen(ip); i++){
+		if (!( (ip[i] == '.') || /* IPv4 dots */
+			   (ip[i] == ':') || /* IPv6 */
+			   (ip[i] == '/') || /* XXX Is it needed for IPv6? */
+			   (isalnum(ip[i]) > 0) /* Numbers (v4 and v6) and letters (v6) */
+		   )) {
+			rc = i;
+			break;
+		}
+	}
+	return rc;
+}
+
+/* Generate a random alphanum sequence
+ * Example: 10841ee0cc4f (12 chars) */
+int
+_rand(char **rs){
+	// https://books.google.ru/books?id=1nG3BwAAQBAJ
+	// hex.data will be 6x2=12 chars
+	unsigned char buf_rnd[6];
+	gnutls_datum_t rnd = { buf_rnd, sizeof(buf_rnd) };
+	int rc;
+	rc = gnutls_rnd(GNUTLS_RND_NONCE, rnd.data, rnd.size);
+	if (rc != GNUTLS_E_SUCCESS) {
+		printf("Error in gnutls_rnd()");
+		goto out;
+	}
+	gnutls_datum_t hex;
+	rc = gnutls_hex_encode2(&rnd, &hex);
+	if (rc != GNUTLS_E_SUCCESS) {
+		printf("Error in gnutls_hex_encode2()");
+		goto out;
+	}
+	*rs = malloc(hex.size);
+	rc = sprintf(*rs, "%s", hex.data);
+	if (rc < 0)  {
+		printf("Error in sprintf()");
+		goto out;
+	}
+out:
+	return rc;
+}
+
+int
+main(){
+	int rc = 0;
+	int http_code = HTTP_ERROR;
+	if (access(DIR, W_OK) != 0) {
+		fprintf(stderr, "Directory %s does not exist or is not writable\n", DIR);
+		http_code = HTTP_ERROR;
+		goto out;
+	}
+	// first chdir() and then chroot()!
+	if (!( chdir(DIR) == 0 && chroot(DIR) == 0 )) {
+		fprintf(stderr, "Cannot chdir and chroot into %s\n", DIR);
+		http_code = HTTP_ERROR;
+		goto out;
+	}
+	while(FCGI_Accept() >= 0) {
+		char *ip = getenv("REMOTE_ADDR");
+		if (ip == NULL) {
+			fprintf(stderr, "%s\n", "Env REMOTE_ADDR is not set");
+			http_code = HTTP_ERROR;
+			goto fcgi_out;
+		}
+		rc = _verify_ip(ip);
+		if (rc != 0) {
+			fprintf(stderr, "%s\n", "Incorrect REMOTE_ADDR");
+			http_code = HTTP_BAD_REQUEST;
+			goto fcgi_out;
+		}
+		char *hex;
+		int max_try = 10;
+		for (int i = 1; i <= max_try; i++) {
+			if (i == max_try) {
+				fprintf(stderr, "%s\n", "Error in random file loop");
+				http_code = HTTP_ERROR;
+				goto fcgi_out;
+			}
+			rc = _rand(&hex);
+			if (rc < 0) {
+				fprintf(stderr, "Error in _rand()");
+				continue;
+			}
+			if (access(hex, F_OK) != 0) {
+				break;
+			}
+		}
+		FCGI_FILE *d = fopen(hex, "w");
+		fprintf(d, "%s", ip);
+		fclose(d);
+		http_code = HTTP_OK;
+fcgi_out:
+		printf("Status: %d\n", http_code);
+		printf("Content-Type: text/plain; charset=utf-8\n");
+		if (http_code == HTTP_OK) {
+			printf("%s\n", "OK");
+			rc = 0;
+		} else {
+			rc = 1;
+		}
+	}
+out:
+	return rc;
+}