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Make IP and UDP checksum calculations for sending raw packets much less

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confusing.  They are now calculated without mutating the underlying
structures so that there is no longer any tricky operation order dependence.
This commit is contained in:
Nicholas J. Kain
2011-06-26 17:50:35 -04:00
parent 0581cc9aed
commit 057211d5f5
1 changed files with 15 additions and 9 deletions
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24
ndhc/packet.c
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@@ -366,13 +366,22 @@ static int send_dhcp_raw(struct dhcpmsg *payload)
size_t padding = DHCP_OPTIONS_BUFSIZE - 1 - endloc; size_t padding = DHCP_OPTIONS_BUFSIZE - 1 - endloc;
size_t iud_len = sizeof(struct ip_udp_dhcp_packet) - padding; size_t iud_len = sizeof(struct ip_udp_dhcp_packet) - padding;
size_t ud_len = sizeof(struct udp_dhcp_packet) - padding; size_t ud_len = sizeof(struct udp_dhcp_packet) - padding;
// UDP checksumming needs a temporary pseudoheader with a fake length.
struct iphdr ph = {
.saddr = INADDR_ANY,
.daddr = INADDR_BROADCAST,
.protocol = IPPROTO_UDP,
.tot_len = htons(ud_len),
};
struct ip_udp_dhcp_packet iudmsg = { struct ip_udp_dhcp_packet iudmsg = {
.ip = { .ip = {
.protocol = IPPROTO_UDP,
.saddr = INADDR_ANY, .saddr = INADDR_ANY,
.daddr = INADDR_BROADCAST, .daddr = INADDR_BROADCAST,
.tot_len = htons(ud_len), .protocol = IPPROTO_UDP,
.tot_len = htons(iud_len),
.ihl = sizeof iudmsg.ip >> 2,
.version = IPVERSION,
.ttl = IPDEFTTL,
}, },
.udp = { .udp = {
.source = htons(DHCP_CLIENT_PORT), .source = htons(DHCP_CLIENT_PORT),
@@ -381,12 +390,9 @@ static int send_dhcp_raw(struct dhcpmsg *payload)
}, },
.data = *payload, .data = *payload,
}; };
iudmsg.udp.check = net_checksum(&iudmsg, iud_len); uint16_t udpcs = net_checksum(&iudmsg.udp, ud_len);
// Set the true IP packet length for the final packet. uint16_t phcs = net_checksum(&ph, sizeof ph);
iudmsg.ip.tot_len = htons(iud_len); iudmsg.udp.check = net_checksum_add(udpcs, phcs);
iudmsg.ip.ihl = sizeof iudmsg.ip >> 2;
iudmsg.ip.version = IPVERSION;
iudmsg.ip.ttl = IPDEFTTL;
iudmsg.ip.check = net_checksum(&iudmsg.ip, sizeof iudmsg.ip); iudmsg.ip.check = net_checksum(&iudmsg.ip, sizeof iudmsg.ip);
r = safe_sendto(fd, (const char *)&iudmsg, iud_len, 0, r = safe_sendto(fd, (const char *)&iudmsg, iud_len, 0,