udp-redirect.c File Reference

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <fcntl.h>
#include <poll.h>
#include <string.h>
#include <math.h>
#include <netdb.h>
#include <time.h>

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Classes
struct	settings
struct	statistics

Macros
#define	UDP_REDIRECT_VERSION   "2.2.0"
#define	STATISTICS_DELAY_SECONDS   60
#define	NETWORK_BUFFER_SIZE   65535
#define	EOK   0
	Readability: errno value for OK.
#define	MAX_ERRNO   256
#define	ERRNO_IGNORE_INIT(X)
#define	ERRNO_IGNORE_SET(X, Y)
#define	ERRNO_IGNORE_CHECK(X, Y)
#define	PARSE_PORT(src, dest, label)
#define	DEBUG(debug_level_local, debug_level_message, fmt, ...)
#define	HUMAN_READABLE_FORMAT   "%.1lf%c"
#define	HRF   HUMAN_READABLE_FORMAT
#define	HUMAN_READABLE(X)
#define	HUMAN_READABLE_SIZES   { ' ', 'K', 'M', 'G', 'T', 'P', 'E' }
#define	HUMAN_READABLE_SIZES_COUNT   7

Enumerations
enum	DEBUG_LEVEL { DEBUG_LEVEL_ERROR = 0 , DEBUG_LEVEL_INFO = 1 , DEBUG_LEVEL_VERBOSE = 2 , DEBUG_LEVEL_DEBUG = 3 }
	The available debug levels. More...

Functions
static int	socket_setup (const int debug_level, const char *desc, const char *xaddr, const int xport, const char *xif, int xfamily, struct sockaddr_storage *xsock_name)
static char *	resolve_host (int debug_level, const char *host)
static int	parse_addr (const char *str, int port, struct sockaddr_storage *out)
static const char *	addr_tostring (const struct sockaddr_storage *sa, char *buf, size_t len)
static int	addr_port (const struct sockaddr_storage *sa)
static socklen_t	addr_len (const struct sockaddr_storage *sa)
static int	addr_is_unset (const struct sockaddr_storage *sa)
static int	addr_equal (const struct sockaddr_storage *a, const struct sockaddr_storage *b)
void	settings_initialize (struct settings *s)
void	usage (const char *argv0, const char *message)
void	statistics_initialize (struct statistics *st)
double	int_to_human_value (double value)
char	int_to_human_char (double value)
void	statistics_display (int debug_level, struct statistics *st, time_t now)
int	main (int argc, char *argv[])
static double	int_to_human_scale (double value, int *count_out)

Variables
static struct option	longopts []

Detailed Description

Author

Dan Podeanu pdan@.nosp@m.esyn.nosp@m.c.org

Version

2.2.0

LICENSE

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

DESCRIPTION

A single-file, high-performance UDP packet redirector supporting IPv4 and IPv6. Useful when layer-level redirection (e.g., firewall rules) is impractical — common use cases include Wireguard VPN, DNS, and game servers.

Design overview:

• Two non-blocking UDP sockets: a listen socket (faces the local client) and a send socket (faces the remote endpoint).
• A poll(2)-based main loop with a 1-second timeout drives all I/O; no threads or dynamic allocation occur inside the loop.
• Packets arriving on the listen socket are forwarded to the connect endpoint; replies arriving on the send socket are forwarded back to the most-recently-seen listen-side source.
• Optional strict-mode flags limit which sources are accepted on each side.
• Optional 60-second windowed statistics are printed to stderr.

Targets: Linux x86-64 and macOS/Darwin arm64.

Macro Definition Documentation

DEBUG

#define DEBUG	(		debug_level_local,
			debug_level_message,
			fmt,
			... )

Value:

do { \
    if ((debug_level_local) >= (debug_level_message)) { \
        fprintf(stderr, "%s:%d:%lld:%s(): " fmt "\n", __FILE__, \
            __LINE__, (long long)(time(NULL)), __func__, ##__VA_ARGS__); \
    } \
} while (0)

Standard debug macro requiring a locally defined debug level. Adapted from the excellent https://github.com/jleffler/soq/blob/master/src/libsoq/debug.h

Note that VA_ARGS is a GCC extension; used for convenience to support DEBUG without format arguments.

