Basic structure in place: can daemonise, listen on a Unix domain

socket, accept new connections, open a network interface using libpcap
and forward (all) traffic.

src/util/hijack.c

@@ -0,0 +1,469 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <stdarg.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <signal.h>
+#include <net/if.h>
+#include <sys/select.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/un.h>
+#include <syslog.h>
+#include <getopt.h>
+#include <pcap.h>
+
+#define SNAPLEN 1600
+
+struct hijack {
+	pcap_t *pcap;
+	int fd;
+};
+
+struct hijack_listener {
+	struct sockaddr_un sun;
+	int fd;
+};
+
+struct hijack_options {
+	char interface[IF_NAMESIZE];
+	int daemonise;
+};
+
+static int daemonised = 0;
+
+/**
+ * Log error message
+ *
+ */
+static void logmsg ( int level, const char *format, ... ) {
+	va_list ap;
+
+	va_start ( ap, format );
+	if ( daemonised ) {
+		vsyslog ( ( LOG_DAEMON | level ), format, ap );
+	} else {
+		vfprintf ( stderr, format, ap );
+	}
+	va_end ( ap );
+}
+
+/**
+ * Open pcap device
+ *
+ */
+static int hijack_open ( const char *interface, struct hijack *hijack ) {
+	char errbuf[PCAP_ERRBUF_SIZE];
+
+	/* Open interface via pcap */
+	errbuf[0] = '\0';
+	hijack->pcap = pcap_open_live ( interface, SNAPLEN, 1, 0, errbuf );
+	if ( ! hijack->pcap ) {
+		logmsg ( LOG_ERR, "Failed to open %s: %s\n",
+			 interface, errbuf );
+		goto err;
+	}
+	if ( errbuf[0] )
+		logmsg ( LOG_WARNING, "Warning: %s\n", errbuf );
+
+	/* Set capture interface to non-blocking mode */
+	if ( pcap_setnonblock ( hijack->pcap, 1, errbuf ) < 0 ) {
+		logmsg ( LOG_ERR, "Could not make %s non-blocking: %s\n",
+			 interface, errbuf );
+		goto err;
+	}
+
+	/* Get file descriptor for select() */
+	hijack->fd = pcap_get_selectable_fd ( hijack->pcap );
+	if ( hijack->fd < 0 ) {
+		logmsg ( LOG_ERR, "Cannot get selectable file descriptor "
+			 "for %s\n", interface );
+		goto err;
+	}
+
+	return 0;
+
+ err:
+	if ( hijack->pcap )
+		pcap_close ( hijack->pcap );
+	return -1;
+}
+
+/**
+ * Close pcap device
+ *
+ */
+static void hijack_close ( struct hijack *hijack ) {
+	pcap_close ( hijack->pcap );
+}
+
+/**
+ * Forward data from hijacker
+ *
+ */
+static ssize_t forward_from_hijacker ( struct hijack *hijack, int fd ) {
+	char buf[SNAPLEN];
+	ssize_t len;
+
+	len = read ( fd, buf, sizeof ( buf ) );
+	if ( len < 0 ) {
+		logmsg ( LOG_ERR, "read from hijacker failed: %s\n",
+			 strerror ( errno ) );
+		return -1;
+	}
+
+	if ( len == 0 )
+		return 0;
+
+	if ( write ( hijack->fd, buf, len ) != len ) {
+		logmsg ( LOG_ERR, "write to hijacked port failed: %s\n",
+			 strerror ( errno ) );
+		return -1;
+	}
+
+	logmsg ( LOG_INFO, "forwarded %zd bytes from hijacker\n", len );
+
+	return len;
+};
+
+/**
+ * Forward data to hijacker
+ *
+ */
+static ssize_t forward_to_hijacker ( int fd, struct hijack *hijack ) {
+	char buf[SNAPLEN];
+	ssize_t len;
+
+	len = read ( hijack->fd, buf, sizeof ( buf ) );
+	if ( len < 0 ) {
+		logmsg ( LOG_ERR, "read from hijacked port failed: %s\n",
+			 strerror ( errno ) );
+		return -1;
+	}
+
+	if ( len == 0 )
+		return 0;
+
+	if ( write ( fd, buf, len ) != len ) {
+		logmsg ( LOG_ERR, "write to hijacker failed: %s\n",
+			 strerror ( errno ) );
+		return -1;
+	}
+
+	logmsg ( LOG_INFO, "forwarded %zd bytes to hijacker\n", len );
+
+	return len;
+};
+
+
+/**
+ * Run hijacker
+ *
+ */
+static int run_hijacker ( const char *interface, int fd ) {
