Some header files are missing header guards
I suggest a CI step involving a python script that checks for missing header guards.
By convention, all or most header files should have a header guard to prevent including their content twice which can result in unforeseen errors that are difficult to figure out.
Here's a low-effort (and not entirely correct) attempt at determining which files are missing the header guards.
$ find . -type f -name '*.h' | xargs grep -L ifndef
./premium/src/hooks/dhcp/lease_query/tests/blq_utils.h
./premium/src/hooks/dhcp/forensic_log/subnets_user_context.h
./premium/src/hooks/dhcp/subnet_cmds/tests/subnet_cmds_unittest.h
./premium/src/hooks/dhcp/ddns_tuning/libloadtests/callout_unittests.h
./src/hooks/dhcp/lease_cmds/tests/lease_cmds_unittest.h
./src/hooks/dhcp/high_availability/tests/ha_test.h
./src/bin/dhcp6/tests/callout_library_common.h
./src/bin/dhcp4/tests/callout_library_common.h
./src/lib/util/tests/memory_segment_common_unittest.h
./src/lib/util/unittests/interprocess_util.h
./src/lib/asiodns/logger.h
./src/lib/dns/rdata/any_255/tsig_250.h
./src/lib/dns/rdata/template.h
./src/lib/dns/rdata/in_1/aaaa_28.h
./src/lib/dns/rdata/in_1/srv_33.h
./src/lib/dns/rdata/in_1/a_1.h
./src/lib/dns/rdata/in_1/dhcid_49.h
./src/lib/dns/rdata/ch_3/a_1.h
./src/lib/dns/rdata/generic/ptr_12.h
./src/lib/dns/rdata/generic/hinfo_13.h
./src/lib/dns/rdata/generic/tkey_249.h
./src/lib/dns/rdata/generic/rp_17.h
./src/lib/dns/rdata/generic/mx_15.h
./src/lib/dns/rdata/generic/spf_99.h
./src/lib/dns/rdata/generic/ns_2.h
./src/lib/dns/rdata/generic/nsec3param_51.h
./src/lib/dns/rdata/generic/dlv_32769.h
./src/lib/dns/rdata/generic/soa_6.h
./src/lib/dns/rdata/generic/caa_257.h
./src/lib/dns/rdata/generic/cname_5.h
./src/lib/dns/rdata/generic/rrsig_46.h
./src/lib/dns/rdata/generic/tlsa_52.h
./src/lib/dns/rdata/generic/dname_39.h
./src/lib/dns/rdata/generic/nsec_47.h
./src/lib/dns/rdata/generic/nsec3_50.h
./src/lib/dns/rdata/generic/ds_43.h
./src/lib/dns/rdata/generic/dnskey_48.h
./src/lib/dns/rdata/generic/naptr_35.h
./src/lib/dns/rdata/generic/txt_16.h
./src/lib/dns/rdata/generic/afsdb_18.h
./src/lib/dns/rdata/generic/opt_41.h
./src/lib/dns/rdata/generic/sshfp_44.h
./src/lib/dns/rdata/generic/minfo_14.h
./src/lib/dns/rdata/hs_4/a_1.h
./src/lib/cryptolink/openssl_common.h
./src/lib/cryptolink/botan_common.h