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Date: Sun, 14 Jun 2026 08:55:56 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: Fernando Fernandez Mancera <fmancera@suse.de>
Cc: netdev@vger.kernel.org, dsahern@kernel.org, davem@davemloft.net,
 edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org
Subject: Re: [PATCH iproute2-next] ipaddress: add support for showing IPv4
 devconf attributes
Message-ID: <20260614085556.1efda1eb@phoenix.local>
In-Reply-To: <3e4a425e-0f58-48dc-a2bc-88fd6eb4a302@suse.de>
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On Sat, 13 Jun 2026 09:22:38 +0200
Fernando Fernandez Mancera <fmancera@suse.de> wrote:

> On 6/13/26 8:41 AM, Fernando Fernandez Mancera wrote:
> > On 6/13/26 4:29 AM, Stephen Hemminger wrote: =20
> >> On Sat, 13 Jun 2026 01:17:22 +0200
> >> Fernando Fernandez Mancera <fmancera@suse.de> wrote:
> >> =20
> >>> tatic void print_inet(FILE *fp, struct rtattr *inet_attr)
> >>> +{
> >>> +=C2=A0=C2=A0=C2=A0 struct rtattr *tb[IFLA_INET_MAX + 1];
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0 parse_rtattr_nested(tb, IFLA_INET_MAX, inet_attr);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0 if (tb[IFLA_INET_CONF] && show_details) {
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 int *conf =3D RTA_DATA(tb=
[IFLA_INET_CONF]);
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 int max_elements =3D RTA_=
PAYLOAD(tb[IFLA_INET_CONF]) /=20
> >>> sizeof(int);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_FORWARDING)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "forwarding", "forwarding %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_FO=
RWARDING - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_MC_FORWARDING)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "mc_forwarding", "mc_forwarding=20
> >>> %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_MC=
_FORWARDING - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_PROXY_ARP)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "proxy_arp", "proxy_arp %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_PR=
OXY_ARP - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ACCEPT_REDIRECTS)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "accept_redirects",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "accept_redirects %s=
 ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AC=
CEPT_REDIRECTS - 1] ?=20
> >>> "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_SECURE_REDIRECTS)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "secure_redirects",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "secure_redirects %s=
 ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_SE=
CURE_REDIRECTS - 1] ?=20
> >>> "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_SEND_REDIRECTS)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "send_redirects",=20
> >>> "send_redirects %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_SE=
ND_REDIRECTS - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_SHARED_MEDIA)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "shared_media", "shared_media %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_SH=
ARED_MEDIA - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_RP_FILTER)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "rp_filter", "rp_filter %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_RP_FILTER - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ACCEPT_SOURCE_ROUTE)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "accept_source_route",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "accept_source_route=
 %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AC=
CEPT_SOURCE_ROUTE - 1] ?=20
> >>> "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_BOOTP_RELAY)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "bootp_relay", "bootp_relay %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_BO=
OTP_RELAY - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_LOG_MARTIANS)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "log_martians", "log_martians %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_LO=
G_MARTIANS - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_TAG)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "tag", "tag %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_TAG - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARPFILTER)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "arpfilter", "arpfilter %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AR=
PFILTER - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_MEDIUM_ID)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "medium_id", "medium_id %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_MEDIUM_ID - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_NOXFRM)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "noxfrm", "noxfrm %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_NO=
XFRM - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_NOPOLICY)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "nopolicy", "nopolicy %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_NO=
POLICY - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_FORCE_IGMP_VERSION)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "force_igmp_version",=20
> >>> "force_igmp_version %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_FORCE_IGMP_VERSION -=
 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARP_ANNOUNCE)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "arp_announce", "arp_announce %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_ARP_ANNOUNCE - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARP_IGNORE)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "arp_ignore", "arp_ignore %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_ARP_IGNORE - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_PROMOTE_SECONDARIES)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "promote_secondaries",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "promote_secondaries=
