[conntrack-tools PATCH v2] conntrack.8: Document --stats counters

[Phil Sutter <phil@nwl.cc>]

Provide a brief description of each counter's meaning based on
code-analysis in kernel's nf_conntrack_core.c and feedback from
netfilter-devel list.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
Changes since v1:
- Update descriptions as per feedback from Florian
---
 conntrack.8 | 42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/conntrack.8 b/conntrack.8
index 2bfd80e5d6aa4..bc78c4823881c 100644
--- a/conntrack.8
+++ b/conntrack.8
@@ -108,7 +108,47 @@ Flush the whole given table
 Show the table counter.
 .TP
 .BI "-S, --stats "
-Show the in-kernel connection tracking system statistics.
+Show the in-kernel connection tracking system statistics. The returned values
+for each CPU are:
+.RS
+.TP
+.B found
+Number of times a tuple was already found and had to be adjusted  when setting
+up a new NAT mapping.
+.TP
+.B invalid
+Number of invalid (e.g., malformed or non-IP) packets encountered.
+.TP
+.B insert
+Number of conntrack entries manually inserted (via netlink or eBPF).
+.TP
+.B insert_failed
+Number of new connections dropped because of unresolvable clashes with existing
+entries.
+.TP
+.B drop
+Number of packets dropped due to memory pressure.
+.TP
+.B early_drop
+Number of connections dropped in an attempt to recover from a full conntrack
+table.
+.TP
+.B error
+Number of invalid ICMP/ICMPv6 packets received.
+.TP
+.B search_restart
+Number of table lookups which had to be restarted. In rare cases a lookup may
+encounter an already deleted entry which causes a search restart.
+.TP
+.B clash_resolve
+Number of entry insert clashes resolved. These happen frequently with DNS
+traffic and thus not neccessarily indicate a problem.
+.TP
+.B chaintoolong
+Number of oversized hash bucket encounters upon inserting a new conntrack
+entry. This is a fatal problem for conntrack and it will drop the packet as a
+consequence.
+.RE
 .TP
 .BI "-R, --load-file "
 Load entries from a given file. To read from stdin, "\-" should be specified.
-- 
2.54.0