packet.7: Describe SOCK_PACKET netif name length issues and workarounds.
Subsystems (3)
PATCH 1/1 packet.7: Describe SOCK_PACKET netif name length issues and workarounds.
Describe the issues with SOCK_PACKET possibly truncating network interface names in results, solutions and possible workarounds.
While the issue is know for a long time it appears to have never been documented properly and is has started to bite software antiques badly since the introduction of Predictable Network Interface Names. So let's document it.
--- man7/packet.7 | 31 ++++++++++++++++++++++++++++++- 1 file changed, 30 insertions(+), 1 deletion(-)
man7/packet.7 +30 -1
@@ -627,6 +627,34 @@ extension is an ugly hack and should be replaced by a control message. There is currently no way to get the original destination address of packets via .BR SOCK_DGRAM . +.PP +The +.I spkt_device +field of +.I sockaddr_pkt +has a size of 14 bytes which is less than the constant +.B IFNAMSIZ +defined in +.I <net/if.h> +which is 16 bytes and describes the system limit for a network interface +name. This means the names of network devices longer than 14 bytes will be +truncated to fit into +.I spkt_device . +All these lengths include the terminating null byte (\(aq\e0\(aq)). +.PP +Issues from this with old code typically show up with very long interface +names used by the +.B Predictable Network Interface Names +feature enabled by default in many modern Linux distributions. +.PP +The preferred solution is to rewrite code to avoid +.BR SOCK_PACKET . +Possible user solutions are to disable +.B Predictable Network Interface Names +or to rename the interface to a name of at most 13 bytes, for example using +the +.BR IP (8) +tool. .\" .SH CREDITS .\" This man page was written by Andi Kleen with help from Matthew Wilcox. .\" AF_PACKET in Linux 2.2 was implemented
@@ -637,7 +665,8 @@ packets via .BR capabilities (7), .BR ip (7), .BR raw (7), -.BR socket (7) +.BR socket (7), +.BR ip (8), .PP RFC\ 894 for the standard IP Ethernet encapsulation. RFC\ 1700 for the IEEE 802.3 IP encapsulation.
Hello Ralf,
Quoted 9 lines · Ralf Baechle
Describe the issues with
SOCK_PACKETpossibly truncating network interface names in results, solutions and possible workarounds.While the issue is know for a long time it appears to have never been documented properly and is has started to bite software antiques badly since the introduction of Predictable Network Interface Names. So let's document it.
Signed-off-by Ralf Baechle <ralf-6z/[email protected]>
Thanks for the patch!
Please see a few comments below.
Thanks,
Alex
man7/packet.7 extension is an ugly hack and should be replaced by a control message. · line 629
Quoted 12 lines
--- man7/packet.7 | 31 ++++++++++++++++++++++++++++++- 1 file changed, 30 insertions(+), 1 deletion(-)man7/packet.7 +0
-0@@ -627,6 +627,34 @@ extension is an ugly hack and should be replaced by a control message. There is currently no way to get the original destination address of packets via .BR SOCK_DGRAM .
Since the bug spreads across multiple paragraphs, maybe consider adding a subsection of BUGS to keep it organized?
man7/packet.7 extension is an ugly hack and should be replaced by a control message. · line 639
Quoted 10 lines
man7/packet.7 +10
-0+.PP +The +.I spkt_device +field of +.I sockaddr_pkt +has a size of 14 bytes which is less than the constant +.B IFNAMSIZ +defined in +.I <net/if.h> +which is 16 bytes and describes the system limit for a network interface
See the following extract from man-pages(7):
$ man 7 man-pages | sed -n '/Use semantic newlines/,/^$/p';
Use semantic newlines
In the source of a manual page, new sentences should be
started on new lines, and long sentences should split into
lines at clause breaks (commas, semicolons, colons, and so
on). This convention, sometimes known as "semantic new‐
lines", makes it easier to see the effect of patches, which
often operate at the level of individual sentences or sen‐
tence clauses.Quoted 7 lines
man7/packet.7 +7
-0+name. This means the names of network devices longer than 14 bytes will be +truncated to fit into +.I spkt_device . +All these lengths include the terminating null byte (\(aq\e0\(aq)). +.PP +Issues from this with old code typically show up with very long interface +names used by the
man7/packet.7 extension is an ugly hack and should be replaced by a control message. · line 656
Quoted 10 lines
man7/packet.7 +10
-0+.B Predictable Network Interface Names +feature enabled by default in many modern Linux distributions. +.PP +The preferred solution is to rewrite code to avoid +.BR SOCK_PACKET . +Possible user solutions are to disable +.B Predictable Network Interface Names +or to rename the interface to a name of at most 13 bytes, for example using +the +.BR IP (8)
You probably meant s/IP/ip/
Quoted 15 lines
man7/packet.7 +3
-1+tool. .\" .SH CREDITS .\" This man page was written by Andi Kleen with help from Matthew Wilcox. .\" AF_PACKET in Linux 2.2 was implemented@@ -637,7 +665,8 @@ packets via .BR capabilities (7), .BR ip (7), .BR raw (7), -.BR socket (7) +.BR socket (7), +.BR ip (8), .PP RFC\ 894 for the standard IP Ethernet encapsulation. RFC\ 1700 for the IEEE 802.3 IP encapsulation.
Signature · 3 lines
Alejandro Colomar Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/ http://www.alejandro-colomar.es/
Quoted 11 lines · Alejandro Colomar (man-pages)
Hello Ralf,
Quoted 8 lines · Ralf Baechle
Describe the issues with
SOCK_PACKETpossibly truncating network interface names in results, solutions and possible workarounds.While the issue is know for a long time it appears to have never been documented properly and is has started to bite software antiques badly since the introduction of Predictable Network Interface Names. So let's document it.
Quoted 4 lines
Quoted 1 line
Signed-off-by Ralf Baechle <ralf-6z/[email protected]>Thanks for the patch!
Please see a few comments below.
I'll make an updated version in a week or so. Until then I'm goning to be offline on civil defence duty in German's flood disaster region ...
Ralf