Add the ability to specify destination port in DHCP-relay mode.

This change also removes a previous bug
where --dhcp-alternate-port would affect the port used
to relay _to_ as well as the port being listened on.
The new feature allows configuration to provide bug-for-bug
compatibility, if required. Thanks to Damian Kaczkowski
for the feature suggestion.

CHANGELOG

@@ -58,6 +58,14 @@ version 2.87
 	Fix write-after-free error in DHCPv6 server code.
 	CVE-2022-0934 refers.
 
+	Add the ability to specify destination port in
+	DHCP-relay mode. This change also removes a previous bug
+	where --dhcp-alternate-port would affect the port used
+	to relay _to_ as well as the port being listened on.
+	The new feature allows configuration to provide bug-for-bug
+	compatibility, if required. Thanks to Damian Kaczkowski 
+	for the feature suggestion.
+
 
 version 2.86
 	Handle DHCPREBIND requests in the DHCPv6 server code.

man/dnsmasq.8

@@ -1342,17 +1342,20 @@ DHCP options. This make extra space available in the DHCP packet for
 options but can, rarely, confuse old or broken clients. This flag
 forces "simple and safe" behaviour to avoid problems in such a case.
 .TP
-.B --dhcp-relay=<local address>[,<server address>][,<interface]
+.B --dhcp-relay=<local address>[,<server address>[#<server port>]][,<interface]
 Configure dnsmasq to do DHCP relay. The local address is an address
 allocated to an interface on the host running dnsmasq. All DHCP
 requests arriving on that interface will we relayed to a remote DHCP
 server at the server address. It is possible to relay from a single local
 address to multiple remote servers by using multiple \fB--dhcp-relay\fP
 configs with the same local address and different server
 addresses. A server address must be an IP literal address, not a
-domain name. If the server address is ommitted, the request will be
+domain name. If the server address is omitted, the request will be
 forwarded by broadcast (IPv4) or multicast (IPv6). In this case the interface
-must be given and not be wildcard.
+must be given and not be wildcard. The server address may specify a non-standard
+port to relay to. If this is used then \fB--dhcp-proxy\fP should likely also be set,
+otherwise parts of the DHCP conversation which do not pass through the relay
+will be delivered to the wrong port.
 
 Access control for DHCP clients has the same rules as for the DHCP
 server, see \fB--interface\fP, \fB--except-interface\fP, etc. The optional

src/dhcp-common.c

@@ -1017,15 +1017,22 @@ void log_relay(int family, struct dhcp_relay *relay)
 {
   int broadcast = relay->server.addr4.s_addr == 0;
   inet_ntop(family, &relay->local, daemon->addrbuff, ADDRSTRLEN);
-  inet_ntop(family, &relay->server, daemon->namebuff, ADDRSTRLEN); 
+  inet_ntop(family, &relay->server, daemon->namebuff, ADDRSTRLEN);
+
+  if (family == AF_INET && relay->port != DHCP_SERVER_PORT)
+    sprintf(daemon->namebuff + strlen(daemon->namebuff), "#%u", relay->port);
 
 #ifdef HAVE_DHCP6
   struct in6_addr multicast;
 
   inet_pton(AF_INET6, ALL_SERVERS, &multicast);
 
   if (family == AF_INET6)
-    broadcast = IN6_ARE_ADDR_EQUAL(&relay->server.addr6, &multicast);
+    {
+      broadcast = IN6_ARE_ADDR_EQUAL(&relay->server.addr6, &multicast);
+      if (relay->port != DHCPV6_SERVER_PORT)
+	sprintf(daemon->namebuff + strlen(daemon->namebuff), "#%u", relay->port);
+    }
 #endif
 
 

src/dhcp.c

@@ -1121,7 +1121,7 @@ static int relay_upstream4(int iface_index, struct dhcp_packet *mess, size_t sz)
 
 	to.sa.sa_family = AF_INET;
 	to.in.sin_addr = relay->server.addr4;
-	to.in.sin_port = htons(daemon->dhcp_server_port);
+	to.in.sin_port = htons(relay->port);
 
 	/* Broadcasting to server. */
 	if (relay->server.addr4.s_addr == 0)

src/dnsmasq.h

@@ -1085,6 +1085,7 @@ struct dhcp_relay {
   union all_addr local, server;
   char *interface; /* Allowable interface for replies from server, and dest for IPv6 multicast */
   int iface_index; /* working - interface in which requests arrived, for return */
+  int port;        /* Port of relay we forward to. */
 #ifdef HAVE_SCRIPT
   struct snoop_record {
     struct in6_addr client, prefix;

src/option.c

@@ -4333,6 +4333,11 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 	  {
 	    if (inet_pton(AF_INET, arg, &new->local))
 	      {
+		char *hash = split_chr(two, '#');
+
+		if (!hash || !atoi_check16(hash, &new->port))
+		  new->port = DHCP_SERVER_PORT;
+
 		if (!inet_pton(AF_INET, two, &new->server))
 		  {
 		    new->server.addr4.s_addr = 0;
@@ -4351,6 +4356,11 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 #ifdef HAVE_DHCP6
 	    else if (inet_pton(AF_INET6, arg, &new->local))
 	      {
+		char *hash = split_chr(two, '#');
+
+		if (!hash || !atoi_check16(hash, &new->port))
+		  new->port = DHCPV6_SERVER_PORT;
+
 		if (!inet_pton(AF_INET6, two, &new->server))
 		  {
 		    inet_pton(AF_INET6, ALL_SERVERS, &new->server.addr6);

src/rfc3315.c

@@ -2170,7 +2170,7 @@ int relay_upstream6(int iface_index, ssize_t sz,
 
 	to.sa.sa_family = AF_INET6;
 	to.in6.sin6_addr = relay->server.addr6;
-	to.in6.sin6_port = htons(DHCPV6_SERVER_PORT);
+	to.in6.sin6_port = htons(relay->port);
 	to.in6.sin6_flowinfo = 0;
 	to.in6.sin6_scope_id = 0;