EOK

#define EOK   0

Readability: errno value for OK.

ERRNO_IGNORE_CHECK

#define ERRNO_IGNORE_CHECK	(		X,
			Y )

Value:

((Y) >= 0 && (Y) < MAX_ERRNO && (X)[(Y)] == 1)

Check if errno is in declared set

ERRNO_IGNORE_INIT

#define ERRNO_IGNORE_INIT

(

X

)

Value:

memset((X), 0, MAX_ERRNO * sizeof(unsigned char))

Initialize errno ignore set

ERRNO_IGNORE_SET

#define ERRNO_IGNORE_SET	(		X,
			Y )

Value:

if ((Y) >= 0 && (Y) < MAX_ERRNO) (X)[(Y)] = 1

Set errno ignore boolean for a specific errno

HRF

#define HRF   HUMAN_READABLE_FORMAT

Shortcut for the above, for shorter printf

HUMAN_READABLE

#define HUMAN_READABLE

(

X

)

Value:

int_to_human_value((X)), int_to_human_char((X))

Hardcoded invocations of int to human readable, compatible with HUMAN_READABLE_FORMAT in printf

HUMAN_READABLE_FORMAT

#define HUMAN_READABLE_FORMAT   "%.1lf%c"

Hardcoded printf format for a human readable value made of a float and a char

HUMAN_READABLE_SIZES

#define HUMAN_READABLE_SIZES   { ' ', 'K', 'M', 'G', 'T', 'P', 'E' }

The human readable size suffixes

HUMAN_READABLE_SIZES_COUNT

#define HUMAN_READABLE_SIZES_COUNT   7

The number of human readable size suffixes

MAX_ERRNO

#define MAX_ERRNO   256

Maximum known errno, used to ignore harmless sendto / recvfrom errors

NETWORK_BUFFER_SIZE

#define NETWORK_BUFFER_SIZE   65535

The size of the network buffer used for receiving / sending packets

PARSE_PORT

#define PARSE_PORT	(		src,
			dest,
			label )

Value:

do { \
    char *_endptr; \
    long _val; \
    errno = EOK; \
    _val = strtol((src), &_endptr, 10); \
    if (errno != EOK || _endptr == (src) || *_endptr != '\0' || _val < 0 || _val > 65535) { \
        DEBUG(debug_level, DEBUG_LEVEL_ERROR, "Invalid %s: %s", (label), (src)); \
        exit(EXIT_FAILURE); \
    } \
    (dest) = (int)_val; \
} while (0)

Parse a port number from a string (optarg). Sets dest to the parsed value, or prints label and exits on error. Validates that the string is a pure decimal integer in [0, 65535].

STATISTICS_DELAY_SECONDS

#define STATISTICS_DELAY_SECONDS   60

The delay in seconds between displaying statistics

UDP_REDIRECT_VERSION

#define UDP_REDIRECT_VERSION   "2.2.0"

The udp-redirect version

Enumeration Type Documentation

DEBUG_LEVEL

enum DEBUG_LEVEL

The available debug levels.

Enumerator
DEBUG_LEVEL_ERROR	Error messages.
DEBUG_LEVEL_INFO	Informational messages.
DEBUG_LEVEL_VERBOSE	Verbose messages.
DEBUG_LEVEL_DEBUG	Debug messages.

Function Documentation

addr_equal()

int addr_equal ( const struct sockaddr_storage * a, const struct sockaddr_storage * b )

static

Return 1 if both sockaddr_storage values have the same family, address, and port.

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addr_is_unset()

int addr_is_unset ( const struct sockaddr_storage * sa )

static

Return 1 if the address is the zero-initialised "unset" sentinel (ss_family == 0).

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addr_len()

socklen_t addr_len ( const struct sockaddr_storage * sa )

static

Return the wire size of a sockaddr_storage appropriate for its address family.

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addr_port()

int addr_port ( const struct sockaddr_storage * sa )

static

Return the port from a sockaddr_storage in host byte order.

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addr_tostring()

const char * addr_tostring ( const struct sockaddr_storage * sa, char * buf, size_t len )

static

Format the address portion of a sockaddr_storage into buf using inet_ntop.

