odhcpd: add support for dhcpv6_pd_min_len parameter

[project/odhcpd.git] / src / dhcpv6-ia.c

diff --git a/src/dhcpv6-ia.c b/src/dhcpv6-ia.c
index 99fd2fda61cab453158ef8e4ea05fe903d210994..41c9f30fbcc9cb6c7e9cba895626a035569cd4d3 100644 (file)
--- a/src/dhcpv6-ia.c
+++ b/src/dhcpv6-ia.c
@@ -1332,6 +1332,31 @@ ssize_t dhcpv6_ia_handle_IAs(uint8_t *buf, size_t buflen, struct interface *ifac
 
                        if (reqlen > 64)
                                reqlen = 64;
+
+                       /*
+                        * A requesting router can include a desired prefix length for its
+                        * delegation.  The delegating router (us) is not required to honor
+                        * the hint (RFC3633, section 11.2, we MAY choose to use the
+                        * information in the option; RFC8168, section 3.2 has several SHOULDs
+                        * about desired choices for selecting a prefix to delegate).
+                        *
+                        * We support a policy setting to conserve prefix space, which purposely
+                        * assigns prefixes that might not match the requesting router's hint.
+                        *
+                        * If the minimum prefix length is set in this interface's
+                        * configuration, we use it as a floor for the requested (hinted)
+                        * prefix length.  This allows us to conserve prefix space so that
+                        * any single router can't grab too much of it.  Consider if we have
+                        * an interface with a /56 prefix.  A requesting router could ask for
+                        * a /58 and take 1/4 of our total address space.  But if we set a
+                        * minimum of /60, we can limit each requesting router to get only
+                        * 1/16 of our total address space.
+                        */
+                       if (iface->dhcpv6_pd_min_len && reqlen < iface->dhcpv6_pd_min_len) {
+                           syslog(LOG_INFO, "clamping requested PD from %d to %d",
+                                  reqlen, iface->dhcpv6_pd_min_len);
+                           reqlen = iface->dhcpv6_pd_min_len;
+                       }
                } else if (is_na) {
                        uint8_t *sdata;
                        uint16_t stype, slen;