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Handle the case of a DNS server on localhost

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By default, passt detects the nameserver used by the host system by reading
/etc/resolv.conf, and advertises that to the guest via DHCP.  However this
breaks down if the host's nameserver is local (on 127.0.0.1 or ::1);
connecting to localhost on the guest won't reach the host's nameserver.

Using a local nameserver is a reasonably common case when using dnsmasq
or similar to merge name resolution on a home network with name resolution
from an organization-private VPN.

We already have the gateway mapping support to allow reaching host-local
services from the guest via the address of the default gateway.  Add code
to detect the case of a local DNS server and use the gateway mapping to
advertise it usefully to the guest.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
David Gibson 2022-07-06 17:28:56 +10:00 committed by Stefano Brivio
parent 4d777144fd
commit aa8603e6a9
1 changed files with 16 additions and 0 deletions
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16
conf.c
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@ -350,6 +350,14 @@ static void get_dns(struct ctx *c)
if (!dns4_set &&
dns4 - &c->dns4[0] < ARRAY_SIZE(c->dns4) - 1 &&
inet_pton(AF_INET, p + 1, dns4)) {
/* We can only access local addresses via the gw redirect */
if (ntohl(*dns4) >> IN_CLASSA_NSHIFT == IN_LOOPBACKNET) {
if (c->no_map_gw) {
*dns4 = 0;
continue;
}
*dns4 = c->gw4;
}
dns4++;
*dns4 = 0;
}
@ -357,6 +365,14 @@ static void get_dns(struct ctx *c)
if (!dns6_set &&
dns6 - &c->dns6[0] < ARRAY_SIZE(c->dns6) - 1 &&
inet_pton(AF_INET6, p + 1, dns6)) {
/* We can only access local addresses via the gw redirect */
if (IN6_IS_ADDR_LOOPBACK(dns6)) {
if (c->no_map_gw) {
memset(dns6, 0, sizeof(*dns6));
continue;
}
memcpy(dns6, &c->gw6, sizeof(*dns6));
}
dns6++;
memset(dns6, 0, sizeof(*dns6));
}