+	struct hijack hijack;
+	fd_set fdset;
+	int max_fd;
+	ssize_t len;
+
+	memset ( &hijack, 0, sizeof ( hijack ) );
+
+	logmsg ( LOG_INFO, "new connection for %s\n", interface );
+
+	if ( hijack_open ( interface, &hijack ) < 0 )
+		goto err;
+	
+	/* Do the forwarding */
+	max_fd = ( ( fd > hijack.fd ) ? fd : hijack.fd );
+	while ( 1 ) {
+		/* Wait for available data */
+		FD_ZERO ( &fdset );
+		FD_SET ( fd, &fdset );
+		FD_SET ( hijack.fd, &fdset );
+		if ( select ( ( max_fd + 1 ), &fdset, NULL, NULL, 0 ) < 0 ) {
+			logmsg ( LOG_ERR, "select failed: %s\n",
+				 strerror ( errno ) );
+			goto err;
+		}
+		if ( FD_ISSET ( fd, &fdset ) ) {
+			len = forward_from_hijacker ( &hijack, fd );
+			if ( len < 0 )
+				goto err;
+			if ( len == 0 )
+				break;
+		}
+		if ( FD_ISSET ( hijack.fd, &fdset ) ) {
+			len = forward_to_hijacker ( fd, &hijack );
+			if ( len < 0 )
+				goto err;
+			if ( len == 0 )
+				break;
+		}
+	}
+
+	hijack_close ( &hijack );
+	logmsg ( LOG_INFO, "closed connection for %s\n", interface );
+
+	return 0;
+
+ err:
+	if ( hijack.pcap )
+		hijack_close ( &hijack );
+	return -1;
+}
+
+/**
+ * Open listener socket
+ *
+ */
+static int open_listener ( const char *interface,
+			   struct hijack_listener *listener ) {
+	
+	/* Create socket */
+	listener->fd = socket ( PF_UNIX, SOCK_SEQPACKET, 0 );
+	if ( listener->fd < 0 ) {
+		logmsg ( LOG_ERR, "Could not create socket: %s\n",
+			 strerror ( errno ) );
+		goto err;
+	}
+
+	/* Bind to local filename */
+	listener->sun.sun_family = AF_UNIX,
+	snprintf ( listener->sun.sun_path, sizeof ( listener->sun.sun_path ),
+		   "/var/run/hijack-%s", interface );
+	if ( bind ( listener->fd, ( struct sockaddr * ) &listener->sun,
+		    sizeof ( listener->sun ) ) < 0 ) {
+		logmsg ( LOG_ERR, "Could not bind socket to %s: %s\n",
+			 listener->sun.sun_path, strerror ( errno ) );
+		goto err;
+	}
+
+	/* Set as a listening socket */
+	if ( listen ( listener->fd, 0 ) < 0 ) {
+		logmsg ( LOG_ERR, "Could not listen to %s: %s\n",
+			 listener->sun.sun_path, strerror ( errno ) );
+		goto err;
+	}
+
+	return 0;
+	
+ err:
+	if ( listener->fd >= 0 )
+		close ( listener->fd );
+	return -1;
+}
+
+/**
+ * Listen on listener socket
+ *
+ */
+static int listen_for_hijackers ( struct hijack_listener *listener,
+				  const char *interface ) {
+	int fd;
+	pid_t child;
+	int rc;
+
+	logmsg ( LOG_INFO, "Listening on %s\n", listener->sun.sun_path );
+
+	while ( 1 ) {
+		/* Accept new connection */
+		fd = accept ( listener->fd, NULL, 0 );
+		if ( fd < 0 ) {
+			logmsg ( LOG_ERR, "accept failed: %s\n",
+				 strerror ( errno ) );
+			goto err;
+		}
+
+		/* Fork child process */
+		child = fork();
+		if ( child < 0 ) {
+			logmsg ( LOG_ERR, "fork failed: %s\n",
+				 strerror ( errno ) );
+			goto err;
+		}
+		if ( child == 0 ) {
+			/* I am the child; run the hijacker */
+			rc = run_hijacker ( interface, fd );
+			close ( fd );
+			exit ( rc );
+		}
+		
+		close ( fd );
+	}
+
+	return 0;
+
+ err:
+	return -1;
+}
+
+/**
+ * Close listener socket
+ *
+ */
+static void close_listener ( struct hijack_listener *listener ) {
+	close ( listener->fd );
+	unlink ( listener->sun.sun_path );
+}
+
+/**
+ * Print usage
+ *
+ */
+static void usage ( char **argv ) {
+	logmsg ( LOG_ERR,
+		 "Usage: %s [options]\n"
+		 "\n"
+		 "Options:\n"
+		 "  -h|--help               Print this help message\n"
