[PATCH v8 11/14] dhcp: Select address for DHCP distribution

[Jon Maloy <jmaloy@redhat.com>]

We introduce a CONF_ADDR_DHCPOFFER flag to mark if an added address is
eligible for DHCP advertisement. By doing this once and for all
in the fwd_set_addr() function, the DHCP code only needs to check
for this flag to know that all criteria for advertisement are
fulfilled. Hence, we update the code in dhcp.c correspondingly.

We also let the conf_print() function use this flag to determine
and print the selected address.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

---
v6: -Split off from a commit handling both DHCP and DHCPv6

v7: -Modified DHCP advertisement eligibility criteria IPv4 addresses:
     We now permit link-local addresses to be eligible if they were
     configured by the user.
    -Adapted to previous changes in this series

v8: - Adapted to previous changes in this series
    - Simplified DHCP eligibility branch in fwd_set_addr() (David)
    - Moved this commit to earlier in the series to avoid
      a technical migration protocol change (David).
    - Renamed CONF_ADDR_DHCP to CONF_ADDR_DHCPOFFER (Stefano)
---
 conf.c  |  5 +++--
 dhcp.c  | 12 ++++++------
 fwd.c   |  9 +++++++++
 passt.h |  1 +
 4 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/conf.c b/conf.c
index 69c1c439..f30c238e 100644
--- a/conf.c
+++ b/conf.c
@@ -47,6 +47,7 @@
 #include "lineread.h"
 #include "isolation.h"
 #include "log.h"
+#include "fwd.h"
 #include "vhost_user.h"
 #include "epoll_ctl.h"
 #include "conf.h"
@@ -832,8 +833,8 @@ static void conf_print(const struct ctx *c)
 			     inet_ntop(AF_INET, &c->ip4.map_host_loopback,
 				       buf, sizeof(buf)));
 
-		a = fwd_get_addr(c, AF_INET, CONF_ADDR_ANY, 0);
-		if (a && !c->no_dhcp) {
+		a = fwd_get_addr(c, AF_INET, CONF_ADDR_DHCPOFFER, 0);
+		if (a) {
 			uint32_t mask;
 			int plen = 0;
 
diff --git a/dhcp.c b/dhcp.c
index 727485f7..a7a9765a 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -31,6 +31,8 @@
 #include "passt.h"
 #include "tap.h"
 #include "log.h"
+#include "fwd.h"
+#include "conf.h"
 #include "dhcp.h"
 
 /**
@@ -302,19 +304,18 @@ static void opt_set_dns_search(const struct ctx *c, size_t max_len)
  */
 int dhcp(const struct ctx *c, struct iov_tail *data)
 {
+	struct in_addr addr, mask, dst;
 	char macstr[ETH_ADDRSTRLEN];
 	const struct guest_addr *a;
 	size_t mlen, dlen, opt_len;
-	struct in_addr mask, dst;
 	struct ethhdr eh_storage;
 	struct iphdr iph_storage;
 	struct udphdr uh_storage;
+	const struct udphdr *uh;
 	const struct ethhdr *eh;
 	const struct iphdr *iph;
-	const struct udphdr *uh;
 	struct msg m_storage;
 	struct msg const *m;
-	struct in_addr addr;
 	struct msg reply;
 	unsigned int i;
 	int plen;
@@ -347,9 +348,8 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
 	    m->op != BOOTREQUEST)
 		return -1;
 
-	a = fwd_get_addr(c, AF_INET, CONF_ADDR_USER, 0);
-	if (!a)
-		a = fwd_get_addr(c, AF_INET, CONF_ADDR_ANY, 0);
+	/* Select address to offer */
+	a = fwd_get_addr(c, AF_INET, CONF_ADDR_DHCPOFFER, 0);
 	if (!a)
 		return -1;
 
diff --git a/fwd.c b/fwd.c
index e4090f4d..669fc237 100644
--- a/fwd.c
+++ b/fwd.c
@@ -292,6 +292,15 @@ void fwd_set_addr(struct ctx *c, const union inany_addr *addr,
 		return;
 	}
 
+	/* Determine advertisement eligibility */
+	if ((flags & CONF_ADDR_USER) ||
+	    (flags & CONF_ADDR_HOST && !(flags & CONF_ADDR_LINKLOCAL))) {
+		if (inany_v4(addr)) {
+			if (!c->no_dhcp)
+				flags |= CONF_ADDR_DHCPOFFER;
+		}
+	}
+
 	/* Add to head or tail, depending on flag */
 	if (flags & CONF_ADDR_OBSERVED) {
 		memmove(&head[1], &head[0], c->addr_count * sizeof(*a));
diff --git a/passt.h b/passt.h
index 005f7631..b323bd48 100644
--- a/passt.h
+++ b/passt.h
@@ -82,6 +82,7 @@ struct guest_addr {
 #define CONF_ADDR_GENERATED	BIT(2)		/* Generated by PASST/PASTA */
 #define CONF_ADDR_LINKLOCAL	BIT(3)		/* Link-local address */
 #define CONF_ADDR_OBSERVED	BIT(4)		/* Seen in guest traffic */
+#define CONF_ADDR_DHCPOFFER	BIT(5)		/* Advertise via DHCP (IPv4) */
 #define CONF_ADDR_ANY		0xff		/* Match any flag */
 };
 
-- 
2.52.0