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[Fix] Preserve req->pos during reply validation to prevent packet truncation #5739

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Merged
vstakhov merged 2 commits into from
Nov 10, 2025
Merged

[Fix] Preserve req->pos during reply validation to prevent packet truncation #5739

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1833877
[Fix] Preserve req->pos during reply validation to prevent packet tru…
vstakhov Nov 10, 2025
c774de1
[Fix] Regenerate DNS transaction ID before copying packet to TCP buffer
vstakhov Nov 10, 2025
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30 changes: 17 additions & 13 deletions contrib/librdns/resolver.c
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Original file line number Diff line number Diff line change
Expand Up @@ -220,17 +220,20 @@ rdns_parse_reply(uint8_t *in, int r, struct rdns_request *req,
* Now we have request and query data is now at the end of header, so compare
* request QR section and reply QR section
*/
unsigned int saved_pos = req->pos;
req->pos = sizeof(struct dns_header);
pos = in + sizeof(struct dns_header);
t = r - sizeof(struct dns_header);
for (i = 0; i < (int) qdcount; i++) {
if ((npos = rdns_request_reply_cmp(req, pos, t)) == NULL) {
rdns_info("DNS request with id %d is for different query, ignoring", (int) req->id);
req->pos = saved_pos;
return false;
}
t -= npos - pos;
pos = npos;
}
req->pos = saved_pos;
/*
* Now pos is in answer section, so we should extract data and form reply
*/
Expand Down Expand Up @@ -481,19 +484,6 @@ rdns_reschedule_req_over_tcp(struct rdns_request *req, struct rdns_server *serv)
return false;
}

oc->write_buf = ((unsigned char *) oc) + sizeof(*oc);
memcpy(oc->write_buf, req->packet, req->pos);
oc->next_write_size = htons(req->pos);

DL_APPEND(ioc->tcp->output_chain, oc);

if (ioc->tcp->async_write == NULL) {
ioc->tcp->async_write = resolver->async->add_write(
resolver->async->data,
ioc->sock, ioc);
}

req->state = RDNS_REQUEST_TCP;
/* Switch IO channel from UDP to TCP */

@github-actions github-actions Bot Nov 10, 2025

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Bug: by moving the channel switch before allocating oc, the failure path returns false after we have already removed the request from the UDP hash, changed req->io, and potentially regenerated the id. Because we exit before assigning kh_value, re-arming the timer, or releasing old_ioc, the request is left detached while the old channel keeps its ref. Please defer the switch until after calloc succeeds (as before) or add explicit rollback on error.

rdns_request_remove_from_hash(req);
req->io = ioc;
Expand All @@ -515,6 +505,20 @@ rdns_reschedule_req_over_tcp(struct rdns_request *req, struct rdns_server *serv)
}
}

oc->write_buf = ((unsigned char *) oc) + sizeof(*oc);
memcpy(oc->write_buf, req->packet, req->pos);
oc->next_write_size = htons(req->pos);

DL_APPEND(ioc->tcp->output_chain, oc);

if (ioc->tcp->async_write == NULL) {
ioc->tcp->async_write = resolver->async->add_write(
resolver->async->data,
ioc->sock, ioc);
}

req->state = RDNS_REQUEST_TCP;

req->async_event = resolver->async->add_timer(resolver->async->data,
req->timeout, req);

Expand Down