Adding new socket options - specifically socket.IPV6_HDRINCL

Rosuav · 2025-11-20T06:46:33.185Z

On Linux, you can create a raw IPv4 socket and send something out with your own hand-crafted IP header. To do this, you need to sock.setsockopt(socket.AF_INET, socket.IP_HDRINCL, 1) to prevent the normal IP header from being added.

On IPv6 sockets, this can still be done, but there’s no socket.IPV6_HDRINCL constant. The number to use is 36 (not the same as IP_HDRINCL), and calling setsockopt does have the correct effect, since the underlying kernel does support this.

How do new socket option constants get added? Can this one get added? For the time being, I’m just using the raw integer, but that’s ugly.

encukou · 2025-11-20T07:25:34.508Z

How do new socket option constants get added?

See #120058 for example. Ping me for review, or if you need any help.

Rosuav · 2025-11-20T07:37:07.498Z

Petr Viktorin:

See #120058 for example.

Ah cool, thanks. Will get to that shortly!

Rosuav · 2025-11-21T10:19:25.641Z

Issue: Add socket constant IPV6_HDRINCL and maybe others · Issue #141817 · python/cpython · GitHub
PR: gh-141817: Add IPV6_HDRINCL constant to the socket module by Rosuav · Pull Request #141818 · python/cpython · GitHub

I made a script to search the Linux header files for more constants. How many of these should also be added? There are a good many. ^[1]