 %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_PR=
OMOTE_SECONDARIES - 1] ?=20
> >>> "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARP_ACCEPT)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "arp_accept", "arp_accept %d ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_ARP_ACCEPT - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARP_NOTIFY)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "arp_notify", "arp_notify %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AR=
P_NOTIFY - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ACCEPT_LOCAL)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "accept_local", "accept_local %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AC=
CEPT_LOCAL - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_SRC_VMARK)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "src_vmark", " src_vmark %s",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_SR=
C_VMARK - 1] ? "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_PROXY_ARP_PVLAN)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "proxy_arp_pvlan",=20
> >>> "proxy_arp_pvlan %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_PR=
OXY_ARP_PVLAN - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ROUTE_LOCALNET)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "route_localnet",=20
> >>> "route_localnet %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_RO=
UTE_LOCALNET - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_BC_FORWARDING)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "bc_forwarding", "bc_forwarding=20
> >>> %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_BC=
_FORWARDING - 1] ? "on" :=20
> >>> "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D=20
> >>> IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "igmpv2_unsolicited_report_interval",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "igmpv2_unsolicited_report_interval %d=
 ",
> >>> +                 =20
> >>> conf[IPV4_DEVCONF_IGMPV2_UNSOLICITED_REPORT_INTERVAL - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D=20
> >>> IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_int(PRINT_ANY, "igmpv3_unsolicited_report_interval",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "igmpv3_unsolicited_report_interval %d=
 ",
> >>> +                 =20
> >>> conf[IPV4_DEVCONF_IGMPV3_UNSOLICITED_REPORT_INTERVAL - 1]);
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_IGNORE_ROUTES_WITH_LINKDOWN)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "ignore_routes_with_linkdown",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "ignore_routes_with_=
linkdown %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_IG=
NORE_ROUTES_WITH_LINKDOWN -=20
> >>> 1] ?
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_DROP_UNICAST_IN_L2_MULTICAST)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "drop_unicast_in_l2_multicast",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "drop_unicast_in_l2_=
multicast %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_DR=
OP_UNICAST_IN_L2_MULTICAST=20
> >>> - 1] ?
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_DROP_GRATUITOUS_ARP)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "drop_gratuitous_arp",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "drop_gratuitous_arp=
 %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_DR=
OP_GRATUITOUS_ARP - 1] ?=20
> >>> "on" : "off");
> >>> +
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (max_elements >=3D IPV=
4_DEVCONF_ARP_EVICT_NOCARRIER)
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 p=
rint_string(PRINT_ANY, "arp_evict_nocarrier",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 "arp_evict_nocarrier=
 %s ",
> >>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 conf[IPV4_DEVCONF_AR=
P_EVICT_NOCARRIER - 1] ?=20
> >>> "on" : "off");
> >>> +=C2=A0=C2=A0=C2=A0 }
> >>> +}
> >>> + =20
> >> There are three different ways to display a flag value in JSON used in=
=20
> >> iproute2.
> >> This one is my least favorite.
> >>
> >> The three ways are:
> >> =C2=A0=C2=A0=C2=A0 - print_bool
> >> =C2=A0=C2=A0=C2=A0 - print_null (only if on)
> >> =C2=A0=C2=A0=C2=A0 - print_string
> >>
> >> I would use the print_null pattern but print_bool would also be ok.
> >> =20
> >=20
> > Thanks for the suggestion Stephen, I would pick print_bool in this case.
> >=20
> > If one of the options evolves to supporting something else we could=20
> > easily adapt it without breaking compatibility if we use print_bool. If=
=20
> > we use print_null I don't think we could do that.
> >  =20
>=20
> Hm. I am actually not so sure about this..
>=20
> the current print_string approach matches the setter and also the=20
> netconf side. While print_bool would be easier to parse for JSON, it=20
> looks not so good for command line output.
>=20
> print_null presents a different problem, users would need to make sure=20
> their parsing is working with an iproute2 version that support these new=
=20
> attributes.
>=20
> So I am not so sure what is the best option here.

None of this is a hard requirement. The requirement is that the output
be valid JSON. The other common practices are:
  - non JSON output of display should match the input command line
    there were even some user tools that depended on this to do save/restor=
e of state
  - JSON output should be easy to parse in python.

If user is using python to read JSON (which seems like the most common).
then bool or presence allows for use in conditional.



>>> import json
>>> json.loads('{"forwarding": true}')["forwarding"]
True                          # <class 'bool'>
>>> json.loads('{"forwarding": "on"}')["forwarding"]
'on'        =20