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Detect address conflicts, simplified #3

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thom-nic wants to merge 2 commits into from
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Detect address conflicts, simplified #3

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fd2823e
mdnsd.c: fix conflict detection to filter out all assigned IP addresses
thom-nic Jan 13, 2026
b2ebe28
fix whitespace
thom-nic Jan 13, 2026
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38 changes: 34 additions & 4 deletions libmdnsd/mdnsd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,7 @@
#include <stdbool.h>
#include <time.h>
#include <errno.h>
#include <ifaddrs.h>

#define SPRIME 109 /* Size of query/publish hashes */
#define LPRIME 1009 /* Size of cache hash */
Expand Down Expand Up @@ -675,6 +676,34 @@ static int _r_out(mdns_daemon_t *d, struct message *m, mdns_record_t **list)
return ret;
}

/* Check if an IPv4 address belongs to this host (any interface) */
static bool _is_local_ipv4(mdns_daemon_t *d, struct in_addr ip)
{
/* Always consider the primary configured address as local */
if (ip.s_addr == d->addr.s_addr)
return true;

struct ifaddrs *ifa = NULL;

if (getifaddrs(&ifa) != 0)
return false;

for (struct ifaddrs *it = ifa; it; it = it->ifa_next) {
if (!it->ifa_addr)
continue;
if (it->ifa_addr->sa_family != AF_INET)
continue;

struct sockaddr_in *sin = (struct sockaddr_in *)it->ifa_addr;
if (sin->sin_addr.s_addr == ip.s_addr) {
freeifaddrs(ifa);
return true;
}
}

freeifaddrs(ifa);
return false;
}

mdns_daemon_t *mdnsd_new(int class, int frame)
{
Expand Down Expand Up @@ -869,6 +898,10 @@ int mdnsd_in(mdns_daemon_t *d, struct message *m, struct in_addr ip, unsigned sh

gettimeofday(&d->now, 0);

/* Ignore packets originated from any of our local IPv4 addresses */
if (_is_local_ipv4(d, ip))
return 0;

if (m->header.qr == 0) {
/* Process each query */
for (i = 0; i < m->qdcount; i++) {
Expand All @@ -883,7 +916,7 @@ int mdnsd_in(mdns_daemon_t *d, struct message *m, struct in_addr ip, unsigned sh
if (!r)
continue;

/* Service enumeratio/discovery prepeare to send all matching records */
/* Service enumeration/discovery prepeare to send all matching records */
if (!strcmp(m->qd[i].name, DISCO_NAME)) {
d->disco = 1;
while (r) {
Expand Down Expand Up @@ -961,9 +994,6 @@ int mdnsd_in(mdns_daemon_t *d, struct message *m, struct in_addr ip, unsigned sh
INFO("Got Answer: Name: %s, Type: %d", m->an[i].name, m->an[i].type);
r = _r_next(d, NULL, m->an[i].name, m->an[i].type);
if (r && r->unique && r->modified && _a_match(&m->an[i], &r->rr)) {
/* double check, is this actually from us, looped back? */
if (ip.s_addr == d->addr.s_addr)
continue;
_conflict(d, r);
}

Expand Down
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