diff --git a/Makefile b/Makefile
index 01670d1..5824015 100644
--- a/Makefile
+++ b/Makefile
@@ -377,5 +377,8 @@ extract_fuzz_telegram_seeds:
 	@rm -f fuzz_testcases/telegrams/seed_*
 	@SEED=1; while read -r line; do echo "$${line}" | xxd -r -p - > "fuzz_testcases/telegrams/seed_$${SEED}"; SEED=$$((SEED + 1)); done < $(BUILD)/seeds; echo "Extracted $${SEED} seeds from simulations."
 
+relay: utils/relay.c
+	gcc -g utils/relay.c -o relay -O0 -ggdb -fsanitize=address -fno-omit-frame-pointer -fprofile-arcs -ftest-coverage
+
 # Include dependency information generated by gcc in a previous compile.
 include $(wildcard $(patsubst %.o,%.d,$(METER_OBJS)))
diff --git a/utils/relay.c b/utils/relay.c
new file mode 100644
index 0000000..e240a50
--- /dev/null
+++ b/utils/relay.c
@@ -0,0 +1,250 @@
+/*
+ Copyright (C) 2021 Fredrik Öhrström
+
+ 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 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+// Utility to relay serial port SND_NR telegrams using mosquitto pub.
+
+#define DEBUG 0
+
+#define HOMEDIR "/home/yourdir"
+
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <termios.h>
+#include <memory.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+typedef unsigned char uchar;
+
+typedef uchar bool;
+#define true 1
+#define false 0
+
+int open_serial(const char *tty, int baud_rate)
+{
+    int rc = 0;
+    speed_t speed = 0;
+    struct termios tios;
+    //int DTR_flag = TIOCM_DTR;
+
+    int fd = open(tty, O_RDWR | O_NOCTTY | O_NONBLOCK);
+    if (fd == -1) {
+        usleep(1000*1000);
+        fd = open(tty, O_RDWR | O_NOCTTY | O_NONBLOCK);
+        if (fd == -1) goto err;
+    }
+
+    switch (baud_rate)
+    {
+    case 300:   speed = B300;  break;
+    case 600:   speed = B600;  break;
+    case 1200:   speed = B1200;  break;
+    case 2400:   speed = B2400;  break;
+    case 4800:   speed = B4800;  break;
+    case 9600:   speed = B9600;  break;
+    case 19200:  speed = B19200; break;
+    case 38400:  speed = B38400; break;
+    case 57600:  speed = B57600; break;
+    case 115200: speed = B115200;break;
+    default:
+        goto err;
+    }
+
+    memset(&tios, 0, sizeof(tios));
+
+    rc = cfsetispeed(&tios, speed);
+    if (rc < 0) goto err;
+    rc = cfsetospeed(&tios, speed);
+    if (rc < 0) goto err;
+
+
+    // CREAD=Enable receive CLOCAL=Ignore any Carrier Detect signal.
+    tios.c_cflag |= (CREAD | CLOCAL);
+    tios.c_cflag &= ~CSIZE;
+    tios.c_cflag |= CS8;
+    tios.c_cflag &=~ CSTOPB;
+    // Disable parity bit.
+    tios.c_cflag &=~ PARENB;
+
+    tios.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
+    tios.c_iflag &= ~INPCK;
+
+    tios.c_iflag &= ~(IXON | IXOFF | IXANY);
+
+    tios.c_oflag &=~ OPOST;
+    tios.c_cc[VMIN] = 0;
+    tios.c_cc[VTIME] = 0;
+
+    rc = tcsetattr(fd, TCSANOW, &tios);
+    if (rc < 0) goto err;
+
+
+    // This code can toggle DTR... maybe necessary
+    // for the pl2303 usb2serial driver/device.
