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SubjectRe: consistent ioctl for getting all net interfaces?
On Sat, 22 May 2004, Joshua Kwan wrote:
> I'm interested in not having to parse /proc/net/dev to get a list of all
> available (not necessarily even up) interfaces on the system. I
> investigated the ioctl SIOCGIFCONF, but it seems to behave differently on
> 2.4 and 2.6 series kernels, e.g. sometimes it won't return all interfaces.
>
> Is there some end-all ioctl that does what I want, or am I forever doomed
> to process /proc/net/dev (in C, no less..)?
>
> Please CC me on replies, I don't read this list very often any more.

Of course this is possible and here is the solution I wrote some time ago
(ioctl-based).

Note that a more simple solution is also possible but is less portable
(because will depend on glibc version).

/* TCPCAP endpoint, just an opaque handle for applications */
struct tcpcap {
int fd; /* socket file descriptor */
struct timeval *ts; /* user supplied addr of timestamp */
struct sockaddr_ll *from; /* user supplied addr of extra info */
int nports; /* number of bits set in ->ports */
int maxport; /* highest port number set in ->ports */
unsigned char *ports; /* ports currently set in ->lsf */
unsigned char *setports; /* pending ports to be added */
unsigned char *clrports; /* pending ports to be removed */
int recv_buflen; /* socket receive buffer size */
struct sock_fprog *lsf; /* compiled filter program */
struct sock_filter *lsf_insns; /* the actual LSF instructions */
int snaplen; /* length of part of each packet */
int pkt_count; /* number of packets seen */
int promisc; /* set interface(s) to promisc. mode */
struct tcpcap_if *iface; /* list of network interfaces */
int ifcount; /* number of elements in ->iface[] */
};


/*
* internal helper: get the list of all IPv4 up interfaces
* and record their name and IP address into pcap->iface[]
* array. Also set promiscuous mode as requested via pcap->promisc.
*/
static int walkiflist(struct tcpcap *pcap)
{
struct ifconf ifc;
struct ifreq *ifreqs, *ifr;
int fd, rq_len, nifs, i, ret = 0;

/* this is a helper datagram socket which we must create
* because the actual packet socket is created later on,
* at tcpcap_start() time.
*/
fd = socket(PF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
DPRINTF("socket(), errno=%d (%s)\n",
errno, strerror(errno));
return TERR_SOCKET;
}

ifc.ifc_buf = NULL;
rq_len = 4*sizeof(struct ifreq);
do {
ifc.ifc_len = rq_len;
ifc.ifc_buf = realloc(ifc.ifc_buf, ifc.ifc_len);
if (ifc.ifc_buf == NULL) {
DPRINTF("ifc.buf = realloc() failed\n");
ret = TERR_REALLOC;
goto outclose;
}
if(ioctl(fd, SIOCGIFCONF, &ifc) < 0) {
DPRINTF("ioctl(SIOCGIFCONF), errno=%d (%s)\n",
errno, strerror(errno));
if (ifc.ifc_buf)
free(ifc.ifc_buf);
ret = TERR_IOCTL;
goto outclose;
}
rq_len *= 2;
} while (rq_len < sizeof(struct ifreq) + ifc.ifc_len);

nifs = ifc.ifc_len / sizeof(struct ifreq);
ifreqs = realloc(ifc.ifc_buf, nifs*sizeof(struct ifreq));
if (ifreqs == NULL) {
DPRINTF("ifreqs = realloc()\n");
ret = TERR_REALLOC;
free(ifc.ifc_buf);
goto outclose;
}

/* allocate enough space for the maximum number of interfaces */
pcap->iface = zalloc(nifs*sizeof(struct tcpcap_if));
if (pcap->iface == NULL) {
DPRINTF("pcap->iface = zalloc() failed\n");
ret = TERR_ZALLOC;
goto outfree;
}

/* look through what we found and select only the 'interesting' ones */
for (ifr = ifreqs, i=0; i<nifs; ifr++, i++) {
struct sockaddr_in *addr;

/* only interested in IPv4 */
if (ifr->ifr_addr.sa_family != AF_INET)
continue;

/* not interested in loopback */
if (!strncmp(ifr->ifr_name, "lo", 2))
continue;

/* request flags because SIOCGIFCONF only
* initialized name, family and address
*/
if(ioctl(fd, SIOCGIFFLAGS, ifr) < 0) {
DPRINTF("ioctl(SIOCGIFFLAGS), errno=%d (%s)\n",
errno, strerror(errno));
ret = TERR_IOCTL;
goto outfree;
}

/* not interested in down interfaces */
if (!(ifr->ifr_flags & IFF_UP))
continue;

/* OK, this interface passed all our criteria, so
* record it into pcap->iface[] array
*/
pcap->iface[pcap->ifcount].ifname =strdup(ifr->ifr_name);
addr = (struct sockaddr_in *)&(ifr->ifr_addr);
pcap->iface[pcap->ifcount].addr = htonl(addr->sin_addr.s_addr);

/* if required set this interface into promisc. mode,
* unless it is already in promiscuous mode.
*/
if (pcap->promisc && !(ifr->ifr_flags & IFF_PROMISC)) {

/* enable promiscuous mode */
ifr->ifr_flags |= IFF_PROMISC;

/* set interface flags */
if (ioctl(fd, SIOCSIFFLAGS, ifr) == -1) {
DPRINTF("ioctl(SIOCSIFFLAGS), errno=%d (%s)\n",
errno, strerror(errno));
ret = TERR_IOCTL;
goto outfree;
}

/* record the fact that we modified this interface */
pcap->iface[pcap->ifcount].promisc = 1;
DPRINTF("Enabled promisc. mode on %s (0x%x)\n",
pcap->iface[pcap->ifcount].ifname,
pcap->iface[pcap->ifcount].addr);
}

pcap->ifcount++;
}

outfree:
free(ifreqs);

outclose:
close(fd);

return ret;
}

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