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Check for carrier before sendto() or write() on interface fd.

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Linux will quietly proceed as if the data were sent even if the carrier
is down and nothing actually happened.  There is still a tiny race
condition where the carrier could drop between the check and the actual
write, but we really can't do anything about that and it is a very
small race.
This commit is contained in:
Nicholas J. Kain
2015-02-13 21:53:15 -05:00
parent 911d4cc58e
commit b4b6ed8fd5
4 changed files with 44 additions and 2 deletions
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28
src/dhcp.c
View File

@ -31,6 +31,7 @@
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <netpacket/packet.h>
@ -107,6 +108,11 @@ static ssize_t send_dhcp_unicast(struct client_state_t *cs,
}
size_t payload_len =
sizeof *payload - (sizeof payload->options - 1 - endloc);
if (!check_carrier(fd)) {
log_error("%s: (%s) carrier down; write would fail",
client_config.interface, __func__);
goto out_fd;
}
ret = safe_write(fd, (const char *)payload, payload_len);
if (ret < 0 || (size_t)ret != payload_len)
log_error("%s: (%s) write failed: %d", client_config.interface,
@ -224,6 +230,22 @@ static ssize_t get_raw_packet(struct client_state_t *cs,
return l;
}
int check_carrier(int fd)
{
struct ifreq ifr;
memset(&ifr, 0, sizeof ifr);
memcpy(ifr.ifr_name, client_config.interface,
sizeof client_config.interface);
if (ioctl(fd, SIOCGIFFLAGS, &ifr) == -1) {
log_error("%s: (%s) ioctl failed: %s", client_config.interface,
__func__, strerror(errno));
return 0;
}
if ((ifr.ifr_flags & IFF_RUNNING) && (ifr.ifr_flags & IFF_UP))
return 1;
return 0;
}
// Broadcast a DHCP message using a raw socket.
static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
{
@ -282,6 +304,11 @@ static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
.sll_halen = 6,
};
memcpy(da.sll_addr, "\xff\xff\xff\xff\xff\xff", 6);
if (!check_carrier(fd)) {
log_error("%s: (%s) carrier down; sendto would fail",
client_config.interface, __func__);
goto carrier_down;
}
ret = safe_sendto(fd, (const char *)&iudmsg, iud_len, 0,
(struct sockaddr *)&da, sizeof da);
if (ret < 0 || (size_t)ret != iud_len) {
@ -292,6 +319,7 @@ static ssize_t send_dhcp_raw(struct dhcpmsg *payload)
log_error("%s: (%s) sendto short write: %z < %zu",
client_config.interface, __func__, ret, iud_len);
}
carrier_down:
close(fd);
return ret;
}