At the moment my program is making a system() call to ifconfig to do this.
It seems a bit messy - maybe ifconfig is not on the path, or in some non-standard location. And then I'd need to check for the iproute2 equivalent in case of failure.
Is there a way to set this programmatically using C?
You can set the SIOCSIFMTU field in an ioctl call, something like this:
struct ifreq ifr;
ifr.ifr_addr.sa_family = AF_INET;//address family
strncpy(ifr.ifr_name, "eth0", sizeof(ifr.ifr_name));//interface name where you want to set the MTU
ifr.ifr_mtu = 9100; //your MTU size here
if (ioctl(sockfd, SIOCSIFMTU, (caddr_t)&ifr) < 0)
//failed to set MTU. handle error.
The above code will set the MTU of a device(as in ifr.name) using ifr_mtu field in the ifreq structure.