Returns

buf on success, "?" on failure.

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int_to_human_char()

char int_to_human_char

(

double

value

)

Convert a value to human readable (i.e., 1500 = 1.5K). Divide by 1000, not 1024.

Parameters

[in]

value

The value to be converted

Returns

The character (K, M, G, etc.) portion of the human readable value.

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int_to_human_scale()

double int_to_human_scale ( double value, int * count_out )

static

Shared scaling helper: divides value by 1000 repeatedly until it is <= 1000 or the maximum prefix is reached.

Parameters

[in]	value	The raw value to scale.
[out]	count_out	The number of divisions performed (index into the human-readable suffix array).

Returns

The scaled value.

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int_to_human_value()

double int_to_human_value

(

double

value

)

Convert a value to human readable (i.e., 1500 = 1.5K). Divide by 1000, not 1024.

Parameters

[in]

value

The value to be converted

Returns

The numeric portion of the human readable value.

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main()

int main	(	int	argc,
		char *	argv[] )

Main program function.

Parameters

[in]	argc	The count of arguments, including the program name.
[in]	argv	The program arguments

Returns

The program return code.

Accept the packet IF:

• There's no previous endpoint, OR
• There is a previous endpoint, but we are not in strict mode, OR
• The previous endpoint matches the current endpoint

Accept the packet IF:

• The listen socket has received a packet, so we know the endpoint, AND
• The packet was received from the connect endpoint, OR
• We are not in strict mode

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parse_addr()

int parse_addr ( const char * str, int port, struct sockaddr_storage * out )

static

Parse an IPv4 or IPv6 address string into a sockaddr_storage. Sets ss_family, the address field, and the port.

Parameters

[in]	str	Dotted-quad IPv4 or colon-separated IPv6 address string.
[in]	port	Port number in host byte order.
[out]	out	Zeroed sockaddr_storage to fill.

Returns

AF_INET or AF_INET6 on success, -1 if the string is not a valid address.

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resolve_host()

char * resolve_host ( int debug_level, const char * host )

static

Resolve a host to an IP address.

Parameters

[in]	debug_level	The debug level to be used for the DEBUG() macro
[in]	host	The host to resolve

Returns

A newly heap-allocated NUL-terminated string containing the resolved IP address (IPv4 dotted-quad or IPv6 colon-separated). The caller owns the buffer and must free() it when done. On failure the function prints a diagnostic and calls exit(EXIT_FAILURE).

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settings_initialize()

void settings_initialize

(

struct settings *

s

)

Initialize settings.

Parameters

[out]

s

The settings structure to initialize.

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socket_setup()

int socket_setup ( const int debug_level, const char * desc, const char * xaddr, const int xport, const char * xif, int xfamily, struct sockaddr_storage * xsock_name )

static

Creates a UDP socket on the specified address, port and interface, returning the socket and the socket name (if either arguments were NULL or 0).

Parameters

[in]	debug_level	The debug level to be used for the DEBUG() macro
[in]	desc	The caller description, added to debug messages
[in]	xaddr	The address for the socket to be created, or NULL for ANY
[in]	xport	The port for the socket to be created, or 0 for random (decided by bind())
[in]	xif	The OS interface name to bind to, or NULL for all interfaces.
[in]	xfamily	AF_INET or AF_INET6; used only when xaddr is NULL.
[out]	xsock_name	The name of the socket created.

Returns

The socket file descriptor as integer.

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statistics_display()

void statistics_display	(	int	debug_level,
		struct statistics *	st,
		time_t	now )

Display the stored statistics

Parameters

[in]	debug_level	The debug level to be used for the DEBUG() macro
[in]	st	The statistics structure
[in]	now	The current time

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statistics_initialize()

void statistics_initialize

(

struct statistics *

st

)

Initialize statistics.

Parameters

[out]

st

The statistics structure to initialize.

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usage()

void usage	(	const char *	argv0,
		const char *	message )

Displays the program usage and exit with error.

Parameters

[in]	argv0	The program name as started, e.g., /usr/local/bin/udp-redirect
[in]	message	The error message, or NULL

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Variable Documentation

longopts

struct option longopts[]

static

Command line options.