#ifndef _LINUX_IN_H
Found a define: IP_ROUTER_ALERT 5 
Found a define: IP_PKTOPTIONS 9 
Found a define: IP_MTU_DISCOVER 10 
Found a define: IP_MTU 14 
Found a define: IP_IPSEC_POLICY 16 
Found a define: IP_XFRM_POLICY 17 
Found a define: IP_PASSSEC 18 
Found a define: IP_ORIGDSTADDR 20 
Found a define: IP_MINTTL 21 
Found a define: IP_NODEFRAG 22 
Found a define: IP_CHECKSUM 23 
Found a define: IP_RECVFRAGSIZE 25 
Found a define: IP_RECVERR_RFC4884 26 
Found a define: IP_PMTUDISC_DONT 0 /* Never send DF frames */
Found a define: IP_PMTUDISC_WANT 1 /* Use per route hints	*/
Found a define: IP_PMTUDISC_DO 2 /* Always DF		*/
Found a define: IP_PMTUDISC_PROBE 3 /* Ignore dst pmtu      */
Found a define: IP_PMTUDISC_INTERFACE 4 
Found a define: IP_PMTUDISC_OMIT 5 
Found a define: IP_MSFILTER 41 
Found a define: MCAST_JOIN_GROUP 42 
Found a define: MCAST_BLOCK_SOURCE 43 
Found a define: MCAST_UNBLOCK_SOURCE 44 
Found a define: MCAST_LEAVE_GROUP 45 
Found a define: MCAST_JOIN_SOURCE_GROUP 46 
Found a define: MCAST_LEAVE_SOURCE_GROUP 47 
Found a define: MCAST_MSFILTER 48 
Found a define: IP_MULTICAST_ALL 49 
Found a define: IP_UNICAST_IF 50 
Found a define: IP_LOCAL_PORT_RANGE 51 
Found a define: IP_PROTOCOL 52 
Found a define: MCAST_EXCLUDE 0 
Found a define: MCAST_INCLUDE 1 
#if  __UAPI_DEF_SOCKADDR_IN
Found a define: __SOCK_SIZE__ 16 /* sizeof(struct sockaddr)	*/
#endif
#if __UAPI_DEF_IN_CLASS
Found a define: IN_CLASSA_NSHIFT 24 
Found a define: IN_CLASSA_MAX 128 
Found a define: IN_CLASSB_NSHIFT 16 
Found a define: IN_CLASSB_MAX 65536 
Found a define: IN_CLASSC_NSHIFT 8 
Found a define: IN_CLASSE_NSHIFT 0 
Found a define: IN_LOOPBACKNET 127 
#endif
#endif /* _LINUX_IN_H */
#ifndef _LINUX_IN6_H
Found a define: IPV6_FL_A_GET 0 
Found a define: IPV6_FL_A_PUT 1 
Found a define: IPV6_FL_A_RENEW 2 
Found a define: IPV6_FL_F_CREATE 1 
Found a define: IPV6_FL_F_EXCL 2 
Found a define: IPV6_FL_F_REFLECT 4 
Found a define: IPV6_FL_F_REMOTE 8 
Found a define: IPV6_FL_S_NONE 0 
Found a define: IPV6_FL_S_EXCL 1 
Found a define: IPV6_FL_S_PROCESS 2 
Found a define: IPV6_FL_S_USER 3 
Found a define: IPV6_FL_S_ANY 255 
#if __UAPI_DEF_IPPROTO_V6
Found a define: IPPROTO_MH 135 /* IPv6 mobility header		*/
#endif /* __UAPI_DEF_IPPROTO_V6 */
Found a define: IPV6_TLV_PAD1 0 
Found a define: IPV6_TLV_PADN 1 
Found a define: IPV6_TLV_ROUTERALERT 5 
Found a define: IPV6_TLV_CALIPSO 7 /* RFC 5570 */
Found a define: IPV6_TLV_IOAM 49 /* RFC 9486 */
Found a define: IPV6_TLV_JUMBO 194 
Found a define: IPV6_TLV_HAO 201 /* home address option */
#if __UAPI_DEF_IPV6_OPTIONS
Found a define: IPV6_ADDRFORM 1 
Found a define: IPV6_2292PKTINFO 2 
Found a define: IPV6_2292HOPOPTS 3 
Found a define: IPV6_2292DSTOPTS 4 
Found a define: IPV6_2292RTHDR 5 
Found a define: IPV6_2292PKTOPTIONS 6 
Found a define: IPV6_2292HOPLIMIT 8 
Found a define: IPV6_AUTHHDR 10 /* obsolete */
Found a define: IPV6_FLOWINFO 11 
Found a define: IPV6_ADD_MEMBERSHIP 20 
Found a define: IPV6_DROP_MEMBERSHIP 21 
Found a define: IPV6_ROUTER_ALERT 22 
Found a define: IPV6_MTU_DISCOVER 23 
Found a define: IPV6_MTU 24 
Found a define: IPV6_JOIN_ANYCAST 27 
Found a define: IPV6_LEAVE_ANYCAST 28 
Found a define: IPV6_MULTICAST_ALL 29 
Found a define: IPV6_ROUTER_ALERT_ISOLATE 30 
Found a define: IPV6_RECVERR_RFC4884 31 
Found a define: IPV6_PMTUDISC_DONT 0 
Found a define: IPV6_PMTUDISC_WANT 1 
Found a define: IPV6_PMTUDISC_DO 2 
Found a define: IPV6_PMTUDISC_PROBE 3 
Found a define: IPV6_PMTUDISC_INTERFACE 4 
Found a define: IPV6_PMTUDISC_OMIT 5 
Found a define: IPV6_FLOWLABEL_MGR 32 
Found a define: IPV6_FLOWINFO_SEND 33 
Found a define: IPV6_IPSEC_POLICY 34 
Found a define: IPV6_XFRM_POLICY 35 
Found a define: IPV6_HDRINCL 36 
#endif
#if 0	/* not yet */
Found a define: IPV6_USE_MIN_MTU 63 
#endif
Found a define: IPV6_AUTOFLOWLABEL 70 
Found a define: IPV6_ADDR_PREFERENCES 72 
Found a define: IPV6_MINHOPCOUNT 73 
Found a define: IPV6_ORIGDSTADDR 74 
Found a define: IPV6_TRANSPARENT 75 
Found a define: IPV6_UNICAST_IF 76 
Found a define: IPV6_RECVFRAGSIZE 77 
Found a define: IPV6_FREEBIND 78 
#endif /* _LINUX_IN6_H */

I’m happy to add any more that are of interest, while we’re doing the whole adding of constants thing.

One might even say there are a thundering lot. ↩︎

vstinner · 2025-11-22T17:15:55.206Z

What is the purpose of these constants? Are they commonly used? Usually I prefer to only add constants when someone comes with an use case.

Rosuav · 2025-11-22T20:14:06.396Z

Victor Stinner:

What is the purpose of these constants? Are they commonly used?

I have no idea - but then, up until a week ago, that would have been my response about IPV6_HDRINCL too.

Victor Stinner:

Usually I prefer to only add constants when someone comes with an use case.

That’s fair. The downside is, not everyone knows how to get more constants added, and maybe won’t even know that the constants are missing. It’s like when Alice noted that the Hatter was missing a few parts, and he said that, things being what they were, he hardly missed them. ^[1]

At the time, his head wasn’t even attached to his torso, never mind his arms and legs. It all makes sense in context… ↩︎