1 files changed, 96 insertions, 0 deletions
diff --git a/netcheck.c b/netcheck.c
new file mode 100644
index 0000000..4900e0f
--- /dev/null
+++ b/netcheck.c
@@ -0,0 +1,96 @@
+//
+// Created by Keuin on 2021/12/29.
+//
+
+#include <netdb.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include "logging.h"
+#include "netcheck.h"
+
+/**
+ * Check network availability.
+ * @return Zero if success, non-zero if failed.
+ */
+int check_network(void *logger) {
+#define RETURN(r) do { rv = (r); goto RET; } while(0)
+#define READ_SIZE 32
+
+    int sock = -1, rv = 0;
+    struct sockaddr_in serv_addr;
+    const char *msg = "GET / HTTP/1.1\r\n"
+                      "Host: www.gov.cn\r\n"
+                      "\r\n";
+
+    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
+        perror("socket()");
+        log_error(logger, "socket() failed.");
+        RETURN(-1);
+    }
+
+    const struct hostent *host = gethostbyname("www.gov.cn"); // TODO cache this
+
+    if (!host) {
+        herror("gethostbyname()");
+        log_error(logger, "Cannot resolve test host.");
+        RETURN(-1);
+    }
+
+    if (host->h_length <= 0) {
+        log_error(logger, "Test host does not have at least one address.");
+        RETURN(-1);
+    }
+
+    serv_addr.sin_family = AF_INET;
+    serv_addr.sin_port = htons(80);
+    serv_addr.sin_addr = **((struct in_addr **) host->h_addr_list);
+
+    if (connect(sock, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) {
+        log_error(logger, "connect() failed.");
+        RETURN(-1);
+    }
+
+    ssize_t ss = 0; // send size
+    while (ss < strlen(msg)) {
+        ssize_t rd = send(sock, msg, strlen(msg), 0);
+        if (rd < 0) {
+            perror("send()");
+            log_error(logger, "send() failed.");
+            RETURN(-1);
+        }
+        ss += rd;
+    }
+
+    log_debug(logger, "HTTP request is sent. Reading response.");
+    char response[READ_SIZE] = {0};
+    ssize_t rs = 0; // receive size
+    while (rs < READ_SIZE) {
+        ssize_t rd = read(sock, response, READ_SIZE - rs);
+        if (rd < 0) {
+            perror("read()");
+            log_error(logger, "read() failed.");
+            RETURN(-1);
+        } else {
+            rs += rd;
+        }
+    }
+
+    const char *expected = "HTTP/1.1 200 OK\r\n";
+    if (memcmp(response, expected, strlen(expected)) != 0) {
+        char buf[64];
+        snprintf(buf, 63, "Unexpected response: %s", response);
+        log_error(logger, buf);
+        RETURN(-1);
+    }
+    RETURN(0);
+
+    RET:
+    if (sock >= 0) close(sock);
+    return rv;
+#undef RETURN
+#undef READ_SIZE
+}
+\ No newline at end of file