Injection locking properties of distributed feedback semiconductor lasers are studied systematically. Due to the high side mode suppression, these devices show different locking properties when compared to lasers having Fabry-Perot structures. The main result is the identification of four regimes for different injection levels. In particular, a symmetrical locking band at low optical injection level is confirmed. The presence of this symmetrical band can be exploited in some applications: as examples, the measurement of the linewidth enhancement factor alpha and the PSK modulation capability are reported.