warning: /etc/hosts.allow, line : host name/address mismatch

[tcpd]

とあるサーバで以下のLOGが出力されていた

warning: /etc/hosts.allow, line 11: host name/address mismatch: 123.31.2.155 != xxx.xxxx.jp

xxx.xxxx.jpはhosts.allowで許可しているドメイン。
IPを調べてみるとベトナムからのアクセスの模様。
どういう機能なのかマニュアルを見てみる

$ man tcpd

該当箇所抜粋

tcpd verifies the client host name that is returned by the address->name DNS server by  looking  at  the  host
name and address that are returned by the name->address DNS server.  If any discrepancy is detected, tcpd con‐
cludes that it is dealing with a host that pretends to have someone elses host name.

If the sources are compiled with -DPARANOID, tcpd will drop the connection in case of a host name/address mis‐
match.   Otherwise, the hostname can be matched with the PARANOID wildcard, after which suitable action can be
taken.

SEE ALSO
       hosts_access(5), format of the tcpd access control tables.

PARANOIDという機能らしい。
IPアドレスを名前解決して不一致だったアクセスを弾くようだ。
host_accessのマニュアルも見れと書いてあるのでこちらも確認

$ man 5 hosts_access

該当箇所抜粋

PARANOID
Matches  any  host  whose name does not match its address.  When tcpd is built with -DPARANOID (defaultmode), it drops requests from such clients even before looking at the  access  control  tables.   Build without -DPARANOID when you want more control over such requests.

デフォルトではPARANOIDは有効になっているようだ。