[PATCH v2 2/2] fwd: Broaden what we consider for DNS specific forwarding rules

[David Gibson <david@gibson.dropbear.id.au>]

passt/pasta has options to redirect DNS requests from the guest to a
different server address on the host side.  Currently, however, only UDP
packets to port 53 are considered "DNS requests".  This ignores DNS
requests over TCP - less common, but certainly possible.  It also ignores
encrypted DNS requests on port 853.

Extend the DNS forwarding logic to handle both of those cases.

Link: https://github.com/containers/podman/issues/23239

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 fwd.c   | 18 ++++++++++++++++--
 passt.1 | 10 +++++-----
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/fwd.c b/fwd.c
index c323aba7..dea36f6c 100644
--- a/fwd.c
+++ b/fwd.c
@@ -156,6 +156,20 @@ void fwd_scan_ports_init(struct ctx *c)
 	}
 }
 
+/**
+ * is_dns_flow() - Determine if flow appears to be a DNS request
+ * @proto:	Protocol (IP L4 protocol number)
+ * @ini:	Flow address information of the initiating side
+ *
+ * Return: true if the flow appears to be directed at a dns server, that is a
+ *         TCP or UDP flow to port 53 (domain) or port 853 (domain-s)
+ */
+static bool is_dns_flow(uint8_t proto, const struct flowside *ini)
+{
+	return ((proto == IPPROTO_UDP) || (proto == IPPROTO_TCP)) &&
+		((ini->fport == 53) || (ini->fport == 853));
+}
+
 /**
  * fwd_nat_from_tap() - Determine to forward a flow from the tap interface
  * @c:		Execution context
@@ -169,10 +183,10 @@ void fwd_scan_ports_init(struct ctx *c)
 uint8_t fwd_nat_from_tap(const struct ctx *c, uint8_t proto,
 			 const struct flowside *ini, struct flowside *tgt)
 {
-	if (proto == IPPROTO_UDP && ini->fport == 53 &&
+	if (is_dns_flow(proto, ini) &&
 	    inany_equals4(&ini->faddr, &c->ip4.dns_match))
 		tgt->eaddr = inany_from_v4(c->ip4.dns_host);
-	else if (proto == IPPROTO_UDP && ini->fport == 53 &&
+	else if (is_dns_flow(proto, ini) &&
 		   inany_equals6(&ini->faddr, &c->ip6.dns_match))
 		tgt->eaddr.a6 = c->ip6.dns_host;
 	else if (!c->no_map_gw && inany_equals4(&ini->faddr, &c->ip4.gw))
diff --git a/passt.1 b/passt.1
index abc13d51..81789cc4 100644
--- a/passt.1
+++ b/passt.1
@@ -244,11 +244,11 @@ usage of DNS addresses altogether.
 
 .TP
 .BR \-\-dns-forward " " \fIaddr
-Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the first
-configured DNS resolver (with corresponding IP version). Mapping is limited to
-UDP traffic directed to port 53, and DNS answers are translated back with a
-reverse mapping.
-This option can be specified zero to two times (once for IPv4, once for IPv6).
+Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the
+first configured DNS resolver (with corresponding IP version). Maps
+only UDP and TCP traffic to port 53 or port 853.  Replies are
+translated back with a reverse mapping.  This option can be specified
+zero to two times (once for IPv4, once for IPv6).
 
 .TP
 .BR \-S ", " \-\-search " " \fIlist
-- 
@@ -244,11 +244,11 @@ usage of DNS addresses altogether.
 
 .TP
 .BR \-\-dns-forward " " \fIaddr
-Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the first
-configured DNS resolver (with corresponding IP version). Mapping is limited to
-UDP traffic directed to port 53, and DNS answers are translated back with a
-reverse mapping.
-This option can be specified zero to two times (once for IPv4, once for IPv6).
+Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the
+first configured DNS resolver (with corresponding IP version). Maps
+only UDP and TCP traffic to port 53 or port 853.  Replies are
+translated back with a reverse mapping.  This option can be specified
+zero to two times (once for IPv4, once for IPv6).
 
 .TP
 .BR \-S ", " \-\-search " " \fIlist
-- 
2.45.2