+		 "  -i|--interface intf     Use specified network interface\n"
+		 "  -n|--nodaemon           Run in foreground\n",
+		 argv[0] );
+}
+
+/**
+ * Parse command-line options
+ *
+ */
+static int parse_options ( int argc, char **argv,
+			   struct hijack_options *options ) {
+	static struct option long_options[] = {
+		{ "interface", 1, NULL, 'i' },
+		{ "nodaemon", 0, NULL, 'n' },
+		{ "help", 0, NULL, 'h' },
+		{ },
+	};
+	int c;
+
+	/* Set default options */
+	memset ( options, 0, sizeof ( *options ) );
+	strncpy ( options->interface, "eth0", sizeof ( options->interface ) );
+	options->daemonise = 1;
+
+	/* Parse command-line options */
+	while ( 1 ) {
+		int option_index = 0;
+		
+		c = getopt_long ( argc, argv, "i:hn", long_options,
+				  &option_index );
+		if ( c < 0 )
+			break;
+
+		switch ( c ) {
+		case 'i':
+			strncpy ( options->interface, optarg,
+				  sizeof ( options->interface ) );
+			break;
+		case 'n':
+			options->daemonise = 0;
+			break;
+		case 'h':
+			usage( argv );
+			return -1;
+		case '?':
+			/* Unrecognised option */
+			return -1;
+		default:
+			logmsg ( LOG_ERR, "Unrecognised option '-%c'\n", c );
+			return -1;
+		}
+	}
+
+	/* Check there's nothing left over on the command line */
+	if ( optind != argc ) {
+		usage ( argv );
+		return -1;
+	}
+
+	return 0;
+}
+
+/**
+ * Daemonise
+ *
+ */
+static int daemonise ( const char *interface ) {
+	char pidfile[16 + IF_NAMESIZE + 4]; /* "/var/run/hijack-<intf>.pid" */
+	char pid[16];
+	int pidlen;
+	int fd = -1;
+
+	/* Daemonise */
+	if ( daemon ( 0, 0 ) < 0 ) {
+		logmsg ( LOG_ERR, "Could not daemonise: %s\n",
+			 strerror ( errno ) );
+		goto err;
+	}
+	daemonised = 1; /* Direct messages to syslog now */
+
+	/* Open pid file */
+	snprintf ( pidfile, sizeof ( pidfile ), "/var/run/hijack-%s.pid",
+		   interface );
+	fd = open ( pidfile, ( O_WRONLY | O_CREAT | O_TRUNC ),
+		    ( S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH ) );
+	if ( fd < 0 ) {
+		logmsg ( LOG_ERR, "Could not open %s for writing: %s\n",
+			 pidfile, strerror ( errno ) );
+		goto err;
+	}
+
+	/* Write pid to file */
+	pidlen = snprintf ( pid, sizeof ( pid ), "%d\n", getpid() );
+	if ( write ( fd, pid, pidlen ) != pidlen ) {
+		logmsg ( LOG_ERR, "Could not write %s: %s\n",
+			 pidfile, strerror ( errno ) );
+		goto err;
+	}
+
+	close ( fd );
+	return 0;
+
+ err:
+	if ( fd >= 0 )
+		close ( fd );
+	return -1;
+}
+
+int main ( int argc, char **argv ) {
+	struct hijack_options options;
+	struct hijack_listener listener;
+	struct sigaction sigchld;
+
+	/* Parse command-line options */
+	if ( parse_options ( argc, argv, &options ) < 0 )
+		exit ( 1 );
+
+	/* Set up syslog connection */
+	openlog ( basename ( argv[0] ), LOG_PID, LOG_DAEMON );
+
+	/* Set up listening socket */
+	if ( open_listener ( options.interface, &listener ) < 0 )
+		exit ( 1 );
+
+	/* Daemonise on demand */
+	if ( options.daemonise ) {
+		if ( daemonise ( options.interface ) < 0 )
+			exit ( 1 );
+	}
+
+	/* Avoid creating zombies */
+	memset ( &sigchld, 0, sizeof ( sigchld ) );
+	sigchld.sa_handler = SIG_IGN;
+	sigchld.sa_flags = SA_NOCLDWAIT;
+	if ( sigaction ( SIGCHLD, &sigchld, NULL ) < 0 ) {
+		logmsg ( LOG_ERR, "Could not set signal handler: %s",
+			 strerror ( errno ) );
+		exit ( 1 );
+	}
+
+	/* Listen for hijackers */
+	if ( listen_for_hijackers ( &listener, options.interface ) < 0 )
+		exit ( 1 );
+
+	close_listener ( &listener );
+	
+	return 0;
+}