Permission denied in Kannel server

I already solve it

The serial port, where the modem is, has permissions for the dialout group.

# ls -la /dev/ttyS*
crw-rw---- 1 root dialout 4, 64 jul 21 17:48 /dev/ttyS0
crw-rw---- 1 root dialout 4, 65 ene 21  2020 /dev/ttyS1
crw-rw---- 1 root dialout 4, 66 ene 21  2020 /dev/ttyS2
crw-rw---- 1 root dialout 4, 67 ene 21  2020 /dev/ttyS3

I only included user kannel in group dialout

# usermod -a -G dialout kannel

# cat /etc/group | grep dialout
dialout:x:18:kannel

Thanks anyway

SELinux is enabled preventing network access of the service to another. You need to allow service "A" to access service "B" local socket or port.

Consult details in CentOS documentation

Command Audit2Allow should help you figure out what's going on, more about on RedHat documentation

I'd recommend to learn the SELinux instead of disabling it.