+    //rc = ioctl(fd, TIOCMBIC, &DTR_flag);
+    //if (rc != 0) goto err;
+
+    return fd;
+
+err:
+    if (fd > 0)
+    {
+        close(fd);
+        fd = -1;
+    }
+    return fd;
+}
+
+void manufacturerFlag(uchar lo, uchar hi, uchar *mfct)
+{
+    int m_field = (hi*256)+lo;
+    mfct[0] = (m_field/1024)%32+64;
+    mfct[1] = (m_field/32)%32+64;
+    mfct[2] = (m_field)%32+64;
+}
+
+bool find_telegram(uchar *buf, size_t *len)
+{
+    size_t l = *len;
+    if (*len < 10) return false;
+    size_t tl = buf[0];
+    if (buf[1] != 0x44) return false;
+    if (tl+1 > l) return false;
+    size_t hl = 2*(tl+1)+1;
+    uchar hex[hl];
+    for (size_t i=0; i<tl+1; ++i)
+    {
+        sprintf(hex+i*2, "%02x", buf[i]);
+    }
+    hex[hl-1] = 0;
+    *len = l-tl-1;
+
+    uchar id[9];
+    sprintf(id,   "%02x", buf[7]);
+    sprintf(id+2, "%02x", buf[6]);
+    sprintf(id+4, "%02x", buf[5]);
+    sprintf(id+6, "%02x", buf[4]);
+    id[8] = 0;
+
+    uchar mfct[4];
+    manufacturerFlag(buf[2], buf[3], mfct);
+    mfct[3] = 0;
+
+    char t[21];
+    snprintf(t, sizeof(t), "wmbusmeters/%s", id);
+    char *program = "mosquitto_pub";
+    const char *argv[8];
+    argv[0] = program;
+    argv[1] = "-h";
+    argv[2] = "localhost";
+    argv[3] = "-t";
+    argv[4] = t;
+    argv[5] = "-m";
+    argv[6] = hex;
+    argv[7] = 0;
+
+    printf("Relaying id:%s mft:%s t:%s \"%s\"\n", id, mfct, t, hex);
+
+    const char *env[2];
+    env[0] = "HOME=" HOMEDIR;
+    env[1] = 0;
+
+    pid_t pid = fork();
+    int status;
+    if (pid == 0) {
+        close(0); // Close stdin
+#if (defined(__APPLE__) && defined(__MACH__)) || defined(__FreeBSD__)
+        execve(program, (char*const*)&argv[0], (char*const*)&env[0]);
+#else
+        execvpe(program, (char*const*)&argv[0], (char*const*)&env[0]);
+#endif
+        perror("Execvp failed:");
+    } else {
+        if (pid == -1) {
+            fprintf(stderr, "(shell) could not fork!\n");
+        }
+        waitpid(pid, &status, 0);
+        if (WIFEXITED(status)) {
+            // Child exited properly.
+            int rc = WEXITSTATUS(status);
+            if (rc != 0) {
+                fprintf(stderr, "(shell) %s exited with non-zero return code: %d\n", program, rc);
+            }
+        }
+    }
+    return true;
+}
+
+int main()
+{
+    uchar buf[512];
+    size_t len = 0;
+    int count_to_clear = 0;
+    int fd = open_serial("/dev/ttyUSB0", 19200);
+
+    if (fd == -1)
+    {
+        fprintf(stderr, "Could not open serial port.\n");
+    }
+
+    for (;;)
+    {
+        if (len >= sizeof(buf))
+        {
+            // Oups, a lot of random data fills up the buffer with no telegrams!
+            len = 0;
+            count_to_clear = 0;
+            if (DEBUG) fprintf(stderr, "overflow clear\n");
+        }
+        if (DEBUG) fprintf(stderr, "Reading...");
+        ssize_t n = read(fd, buf+len, sizeof(buf)-len);
+        if (n <= 0)
+        {
+            if (errno == EBADF)
+            {
+                fprintf(stderr, "Could not read from serial port.\n");
+                break;
+            }
+            sleep(1);
+            count_to_clear++;
+            if (count_to_clear >= 2)
+            {
+                // After 2 seconds of no traffic, we clear the buffer.
+                // Any existing telegrams should have been found already.
+                len = 0;
+                count_to_clear = 0;
+                if (DEBUG) fprintf(stderr, "timeout clear\n");
+            }
+            else
+            {
+                if (DEBUG) fprintf(stderr, "no data.\n");
+            }
+            continue;
+        }
+        len+=n;
+        if (DEBUG) fprintf(stderr, "received data, buffer len %zu\n", len);
+        bool ok = find_telegram(buf, &len);
+        if (!ok)
+        {
+            if (DEBUG) fprintf(stderr, "No telegram found in data.");
+        }
+    }
+    close(fd);
+}